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As explained in How to Write a Tree Expansion Listener, you can use a tree-will-expand listener to prevent a treenode from expanding or collapsing. To be notified just after an expansion or collapse occurs, you should use a tree expansion listener instead.
The following applet adds a tree-will-expand listener to the
TreeExpandEventDemo
example discussed in How to Write a Tree Expansion Listener. The new code demonstrates the ability of tree-will-expand listeners to veto node expansions and collapses: it asks for confirmation each time you try to expand a node.
This is a picture of the applet's GUI. To run the applet, click the picture. The applet will appear in a new browser window.
Try this:
- Click the graphic to the left of the Potrero Hill node. This tells the tree that you want to expand the node.
A dialog will appear asking you whether you really want to expand the node.- Click "Expand" or dismiss the dialog.
Messages in the text area tell you that both a tree-will-expand event and a tree expanded event have occurred. At the end of each message is the path to the expanded node.- Try to expand another node, but this time press the "Cancel Expansion" button in the dialog.
The node does not expand. Messages in the text area tell you that a tree-will-expand event occurred, and that you cancelled a tree expansion.- Collapse the Potrero Hill node.
The node collapses without a dialog appearing, because the event handler'streeWillCollapse
method lets the collapse occur, uncontested.The following snippet shows the code that this program adds to
TreeExpandEventDemo
. The bold line prevents the tree expansion from happening. You can find all the applet's source code inTreeExpandEventDemo.2java
.
public class TreeExpandEventDemo2 ... { ... class DemoArea ... implements ... TreeWillExpandListener { ... public DemoArea() { ... tree.addTreeWillExpandListener(this); ... } ... // Required by TreeWillExpandListener interface. public void treeWillExpand(TreeExpansionEvent e) throws ExpandVetoException { saySomething("Tree-will-expand event detected", e); //...show a dialog... if (/* user said to cancel the expansion */) { //Cancel expansion. saySomething("Tree expansion cancelled", e); throw new ExpandVetoException(e); } } // Required by TreeWillExpandListener interface. public void treeWillCollapse(TreeExpansionEvent e) { saySomething("Tree-will-collapse event detected", e); } ... } }
TheTreeWillExpandListener
interface contains two methods:
See The Tree Expansion Event API for information about the
void treeWillCollapse(TreeExpansionEvent)
- Called just before a tree node collapses. To prevent the collapse from occurring, your implementation of this method should throw a
ExpandVetoException
event.
void treeWillExpand(TreeExpansionEvent)
- Called just before a tree node expands. To prevent the expansion from occurring, your implementation of this method should throw a
ExpandVetoException
event.
TreeExpansionEvent
argument for the preceding methods.
TreeExpandEventDemo2
,
featured in this section,
is our only example that uses a tree-will-expand listener.
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