安卓开发 自定义ScrollView 实现滚动，停止状态的监听

Scrollview或者NestScrollView没有提供对其状态的监听（类似Recyclerview的OnScrollListener.onScrollSateChanged接口），只能自己实现。

在百度以后，发现大多数采用的都是新开一个线程，延迟一小段时间来判断是否已经暂停，都是直接在onTouchEvent()的MotionEvent.ACTION_UP中判断是否已经暂停，没有对Fling状态进行判断，所以对于停止的状态是不准确的。

在他们的基础上，本文将对Fling状态进行判断，能得到正确的停止时机。

---

到达暂停的路径只有两种：

1.ACTION_DOWN->MOVE->UP->停止

2.ACTION_DOWN->MOVE->UP->FLING->停止

本例子采用两个线程来判断是否停止

如何判断停止：

开监听线程，并且在滚动的时候一直更新改线程中的最后刷新时间，当最后（线程当前时间=刷新时间(外部更新)+200ms）时，判定为停止。

满足上述条件的那一秒可以发送多条SCROLL_STATE_IDLE的监听，但是我们只需要一条，所以需要标志位来控制每次停止只发送一次监听。

对于第一种情况：

MotionUpThread用于判断情况一的停止，MotionUp的时候刷新MotionUpThread中的计时，如果200ms后没有移动，则认为是暂停，发送停止回调。

对于第二种情况：

FlingThread用于判断情况二的停止，MotionUp以后，继续Fling，此时isScroll为true，MotionUpThread不会发送停止的回调。而FlingThread在最后的Fling滚动结束后200ms会发送停止回调。

 

经过测试，打开关闭该只有该布局的网页，会使内存增加，但是再检查问题的时候也一直没有找到是哪里泄露了，如果有人测试并且发现了问题在哪，请留言告诉我一起交流交流，万分感谢。

---

以下为代码

public class ScrollViewWithListener extends NestedScrollView {
    private OnScrollListener mScrollListener;
    // 是否在触摸状态
    private boolean inTouch = false;
    private FlingThread mFlingThread;
    private MotionUpThread mMotionUpThread;
    private MyRunnable mRunnable;
    //避免一秒内发送多次回调
    private boolean isMotionUpRun = false;
    private boolean isFlingRun = false;
    //判断是否在滚动
    private boolean isScroll = false;
    private boolean isDestory = false;

    public ScrollViewWithListener(Context context) {
        this(context, null);
    }

    public ScrollViewWithListener(Context context, AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public ScrollViewWithListener(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        mFlingThread = new FlingThread(this);
        mFlingThread.start();
        mMotionUpThread = new MotionUpThread(this);
        mMotionUpThread.start();
        mRunnable = new MyRunnable(this);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        int action = event.getAction();
        switch (action) {
            case MotionEvent.ACTION_DOWN:
                isFlingRun = false;
                break;
            case MotionEvent.ACTION_MOVE:
                inTouch = true;
                isScroll = true;
                break;
            case MotionEvent.ACTION_UP:
                inTouch = false;
                isScroll = false;
                mMotionUpThread.setRefreshTime(System.currentTimeMillis());
                break;
            case MotionEvent.ACTION_CANCEL:
                inTouch = false;
                isScroll = false;
                mMotionUpThread.setRefreshTime(System.currentTimeMillis());
                break;
        }
        return super.onTouchEvent(event);
    }


    @Override
    protected void onScrollChanged(int l, int t, int oldl, int oldt) {
        super.onScrollChanged(l, t, oldl, oldt);
        if (inTouch) {
            // 有手指触摸，并且位置有滚动 为滚动状态
            if (mScrollListener != null) {
                mScrollListener.onScrollStateChanged(this, OnScrollListener.SCROLL_STATE_TOUCH_SCROLL, isBottom());
            }
        } else {
            isScroll = true;
            // 没有手指触摸，并且位置有滚动 为fling状态
            if (mScrollListener != null) {
                mScrollListener.onScrollStateChanged(this, OnScrollListener.SCROLL_STATE_FLING, isBottom());
            }
            mFlingThread.setRefreshTime(System.currentTimeMillis());
        }

        if (mScrollListener != null) {
            mScrollListener.onScroll(l, t, oldl, oldt);
        }
    }

    static class FlingThread extends Thread {
        private WeakReference scrollViewWithListenerWeakReference;
        private long refreshTime;

        public FlingThread(ScrollViewWithListener scrollViewWithListenerWeakReference) {
            this.scrollViewWithListenerWeakReference = new WeakReference<>(scrollViewWithListenerWeakReference);
        }

        public void setRefreshTime(long refreshTime) {
            ScrollViewWithListener context = scrollViewWithListenerWeakReference.get();
            if (context != null) {
                this.refreshTime = refreshTime;
            }
        }

