The K Desktop Environment

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3. OpenParts

3.1 Introduction

The goals of the OpenParts technology, based upon KOM, are:

To simplify the act of understanding OpenParts I will give a short example situation:

Imagine you have an a word processor and a formula editor, both being separate applications. If you now want to insert a formula into your word processor document by using your formula editor application this arises several problems: You can of course embed the formula editor's main widget window via swallowing by using XReparentWindow and friends, or easier by using QXEmbed. But then how do you want to edit your formula without having access to the formula editor's menubar / toolbar? In any way it would look ugly if these are part of the formula window. Wouldn't it be nice if the menus / toolbars of your word processor application would get replaced by the formula editor ones, except for some general menu /toolbar items? And when you go back to your text document the old menus / toolbars come back again.

Well, this is a perfect job for OpenParts :-) .

OpenParts solves the above described problem by introducing a new sytem of visual components and a new way of creating shared GUI elements, such as menus or toolbars, dynamically on demand.

In the implementation of OpenParts every element consists usually of two classes, the interface implementation, where the class name ends with "If", and the Qt/KDE object. So for example the OpenParts StatusBar element is represented by two classes: OPStatusBar, being derived from KStatusBar, handles the Qt/KDE specific extensions, and OPStatusBarIf which is responsible for providing an implementation of the actual OpenPartsUI::StatusBar interface by "translating" the interface functionality into Qt/KDE function calls.

Since every Qt/KDE object in OpenParts is most often bound to such an interface, like described above, there is usually an interface() function which returns a reference to the OpenParts interface of the element. In case of the above example OPStatusBar::interface() returns a reference to an OPStatusBarIf object which is directly bound to this OPStatusBar object.

3.2 OPApplication

Similar to KOMApplication, the class OPApplication (derived from KOMApplication) is required when using OpenParts.

3.3 QWidget as Component? - the OpenParts::Part interface

In OpenParts every window which has its own GUI and which is meant to be displayed in a MainWindow is called a Part (just like the formula editor view or the word processor document view in the above described example) , and implements the OpenParts::Part interface by deriving from the class OPPartIf.

If you want to make a widget class a full-featured Part component then you have to handle some things different than you might be used to, in regard to the standard Qt/KDE widget framework. In fact now a widget is no more a simple window in which you display some data, no, a Part is much more than this. In particular a Part has, beside it's window (widget, which may of course contain sub-windows or even other Parts (see OPFrame documentation) ) a full-featured GUI, consisting of a menubar with menus, toolbar(s) and a statusbar.

The special thing with the GUI is the way it is created, handled/used and "destroyed" . All this has to be highly dynamic because now the user decides about which Part he wants to have active. OpenParts provides the basic framework for this:

The very first step you have to make is to tell the OpenParts Part Interface (OPPartIf) , the class you have to inherit from, what the actual widget is, because OPartIf does not inherit from QWidget. This gives you the flexibility to separate your Part component from your actualy widget, but you don't have to do this. You can simply multiply inherit from OPPartIf and QWidget or the appropriate widget class. In any way you specify your Part widget by calling setWidget( your_widget_here ) . In most cases, when the Part component is the widget at the same time, you simply call setWidget( this ) :-) . Make sure this call is done in the constructor of your class!

The next important point is that you will want to re-implement the virtual init() function of OPPartIf . This is highly recommended since this function is called after your Part got registered by a MainWindow. The idea behind this function is that at the time the constructor of a Part gets executed, the Part itself is definitely not registered to a MainWindow, yet. But in fact you need to know when your Part gets registered, in order to register your Part at the GUI servant objects, which are only available via the MainWindow's interface. A reference to the MainWindow is available through the m_vMainWindow variable, which will be automatically initialized when the MainWindow registration is done, so don't use this variable before your init() function gets called (m_vMainWindow will be nil anyway) .

Now over to the details of the init() function. Here you should place all initialisation stuff which depends on being registered to a MainWindow. In addition you can do the above mentioned registration at the GUI managing objects. Usually this looks like the following example:

void MyPart::init()
{
  //register at the menubar manager if you want to use/display a menubar
  OpenParts::MenuBarManager_var menuBarManager = m_vMainWindow->menuBarManager();
  if ( !CORBA::is_nil( menuBarManager ) ) //check whether the shell window allows us to have a menubar
    menuBarManager->registerClient( id(), this ); //see chapter about the
              //*barManager objects
              //for further explanations

  //...the same with the toolbar
  OpenParts::ToolBarManager_var toolBarManager = m_vMainWindow->toolBarManager();
  if ( !CORBA::is_nil( toolBarManager ) )
    toolBarManager->registerClient( id(), this );

  //better define a class wide variable, of course
  OpenPartsUI::StatusBar_var m_vMyStatusBar;

  OpenParts::StatusBarManager_var statusBarManager = m_vMainWindow->statusBarManager();
  if ( !CORBA::is_nil( statusBarManager ) )
    m_vMyStatusBar = statusBarManager->registerClient( id() );
}

Note that the registration calls for these three GUI element types are only necessary if you really want to use them. For example if your Part does not want to display any toolbar you should leave out the corresponding call. In addition you might come up with the situation that for example the toolBarManager() call returns a nil reference, which indicates that the MainWindow does not allow its Parts to have a toolbar. Obviously the same applies for the menubar and the statusbar.

