Window System API

Note: See the changes document. Window system implementation changed is major version. Comparing to the older version it has changed its laout significantly, introduced group, and released support of workspaces

Overview

# Javadoc

The Javadoc is present in the `org.openide.windows` package. Most module authors will want to look at `TopComponent` to subclass it, or perhaps at `CloneableTopComponent`.

# Contents

- Overview of the Window System
  - Top components and docking
  - Cloning
  - Groups
- Creating a Top Component
  - Subclassing `TopComponent`
  - Finding modes
  - Creating a cloneable component
  - Handling focus, and the node selection
  - Special support for serialization
- Manipulating Existing Windows
  - Finding modes, top components and groups
  - Giving focus and closing
  - Listening to window system events
- XML Persistence Format and Installation
  - Overview
  - Physical Layout and File Formats
  - Visual Display
  - Programmatic Manipulation
  - Common Scenarios
- UML diagrams
  - Overall structure class diagram
  - Input-Output class diagram

# Window System API

The Window System API provides fairly abstract support for creating windows or window-like container components that may be handled smoothly by the NetBeans window manager implementation.

## Overview of the Window System

As a rule, modules should not create their own top-level windows (e.g. `java.awt.Window`), since these would exist without the knowledge of the IDE's window manager. This window manager is capable of manipulating application windows and panels, including docking and undocking them into tabbed frames, and making the window configuration persistent across sessions. Well-behaved modules will use the API so as to integrate nicely into the feel of the rest of the IDE.

### Top components and docking

A top component is a Swing component (usually a panel or the like, though not necessarily); it might be docked (added into windows system) into one tab of a multi-tabbed window. The top component has to be docked, in order to be manipulated by window system. If the top component isn't docked it can be used as an ordinary Swing JComponent, e.g by adding it another container (possibly docked top component) without window system knowledge about it. Almost everything visible in the IDE is a top component, except the Main Window (which is treated specially because of its centrality), and dialog boxes.

The API provides access to a set of modes, each of which can contain some top components. The API user cannot create modes directly. She can achieve that by specifying them in a XML layer.

Top components have some special trimming, they may have a popup menu of actions (e.g. Save, Close, ...) which can be displayed in the tab. Top components may provide special actions in this list as well.

At any given time, one top component is active. It will be the last docked top component containing focus, and of course will be in the selected tab. This component serves as the base for a number of things in the IDE; for example, the current node selection is controlled by the active component.

### Cloning

Some top components can be cloned, meaning that a new top component created which initially shares the same contents. Exactly how the cloning works depends on the top component, but typically the same data will be referred to and the cloned component will simply be a new view on it. For example, Editor panes can be cloned with the effect that the same file will be open in each view.

### Groups

The IDE starts off with the groups "form", "debugger", etc. to group windows according to their expected applicability. The groups manage opening and closing of top components belonging to the group accodring the user work flow. User can influence which of the top component should be opened at the group opening, or remamained opened at the group closing etc. The group is usualy opened/closed when user starts/finishes some specific taks, like GUI editing and debugging.

The IDE tries to save the current window configuration when it exits, including the positions of all modes, the arrangement of modes, and the components present in each mode; and grouos, their state; and then restore this configuration when next started.

## Creating a Top Component

Creating a custom top component, including assigning it to the proper docking mode, is not generally difficult.

### Subclassing `TopComponent`

To create a simple top component, it suffices to subclass `TopComponent`. This is a subclass of `JComponent`, which means that it is possible to draw on it using a variety of mechanisms; typically, it is treated as a container, and its layout may be set and components added to it.

There are a few general ways in which you can customize the component besides painting subcomponents on it:

- You may set a display name and icon for the component, affecting its appearance in window title bars and tabs. These ought generally be set in the constructor.
- `TopComponent.getHelpCtx()` should be overridden to provide a help link for the component as a whole, if you have any context help. Or, subcomponents may have their own more specific help. By default, context help is taken from the activated node selection, if there is any to be found.
- `TopComponent.getUndoRedo()` allows you to associate undo/redo actions with the component, so that these actions will work properly when your component is focussed.
- `TopComponent.getActions()` permits you to provide a list of actions which will appear in a popup menu, e.g. on a tab for the component. The system should provide a few standard actions such as "Close"; you may want to add more (for example, "Save" for an Editor pane). The Actions API describes how to write these.

