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Help with JScrollPane

Dan Platon

Joined: Jan 27, 2004
Posts: 4
Hello everyone !
I've developed an application to draw workflows (basically just boxes connected with arrows but hey ! it's a workflow !). These drawings are placed onto a panel which is placed on a JScrollPane. To enable scrolling when a box is dragged outside the visible area I use the following code:

The above code was placed in the mouseReleased method and worked fine.
I have recently added a save/load feature for the diagrams, so that a user can load a previously created diagram. The problem is the diagram loads properly but even if it was larger than the actual screen the the scrollbars did not appear (which is normal, because no mouseReleased event occured). Then I put the same code with minor modifications in the paintComponent method of the panel:

Now the scrollbars appear but I cannot use them, because the panel keeps on scrolling to the "farthest" box on the panel.
Could anyone help me please, i'm new in swing !
Craig Wood
Ranch Hand

Joined: Jan 14, 2004
Posts: 1535
Putting your validation code in paintComponent means that it will be called at any time and could result in a undesirable behavior; beyond your control at a minimum.

The JScrollPane checks all of its children and asks for their preferred sizes before it lays them out. So all we need to do is arrange for a suitable Dimension to be returned form our component whenever it is asked for its PS. Whenever changes are made to the GUI (such as adding/removing components or changing graphic boundries) we signal the JScrollPane to lay out its children again by calling revalidate on the child component of its view.
So the technique is:
1 - update size parameters of our component
2 - call revalidate to signal the JScrollPane to do a new layout.

This is taken care of automatically by any scrollpane children who implement Scrollable such as JTextArea.

Here's a demo app. You can drag the rectangle past the boundries of its container to trigger scrollpane updates.

[ January 29, 2004: Message edited by: Craig Wood ]
I agree. Here's the link: http://aspose.com/file-tools
subject: Help with JScrollPane
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