        @Override
        public void run() {
            ScrollViewWithListener context = scrollViewWithListenerWeakReference.get();
            while (context != null && !context.isDestory) {
                if (System.currentTimeMillis() == refreshTime + 200 && !context.isFlingRun) {
                    context.isFlingRun = true;
                    context.removeCallbacks(context.mRunnable);
                    context.postDelayed(context.mRunnable, 20);
                }
            }
            Log.d("testScroll: ", "FlingThread: dead");
        }
    }

    static class MotionUpThread extends Thread {
        private WeakReference scrollViewWithListenerWeakReference;
        private long refreshTime;

        public MotionUpThread(ScrollViewWithListener scrollViewWithListenerWeakReference) {
            this.scrollViewWithListenerWeakReference = new WeakReference<>(scrollViewWithListenerWeakReference);
        }

        public void setRefreshTime(long refreshTime) {
            ScrollViewWithListener context = scrollViewWithListenerWeakReference.get();
            if (context != null) {
                this.refreshTime = refreshTime;
            }
        }

        @Override
        public void run() {
            ScrollViewWithListener context = scrollViewWithListenerWeakReference.get();
            while (context != null && !context.isDestory) {
//                isScroll标志位用于判断抬起后的fling事件
                if (System.currentTimeMillis() == refreshTime + 200 && !context.isMotionUpRun && !context.isScroll) {
                    context.isMotionUpRun = true;
                    context.removeCallbacks(context.mRunnable);
                    context.postDelayed(context.mRunnable, 20);
                }
            }
            Log.d("testScroll: ", "MotionUpThread: dead");
        }
    }

    static class MyRunnable implements Runnable {
        private WeakReference scrollViewWithListenerWeakReference;

        public MyRunnable(ScrollViewWithListener scrollViewWithListenerWeakReference) {
            this.scrollViewWithListenerWeakReference = new WeakReference<>(scrollViewWithListenerWeakReference);
        }

        @Override
        public void run() {
            ScrollViewWithListener context = scrollViewWithListenerWeakReference.get();
            Log.d("testScroll: ", "is stoping");
            if (context != null && context.mScrollListener != null) {
                context.mScrollListener.onScrollStateChanged(context, OnScrollListener.SCROLL_STATE_IDLE, context.isBottom());
                context.isFlingRun = false;
                context.isMotionUpRun = false;
                context.isScroll = false;
            }
            Log.d("testScroll: ", "stop finish");
        }
    }
    
    @SuppressLint("RestrictedApi")
    private boolean isBottom() {
        return getScrollY() + getHeight() >= computeVerticalScrollRange();
    }
    
    /**
     * 设置ScrollView的滚动监听
     *
     * @param scrollListener
     */
    public void setOnScrollListener(OnScrollListener scrollListener) {
        mScrollListener = scrollListener;
    }

    /**
     * 滚动监听事件
     */
    public interface OnScrollListener {
        /**
         * 状态：停止
         */
        int SCROLL_STATE_IDLE = 0;

        /**
         * 状态：手动拖拽
         */
        int SCROLL_STATE_TOUCH_SCROLL = 1;

        /**
         * 状态：惯性滑动
         */

        int SCROLL_STATE_FLING = 2;

        /**
         * 滑动状态回调
         *
         * @param view         当前的scrollView
         * @param scrollState  当前的状态
         * @param arriveBottom 是否到达底部
         */
        void onScrollStateChanged(NestedScrollView view, int scrollState, boolean arriveBottom);

        /**
         * 滑动位置回调
         *
         * @param l
         * @param t
         * @param oldl
         * @param oldt
         */
        void onScroll(int l, int t, int oldl, int oldt);
    }
    @Override
    protected void onDetachedFromWindow() {
        isDestory = true;
        boolean isRemove = removeCallbacks(mRunnable);
        Log.d("testScroll: ", "mRunnable isRemove:" + isRemove);
        Log.d("testScroll: ", "mFlingThread isAlive:" + mFlingThread.isAlive());
        Log.d("testScroll: ", "mMotionUpThread isAlive:" + mMotionUpThread.isAlive());
        super.onDetachedFromWindow();
    }
}