RecyclerView没有提供表项点击事件监听器，只能自己处理。
这是读源码长知识系列的第一篇，该系列的特点是将源码中的设计思想运用到真实项目之中，系列文章目录如下：
方案一：层层传递点击监听回调
RecyclerView表项被ViewHolder持有，而ViewHolder在RecyclerView.Adapter中被构建。所以将点击事件回调注入RecyclerView.Adapter，再传递给ViewHolder，并为itemView设置View.OnClickListener，是最直接的解决方案：
//'定义点击回调'
public interface OnItemClickListener {
void onItemClick(int position);
}
让Adapter持有接口：
public class MyAdapter extends RecyclerView.Adapter<MyViewHolder> {
//'持有接口'
private OnItemClickListener onItemClickListener;
//'注入接口'
public void setOnItemClickListener(OnItemClickListener onItemClickListener) {
this.onItemClickListener = onItemClickListener;
}
@Override
public MyViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.grid_item, null);
return new MyViewHolder(view);
}
//'将接口传递给ViewHolder'
@Override
public void onBindViewHolder(MyViewHolder holder, int position) {
holder.bind(onItemClickListener);
}
}
然后就能在ViewHolder中调用接口：
public class MyViewHolder extends RecyclerView.ViewHolder {
public MyViewHolder(View itemView) {
super(itemView);
}
public void bind(final OnItemClickListener onItemClickListener){
//'为ItemView设置点击事件'
itemView.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
if (onItemClickListener != null) {
onItemClickListener.onItemClick(getAdapterPosition());
}
}
});
}
}
这个方案的优点是简单易懂，但缺点是点击事件的接口经过多方传递：为了给itemView设置点击事件，需要ViewHolder和Adapter的传递（因为不能直接拿到itemView）。这就使它们和点击事件接口耦合在一起，如果点击事件接口改动，这两个类需要跟着一起改。
还有一个缺点是，内存中会多出 N 个 OnClickListener 对象（N为一屏的表项个数）。虽然这也不是一个很大的开销。而且onBindViewHolder()会在列表滚动时多次触发，导致会为同一个表项无谓地多次设置点击监听器。
上面这两点还不是最致命的，在onBindViewHolder()中设置点击监听器还会导致 bug，因为“快照机制”，作为参数传入onItemClick()的索引值是在调用onBindViewHolder()那一刻生成的快照，如果数据发生增删，但因为各种原因没有及时刷新对应位置的视图（onBindViewHolder()没有被再次调用），此时发生的点击事件拿到的索引就是错的。
有没有更解耦且所有表项共用一个点击事件监听器的方案？
从 ListView 源码中找答案
突然想到ListView.setOnItemClickListener()，这不就是所有表项共享的一个监听器吗？看看它是怎么实现的：
public interface OnItemClickListener {
/**
* '第二个参数是被点击的表项'
* @param view The view within the AdapterView that was clicked
* '第三个参数是被点击表项的适配器位置'
* @param position The position of the view in the adapter.
