Resolve "SWT Resource was not properly disposed" for "Statistics"

[ruspl-afed]

• implement dispose for TmfPieChartViewer
• use dispose properly

What it does

The PR introduces proper handling of SWT resources for "Statistics" view

How to test

Open "Statistics" view, do something, close it: the console should have no "SWT Resource was not properly disposed" initiated from TmfStatisticsViewer should be present.

Follow-ups

I plan to fix similar problems for other views

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the instructions in this template

[bhufmann]

Looks good to me. Test and no more such exceptions. Thank!

[ruspl-afed]

Thank you for your review @bhufmann !
We need to get @MatthewKhouzam 's attention somehow to advance with PRs :)
I'll try to find something non-conflicting to fix while this PR is waiting.

[PatrickTasse]

It's actually not the intended SWT design to require dispose() to also dispose the children.

The method Widget.dispose() is not a cleanup method, it's a trigger for the application to remove a widget (and all its children). Internally it's the Widget.release() method that gets called recursively, automatically.

If a widget has cleanup to do, it should be done in a DisposeListener. This will always be called when the widget is removed, while it's not guaranteed that dispose() will ever be called, it might only be called on one of the widget's parents.

The real source of the problem is the implementation in org.eclipse.swtchart.Chart. It holds some system resources (e.g. Font in the AxisTitle) that are only released if Chart.dispose() is called, which is not normally called. Because of this implementation, we have to make sure to call Chart.dispose() on our child pie charts.

However, to avoid repeating the mistake, we should do this in a DisposeListener added in the constructor. There is already one, but it should be added to 'this', not to the parent. The disposal of the two pie charts can be moved to that listener. Then this overload of dispose() is no longer needed.

[PatrickTasse]

Just a clarification, it works currently since TmfPieChartViewer is only a child of TmfStatisticsViewer, but I want to protect in case it is ever put as a child of any other Composite that might not call dispose().

[ruspl-afed]

I am very grateful for your comments @PatrickTasse
Please give me some time to revisit SWT API and find better formulation for this change.