### Finding modes

Finding modes is straightforward.

You cannot create mode directly, however you can specify new mode in XML layer. Then you can find such a mode by call `WindowManager.findMode(String)` . Do not keep references to modes, Rather use the above method to access them. Modes could be removed from window system inbetween, thus manipulating with such a mode wouldn't achieve desired goal.

To add top components to a mode, just call `Mode.dockInto(TopComponent)`. For example:

    public static class MyComponent extends TopComponent {
        public MyComponent(Object data, Image icon) {
            setName(NbBundle.getMessage(This.class, "LBL_widget_tree"));
            setIcon(icon);
            setLayout(new BorderLayout());
            add(new JTree(createModel(data)), BorderLayout.CENTER);
        }
        public Action[] getActions() {
            List[] actions = Arrays.asList(super.getctions());
            actions.add(new WidgetReparseAction());
            return (Action[])actions.toArray(new Action[0]);
        }
    }

    // ...
    TopComponent myComponent1 = new MyComponent(firstData, firstIcon);
    TopComponent myComponent2 = new MyComponent(secondData, secondIcon);
    Mode myMode = WindowManager.getDefault().findMode("myWidgetsMode");
    if (myMode != null) {
        myMode.dockInto(myComponent);
        myMode.dockInto(myComponent2);
    } else {
        // Mode was not found, i.e. it is not specified in XML layer, or some problem occured.
        // Then the TopComponent will be opened in default editor mode.
    }
    myComponent.open();
    myComponent2.open();
    myComponent2.requestActive();

### Creating a cloneable component

Just by subclassing `CloneableTopComponent`, you can create a component which is capable of cloning itself, like the Editor does with the popup action "Clone View". If `Object.clone()` takes care of all of your instance state satisfactorily, then you need do little more; otherwise, you can override `CloneableTopComponent.createClonedObject()` to specify exactly what fields should be shared with the original top component. Typically all clones of a component should share any underlying data object (e.g. edited files), but they may each have different display parameters or other noncritical settings.

You may specify how these cloned windows act when they are closed, by overriding `CloneableTopComponent.closeLast()`. (There are more general methods for all top components pertaining to closing them; this method is specific to cloneable top components.) It will be called when the last clone of a component is about to be closed. Components keeping some sort of user data, such as the Editor, should offer to save it here, and also shut down the editing system for that file. If you do not wish to close the last clone (for example, Cancel was pressed on a save dialog), just return `false`.

Other methods allow you to keep track of when new clones are created, and to find the sister clones of a top component, if that is needed. As an example, you could provide a component action which would cause all visible clones to display a different part of the content of the data simultaneously, so the user could use the screen more effectively.

### Handling focus, and the node selection

There are several more ways in which top components can interact with the desktop smoothly.

#### Focus/activation

Focus works naturally with top components: the focussed multitabbed window has the activated component on its selected tab.

You can explicitly request that a top component be activated by calling `TopComponent.requestActive()`. (It should be opened first.) `TopComponent.getRegistry()` and then `TopComponent.Registry.getActivated()` correspondingly finds the last-activated component.

#### The node selection

Finally, each top component may have associated with it a node selection, which is simply a list of `Node`s that it decides to treat as "active". The node selection will have effects on other parts of the system - for example, `NodeAction`s and `CookieAction`s pay attention to it.

The selection may be set using `TopComponent.setActivatedNodes(...)`.

Explorer views embedded in as described at `ExplorerUtils` automatically make the node selection track user selections in the view, as you would expect. Also, any top component associated with a data object will automatically select that object's node delegate, which is usually the intuitive behavior as well.

