ListView,ScrollView,RecyclerView上下滑动监听

在项目中有这样需求要对ListView或ScrollView或RecyclerView滚动进行监听，来做一些处理，下面来看对应实现

一：Listview上下滑动监听

通过实现AbsListView.OnScrollListener接口onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount)实现（通过判断firstItem与前一次的差值比较）

下面是对接口封装处理

abstract class AbsListViewScrollDetector implements AbsListView.OnScrollListener {
    private int mLastScrollY;
    private int mPreviousFirstVisibleItem;
    private AbsListView mListView;
    private int mScrollThreshold;

    abstract void onScrollUp();

    abstract void onScrollDown();

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

    @Override
    public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount) {
        if(totalItemCount == 0) return;
        if (isSameRow(firstVisibleItem)) {
            int newScrollY = getTopItemScrollY();
            boolean isSignificantDelta = Math.abs(mLastScrollY - newScrollY) > mScrollThreshold;
            if (isSignificantDelta) {
                if (mLastScrollY > newScrollY) {
                    onScrollUp();
                } else {
                    onScrollDown();
                }
            }
            mLastScrollY = newScrollY;
        } else {
            if (firstVisibleItem > mPreviousFirstVisibleItem) {
                onScrollUp();
            } else {
                onScrollDown();
            }

            mLastScrollY = getTopItemScrollY();
            mPreviousFirstVisibleItem = firstVisibleItem;
        }
    }

    public void setScrollThreshold(int scrollThreshold) {
        mScrollThreshold = scrollThreshold;
    }

    public void setListView(AbsListView listView) {
        mListView = listView;
    }

    private boolean isSameRow(int firstVisibleItem) {
        return firstVisibleItem == mPreviousFirstVisibleItem;
    }

    private int getTopItemScrollY() {
        if (mListView == null || mListView.getChildAt(0) == null) return 0;
        View topChild = mListView.getChildAt(0);
        return topChild.getTop();
    }
}

这里分为两种情况处理：

1，比较两次firstItem差值进行判断 ，这种情况比较容易考虑到，还有另外一种情况2

2，如果当滑动距离较小时（或者itemView高度较大）为一个ItemView，需要判断Itemview距离顶部距离判断进行比较处理

二：ScrollView滚动上下滚动监听

ScrollView并没有提供像ListViewt那样监听器，那如何实现呢？答案就是继承ScrollView 重新onScrollChanged方法判断，并提供监听回调

具体实现：

public class ObservableScrollView extends ScrollView {

    public interface OnScrollChangedListener {
        void onScrollChanged(ScrollView who, int l, int t, int oldl, int oldt);
    }

    private OnScrollChangedListener mOnScrollChangedListener;

    public ObservableScrollView(Context context) {
        super(context);
    }

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

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

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

    public void setOnScrollChangedListener(OnScrollChangedListener listener) {
        mOnScrollChangedListener = listener;
    }
}

即当我们使用ScollView时，使用ObservableScrollView替换即可，这里需要注意的是onScrollChanged方法并不是ScrollView独有定义的，SrollView继承FrameLayout

FrameLayout继承ViewGroup,也不在ViewGroup，FrameLayout，而是来源于View基类，源码说明如下：

/**
     * This is called in response to an internal scroll in this view (i.e., the
     * view scrolled its own contents). This is typically as a result of
     * {@link #scrollBy(int, int)} or {@link #scrollTo(int, int)} having been
     * called.
     *
     * @param l Current horizontal scroll origin.
     * @param t Current vertical scroll origin.
     * @param oldl Previous horizontal scroll origin.
     * @param oldt Previous vertical scroll origin.
     */
    protected void onScrollChanged(int l, int t, int oldl, int oldt) {
        if (AccessibilityManager.getInstance(mContext).isEnabled()) {
            postSendViewScrolledAccessibilityEventCallback();
        }

        mBackgroundSizeChanged = true;

        final AttachInfo ai = mAttachInfo;
        if (ai != null) {
            ai.mViewScrollChanged = true;
        }

        if (mListenerInfo != null && mListenerInfo.mOnScrollChangeListener != null) {
            mListenerInfo.mOnScrollChangeListener.onScrollChange(this, l, t, oldl, oldt);
        }
    }

即当调用SrollBy(),或ScrollTo()是调用，而ScrollView内部就是调用这两方法，所有可以通过重写此方法实现

下面实现是监听实现：

abstract class ScrollViewScrollDetector implements ObservableScrollView.OnScrollChangedListener {
    private int mLastScrollY;
    private int mScrollThreshold;

    abstract void onScrollUp();

    abstract void onScrollDown();

    @Override
    public void onScrollChanged(ScrollView who, int l, int t, int oldl, int oldt) {
        boolean isSignificantDelta = Math.abs(t - mLastScrollY) > mScrollThreshold;
        if (isSignificantDelta) {
            if (t > mLastScrollY) {
                onScrollUp();
            } else {
                onScrollDown();
            }
        }
        mLastScrollY = t;
    }

    public void setScrollThreshold(int scrollThreshold) {
        mScrollThreshold = scrollThreshold;
    }
}

三：RecyclerView实现滚动方向判断

比较简单，因为系统以提供对应接口RecyclerView.OnScrollListener ，滚动dx,dy已给出

实现如下：

abstract class RecyclerViewScrollDetector extends RecyclerView.OnScrollListener {
    private int mScrollThreshold;

    abstract void onScrollUp();

    abstract void onScrollDown();

    @Override
    public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
        boolean isSignificantDelta = Math.abs(dy) > mScrollThreshold;
        if (isSignificantDelta) {
            if (dy > 0) {
                onScrollUp();
            } else {
                onScrollDown();
            }
        }
    }

    public void setScrollThreshold(int scrollThreshold) {
        mScrollThreshold = scrollThreshold;
    }
}

说明，mScrollThreshold的大小这里设置4dp,可以根据具体环境进行调整。