Android MultiTouch不起作用

Question

 @Override 
public boolean onTouchEvent(MotionEvent event)
{ 
    synchronized (getHolder())
    {   
        int aktion = event.getAction();
        if (aktion == MotionEvent.ACTION_DOWN)
        {
            touched = true;
            bar.shareTouch(event.getX(), event.getY(), touched);
        }
        else if (aktion == MotionEvent.ACTION_UP)
        {
            touched = false;
            bar.shareTouch(event.getX(), event.getY(), touched);
        }
            if (aktion == MotionEvent.ACTION_POINTER_DOWN)
        {
            touched2 = true;
            bar.shareTouch2(event.getX(), event.getY(), touched2);
        }
        else if (aktion == MotionEvent.ACTION_POINTER_UP) 
        {
            touched2 = false;
            bar.shareTouch2(event.getX(), event.getY(), touched2);
        }
    }
    return true;
}

This is a Code to chech if first Finger is going onto the screen or leaving it. The same for another finger.

public void shareTouch2(float xTouch2, float yTouch2, boolean touched2)
{
    if (xTouch2 <= gameViewWidth/2) {
        if (touched2 == true) {
            touchedLeft2 = true;
        }
        else if(touched2 == false) {
            touchedLeft2 = false;
        }
    }
    else if (xTouch2 > gameViewWidth/2) {
        if (touched2 == true) {
            touchedRight2 = true;
        }
        else if(touched2 == false) {
            touchedRight2 = false;
        }
    }
}

---

public void shareTouch(float xTouch, float yTouch, boolean touched)
{
    if (xTouch <= gameViewWidth/2) {
        if (touched == true) {
        touchedLeft = true;
        }
        else if(touched == false) {
            touchedLeft = false;
        }
    }
    else if (xTouch > gameViewWidth/2) {
        if (touched == true) {
            touchedRight = true;
        }
        else if(touched == false) {
            touchedRight = false;
        }
    }

}

private void moveRight()
{
    x += 3;
}

private void moveLeft()
{
    x -= 3;
}

---

private void checkTouch() {
    if ((touchedLeft == true && touchedRight2 == false) || (touchedLeft2 == true && touchedRight == false)) {
        moveLeft();
    }
    else if ((touchedLeft == false && touchedRight2 == true) || (touchedLeft2 == false && touchedRight == true)) {
        moveRight();
    }
    else if ((touchedLeft == true && touchedRight2 == true) || (touchedLeft2 == true && touchedRight == true)) {

    }
}

The checkTouch() is called in the onDraw() Method. Now if I place a finger on the right side of the screen it moves right. Same for left. But if I touch left and the right without removing the left finger the Object still moves left although it should stop. Now when I leave the left finger it still moves left although it should move right.

I hope you understand my problem.

Hope you can help

Answer 1

You can't just assume the ACTION_POINTER_UP Event will only be sent for the same finger ACTION_POINTER_DOWN was sent.

Just as the ACTION_DOWN event is always fired for the first finger to touch down, the ACTION_UP event is only fired for the last finger to lift (ie when there are no more fingers on the screen). All others will get an ACTION_POINTER_UP event, even if it's the finger that started the gesture.

However, the Android documentation does specify

Each pointer has a unique id that is assigned when it first goes down [and] remains valid until the pointer eventually goes up.

So in theory, your first and second finger should still have the same ID, regardless whether the Event that reports them.

The solution is simple, then: Use getPointerCount() and getPointerID() to keep track of your fingers.

This may be easier if you refactor your code to account for more than two fingers by replacing the Booleans touchedLeft and touchedLeft2 with a single counter, eg fingersOnLeftSide - which has the neat side effect of only needing one shareTouch() method and otherwise reducing redundancy in your code.

EDIT:

Disclaimer: This is just off the top of my head, untested and written without knowledge of any of your code other than what you posted. Not necessarily the best way to solve your problem, but the shortest I could think of.

This is the Event handler:

public boolean onTouchEvent(MotionEvent event)
{
    synchronized (getHolder())
    {   
        int aktion = event.getAction();
        if (aktion == MotionEvent.ACTION_DOWN || aktion == MotionEvent.ACTION_POINTER_DOWN
        || aktion == MotionEvent.ACTION_UP || aktion == MotionEvent.ACTION_POINTER_UP)
        {
            bar.clearTouches();
            for (int i = 0; i < event.getPointerCount(); i++) {
                bar.shareTouch(event.getX(i)); // we don't need the y coordinate anyway
            }
        }
    }
    return true;
}

The rewritten shareTouch() to go along with it, as well as clearTouches() I introduced to make this simpler:

private void shareTouch(float x) 
{
    if (x < gameViewWidth/2) {
        fingersLeft++;
    } else {
        fingersRight++;
    }
}

private void clearTouches() 
{
    fingersLeft = 0;
    fingersRight = 0;
}

And finally, the new checkTouch() :

public void checkTouch()
{
    if (fingersLeft > fingersRight) {
        moveLeft();
    } else if (fingersRight > fingersLeft) {
        moveRight();
    }
}