### Special support for serialization

If you are writing a top component with a complex configuration or other instance state not associated with a data object, you may want to specially support its serialization, so that the same configuration will be restored after IDE restart. This is not difficult to do; you just need to hook into the externalization methods of `TopComponent`:

    public class FlippableView extends TopComponent {
        public static int HORIZ_ORIENT = 1;
        public static int VERT_ORIENT = 2;
        private int orient = HORIZ_ORIENT;
        public int getOrientation() { /* ... */ }
        public void setOrientation(int ornt) { /* ... */ }
        public void writeExternal(ObjectOutput oo) throws IOException {
            super.writeExternal(oo);
            oo.writeInt(getOrientation());
        }
        public void readExternal(ObjectInput oi) throws IOException, ClassNotFoundException {
            super.readExternal(oi);
            setOrientation(oi.readInt());
        }
    }

It is desirable to store as much of your component's configuration as possible (if you can do so safely, i.e. without triggering an exception typically during `readExternal`). For example, Editor windows will store the open file and cursor position; Explorer windows, the current node selection and expanded path; etc. If part of the configuration you wish to store is some piece of serializable data that you are not completely confident can be deserialized without error, please instead store a `NbMarshalledObject` wrapping the data, which will protect the object streams from being corrupted just because of one component. For example:

    // Possible that you may break serialization of this class accidentally:
    private MySerObject state;
    // ...
    public void writeExternal(ObjectOutput oo) throws IOException {
        super.writeExternal(oo);
        Object toWrite;
        try {
            toWrite = new NbMarshalledObject(state);
        } catch (Exception e) {
            ErrorManager.getDefault().notify(ErrorManager.WARNING, e);
            toWrite = null;
        }
        oo.writeObject(toWrite);
    }
    public void readExternal(ObjectInput oi) throws IOException, ClassNotFoundException {
        super.readExternal(oi);
        NbMarshalledObject read =(NbMarshalledObject)oi.readObject();
        if (read != null) {
            try {
                state = (MySerObject)read.get();
            } catch (Exception e) {
                ErrorManager.getDefault().notify(ErrorManager.WARNING, e);
            }
        }
    }

This example assumes that your component can survive a restart even without setting this piece of its state correctly (it can just use some default settings). If the component cannot be validly recreated without this information, still use an `NbMarshalledObject`, but do not throw the original exception from it to the `writeExternal` or `readExternal` callers - this will make the whole window system stream be treated as corrupt and discarded! Instead, use:

    try {
        // new NbMarshalledObject(obj) or nbmo.get()
    } catch (Exception e) {
        throw new SafeException(e);
    }

This will cause your top component to not be stored or loaded, but other components in the system will be unaffected.

The default implementation of the read and write methods must always be called. It stores the name and some internal information pertaining to the Window System. You must save the icon yourself, though most users will set the icon in the constructor. Remember that a `TopComponent` must have a default constructor in order to be deserialized. In older versions of the IDE this needed to be public; this is no longer necessary.

Persistent singleton implementations (i.e. where only one instance of the class should exist in the IDE) are possible; just remember to assign the default instance both in the default constructor, and also in the `readResolve` method. To force deserialization of singleton instance by window system method `WindowManager.findTopComponent(String)` must be used. Pass unique TopComponent ID (name of settings file) as parameter to this method. To be able to create singleton instance 2 public static accessor methods must be provided: First eg. `getDefault` reserved for window system will be given as creation method in settings file and also called from `readResolve`. `getDefault` creates singleton instance using constructor as usual in common singleton accessor. Second eg. `findDefault` will be used as normal accessor method to get correctly deserialized singleton instance by window system. It can be used to access singleton instance for example by view action. `findDefault` will call `WindowManager.findTopComponent`. As `WindowManager.findTopComponent` can return `null` `findDefault` should handle such case somehow eg. by calling of `getDefault`. There is a simple example module where persistent singleton instance is defined in module layer as described here.

It is possible to use `writeReplace` and `readResolve` as well for some advanced uses - but be very careful to resolve to a subclass of `TopComponent`, and to always invoke the default implementations of `readExternal` and `writeExternal`.

#### TopComponent persistence type

It is recommended to explicitely set the persistence type of a TopComponent subclass by overriding API method `TopComponent.getPersistenceType`.

