Friday, April 15, 2011

Why does an SWT Composite sometimes require a call to resize() to layout correctly?

Sometimes we encounter an SWT composite that absolutely refuses to lay itself out correctly. Often we encounter this when we have called dispose on a composite, and then replaced it with another; although it does not seem to be strictly limited to this case.

When we run into this problem, about 50% of the time, we can call pack() and layout() on the offending composite, and all will be well. About 50% of the time, though, we have to do this:

Point p = c.getSize();
c.setSize(p.x+1, p.y+1);

We've had this happen with just about every combination of layout managers and such.

I wish I had a nice, simple, reproduceable case, but I don't. I'm hoping that someone will recognize this problem and say, "Well, duh, you're missing xyz...."

From stackoverflow
  • Looks to me like the layout's cache is outdated and needs to be refreshed.

    Layouts in SWT support caches, and will usually cache preferred sizes of the Controls, or whatever they like to cache:

    public abstract class Layout {
        protected abstract Point computeSize (Composite composite, int wHint, int hHint, boolean flushCache);
        protected boolean flushCache (Control control) {...}
        protected abstract void layout (Composite composite, boolean flushCache);

    I'm relatively new to SWT programming (former Swing programmer), but encountered similar situations in which the layout wasn't properly updated. I was usually able to resolve them using the other layout methods that will also cause the layout to flush its cache:

    layout(boolean changed)
    layout(boolean changed, boolean allChildren)

    Hope that helps...

    Jared : +1 and accepted. Wish I could +10 this one :P
  • A composite's layout is responsible for laying out the children of that composite. So if the composite's size does not change, but the relative positions and sizes of the children need to be updated, you call layout() on the composite. If, however, the size or position of the composite itself needs to be updated, you will have to call layout() on its parent composite (and so on, until you reach the shell).

    A rule of thumb: If you have added or removed a control, or otherwise done something that requires a relayout, walk up the widget hierarchy until you find a composite with scrollbars and call layout() on it. The reason for stopping at the composite with scrollbars is that its size will not change in response to the change - its scrollbars will "absorb" that.

    Note that if the change requiring a layout is not a new child, or a removed child, you should call Composite.changed(new Control[] {changedControl}) before calling layout.

  • In the meantime I learned a little more about SWT's shortcomings when changing or resizing parts of the control hierarchy at runtime. ScrolledComposites and ExpandBars need also to be updated explicitely when the should adapt their minimal or preferred content sizes.

    I wrote a little helper method that revalidates the layout of a control hierarchy for a control that has changed:

    public static void revalidateLayout (Control control) {
     Control c = control;
     do {
      if (c instanceof ExpandBar) {
       ExpandBar expandBar = (ExpandBar) c;
       for (ExpandItem expandItem : expandBar.getItems()) {
         .setHeight(expandItem.getControl().computeSize(expandBar.getSize().x, SWT.DEFAULT, true).y);
      c = c.getParent();
     } while (c != null && c.getParent() != null && !(c instanceof ScrolledComposite));
     if (c instanceof ScrolledComposite) {
      ScrolledComposite scrolledComposite = (ScrolledComposite) c;
      if (scrolledComposite.getExpandHorizontal() || scrolledComposite.getExpandVertical()) {
        .setMinSize(scrolledComposite.getContent().computeSize(SWT.DEFAULT, SWT.DEFAULT, true));
      } else {
     if (c instanceof Composite) {
      Composite composite = (Composite) c;
      composite.layout(true, true);
    Jared : Yeah - I've had to do that before - it's a pain. It wouldn't hurt to phrase this as a separate question and answer, for searchability's sake.


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