OpenParts makes use of KOM events to tell a Part about the construction/destruction of it's GUI. These are in particular:

The attached argument is a OpenPartsUI::MenuBar

The attached argument is a OpenPartsUI::ToolBarFactory

Depending on whether a Part wants to display a menubar and/or toolbar, the managing objects emit these two events to it. In regard to your implementation this means that you have to re-implement the event() function (remember: A Part is a full-featured KOM Component) .

The attached event arguments indicate whether the toolbar(s) or the menubar are to be created or cleared. Check these arguments against CORBA::is_nil() and you know :-) .

The OpenParts StatusBar is handled different compared to the MenuBar/ToolBar(s) . In fact it is easier: When registering at the OpenParts StatusBarManager you receive your OpenParts::StatusBar as return value. You can then use the StatusBar everywhere in your Part, independend from whether it is visible (active) or not.

In the init() function a lot of registration stuff is done, and corresponding to this in the cleanUp() function (see chapter about KOM::Base) you have to unregister from the GUI servant objects and free all appropriate references, following KOM's model of symmetric references and connections. Usually the code looks like this:

void MyPart::cleanUp()
{
  if ( m_bIsClean )
    return;

  //unregister our menubar
  OpenParts::MenuBarManager_var menuBarManager = m_vMainWindow->menuBarManager();
  if ( !CORBA::is_nil( menuBarManager ) )
    menuBarManager->unregisterClient( id() );

  //...the same with the toolbar
  OpenParts::ToolBarManager_var toolBarManager = m_vMainWindow->toolBarManager();
  if ( !CORBA::is_nil( toolBarManager ) )
    toolBarManager->unregisterClient( id() );

  OpenParts::StatusBarManager_var statusBarManager = m_vMainWindow->statusBarManager();
  if ( !CORBA::is_nil( statusBarManager ) )
    statusBarManager->unregisterClient( id() );

  //free other references here
  //...

  //this is IMPORTANT!!!
  //Always call the cleanUp() method of the base class when you're done!
  OPPartIf::cleanUp();
}

3.4 Part Children

A Part Child is a usual Part with three extra features:

  1. A Part Child has a Parent Part assigned.
  2. It does not have a GUI and it therefore does not receive any GUI creation events.
  3. The Parent Child receives events whenever there are any child related focus changes. Since the Part Child functionality is integrated in the OpenParts::Part interface and it's implementation, OPPartIf, you don't have to deal with additional classes when using Child Parts. Simply leave out the mapping of the OpenParts GUI events in the Child Part and instead map the Child Part events described below and assign the Parent Part via setParent(). See the interface description of OpenParts::Part , in openparts.idl, for further information about the events and the API in general.

3.5 How to embed a Part - the OPFrame class

Now that you know how to create full-featured Part components it is still unexplained how Parts are really displayed/shown. Since Parts are no simple QWidgets but CORBA objects we need a helping hand here, which is the OPFrame class. In fact OPFrame is a QWidget, but in conjuction with Qt's QXEmbed it embeds the Part's widget window. The usage of OPFrame is really easy, usually the code looks like this:

... somewhere in an application's widget ...
myFrame = new OPFrame( the_parent_widget );
myFrame->attach( a_reference_to_the_part_we_want_to_embed );
myFrame->show();

In addition to the above example you can detach() your Part, which you should usually do on exit. Just have a look at opFrame.h, it is documented.

One last important thing you have to know about OPFrame is that this class internally uses KOM referencing (using a KOMVar variable) to hold the Part. This means that there are two possible situations when using OPFrame :

3.6 The center of OpenParts - the OpenParts::MainWindow interface

Another important component is the so called OpenParts::MainWindow, being derived from a KTMainWindow in the implementation (and therefore the top-level window of your application) and being the shell around visible sub-windows and shared GUI elements.

The MainWindow's functionality is extended by some builtin aggregates, the managing objects for the menu-/tool-/statusbar. These objects are either available directly via the *barManager() methods of the MainWindow's interface or indirectly by being aggregates and therefore available via the components interface repository (getInterface(), supportsInterface(), ...) .

A Part can only be displayed in a MainWindow and the MainWindow has to know about this. So before you can display a Part you have to register it to the MainWindow. This is done by calling the Part Interface's setMainWindow() method (see previous chapter for further information about the process of registration) and this will give the Part a unique ID (which is for example used when addressing the part's GUI via the *bar manager objects) .

