Subtask in ProgressMonitors

Here is a trivial tip. When inspecting a bug, I found a interesting thing on Progress Monitors. We know monitor.worked() increments the progress bar, but how do we change the text to update the current subtask? The initial text is set by the beginTask() method and it should be called only once. I digged into the IProgressMonitor and found the subtask() method:

IRunnableWithProgress operation = new IRunnableWithProgress() {

 public void run(IProgressMonitor monitor) {

  monitor.beginTask("Main task running ...", 5);
  for (int i = 0; i < 5; i++) {
   monitor.subTask("Subtask # " + i + " running.");
   runSubTask(new SubProgressMonitor(monitor, 1), i);
  }
 }

};

Now the question is what happens when the runSubTask() method sets another subTask on the SubProgressMonitor?

private void runSubTask(IProgressMonitor monitor, int subTaskId) {

 monitor.beginTask("Sub task running", 10);
  for (int i = 0; i < 10; i++) {
   monitor.subTask("Inside subtask, " + i + " out of 10");
   // do something here ...
   monitor.worked(1);
   if (monitor.isCanceled())
    throw new OperationCanceledException();
 }
  monitor.done();
 }

}

Basically the SubProgressMonitor's subTask() overwrites the parent's subTask(). Thats the default behaviour. You can customize it with the style bits provided in the SubProgressMonitor:

If you want to append the SubProgressMonitor's subTask info, use the style PREPEND_MAIN_LABEL_TO_SUBTASK:

new SubProgressMonitor(monitor, 1, SubProgressMonitor.PREPEND_MAIN_LABEL_TO_SUBTASK), i);

Else if you want to ignore it altogher then use the SUPPRESS_SUBTASK_LABEL style:

new SubProgressMonitor(monitor, 1, SubProgressMonitor.SUPPRESS_SUBTASK_LABEL), i);