Unable to cancel zooming operation using Job

Question

I am working on an eclipse RCP application. I have a ScalableLayeredPane on which I have around 800 figures.

I need to change the zoom value by using setScale() method.

Also I want to display progress of the zooming using Job.

I am using asyncexec method to call setScale() function. However, my GUI is starving and I can not click on the cancel button also.

Any help on this will be appreciated.

Here is what I am doing.

Job job = new Job("Some job") {

    @Override
    protected IStatus run(IProgressMonitor monitor) {
    // Decide scaling amount


    PlatformUI.getWorkbench().getDisplay().asyncExec(new Runnable() {

    @Override
    public void run() {
        scalableLayeredPaneObj.setScale(scale);                                 }
    });


    // Check if cancel is pressed and return status accordingly     
    if(monitor.isCanceled()) {
        return Status.CANCEL_STATUS;
    }
        return Status.OK_STATUS;
    }
};

job.addJobChangeListener(new JobChangeAdapter() {
    @Override
    public void done(IJobChangeEvent event) {
        if(event.getResult().isOK()) {
            System.out.println("Job done!");
            // Do something
        } else {
            System.out.println("Job cancelled!");
            // Do something else
        }
    }
});

job.setUser(true);
job.schedule();

However, when this particular code is run, GUI becomes unresponsive and cancel button can not be pressed. I am always getting "Job done!" message.

I have also tried using PlatformUI#getWorkbench()#getDisplay()#syncExec method for running UI thread. That didn't work.

What my analysis suggests is setScale operation itself taking a long time thus making GUI unresponsive. I need to call setScale from another thread(if possible) so that Main thread will not hang up.

Answer 1

Issue is related to setScale() method only. Due to large canvas size, setScale() is taking a long time in UI thread.

Basically Job is used to perform non-UI activities in backend. For UI activities, busy indicator is suitable. However, since I wanted to cancel the scaling task, I was using user Job. Issue can be resolved using busy indicator instead and thus no cancel action could be provided.