This method can return one of following constants:

`TopComponent.PERSISTENCE_NEVER`
  The system will not try to serialize your component. Suitable for transient windows that the user would not generally wish to save across IDE restarts as well as for windows containing data which cannot by its nature be persisted correctly.
`TopComponent.PERSISTENCE_ONLY_OPENED`
  The system will only try to serialize your component if it was open at the time. Suitable for many types of windows that should be saved if visible, but for which there is no compelling reason to keep information about the window state (docked mode and so on) when closed.
`TopComponent.PERSISTENCE_ALWAYS`
  The system will try to serialize your component always. Suitable for many types of windows that should be saved even if closed. User can close and reopen such component anytime even after IDE restart. Such components are often singletons and have view action so user is able to open/reopen component.

By default all top components are persisted but it is recommended not to rely on default but to set persistence type explicitely as described above.

## Manipulating Existing Windows

Most manipulation of existing windows should be kept to a minimum, of course, to avoid disrupting the user needlessly. Occasionally it makes sense to do a few things to top components externally.

### Finding modes, top components and groups

A few method calls may be used to find modes, groups and top components:

- `WindowManager` instance is obtained by `WindowManager.getDefault()`.
- The modes in a window systeme may be gotten with `WindowManager.getModes()` or `WindowManager.findMode(String)` (or also `WindowManager.findMode(TopComponent)`).
- Each mode has some top components in it, obtainable with `Mode.getTopComponents()`.
- Groups can be retrieved via `WindowManager.findTopComponentGroup(String)`
- To get the currently selected top component, use `TopComponent.getRegistry()` followed by `TopComponent.Registry.getActivated()`. You can also get all opened components with `TopComponent.Registry.getOpened()`.

### Activating and closing

Any top component may be activated just by calling `TopComponent.requestActive()`.

To close a top component programmatically, call `TopComponent.close()`.

### Listening to window system events

Most interesting aspects of the Window System may be listened to using the Java Event model. Most commonly, this is done to listen to changes in the activated node selection, but other things are possible too.

- All properties of the window manager (i.e. contained modes) may be listened to with `WindowManager.addPropertyChangeListener(PropertyChangeListener)`.
- All properties of the top component registry (i.e. available and selected components, and the node selection) may be listened to with `TopComponent.Registry.addPropertyChangeListener(PropertyChangeListener)`.
- Currently, there is no support for listening to changes in `Mode` nor in `TopComponent`, though both have modifiable properties which in principle there might be a need to listen to.

## XML Persistence Format and Installation

### Overview

The window system layout used by the IDE is based on modes.

The was needed in two areas - modules and initial layout. What does this mean? Modules had limited possibilities to control layout of their modes. Also, the initial layout of system modes was hardcoded in core sources, which was not sufficient due to the nature of this information, which is likely to change frequently. Modifications of initial layout needed to be easier.

#### Justification

Main goals comes directly from the issues that were mentioned in motivation section:

Enhance module control over modes layout

  Modules will have full control over layout of own modes, including frame MDI constraints.

Easy initial layout modifications

  Initial layout modifications should be as easy as possible, without changes in source code.

Less hard-coding in core

  Core implementation currently specifies nearly all initial layout. However, right way to go is to let modules specify complete layout of their modes, such as inspector, palette, debugger etc.

Expert users

  Allow expert users to read, understand, modify, backup and make versions of stored window system content.

From above points, it's clear that description of modes layout should cover all features that window system offers (point 1.) and should be expressed in some human readable form (point 2, 4.). Mentioned requirements fits well into XML layers architecture, which is already supported by Netbeans' Open APIs.

Solution backbone reads like this:

- Each module will have a possibility to define modes layout described in their module layer, expressed in XML document.

- Core itself will define basic layout for explorer and editor modes (etc.), other modes will be specified directly by responsible modules.

- Solution will use advantages of XML layers architecture, modes defined by individual modules and core will be merged into whole modes picture which will define overall content of window system.

- WS implementation will read and understand XML mode layout format.

- WS implementation will make its content persistent using XML layers support too.

Example of simple window system content expressed as merge of modes layouts from different modules is shown on picture below:

Note: The same applies for groups, but those are not dealing with laout, but rather opening and closing of component.

