Makes getting overscroll value from ListViews and other scrollables easy

BlockingOverscrollView.java

package ru.mos.portal.android.design;

import android.content.Context;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.support.v4.view.MotionEventCompat;
import android.support.v4.view.ViewCompat;
import android.util.AttributeSet;
import android.util.Log;
import android.view.MotionEvent;
import android.view.View;
import android.view.ViewConfiguration;
import android.widget.AbsListView;
import android.widget.FrameLayout;

import java.util.ArrayList;
import java.util.List;

/**
 * Makes getting overscroll value from ListViews and other scrollables easy
 * Just add it as parent of whatever scrollable you have.
 * @author me@cab404.ru
 */

public class BlockingOverscrollView extends FrameLayout {
    private static final String TAG = "BlockingOverscrollView";


    private final static int
            STATE_IDLE = 0,
            STATE_DETECTING = 1,
            STATE_PULLING = 2,
            STATE_SETTLING = 3;

    private View mTarget;

    private float mOverscrollDirection;
    private int mState;
    private float mTouchSlop;
    private float mDragStart;
    private float mPullHeight = 500;
    private float mSettlingSpeed = 0.7f;
    private float mSettlingThreshold = 0.01f;
    private float mProgress = 0f;


    public interface DragListener {
        void onDrag(float amount);
    }

    private List<DragListener> dragListeners = new ArrayList<>();

    public void addDragListener(DragListener listener) {
        dragListeners.add(listener);
    }

    public void removeDragListener(DragListener listener) {
        dragListeners.remove(listener);
    }


    public BlockingOverscrollView(@NonNull Context context) {
        super(context, null);
    }

    public BlockingOverscrollView(@NonNull Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
        mTouchSlop = ViewConfiguration.get(context).getScaledTouchSlop();
        Log.d(TAG, "BlockingOverscrollView: " + mTouchSlop);

        mPullHeight *= context.getResources().getDisplayMetrics().density;
    }

    void ensureTarget() {
        mTarget = getChildAt(0);
    }

    private boolean canScrollUp() {
        return ViewCompat.canScrollVertically(mTarget, -1);
    }

    private boolean canScrollDown() {
        return ViewCompat.canScrollVertically(mTarget, 1);
    }


    void onProgressChange() {
        if (Math.abs(mProgress) < mSettlingThreshold)
            mProgress = 0;
        for (DragListener listener : dragListeners) {
            listener.onDrag(mProgress);
        }
    }

    void calculateProgress(float unconsumedY) {
        mProgress = unconsumedY / mPullHeight;
        if (Math.abs(mProgress) > 1f)
            mProgress = Math.signum(mProgress);

        float overkill = Math.abs(unconsumedY) - mPullHeight;
        if (overkill > 0)
            mDragStart -= overkill * Math.signum(mProgress);

        onProgressChange();
    }

    void drag(float y) {
        final float deltaY = mDragStart - y;
        final float deltaYSignum = Math.signum(deltaY);

        switch (mState) {
            case STATE_DETECTING:
                if (mOverscrollDirection != 0 && Math.abs(deltaY) > mTouchSlop)
                    if (deltaYSignum == mOverscrollDirection) {
                        mDragStart = y;
                        mState = STATE_PULLING;
                    }
                break;
            case STATE_PULLING:
                if (deltaYSignum == mOverscrollDirection)
                    calculateProgress(deltaY);
                break;
            case STATE_IDLE:
                mDragStart = y;
                mState = STATE_DETECTING;
                break;
        }
    }

    private void onFinishDrag() {
        if (Math.abs(mProgress) < mSettlingThreshold)
            mState = STATE_IDLE;
        else {
            mState = STATE_SETTLING;
            postSettler();
        }
    }

    // Copied from SwipeRefreshLayout
    @Override
    public void requestDisallowInterceptTouchEvent(boolean b) {
        // if this is a List < L or another view that doesn't support nested
        // scrolling, ignore this request so that the vertical scroll event
        // isn't stolen
        if ((android.os.Build.VERSION.SDK_INT < 21 && mTarget instanceof AbsListView)
                || (mTarget != null && !ViewCompat.isNestedScrollingEnabled(mTarget))) {
            // Nope.
        } else {
            super.requestDisallowInterceptTouchEvent(b);
        }
    }

    @Override
    public boolean onInterceptTouchEvent(MotionEvent ev) {
        ensureTarget();
        final int action = MotionEventCompat.getActionMasked(ev);

        if (mState == STATE_SETTLING) {
            // Intercepting before client does something bad
            return true;
        }

        if (action == MotionEvent.ACTION_DOWN) onFinishDrag();

        if (action == MotionEvent.ACTION_MOVE) {
            if (!canScrollUp())
                mOverscrollDirection = -1;
            else if (!canScrollDown())
                mOverscrollDirection = 1;
            else
                mOverscrollDirection = 0;
            drag(ev.getY());
        }

        return mState == STATE_PULLING;
    }

    @Override
    public boolean onTouchEvent(MotionEvent ev) {
        final int action = ev.getActionMasked();

        if (mState == STATE_SETTLING) {
            // Intercepting before client does something bad
            return true;
        }

        switch (mState) {
            case STATE_PULLING:
                if (action == MotionEvent.ACTION_MOVE) {
                    drag(ev.getY());
                }
                break;

        }
        if (action == MotionEvent.ACTION_UP) onFinishDrag();

        return true;
    }

    Runnable settler = new Runnable() {
        @Override
        public void run() {
            mProgress *= mSettlingSpeed;
            onProgressChange();
            if (Math.abs(mProgress) > mSettlingThreshold)
                postSettler();
            else
                mState = STATE_IDLE;
        }
    };

    public void postSettler() {
        ViewCompat.postOnAnimation(this, settler);
    }

}