Android Gallery Fling: how to calculate the necessary velocity for a fling

Question

I have to implement a gallery which moves to next slide on fling with animation. I have found some solutions here: How to have scrolling animation programmatically

I am using this code:

//scroll forward or backward
 private void scroll(int type){
View selectedV = mG.getSelectedView();
int idx = mG.indexOfChild(selectedV);
switch(type){
    case FORWARD:
default:
    if(idx<mG.getChildCount()-1)
        idx++;
    break;
case BACKWARD:
    if(idx>0)
        idx--;          
    break;
}
//now scrolled view's child idx in gallery is gotten
View nextView = mG.getChildAt(idx);
//(x,y) in scrolled view is gotten
int x = nextView.getLeft()+nextView.getWidth()/2;
int y = nextView.getTop()+nextView.getHeight()/2;
String out = String.format("x=%d, y=%d", x, y);
Log.i(TAG+".scroll", out);

//Kurru's simulating clicking view
MotionEvent event = MotionEvent.obtain(100, 100, MotionEvent.ACTION_DOWN, x, y, 0);
mG.onDown(event); 
boolean res = mG.onSingleTapUp(null);
Log.i(TAG+".scroll", "onSingleTapUp return =" + res);       

}

The problem is that it only works when I have 3 images visible and also apparently it doesn't even work on some devices.

BUT when I have my images shown one at a time (they occupy almost all of the device width) this method doesn't work. That is why I have implemented the following method:

@Override
public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX,
        float velocityY) {

    if(e1 == null || e2 == null) return false;
    if (isScrollingLeft(e1, e2)) { // Check if scrolling left


        if(State.curentZoom==0)
            return super.onFling(e1, e2, State.imgWidthBig*1.1f, 0);
        else {
            scroll(BACKWARD);
            return true;
        }
    } else if (isScrollingRight(e1, e2)) { // Otherwise scrolling right

        if(State.curentZoom==0)
            return super.onFling(e1, e2, (-1)*State.imgWidthBig*1.1f, 0);
        else {
            scroll(FORWARD);
            return true;
        }
    } else
        return false;

}

using code from that other post: How to stop scrolling in a Gallery Widget?

OBJECTIVE: calculate the right velocityX to have a smooth scrolling from one slide to another, either left or right. Velocity is calculated in pixels / second. If the velocity I provide is too small then the image will scroll a bit and get back to the previous one. If the velocity is too big then it will scroll trough more than one image but I need it to go ONE BY ONE and scroll to the next/previous image even the distance is very small. I have found that trying, the best value is slightly larger than the devices width but I wonder if it will be the case for all the devices.

Answer 1

A bit late to the party. AOSP provides 2 classes to help you calculate velocity, VelocityTracker & ViewConfiguration. The tracker consumes MotionEvents and outputs X/Y velocities. While ViewConfiguration declares thresholds for different gesture types.

Below is a simple example using the 2 classes to detect a fling gesture.

    mVelocityTracker = VelocityTracker.obtain();
    mViewConfiguration = ViewConfiguration.get(mContext);

    mListView.setOnTouchListener(new OnTouchListener() {

        @Override
        public boolean onTouch(View v, MotionEvent event) {

            final int action = event.getActionMasked();
            mVelocityTracker.addMovement(event);

            if (action == MotionEvent.ACTION_UP) {
                mVelocityTracker.computeCurrentVelocity(1000, mViewConfiguration.getScaledMaximumFlingVelocity());
                if (mVelocityTracker.getXVelocity() > mViewConfiguration.getScaledMinimumFlingVelocity()) {
                    // horizontal fling!
                    return true;
                }
            }
            return false;
        }
    });