Above picture shows simple situation when modules (in this case form and debugger) define just their modes and nothing more. In reality, modules will want to influence other modes, especially modes created by core implementation.
To support such features, modes layout description format has to be well organized and granularized, as described in next paragraph.

### Physical Layout and File Formats

In first part of this chapter, overall structure of layout configuration is discussed. XML layers architecture uses directory-like structure for storing information. Directory structure itself is expressed in XML syntax, creating picture of 'virtual filesystem' stored in XML file.
Further, exact format of folders and files which hold layout information is specified, together with examples.

#### Storage of Configurations

Physical storage of layout configuration documents is determined by layers support. Until modified, layout configuration files lives in modules that defined them. More exactly, they are read from files which are stored in module jar archives, together with other module classes.

However, modes layout configuration is likely to change frequently during runtime of the system, so then modifications will be stored under (for example) the /system subdirectory of current userdir, as shows example directory tree below. Described mechanism is fully driven by XML layers support provided by Filesystems API and we needn't care about it more from the perspective of window system. (The precise details of where customizations are stored is dependent on the implementation; suffice it to say that the default file system of the repository is responsible for holding the files and applying any changes made.)

#### Directory structure

To effectively use XML layers, it's necessary to define directory structure for modes layout description. Following directory structure is fixed and in fact it represents part of API modules can deal with. It means that this structure will not change in incompatible manner and modules can rely on it. Again, the specification is of resource paths in the system filesystem rather than details of how this filesystem is composed and modified.

Window system is not part of individual projects as it was in previous releases. Window system configuration is defined by modules in folder Windows2 and is stored localy under $NBUSERDIR/system/Windows2Local. It means that default initial configuration defined by modules and customized configuration are separated to different folders.

       See Structure of configuration data.

##### Modes

There exist direct mapping between instances of classes which are defined in traditional winsys API and folders and files shown above. Folders and files named "Mode_X" under `Modes/` define modes (= component containers) belonging to window system, and map to instances of org.openide.windows.Mode (by which it is meant that the corresponding data objects have an appropriate instance cookie).

Naming of folders and files is important, because folder and file names are used as unique identifiers for modes and groups. Basically it means that winsys API methods Mode.getName() will return the same names as used for folders and files.

##### Folder - data file pairs

Folders for window manager, for each mode have to be stored together with their properties, for example display name or icon URL. This is done using "folder - file pair", as evident from example directory tree above. Folders cannot hold their properties directly, that's why each folder is accompanied by file on the same directory level, which has the same name as folder and stores property information for its folder.
Note, window system will recognize only folder - file pairs with exactly same names.

##### Top Components

Components introduce a bit of complexity to the directory structure, because one top component can be a mode(=component containers) and also can by in some group. To handle that sharing correctly, modes and groups contains only references to components, Component's data are stored in separate folder named `Windows2/Components/`. Example of top component reference is file TC_1.wstcref or TC_1.wstcgrp shown above in directory tree. Top component data needs to be referenced through some unique identifiers, treated as unique IDs.

##### Structure Depth

Directory structure of winsys configuration is fine-grained in the sense that each mode and group is represented by a folder, while each presence of top component is represented by special file. Such design will enhance module possibilities of configuring modes ang groups defined by other modules or by core implementation.

Detailed explanation: Imagine that debugger module wants to specify size, location, constraints of debugger window not only on its own debugger mode, but also on output mode. So debugger module specifies folder 'debugger', with proper 'debugger.wsmode' file content, under folder 'output'. Note that debugger module must not define 'execution.wsmode', because debugger module is not defining output mode, debugger is only completing output mode if present. As a result of layers merge done by system, 'debugger.wstcref' will appear as file under 'output'.

If there were no folders for frames, this operation would not be possible, because layering is done on folder and file level, not on file contents level.

Modules now can:

Create their own modes

  Module builds its own modes and layout from the scratch by creating new folder and data file for mode. Module can put own components on such mode. Other modules can 'complete' such mode by their own components.

Create their own groups

  Module builds its own groups and layout from the scratch by creating new folder and data file for group. Module can put own components on such group. Other modules can 'complete' such group by their own components.