The MainWindow, as shell, has full control over all shared GUI elements. This means that it is responsible for

The creation of the *bar managers can be easily done by simply performing a dummy call to *barManager() which usually returns a pointer to the appropriate manager and also creates a new one if it does not exist yet. It is recommended to perform these calls in the constructor of your MainWindow.

The creation/handling of the skeleton GUI is explained later in the chapters about OPMenu(Bar)/OPToolBar .

Your MainWindow emits a Qt signal (activePartChanged) which informs you about a focus change of the active part, meaning whenever the user clicks on a non-active Part and it accepts the focus. Beside the pure informative sense of this signal it is recommended to connect to this signal and perform the following two steps in the slot implementation:

  1. deactivate the previous active Part's GUI by calling the *bar managers clear() function, which will do the job and, beside some internal stuff, emit the GUI events (see previous chapter) to the Part.
  2. activate the new active Part's GUI by calling the *bar managers create() function, which will, similar to the activation, emit events to the Part. This is not really required but it is highly recommended. The following code is usually used for this:
          void NameOfYourMainWindow::slotActivePartChanged( unsigned long old_id,
                                                            unsigned long new_id )
          {
            // clear the menu/tool/statusbar(s)
            menuBarManager()->clear();
            toolBarManager()->clear();
            statusBarManager()->clear();
            // create the new Part's GUI
            menuBarManager()->create( new_id );
            toolBarManager()->create( new_id );
            statusBarManager()->create( new_id );
          }
          
    

Now that the MainWindow handles all the shared "stuff" there is one thing which was not mentioned in this documentation, yet: What about the MainWindow's caption? The OpenParts MainWindow interface allows parts to have their own window captions, but how does OpenParts handle this?

Well, there are two ways:

  1. OPMainWindow provides you a so called AutoCaption mode which automatically changes the MainWindow's caption whenever the active part changes. This is enabled by default.
  2. But sometimes the shell wants to have full control over the window's caption, and this is accomplished by disabling the AutoCaption mode, which leads to the situation that the window's caption is not changed by OpenParts in any way but instead gives you control over it.

3.7 Access shared GUI elements through CORBA - OpenPartsUI and its interfaces

Well, now that we know when we have to construct/destruct a Part's GUI, via the OpenPartsUI events, we have to learn how to really create it, because we don't have the common KMenuBar, KToolBar, etc. classes anymore available. The replacement for them are CORBA Objects, described in openparts_ui.idl which is, together with the corresponding implementations, a part of the partsui module. The interfaces are 98%; similar to the KDE/Qt classes, so they're quite easy to use. Instead of bloating up this documentation with example code I rather suggest reading the tutorials in kdelibs/corba/tutorials .

3.8 Pixmaps and Strings in OpenParts - OPUIUtils

Toolbars and menus are usually beautified with pixmaps, using QPixmap classes. As we now use a CORBA interface to access our GUI elements, QPixmap has become OpenPartsUI::Pixmap for OpenParts applications. OpenPartsUI::Pixmap is just a "stringified" QPixmap, and opUIUtils.(h,cc) contains some easy to use conversion routines.

In addition OPUIUtils contains string conversion routines between CORBA::WChar* and QString. This is necessary since Qt version >=2.0 supports Unicode, via QString, all over the place, and obviously GUI elements like menus or toolbars have been converted to support this. OpenParts has been converted, too, by using "wstring" (CORBA::WChar *) in the interfaces and by using and providing conversion routines. These routines are static member functions of the OPUIUtils class, just like with the pixmap conversion. To simplify the usage, two macros have been defined: Q2C and C2Q . The first one converts a QString into a CORBA::WChar * string and the second one vice-versa.

When converting from QString to CORBA::WChar * the conversion routine allocates memory. To avoid memory leaks it is highly recommended to use CORBA::WString_var variables. Exactly the same applies for QPixmap -> OpenPartsUI::Pixmap conversions: Use OpenPartsUI::Pixmap_var , and you don't have to worry about leaks :-) .

Here's some example code, to show how to do it right:

    ...
    OpenPartsUI::Pixmap_var pm = OPUIUtils::convertPixmap( QPixmap_variable_here );
    someToolBar->insertButton( pm, ... );
    ...
    //use the same pm variable again
    pm = OPUIUtils::convertPixmap( another_qpixmap );
    ...

Similar things have to be done with QString's:

    ...
    CORBA::WString_var text = Q2C( QString_here );
    someMenuBar->insertItem7( text, ... );
    ...
    // or you can write:
    someMenuBar->insertItem7( ( text = Q2C( QString_here ) ) , ...);
    ...

One note left: When you return a "wide string" (CORBA::WChar *) as a function result by using Q2C, make sure not to use CORBA::wstring_dup().

Example:

  return CORBA::string_dup( Q2C( QString_here ) ); //!!!! WRONG!!!!!
  ...
  return Q2C( QString_here ); // RIGHT! because Q2C already allocates the string

That's it! Have fun using KOM/OpenParts :-)

TODO:

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