Question

In R&D for a related view I’m attempting to build, I started a simple project to grasp how to do more of the basic drawing “stuff” before getting ahead of myself.

Like my other question states, there’s a lot of information out for SurfaceView threading and animation, but I feel for this application, that may be overkill. I’ll be showing somewhere between 10 and perhaps hundreds of the final View on screen at once, and from what I gather about SurfaceView it’s usually used as the main view of an Activity (game screen, etc).

This R&D app draws a few circles, and calculates the distance between the target (red lines) and the previous position (white lines):

My onDraw is pretty straightforward, but I’m having quite a hard time figuring out how to implement a Thread or Handler, or any sort of non-UI loop to calculate the values between the target and the main circle.

I have a private static inner Thread that I thought would be easy, but no dice. The latest Frankencode:

private static class DragThread extends Thread {
    private DragTest view;
    private boolean animate;

    private float currentY, targetY, delta, inc;

    public DragThread(View v){
        view = (DragTest)v;
        targetY = view.mTargetData.getY();
        currentY = view.mMainData.getY();
    }
    public void run(){
        while(Math.abs(currentY - targetY) >= 0){
            view.mMainData.setY(currentY + 10f);
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {

            }finally{
                view.postInvalidate();    
            }
        }
        Log.e(TAG, "Loop finished");
    }
}

mMainData and mTargetData are just POJOs holding the positional values of each circle, which my onDraw uses to update the position:

mMainData = new CircleData(startXposition, startYposition, radius);

…and retrieved using standard getters.

Essentially, I know I’ll need to start the loop logic when I lift my finger, but I’ve gotten a few errors ranging from “Thread Already Started”, to freezes and crashes. Handlers give me leak warnings… man. I’m spent.

Here’s the onTouchEvent for my view, with explanation of each event:

@Override
public boolean onTouchEvent(MotionEvent event) {
    int action = event.getAction();

    if(action == MotionEvent.ACTION_DOWN){

        // set some flags for drawing, update "hero" circle Y position
        // based off event data
    }else if(action == MotionEvent.ACTION_UP){

        // reset flags that dictate ghost circle drawing, etc. and invalidate
        // I think I need to start the loop here, but how?! 
        // new Thread().start()? Some sort of Handler? GAH!

    }else if(action == MotionEvent.ACTION_MOVE){
        // update mMainData Y position which is passed into onDraw, invalidate to reflect movement changes
    }        
    return false;
}

Answer 1

Based on this article, the solution was apparently staring me in the face, but I thought it was harder than it really is.

Final solution for the sample application in question is to have the view implement Runnable, set up a new animation Thread when the user lifts their finger, and use a Scroller to compute the offset.

@Override
public boolean onTouchEvent(MotionEvent event) {
    int action = event.getAction();

    if(action == MotionEvent.ACTION_DOWN){
        // ...
    }else if(action == MotionEvent.ACTION_UP){
        // ...

        startAnimation();
        return true;
    }else if(action == MotionEvent.ACTION_MOVE){
        // ...
    }        
    return false;
}
public void run(){
    int start = (int)mMainData.getY();
    int end   = (int)mTargetData.getY();

    mScroller.startScroll(0, start, 0, -Math.round(mMainData.getY() - end), 500);

    while(canAnimate()){
        while(mScroller.computeScrollOffset()){
            mMainData.setY(mScroller.getCurrY());
            postInvalidate();
        }
        stopAnimation();
    }
}
private void startAnimation(){
    mThread = new Thread(this);
    mThread.start();
}
private void stopAnimation(){
    animate = false;
}
private boolean canAnimate(){
    return animate == true;
}