Add components to foreign modes

  Module can install a component into mode that was defined by other module or core implementation. To achieve this, module is required to create folder named by foreign mode, together with file '.wstcref' referencing its component.
  Note that module which is using other module's mode may not define '.wsmode' data file for such 'foreign' mode, because module which define folder and '.wsmode' file for mode then owns that mode and there cannot be multiple modules owning the same mode.

Add components to foreign groups

  Module can install a component into group that was defined by other module or core implementation. To achieve this, module is required to create folder named by foreign group, together with file '.wstcgrp' referencing its component.
  Note that module which is using other module's group may not define '.wsgrp' data file for such 'foreign' group, because module which define folder and '.wsgrp' file for group then owns that group and there cannot be multiple modules owning the same group.

Override winsys elements defined by other modules

  As layers can overlap and hide information between each other, modules can also modify and/or hide winsys layout elements defined in other modules. However, module must depend on other module whose winsys layout it overrides, as is generally required when overriding layer-provided files.

#### Global properties

Window system as a whole stores several properties which have global nature. They are always defined by a folder `Windows/WindowManager/` and data file `Windows/WindowManager.wswmgr`. Core implementation defines default values of global properties for win sys. Format of data file is simple:

    <?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE windowmanager PUBLIC
              "-//NetBeans//DTD Window Manager Properties 2.0//EN"
              "http://www.netbeans.org/dtds/windowmanager-properties2_0.dtd">
    <windowmanager version="2.0">
        <!-- size and location of the main window -->
        <main-window>
             <joined-properties centered-vertically="true" centered-horizontally="true"
                                relative-width="0.8" relative-height="0.8"/>
        </main-window>
        <!-- size of the screen last displayed on -->
        <screen width="1024" height="768"/>
        <!-- reference to active mode -->
        <active-mode name="explorer"/>
        <!-- reference to maximized mode -->
        <maximized-mode name="explorer"/>
        <!-- reference to toolbar configuration -->
        <toolbar configuration="Standard"/>
    </windowmanager>

Modules will typically never need to access these global properties, and modules should never try to change them.

#### Modes

Modules can either define own mode from scratch or reference mode defined by other modules.

##### Mode definition

Module defines mode by creating proper mode folder `Windows2/Modes/MODE_UID/` and data file `Windows2/Modes/MODE_UID.wsmode`.

Here is example of valid mode data file:

    <?xml version="1.0" encoding="UTF-8"?>

    <!DOCTYPE mode PUBLIC
              "-//NetBeans//DTD Mode Properties 2.0//EN"
              "http://www.netbeans.org/dtds/mode-properties2_0.dtd">

    <mode version="2.0">
        <module name="org.netbeans.core.ui/1" spec="1.2" />
        <name unique="explorer" />
        <kind type="view" />
        <state type="joined" />
        <constraints>
            <path orientation="vertical" number="0" weight="0.7" />
            <path orientation="horizontal" number="0" weight="0.25" />
        </constraints>
        <active-tc id="filesystems" />
        <empty-behavior permanent="true" />
    </mode>

##### Mode referencing

Module references mode defined by other module in order to place own components into such mode to complete its layout. Module specifies only mode folder `Windows2/Modes/MODE_UID/` filled with own top component reference files. The mode data file should not be given.

#### Groups

Modules can either define own group from scratch or reference grouo defined by other modules.

##### Group definition

Module defines group by creating proper group folder `Windows2/Groups/GROUP_UID/` and data file `Windows2/Groups/GROUP_UID.wsgrp`.

Here is example of valid group data file:

    <?xml version="1.0" encoding="UTF-8"?>

    <!DOCTYPE group PUBLIC
              "-//NetBeans//DTD Group Properties 2.0//EN"
              "http://www.netbeans.org/dtds/group-properties2_0.dtd">

    <group version="2.0">
        <module name="org.netbeans.modules.form/2" spec="1.7" />
        <name unique="form" />
        <state opened="false" />
    </group>

##### Group referencing

Module references group defined by other module in order to place own components into such group to complete its layout. Module specifies only group folder `Windows2/Groups/GROUP_UID/` filled with own top component reference files. The group data file should not be given.

