Calling LayoutPanel.add(child) from outside the constructor of my Composite Widget causes the child to not be appended to the DOM

Question

I'm just trying to dynamically add children to the parent through a public method. If I call canvasContainer.add(Canvas.createIfSupported()) from inside the constructor, it works as intended, but from inside the insertLayerAt() method, it silently fails, not appending to the DOM.

I've tried using different Panels as the parent element, and different Widgets as the child element, but the problem persists.

Am I approaching this wrong? I can't find anything that says I shouldn't be using the add method from outside the constructor, but then again I might be missing something simpler than that.

My code is below, I'm using GWT 2.4 and MVP4G

public class CanvasView extends Composite implements
        CanvasPresenter.ICanvasView {

    LayoutPanel canvasContainer;

    @Inject
    public CanvasView()
    {
        Document document = Document.getInstance();
        Dimension d = document.getDimension();


        canvasContainer = new LayoutPanel();
        canvasContainer.setPixelSize(d.width, d.height);
        canvasContainer.setStyleName("CanvasController");

        initWidget(canvasContainer);


    }

    public void insertLayerAt(int zpos, Layer layer)
    {

        Canvas canvas = Canvas.createIfSupported();
        if (canvas != null)
        {
            GWT.log("Canvas is supported.", null);              
            canvasContainer.add(canvas);

        }else{
            GWT.log("Canvas isn't supported.", null);
        }
    }


}

Try looking at Chrome's JavaScript log when running the app. You'll probably find that it is, in one case, trying to access an object that has not been created yet. — Rok Strniša, Nov 26 '11 at 03:07
Sorry, I forgot to say, there's nothing being written too the JS log. I also tried picking the app apart with the debugger, but the problem doesn't seem to arise until it's getting into invokeNative etc, which is all a bit too complex for me to understand. — Jim, Nov 26 '11 at 10:55
In the past, I've noticed that many things related to the custom widget do not work when done _before_ `initWidget`. Have you tried moving `setPixelSize` and `setStyleName` after `initWidget`? — Rok Strniša, Nov 26 '11 at 11:22
I've just tried that now, but unfortunately it made no difference. — Jim, Nov 26 '11 at 12:10

score 0 · Answer 1 · answered Dec 02 '11 at 00:03

Using a deferred command helps sometimes when stuff like this happens if the reason is that there's something it's not finished with. I haven't tried this code though, so I have no idea if it works in this case.

public void insertLayerAt(int zpos, Layer layer)
{

    Canvas canvas = Canvas.createIfSupported();
    if (canvas != null)
    {
        GWT.log("Canvas is supported.", null);              
        Scheduler.get().scheduleDeferred(new Scheduler.ScheduledCommand() {         
            public void execute() {
                    canvasContainer.add(canvas);
            }
         });
    }else{
        GWT.log("Canvas isn't supported.", null);
    }
}

Calling LayoutPanel.add(child) from outside the constructor of my Composite Widget causes the child to not be appended to the DOM

1 Answers1