Java Reference
In-Depth Information
This is a cheap operation requiring only six lines. Recompile, create a few elements, and then watch
them disappear with a right button click.
How It Works
After verifying that
highlightElement
is not
null
, we call the
remove()
method that we added in
the
SketchModel
class way back. This will delete the element from the list, so when the view is
repainted it will no longer be displayed. The repaint occurs automatically because the
update()
method for the view - the method we implemented for the
Observer
interface - will be called because
the model has changed. Of course, we must remember to set
highlightElement
to
null
too,
otherwise it could get drawn by a mouse handler even though it is no longer in the model.
Let's do another easy one - Send-to-Back.
Implementing the Send-to-Back Operation
The send-to-back operation is really an extension of the delete operation. We can move an element
from wherever it is in the list by deleting it, then adding it again at the end of the list.
Try It Out - The Send-to-Back Operation
The
actionPerformed()
method in the
SketchView
class has the job of removing the highlighted
element from wherever it is in the model, and then adding it back at the end:
public void actionPerformed(ActionEvent e) {
Object source = e.getSource();
if(source == moveItem) {
// (Process a move...)
} else if(source == deleteItem) {
// Code as inserted here earlier
} else if(source == rotateItem) {
// (Process a rotate)
} else if(source == sendToBackItem) {
if(highlightElement != null) {
theApp.getModel().remove(highlightElement);
theApp.getModel().add(highlightElement);
highlightElement.setHighlighted(false);
highlightElement = null;
repaint();
}
}
}
A little harder this time - eight lines of code. You can try this by drawing a few concentric circles, with
the outermost drawn first. An outer circle will prevent an inner circle from being highlighted, but
applying
Send-to-back
to the outer circle will make the inner circle accessible.