#### Top component references in modes

For top component to appear in mode, reference file needs to be put into mode folder, for example file `Windows2/Modes/MODE_UID/COMP_UID.wstcref`. Keep in mind that component reference file is only a part of the story, to specify complete link to top component through `COMP_UID`, you have to provide file `COMP_UID.settings` or `COMP_UID.ser` and place it into `Windows2/Components/` folder.

Example top component reference file content:

    <?xml version="1.0" encoding="UTF-8" ?>

    <!DOCTYPE tc-ref PUBLIC
              "-//NetBeans//DTD Top Component in Mode Properties 2.0//EN"
              "http://www.netbeans.org/dtds/tc-ref2_0.dtd">

    <tc-ref version="2.0">
        <module name="org.netbeans.core.ui/1" spec="1.2" />
        <tc-id id="filesystems" />
        <state opened="true" />
    </tc-ref>

#### Top component references in groups

For top component to appear in group, reference file needs to be put into group folder, for example file `Windows2/Groups/GROUP_UID/COMP_UID.wstcgrp`. Keep in mind that component reference file is only a part of the story, to specify complete link to top component through `COMP_UID`, you have to provide file `COMP_UID.settings` or `COMP_UID.ser` and place it into `Windows2/Components/` folder.

Example top component reference in group file content:

    <?xml version="1.0" encoding="UTF-8" ?>

    <!DOCTYPE tc-group PUBLIC
              "-//NetBeans//DTD Top Component in Group Properties 2.0//EN"
              "http://www.netbeans.org/dtds/tc-group2_0.dtd">

    <tc-group version="2.0">
        <module name="org.netbeans.modules.form/2" spec="1.7" />
        <tc-id id="ComponentInspector" />
        <open-close-behavior open="true" close="true" />
    </tc-group>

#### Top component data

Stored in folder `Windows2/Components/` as `*.settings` or `*.ser` files, which holds data of top component. Top components are instantiated using information stored in these files. If top component is created dynamically in the code, system will automatically create a new file containing proper information during save operation.

Names of files in `Windows2/Components/` (without extensions) are used to link top components and their references in modes and groups.

In the future other or even arbitrary file extensions and types may be supported for storing top component information, provided that the result of `DataObject.getName()` matches the ID. Also currently all settings for newly created components will be stored in `*.settings` files.

Top component can be divided from instantiation perspective to following types:

1. "Singleton", static top components
   These components are created only once, typically there exist only one instance of component per class. Examples of such components are Filesystems, Runtime, Execution.
2. Dynamic top components
   Some components have dynamic character, which means that new instances of such components are needed during IDE runtime. Another typical characteristic is cloneability. All editors like java source, html, xml or form editors fall into this category.

For static components, specifying their data file and link it with component reference in mode is well enough to describe all life of such component. However, for dynamic top components, situation is more complicated.

#### Defined locations

The window system API does not define any particular mode names; it only defines the structure given names. So the only files whose names are defined here are:

1. `Windows2/WindowManager.wsmgr/`
2. `Windows2/Components`

#### Automatic deinstallation support

Modules that define modes, groups or component references can "mark" these items so that the system can perform automatic removal of these items when the module is uninstalled.

All three types of window system XML files may have module information defined in the same way, using the optional `<module/>` element, which has two attributes `name` and `spec`.

Normally a module will define just the `name` attribute, with its code name base (for example, `org.domain.module`). The windowing elements will be removed if the module thus named is uninstalled (or removed). `name` may also be a full code name (for example, `org.domain.module/1`) and `spec` may be given with the specification version (for example, `1.0`) in which case the windowing element will also be removed if the module is present and enabled, but in an older version than was specified - though this feature is unlikely to be used commonly.

The window system may at its discretion just ignore XML files corresponding to missing modules; or it may actually delete them. In either case, removal of windowing components applies also to subcomponents: so for example a missing mode (or group) implies that any contained modes (or group) are not available either, even if they otherwise would have been.

