ObservableScrollView

ObservableScrollView.java

/*
 * Copyright 2013 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package x.y.z


import android.view.MotionEvent;
import android.content.Context;
import android.util.AttributeSet;
import android.widget.ScrollView;

/**
 * A custom ScrollView that can accept a scroll listener.
 */
public class ObservableScrollView extends ScrollView {

    private int prevScrollY = 0;
    private int lastScrollDirection = 0;

    private static final int DOWN = 1;
    private static final int UP = -1;

    private Callbacks mCallbacks;

    public ObservableScrollView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }


    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        super.onLayout(changed, l, t, r, b);

        if (mCallbacks != null) {
            mCallbacks.onLayoutChanged();
        }
    }

    @Override
    protected void onScrollChanged(int l, int t, int oldl, int oldt) {
        super.onScrollChanged(l, t, oldl, oldt);
        if (mCallbacks != null) {
            detectUpDownScroll(t);
            mCallbacks.onScrollChanged(t);
        }
    }

    @Override
    public boolean onTouchEvent(MotionEvent ev) {
        if (mCallbacks != null) {
            switch (ev.getActionMasked()) {
                case MotionEvent.ACTION_DOWN:
                    mCallbacks.onDownMotionEvent();
                    break;
                case MotionEvent.ACTION_UP:
                case MotionEvent.ACTION_CANCEL:
                    mCallbacks.onUpOrCancelMotionEvent();
                    break;
            }
        }
        return super.onTouchEvent(ev);
    }

    @Override
    public int computeVerticalScrollRange() {
        return super.computeVerticalScrollRange();
    }

    private void detectUpDownScroll(int scrollY) {
        if (mCallbacks == null)
            return;

        if (scrollY == 0)
            return;

        if (Math.abs(prevScrollY - scrollY) < 50)
            return;

        if (prevScrollY < scrollY) {
            if (lastScrollDirection != DOWN) {
                mCallbacks.onScrollDown();
                lastScrollDirection = DOWN;
            }

        } else if (prevScrollY > scrollY) {
            if (lastScrollDirection != UP) {
                mCallbacks.onScrollUp();
                lastScrollDirection = UP;
            }
        }

        prevScrollY = scrollY;
    }

    public void setCallbacks(Callbacks listener) {
        mCallbacks = listener;
    }


    public static interface Callbacks {
        public void onScrollChanged(int scrollY);

        public void onDownMotionEvent();

        public void onUpOrCancelMotionEvent();

        public void onScrollUp();

        public void onScrollDown();

        public void onLayoutChanged();
    }
}