How to detect if a ListView is fast scrolling

Question

Is there a way to detect if a ListView is fast scrolling? That is to say, can we somehow know when the user presses and releases the FastScroller?

Juryrigged

Answer 1

Reflection allows you to do it. By taking a look at the AbsListView source code, there is a FastScroller object which indicates a wrapper around the fast scrolling functionality. Its source code shows an interesting field:

/**
 * Current decoration state, one of:
 * <ul>
 * <li>{@link #STATE_NONE}, nothing visible
 * <li>{@link #STATE_VISIBLE}, showing track and thumb
 * <li>{@link #STATE_DRAGGING}, visible and showing preview
 * </ul>
 */
private int mState;

This field contains the status of the FastScroller object. The solution consists in reading this field's value via reflection, every time the onScroll() and onScrollStateChanged() methods are triggered.

This code implements the solution described above:

private class CustomScrollListener implements OnScrollListener {

    private ListView list;
    private int mState = -1;
    private Field stateField = null;
    private Object mFastScroller;
    private int STATE_DRAGGING;

    public CustomScrollListener() {
        super();

        String fastScrollFieldName = "mFastScroller";
        // this has changed on Lollipop
        if (Build.VERSION.SDK_INT >= 21) {
            fastScrollFieldName = "mFastScroll";
        }

        try {
            Field fastScrollerField = AbsListView.class.getDeclaredField(fastScrollFieldName);
            fastScrollerField.setAccessible(true);
            mFastScroller = fastScrollerField.get(list);

            Field stateDraggingField = mFastScroller.getClass().getDeclaredField("STATE_DRAGGING");
            stateDraggingField.setAccessible(true);
            STATE_DRAGGING = stateDraggingField.getInt(mFastScroller);

            stateField = mFastScroller.getClass().getDeclaredField("mState");
            stateField.setAccessible(true);
            mState = stateField.getInt(mFastScroller);

        } catch (NoSuchFieldException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }
    }

    @Override
    public void onScrollStateChanged(AbsListView view, int scrollState) {

        // update fast scroll state
        try {
            if (stateField != null) {
                mState = stateField.getInt(mFastScroller);
            }
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }


        if (mState == STATE_DRAGGING)) {
            // the user is fast scrolling through the list
        }
    }

    @Override
    public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount) {

        // update fast scroll state
        try {
            if (stateField != null) {
                mState = stateField.getInt(mFastScroller);
            }
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }

    }
}

Upsilon

what is the field "grid" in fastScrollerField.get(grid); Also is there any disadvantages to using this, i assume reflections makes this 'slow' – Mellissamellitz

grid is the GridView that I was using. I'm changing that to list, so to be more clear. – Upsilon

Also assuming this might break if the source is changed from sdk to sdk? and therefore the try catch. seems like a hacky solution – Mellissamellitz

Yes, it is hacky, and yes, it has changed already, but only once. I'm updating the answer with the two variants. IMO, this is the only viable approach. – Upsilon

Right so it works! Brilliant! +1 But like you said it has changed so that the field variable is now "mFastScroll". Got to be a better way hummm – Mellissamellitz

The thing is that now we have RecyclerView as the main list, and that does not support fast scrolling out of the box. Mark Hallison is writing a nice series of articles on this very subject: blog.stylingandroid.com/recyclerview-fastscroll-part-1 – Upsilon

This unfortunately does not work anymore on API 28 (Pie) if using targetSdkVersion=28, because "mState" is on the dark grey-list, resulting in NoSuchFieldException when trying to access that field. – Entertainment

Answer 2

Try using the setOnScrollListener and implement the onScrollStateChanged. The scrollState can be idle, touch scroll, or fling

setOnScrollListener(new OnScrollListener(){
    public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount) {
      // TODO Auto-generated method stub
    }
    public void onScrollStateChanged(AbsListView view, int scrollState) {
      // TODO Auto-generated method stub
      if(scrollState == 2) Log.i("a", "fast scroll");
    }
  });
}

Skardol

This doesn't mean it's fast tho. This will trigger scroll even if the user flings with little acceleration. – Jonejonell

Are you trying to determine how fast it's scrolling? – Skardol

I tried that already and scrolling touching the screen or scrolling the fastScroller return the same scrollState. I am using JazzyListView with a SLIDE_IN effect and have fast scrolling enabled in the ListView. When I fast scroll it does not work quite right because the first or last visible items are many times not visible. I wanted disable the effect when entering the fast scroll mode and re-enable it when leaving it. – Juryrigged

It appears like there isn't a way to determine when fast scroll is used. there is a bug report about it that says a fix is already released, but I can't find anything in the docs about it. – Skardol