### Common Scenarios

Following chapter summarizes procedures that module can do in order to use features of XML mode layout. Each procedure is detailed into steps which module writers are required or supposed to do.

#### Creating new mode

Following section shows creation of new mode defined by module.

1. Specify module layer
   To use module layer properly, specify reference to module layer in module's manifest file using OpenIDE-Module-Layer entry, with relative patch to the xml file describing module's xml layer. In other words, add to the manifest line like this:

       OpenIDE-Module-Layer: org/your_company/your_module/resources/YourModuleLayer.xml

2. Describe mode
   Example of adding new mode, with id "my_own_mode", without any components contained in mode yet:

       <?xml version="1.0" encoding="UTF-8"?>
       <!DOCTYPE filesystem PUBLIC
                 "-//NetBeans//DTD Filesystem 1.0//EN"
                 "http://www.netbeans.org/dtds/filesystem-1_0.dtd">
       <filesystem>
           <folder name="Windows2">
               <folder name="Modes">
                   <!-- configuration file is stored separately -->
                   <file name="my_own_mode.wsmode" url="my_own_mode.wsmode"/>
                   <folder name="my_own_mode"/>
               </folder>
           </folder>
       </filesystem>

   What example shows? Creates folder Windows2/Modes/my_own_mode and inserts description of its properties into file Windows2/Modes/my_own_mode.wsmode, which has format defined in its DTD (see above). Folder name must be unique, fixed name for mode identification.

#### Add components to foreign modes

It's even possible to add own top components into frames defined by other modules or core. To add top component "shower" into frame "bathroom", specify layer like this (ser or settings file shower.ser or shower.settings is expected in folder Windows/Components):

    <?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE filesystem PUBLIC
              "-//NetBeans//DTD Filesystem 1.0//EN"
              "http://www.netbeans.org/dtds/filesystem-1_0.dtd">
    <filesystem>
        <folder name="Windows2">
            <folder name="Modes">
                <!-- foreign mode, don't specify data file -->
                <folder name="bathroom">
                    <!-- configuration file stored separately -->
                    <file name="shower.wstcref" url="shower.wstcref">
                </folder>
            </folder>
        </folder>
    </filesystem>

There are several other notes that developer should be aware of:

1. Don't insert configuration files directly using CDATA sections, keep them in separate files and refer through url (like in above examples). The reason is performance, CDATA sections are not maintained so efficiently by xml parsers system currently uses.
2. Link modes and components together using names of files where data of top components are stored (see section 3.7). For example, if you create file "foo.setting" or "foo.ser" in Windows2/Components directory for your top component, then use name "foo" in top component references.

#### Overriding components and modes defined by other modules

In some cases, it's useful to override mode and components layout defined by other module. To achieve this, the modules in question must have dependency defined.

So let's say we have following example situation:

1. Two modules, A and B
2. Module B depends on module A
3. Module A defined its own mode, called "a_mode" with one component. "a_comp" and module B defines also mode called "a_mode" with one componetn "b_comp" reference.
4. Module B wants to override layout, so that mode "a_mode" will be replaced by its own mode and component "a_comp" will vanish.

Solution:
Module A needs no changes, module B has to assure following:

1. Define mode "a_comp" in foreign mode "a_mode", which will override properties of original mode specified in module A. Important thing is, that file name of new mode must be exactly the same as old one. Layered architecture of xml filesystem assures that file defined by module B will "hide" or "override" original file.
2. Specify special "hiding" file for "a_comp" in folder "a_mode" so that it doesn't appear in module B's mode:
   <file name="a_comp1.wstcref_hidden" />
3. Create own content of mode "b_comp". It means create proper XML for top component references in "a_mode" folder.
4. Remove original mode "a_mode" by specifying "hiding" file for mode itself:
   <file name="a_mode.wsmode_hidden" />
   It's not needed to explicitly hide mode folder or mode components, because mode will not be recognized if *.wsmode file is missing, so hiding *.wsmode is enough.

Similar procedures can be applied when module needs to override only one top component reference entry or hide whole workspace.

## UML Diagrams

### Overall structure class diagram

### Input-Output class diagram

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