android - ACTION_CANCEL while touching

Question

I has the following class that represents a View that is touchable and draw a Slide Bar.

public class SlideBar extends View {
private int progress;
private int max;

private Paint background;
private Paint upground;

private RectF bar;

private boolean firstDraw;

public SlideBar(Context context, AttributeSet attrs) {
    super(context, attrs);
    progress = 0;

    upground = new Paint();
    upground.setColor(Color.parseColor("#C2296C"));

    background = new Paint();
    background.setColor(Color.parseColor("#777777"));
}

private void onFirstDraw() {
    max = getWidth();
    bar = new RectF(0, 19, max, 21);
}

public void onDraw(Canvas canvas) {
    if (!firstDraw) {
        onFirstDraw();
        progress = max;
        firstDraw = true;
    }

    canvas.save();
    canvas.drawRoundRect(bar, 5, 5, background);
    canvas.drawCircle(progress, 20, 9, upground);
    canvas.restore();
}

public void setValue(int value) {
    progress = value;
}

public boolean onTouchEvent(MotionEvent evt) {
    System.out.println(evt.getAction());
    progress = (int) evt.getX();
    invalidate();
    return false;
}
}

But when touching and dragging it, I receive a ACTION_DOWN, some ACTION_MOVEs then receive a ACTION_CANCEL and no further events.

Why it's happens? I don't want to cancel the event and enable it to keep dragging bar.

Answer 1

This will happen when parent container will intercept your touch event. Any ViewGroup that overrides ViewGroup.onInterceptTouchEvent(MotionEvent) can do that (ScrollView or ListView for instance).

Proper way to deal with this is to call ViewParent.requestDisallowInterceptTouchEvent(boolean) method on your parent view once you think you need to keep the motion event.

Here's a quick example (attemptClaimDrag method is taken from android source code):

/**
 * Tries to claim the user's drag motion, and requests disallowing any
 * ancestors from stealing events in the drag.
 */
private void attemptClaimDrag() {
    //mParent = getParent();
    if (mParent != null) {
        mParent.requestDisallowInterceptTouchEvent(true);
    }
}

@Override
public boolean onTouchEvent(MotionEvent event) {
    if (event.getAction() == MotionEvent.ACTION_DOWN) {
        if (iWantToKeepThisEventForMyself(event)) {
            attemptClaimDrag();
        }
        //your logic here
    } else {
        //your logic here
    }
}