Draw on a canvas with delay - "make onDraw() slow down"

Question

I use functions for canvas like drawCircle and drawPoint in android. This works fine.

But the problem now is to draw these different items with a delay, so it looks like an animation.

What kind of mechanism should I use? Have tried with async but I dont like that way of doing it.

Should I use some kind of timer that just draw with an interval or is there other clever ways to do this?

Answer 1

I use this strategy, first I declare a Handler and a Runnable that way:

    private final Observable mObservable = new Observable();
    private final static int TIME_STEP_MS = 5;
    private final Handler mHandler = new Handler();
    private final Runnable mTimeManager = new Runnable() 
    {
        public void run() 
        {
            mObservable.notifyObservers(TIME_STEP_MS);
            mHandler.postDelayed(mTimeManager, TIME_STEP_MS);
        }
    };

Then when I want to start my time manager I just call the mTimeManager.run() and it will start to notify my Observer s (previously added) periodically.

If you need for some reason stop the timer or something you just do that:

    mHandler.removeCallbacks(mTimeManager);

[ EDIT - More complete code ]

Ok than let's make it clearer, first I made a custom Observable object like that [that's optional]:

    private final Observable mObservable = new Observable()
    {
        public void notifyObservers()
        {
            setChanged();
            super.notifyObservers();
        };

        @Override
        public void notifyObservers(Object data) 
        {
            setChanged();
            super.notifyObservers(data);
        };
    };

the reason for that is just because I can't call setChanged() outside Observable class - it's protected, if it's not changed it doesn't notify any observer.

The other declarations keep the same as shown before, now I need to start this TimeManager somewhere, my app is a LiveWallpaper and I make all rendering stuff into a class that extends a Thread but you don't need that necessarily, I made a method called resumeDrawing(), this one is called right after super.start(); at my @Override of public synchronized void start() from Thread class, the method looks like that:

    public void resumeDrawing()
    {
        if (!mTimeManagerRunning) // just a boolean field in my class
        {
            System.err.println("Resuming renderer."); // just for debug
            mTimeManager.run();
            mTimeManagerRunning = true;
        }
        else
        {
            System.err.println("Renderer already running."); // just for debug
        }
    }

and it's dual:

    public void pauseDrawing()
    {
        if (mTimeManagerRunning)
        {
            System.err.println("Pausing renderer.");
            mHandler.removeCallbacks(mTimeManager);
            mTimeManagerRunning = false;
        }
        else
        {
            System.err.println("Renderer already paused.");
        }
    }

Ok, now we can start and stop the time manager, but who's listening? Nobody! so let's add'em: On the constructor of my Renderer I add some Observer s to my mObservable object, one of those is the Renderer itself, so my renderer extends Thread and implements Observer:

    @Override // from Observer interface
    public void update(Observable arg0, Object arg1) 
    {
        mElapsedMsRedraw += (Integer) arg1; 

        if (mElapsedMsRedraw >= mDrawingMsPerFrame)
        {
            mElapsedMsRedraw = 0;
            drawEm(); // refresh the canvas and stuff
        }
    }

to add observers you simply do mObservable.addObserver(THE_OBJECT - Implements Observer)

you can see that I don't re-render my stuff each time I'm notified, that's because I use this TimeManager for other thinks than just refresh the Canvas like updating the position of the objects I want to draw just internally.

So, what you need to slow down the drawing is to change the way your objects change internally while the time passes, I mean your circles and points etc, or you can chance your time step, I recommend the first one.

Was it clearer? I hope it helps.

Answer 2

I would use a timer, or create Animations. You can create Animations that will do all sorts of things including changing transparency over time.

Here's the Android Documentation for Animation Resources