Creating your own Views

Das KDevelop-Programmierhandbuch: Leitfaden zur C++-Anwendungsentwicklung für das K Desktop Environment (KDE) mit Hilfe der KDevelop-IDE in der Version 1.2
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# 4.2. Creating your own Views

Now that you have a general overview of what is already provided, you may notice that for a lot of purposes already existing widgets can be used or combined together. KMail is an example as well as KDevelop itself makes use of library view components to display data.

For applications that use a special file format or have to deal with graphics, you are probably forced to create your own view widget to allow data manipulation. This is realized in our sample by the class `KScribbleView`, already providing a base for a view area.

The inheritance from `QWidget` is necessary to overwrite the virtual methods to process user events, this is probably the most work besides providing popup menus for easier access of certain functions. Also it is likely that you have to implement a set of slots which can be accessed by toolbar buttons or menu bar commands to connect to as well as methods to manipulate variables such as e.g. a painter color.

For completeness, we will repeat the necessary methods:

a) Keyboard events --TAB and Shift-TAB keys:

changes the keyboard input focus from the current widget to the next widget in the focus order. The focus can be set to widgets by calling `setFocusPolicy()` and process the following event handlers:

- `virtual void focusInEvent ( QFocusEvent * )`

- `virtual void focusOutEvent ( QFocusEvent * )`

b) all other keyboard input:

- `virtual void keyPressEvent ( QKeyEvent * )`

- `virtual void keyReleaseEvent ( QKeyEvent * )`

c) mouse movements:

- `virtual void mouseMoveEvent ( QMouseEvent * )`

- `virtual void enterEvent ( QEvent * )`

- `virtual void leaveEvent ( QEvent * )`

d) mouse button actions:

- `virtual void mousePressEvent ( QMouseEvent * )`

- `virtual void mouseReleaseEvent ( QMouseEvent * )`

- `virtual void mouseDoubleClickEvent ( QMouseEvent * )`

e) window events containing the widget:

- `virtual void moveEvent ( QMoveEvent * )`

- `virtual void resizeEvent ( QResizeEvent * )`

- `virtual void closeEvent ( QCloseEvent * )`

When re-implementing these functions, you should watch certain issues to avoid implementation mistakes that will make it almost impossible to change the widget's behavior afterwards:

1. declare your virtual methods as virtual as well and keep the access to protected. This allows code-reuse by inheritance and is consistent.

2. don't hard-code any event-processing which should be made configurable. This counts most for keyboard events which should be realized with keyboard accelerators if any function is called. This even counts for text processing ! (Imagine that a lot of users are familiar with their favorite editor's behavior. If this is configurable, they can use the behavior they like and are used to)

3. forward popup menu highlighting signals to the main widget to enable statusbar help

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Application View Design | Hoch | Configuring Menubars and Toolbars