Shabbir Dhangot Shabbir Dhangot - 1 year ago 341
Android Question

Android : Control Smooth scroll over recycler view

I am using Recyclerview with CardView. I am aware how to control speed on list view. But not for Recyclerview.

I searched a lot in found class name SmoothScroll. How to use that? I have no Idea! Right now Recyclerview by default scroll is fast.


I did it with this

Answer Source

It's unclear what you mean when you say "smoothScroll". You could be referring to the automatic "smoothScrollToPosition" which will automatically scroll to a specified position, you could be talking about manual scrolling and you could be talking about flinging. For the sake of prosperity, I will attempt to answer all of these issues now.

1. Automatic smooth scrolling.

Inside your layout manager, you need to implement the smoothScrollToPosition method:

    public void smoothScrollToPosition(RecyclerView recyclerView, State state, int position)
        // A good idea would be to create this instance in some initialization method, and just set the target position in this method.
        LinearSmoothScroller smoothScroller = new LinearSmoothScroller(getContext())
            public PointF computeScrollVectorForPosition(int targetPosition)
                int yDelta = calculateCurrentDistanceToPosition(targetPosition);
                return new PointF(0, yDelta);

            // This is the important method. This code will return the amount of time it takes to scroll 1 pixel.
            // This code will request X milliseconds for every Y DP units.
            protected float calculateSpeedPerPixel(DisplayMetrics displayMetrics)
                return X / TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, Y, displayMetrics);



In this example, I use a helper method named "calculateCurrentDistanceToPosition". This can be a bit tricky, since it involves keeping track of your current scroll position, and calculating the scroll position of a given y position. You can find an example of how to keep track of the recycler's scroll y here.

Calculating the scroll y of a given position is really dependent on what your recycler is displaying. Assuming all your items are the same height, you can calculate this by performing the following calculation:

targetScrollY = targetPosition * itemHeight

Then, to calculate the distance you need to scroll, simply subtract the current scroll y with the target scroll y:

private int calculateCurrentDistanceToPosition(int targetPosition) {
    int targetScrollY = targetPosition * itemHeight;
    return targetScrollY - currentScrollY;

2. Slowing down manual scrolling.

Once again, you need to edit your layout manager, this time - the scrollVerticallyBy method:

    public int scrollVerticallyBy(int delta, Recycler recycler, State state)
       // write your limiting logic here to prevent the delta from exceeding the limits of your list.

       int prevDelta = delta;
       if (getScrollState() == SCROLL_STATE_DRAGGING)
            delta = (int)(delta > 0 ? Math.max(delta * MANUAL_SCROLL_SLOW_RATIO, 1) : Math.min(delta * MANUAL_SCROLL_SLOW_RATIO, -1));  

       // MANUAL_SCROLL_SLOW_RATIO is between 0 (no manual scrolling) to 1 (normal speed) or more (faster speed).
       // write your scrolling logic code here whereby you move each view by the given delta

        if (getScrollState() == SCROLL_STATE_DRAGGING)
            delta = prevDelta;

        return delta;

Edit: In the above method, I call "getScrollState()". This is a method of RecyclerView. In this implementation, my custom LayoutManager is a nested class of my custom RecyclerView. If this doesn't work for you, you can try to grab the scroll state via some interface pattern.

3. Slow down the fling speed

Here you want to scale down the fling velocity. You will need to override the fling method inside your RecyclerView subclass:

public boolean fling(int velocityX, int velocityY)
     velocityY *= FLING_SCALE_DOWN_FACTOR; // (between 0 for no fling, and 1 for normal fling, or more for faster fling).

     return super.fling(velocityX, velocityY);

It's difficult for me to provide a more tailored solution, since you didn't post any of your code, or provide much information about your setup, but I hope this covers most bases and will help you find the best solution for you.

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