Fast scroller for Android recyclerView based with adjustments for sticky headers

FastScroller

/**
 * FastScroller for RecyclerView with adjustments for sticky headers
 * based off the POC from https://github.com/AndroidDeveloperLB/LollipopContactsRecyclerViewFastScroller
 * the adjustments specifically target the SLiM library (https://github.com/TonicArtos/SuperSLiM)
 **/

import android.annotation.TargetApi;
import android.content.Context;
import android.os.Build;
import android.support.annotation.NonNull;
import android.support.v7.widget.RecyclerView;
import android.util.AttributeSet;
import android.view.LayoutInflater;
import android.view.MotionEvent;
import android.view.View;
import android.widget.LinearLayout;
import android.widget.TextView;

import com.nineoldandroids.animation.Animator;
import com.nineoldandroids.animation.AnimatorListenerAdapter;
import com.nineoldandroids.animation.ObjectAnimator;
import com.nineoldandroids.view.ViewHelper;

import static android.support.v7.widget.RecyclerView.NO_POSITION;
import static android.support.v7.widget.RecyclerView.OnScrollListener;

public class FastScroller extends LinearLayout {
    protected static final int BUBBLE_ANIMATION_DURATION = 100;

    // This to avoid the miscalculations caused by the empty header view at the start of the adapter (it never gets inflated for some reason?)
    public static final int IGNORED_AMOUNT = 1;

    protected TextView bubble;
    protected View handle;
    protected RecyclerView recyclerView;
    protected final ScrollListener scrollListener = new ScrollListener();
    protected int height;
    protected int handleLeeway;

    protected ObjectAnimator currentAnimator = null;

    @TargetApi(Build.VERSION_CODES.HONEYCOMB)
    public FastScroller(final Context context, final AttributeSet attrs, final int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        initialise(context);
    }

    public FastScroller(final Context context) {
        super(context);
        initialise(context);
    }

    public FastScroller(final Context context, final AttributeSet attrs) {
        super(context, attrs);
        initialise(context);
    }

    private void initialise(Context context) {
        setOrientation(HORIZONTAL);
        setClipChildren(false);
        LayoutInflater inflater = LayoutInflater.from(context);
        inflater.inflate(R.layout.recycler_view_fast_scroller__fast_scroller, this, true);
        bubble = (TextView) findViewById(R.id.fast_scroller_bubble);
        handle = findViewById(R.id.fast_scroller_handle);
        bubble.setVisibility(INVISIBLE);
        handleLeeway = getResources().getDimensionPixelSize(R.dimen.base_margin) * 2;
    }

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);
        height = h;
    }

    @Override
    public boolean onTouchEvent(@NonNull MotionEvent event) {
        final int action = event.getAction();
        switch (action) {
            case MotionEvent.ACTION_DOWN:
                if (event.getX() < ViewHelper.getX(handle) - handleLeeway)
                    return false;
                if (currentAnimator != null)
                    currentAnimator.cancel();
                if (bubble.getVisibility() == INVISIBLE)
                    showBubble();
                handle.setSelected(true);
            case MotionEvent.ACTION_MOVE:
                final float y = event.getY();
                setRecyclerViewPosition(y / (1 - event.getSize() / 2));
                return true;
            case MotionEvent.ACTION_UP:
            case MotionEvent.ACTION_CANCEL:
                handle.setSelected(false);
                hideBubble();
                return true;
        }
        return super.onTouchEvent(event);
    }

    public void setRecyclerView(RecyclerView recyclerView) {
        this.recyclerView = recyclerView;
        recyclerView.addOnScrollListener(scrollListener);
    }

    private void setRecyclerViewPosition(float y) {
        if (recyclerView != null) {
            BubbleTextGetter bubbleTextGetter = ((BubbleTextGetter) recyclerView.getAdapter());
            float displayedCount = recyclerView.getHeight() / bubbleTextGetter.getAverageCellSize();
            int itemCount = recyclerView.getAdapter().getItemCount() - IGNORED_AMOUNT;
            float proportion = Math.max(0, Math.min(1, y / (float) height));
            int targetPos = getValueInRange(0, itemCount - 1, (int) (proportion * (itemCount - displayedCount))) + IGNORED_AMOUNT;
            recyclerView.scrollToPosition(targetPos);
            int bubblePos = getValueInRange(0, itemCount - 1, (int) (proportion * (float) itemCount)) + IGNORED_AMOUNT;
            String bubbleText = bubbleTextGetter.getTextToShowInBubble(bubblePos);
            bubble.setText(bubbleText);
            updatePosition();
        }
    }

