Swing Hacks

Animate JTree Drops #27

Chapter 3, Tables and Trees

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145

HACK

The special rendering is a two-step process. First, getTreeCellRendererComponent( )

figures out if the cell to be rendered is the drop target, and if so, it sets a

local

boolean

. It sets another

boolean

to indicate that the drop target cell is a

leaf. Having set these

boolean

s, it returns the superclass’s implementation.

In short order, the renderer’s

paint( )

method is called. In the

paint( )

, you

can use the

boolean

s to apply special rendering. In this version, a drop tar-

get that is a leaf gets a line drawn in its top inset, suggesting that the

dropped item will be inserted before this node. If rendering a drop target

that is a branch—i.e., it’s not a leaf—then the special rendering puts a box

around the component.

Running the Code

The main method, shown in Example 3-17, builds a reorderable tree and

puts it in a

JFrame.

Example 3-17. Testing the drag-and-drop JTree

public static void main (String[] args) {

JTree tree = new DnDJTree( );

DefaultMutableTreeNode root = new DefaultMutableTreeNode("People");

DefaultMutableTreeNode set1 = new DefaultMutableTreeNode("Set 1");

DefaultMutableTreeNode set2 = new DefaultMutableTreeNode("Set 2");

DefaultMutableTreeNode set3 = new DefaultMutableTreeNode("Set 3");

set1.add (new DefaultMutableTreeNode ("Chris"));

set1.add (new DefaultMutableTreeNode ("Kelly"));

set1.add (new DefaultMutableTreeNode ("Keagan"));

set2.add (new DefaultMutableTreeNode ("Joshua"));

set2.add (new DefaultMutableTreeNode ("Kimi"));

set3.add (new DefaultMutableTreeNode ("Michael"));

set3.add (new DefaultMutableTreeNode ("Don"));

set3.add (new DefaultMutableTreeNode ("Daniel"));

root.add (set1);

root.add (set2);

set2.add (set3);

DefaultTreeModel mod = new DefaultTreeModel (root);

tree.setModel (mod);

// expand all

for (int i=0; i<tree.getRowCount( ); i++)

tree.expandRow (i);

// show tree

JScrollPane scroller =

new JScrollPane (tree,

ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS,

ScrollPaneConstants.HORIZONTAL_SCROLLBAR_NEVER);

JFrame frame = new JFrame ("DnD JTree");

frame.getContentPane( ).add (scroller);

frame.pack( );

frame.setVisible(true);

}

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Chapter 3, Tables and Trees

#27 Animate JTree Drops

HACK

Figure 3-12 shows the animation of a drag. In this case, the “Chris” node is

about to be dropped on top of the “Michael” node.

Because the drop is occurring over a leaf, the custom rendering shows a line

on top of the “Michael” node, meaning that “Chris” will be inserted before

“Michael”. The result of the drop is shown in Figure 3-13.

Figure 3-12. Dragging a node within a JTree

Figure 3-13. Dropping a node within a JTree

Animate JTree Drops #27

Chapter 3, Tables and Trees

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147

HACK

In case that wasn’t such a good idea, Figure 3-14 shows the “Chris” node

being dragged back to where it was. In this case, the drag is over the “Set 1”

branch, which causes it to be drawn with a box around it, suggesting that

the drop will be “into” the branch.

The drop makes the dragged node the last child of the “Set 1” branch, as

shown in Figure 3-15.

Figure 3-14. Dragging a node to a branch

Figure 3-15. Dropping a node onto a branch

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Chapter 3, Tables and Trees

#27 Animate JTree Drops

HACK

This code is primarily focused on making a single JTree reorderable. If you

want to make drag-and-drop work between widgets in your GUI, the big-

gest change you’ll need to make is to have the

drop( )

method not remove

the dragged node from the tree’s model, since you can’t assume that the

dragged item is even a node in this tree. A little bit of checking the event

source makes it easy enough to find what scenario you’re dealing with.