How to pause for 5 seconds before drawing the next thing on Android?

Question

Say I want to draw a line, then wait five seconds, then draw another line. I have a method like this:

    public void onDraw(Canvas canvas) {
        int w = canvas.getWidth();
        int h = canvas.getHeight();
        canvas.drawLine(w/2, 0, w/2, h-1, paint);
        // PAUSE FIVE SECONDS
        canvas.drawLine(0, h/2, w-1, h/2, paint);
    }

How do I pause?

Solution 1

Don't wait in onDraw method it's called in the UI thread and you'll block it. Use flags to handle which line will be drawn

boolean shouldDrawSecondLine = false;

public void setDrawSecondLine(boolean flag) {
    shouldDrawSecondLine = flag;
}

public void onDraw(Canvas canvas) {
    int w = canvas.getWidth();
    int h = canvas.getHeight();
    canvas.drawLine(w/2, 0, w/2, h-1, paint);
    if (shouldDrawSecondLine) {
        canvas.drawLine(0, h/2, w-1, h/2, paint);
    }
}

Than use it in your code like this

final View view;
// initialize the instance to your view
// when it's drawn the second line will not be drawn

// start async task to wait for 5 second that update the view
AsyncTask<Void, Void, Void> task = new AsyncTask<Void, Void, Void>() {
    @Override
    protected Void doInBackground(Void... params) {
        try {
            Thread.sleep(5000);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
        }
        return null;
    }

    @Override
    protected void onPostExecute(Void result) {
        view.setDrawSecondLine(true);
        view.invalidate();
        // invalidate cause your view to be redrawn it should be called in the UI thread        
    }
};
task.execute((Void[])null);

Solution 2

you can use a CountDownTimer like this :

public void onDraw(Canvas canvas) {
        int w = canvas.getWidth();
        int h = canvas.getHeight();
        canvas.drawLine(w/2, 0, w/2, h-1, paint);
        // PAUSE FIVE SECONDS
        new CountDownTimer(5000,1000){

            @Override
            public void onTick(long miliseconds){}

            @Override
            public void onFinish(){
               //after 5 seconds draw the second line
               canvas.drawLine(0, h/2, w-1, h/2, paint);
            }
        }.start();

    }

Regards,

Solution 3

Well... you could set a flag in (which would be in some other method) and within your onDraw() based on the value of this flag draw that line.

i.e. maybe something like (though I'm not sure why you would need a pause)

invalidate();
pause/sleep();
//set flag
invalidate();

Comments

[Kalina]

That seems to wait five seconds, and then draw both lines.

[source.rar]

Probably will and in any case you should never delay in your onDraw(). Would make your app feel laggy.

[Kalina]

@source.rar Where and how would I do this, then?

[Egor]

@DeeV, I thought it was meant that all the drawing is executed in a separate thread.

[CodeFusionMobile]

@TheBeatlemaniac All drawing is done on the UI thread, and AFAIK, putting in a delay like that won't draw anything. The screen won't update until the onDraw function returns.

[Kalina]

This requires me to change h, w, and canvas to final variables. When I do that and run it, only the first line is drawn.

[Houcine]

i think you have missed to start the countdowntimer :!!! try to display a msg logcat in the onFinish() method , and see if it is executed or not