    private int getValueInRange(int min, int max, int value) {
        int minimum = Math.max(min, value);
        return Math.min(minimum, max);
    }

    private void setBubbleAndHandlePosition(float proportion) {
        int bubbleHeight = bubble.getHeight();
        int handleHeight = handle.getHeight();

        float y = proportion * (height - handleHeight);
        ViewHelper.setY(handle, y);
        ViewHelper.setY(bubble, getValueInRange(0, height - bubbleHeight, (int) (y - bubbleHeight + handleHeight / 2)));
    }

    private void showBubble() {
        bubble.setVisibility(VISIBLE);
        if (currentAnimator != null)
            currentAnimator.cancel();
        currentAnimator = ObjectAnimator.ofFloat(bubble, "alpha", 0f, 1f).setDuration(BUBBLE_ANIMATION_DURATION);
        currentAnimator.start();
    }

    private void hideBubble() {
        if (currentAnimator != null)
            currentAnimator.cancel();
        currentAnimator = ObjectAnimator.ofFloat(bubble, "alpha", 1f, 0f).setDuration(BUBBLE_ANIMATION_DURATION);
        currentAnimator.addListener(new AnimatorListenerAdapter() {
            @Override
            public void onAnimationEnd(Animator animation) {
                super.onAnimationEnd(animation);
                bubble.setVisibility(INVISIBLE);
                currentAnimator = null;
            }

            @Override
            public void onAnimationCancel(Animator animation) {
                super.onAnimationCancel(animation);
                bubble.setVisibility(INVISIBLE);
                currentAnimator = null;
            }
        });
        currentAnimator.start();
    }

    protected void updatePosition() {
        float proportion;

        propCalc:
        {
            Float firstTop = null, firstHeight = null;
            Float lastTop = null, lastHeight = null;
            float firstPos = NO_POSITION;
            float lastPos = NO_POSITION;
            {
                View firstView = null;
                View lastView = null;
                for (int i = 0, n = recyclerView.getChildCount(); i < n; i++) {
                    View view = recyclerView.getChildAt(i);
                    int position = recyclerView.getChildAdapterPosition(view);

                    if (firstPos == NO_POSITION) {
                        // If the view has a top greater than 0, then there is currently a sticky header at the top, and the top needs to be offset by it
                        if (view.getTop() > 0) {
                            // Find the header view
                            for (int si = 0, sn = recyclerView.getChildCount(); si < sn; si++) {
                                View headerView = recyclerView.getChildAt(si);
                                int headerPos = recyclerView.getChildAdapterPosition(headerView);
                                if (headerPos == position - 1) {
                                    firstPos = headerPos;
                                    firstView = headerView;
                                    firstHeight = (float)headerView.getHeight();
                                    firstTop = view.getTop() - firstHeight;
                                    break;
                                }
                            }
                        }

                        if (firstPos == NO_POSITION) {
                            firstPos = position;
                            firstView = view;
                            firstHeight = (float)view.getHeight();
                            firstTop = (float)firstView.getTop();
                        }
                    }

                    if (lastPos == NO_POSITION || lastPos < position) {
                        lastPos = position;
                        lastView = view;
                        lastHeight = (float)lastView.getHeight();
                        lastTop = (float)lastView.getTop();
                    }
                }

                if (firstView == null || lastView == null) {
                    proportion = 0;
                    break propCalc;
                }

            }


            // Sometimes (once in a blue moon really) the empty top is actually seen, so it does go to 0 for the pos from time to time (rarely)
            if (firstPos >= IGNORED_AMOUNT) {
                firstPos -= IGNORED_AMOUNT;
                lastPos -= IGNORED_AMOUNT;
            }

            float firstViewTopProp = -(firstTop / firstHeight);
            if (Float.isNaN(firstViewTopProp)) firstViewTopProp = 0;
            float lastViewTopProp = ((height - lastTop) / lastHeight);
            if (Float.isNaN(lastViewTopProp)) lastViewTopProp = 0;

            firstPos += firstViewTopProp;
            lastPos += lastViewTopProp;

            float visibleRange = lastPos - firstPos;

            int itemCount = recyclerView.getAdapter().getItemCount() - IGNORED_AMOUNT;

            float firstViewWeight = (itemCount - visibleRange - firstPos) / (itemCount - visibleRange);
            float lastViewWeight = (lastPos - visibleRange) / (itemCount - visibleRange);
            proportion = ((1 - firstViewWeight) + lastViewWeight) / 2;
        }

        setBubbleAndHandlePosition(proportion);
    }

    private class ScrollListener extends OnScrollListener {
        @Override public void onScrolled(RecyclerView rv, int dx, int dy) { updatePosition(); }
    }
}