Friday, January 15, 2010

JComponent repaint(), validate(), pack(), realization and UI delegates

See below for other people's definitions, but
- I think validation means a re-layout of children within a container pane. Required after changes therein. See javadoc.
- I think pack() compacts a container to the minimum size needed to display all children according to their preferred size. It has an important side effect of realizing the host JComponent.
- Repaint() is more common but underlying paint() operation is a low-level (more native), physical operation that takes care of the nitty gritty of displaying a jcomponent via its LAF UI delegate.
- I guess realization means isDisplayable? Note UI delegate objects are instantiated much earlier. [1]

[1] If you do a new JLabel() first thing in main(), and put a breakpoint on ComponentUI.java ctor, you will see that UI delegate objects are instantiated well before setVisible.

JComponent.paint(Graphic) is so sacred that it is EDT-only. Repaint() is usable from any thread, to schedule paint() on EDT.

"The validate method is used to cause a container to lay out its subcomponents again." When changing container's content, you have to call *both* (in either order)

* revalidate()
* repaint() to request a repaint for this container

"The following JComponent instance methods are safe to call from any thread: repaint(), revalidate(), and invalidate(). The repaint() and revalidate() methods queue requests for the event-dispatching thread to call paint() and validate(), respectively. The invalidate() method just marks the host component and all of its direct ancestors as requiring validation."

Pack-and-show is a common idiom. JFrame.pack() is one of the birth methods whereby all child components become realized, whether hidden or visible. JFrame.show() and JFrame.setVisible() actually make them visible. These 2 methods are birth methods too as they makes subcomponents realized and visible. I think pain() is an underlying birth method but is invoked only from other birth methods.

http://java.sun.com/products/jfc/tsc/articles/painting/#background is a detailed article on painting

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