*/
void onItemClick(AdapterView<?> parent, View view, int position, long id);
}
/**
* '注入表项点击监听器'
*/
public void setOnItemClickListener(@Nullable OnItemClickListener listener) {
mOnItemClickListener = listener;
}
这是定义在ListView中的表项点击监听器接口，接口的实例通过setOnItemClickListener()注入并保存在mOnItemClickListener中。
接口参数中有被点击的表项View和其适配器索引，好奇这两个参数是如何从点击事件生成的？沿着mOnItemClickListener向上查找调用链：
public boolean performItemClick(View view, int position, long id) {
final boolean result;
if (mOnItemClickListener != null) {
playSoundEffect(SoundEffectConstants.CLICK);
//'调用点击事件监听器'
mOnItemClickListener.onItemClick(this, view, position, id);
result = true;
} else {
result = false;
}
if (view != null) {
view.sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);
}
return result;
}
mOnItemClickListener只有在performItemClick(View view, int position, long id)中被调用，沿着调用链继续向上查找第一个参数view是如何生成的：
private class PerformClick extends WindowRunnnable implements Runnable {
//'被点击表项的索引值'
int mClickMotionPosition;
@Override
public void run() {
if (mDataChanged) return;
final ListAdapter adapter = mAdapter;
final int motionPosition = mClickMotionPosition;
if (adapter != null && mItemCount > 0 &&
motionPosition != INVALID_POSITION &&
motionPosition < adapter.getCount() && sameWindow() &&
adapter.isEnabled(motionPosition)) {
//'通过motionPosition索引值定位到被点击的View'
final View view = getChildAt(motionPosition - mFirstPosition);
if (view != null) {
performItemClick(view, motionPosition, adapter.getItemId(motionPosition));
}
}
}
}
被点击的view是通过getChildAt(index)获得的，问题就转变成对应的索引值是如何产生的？搜索所有PerformClick.mClickMotionPosition被赋值的地方：
public abstract class AbsListView extends AdapterView<ListAdapter>{
/**
* '接收按下事件表项的位置'
* The position of the view that received the down motion event
*/
int mMotionPosition;
private void onTouchUp(MotionEvent ev) {
switch (mTouchMode) {
case TOUCH_MODE_DOWN:
case TOUCH_MODE_TAP:
case TOUCH_MODE_DONE_WAITING:
//'被AbsListView.mMotionPosition赋值'
final int motionPosition = mMotionPosition;
final View child = getChildAt(motionPosition - mFirstPosition);
if (child != null) {
if (mTouchMode != TOUCH_MODE_DOWN) {
child.setPressed(false);
}
final float x = ev.getX();
final boolean inList = x > mListPadding.left && x < getWidth() - mListPadding.right;
if (inList && !child.hasExplicitFocusable()) {
if (mPerformClick == null) {
mPerformClick = new PerformClick();
}
final AbsListView.PerformClick performClick = mPerformClick;
//'被AbsListView.mMotionPosition赋值'
performClick.mClickMotionPosition = motionPosition;
...
}
}
PerformClick.mClickMotionPosition被赋值的地方只有一个，在AbsListView.onTouchUp()中被AbsListView.mMotionPosition赋值，看着它的注释感觉好像没有找错方向，继续搜索它是在哪里被赋值的：
public abstract class AbsListView extends AdapterView<ListAdapter>{
@Override
public boolean onTouchEvent(MotionEvent ev) {
case MotionEvent.ACTION_POINTER_UP: {
onSecondaryPointerUp(ev);
final int x = mMotionX;
final int y = mMotionY;
//'获得点击表项索引的关键代码'
final int motionPosition = pointToPosition(x, y);
if (motionPosition >= 0) {
// Remember where the motion event started
final View child = getChildAt(motionPosition - mFirstPosition);
mMotionViewOriginalTop = child.getTop();
mMotionPosition = motionPosition;
}
mLastY = y;
break;
}
}
最终在onTouchEvent()中找到了索引值产生的方法pointToPosition()：
/**
* Maps a point to a position in the list.
*
* @param x X in local coordinate
* @param y Y in local coordinate
* @return The position of the item which contains the specified point, or
* {@link #INVALID_POSITION} if the point does not intersect an item.
*/
public int pointToPosition(int x, int y) {
Rect frame = mTouchFrame;
if (frame == null) {
mTouchFrame = new Rect();
frame = mTouchFrame;
}
//'遍历列表表项'
final int count = getChildCount();
for (int i = count - 1; i >= 0; i--) {
final View child = getChildAt(i);
if (child.getVisibility() == View.VISIBLE) {
//'获取表项区域并存储在frame中'
child.getHitRect(frame);
//'如果点击坐标落在表项区域内则返回当前表项的索引'
if (frame.contains(x, y)) {
return mFirstPosition + i;
}
}
}
return INVALID_POSITION;
}
原来是通过遍历表项，判断点击坐标是否落在表项区域内来获取点击表项在列表中的索引。
方案二：将点击坐标转化成表项索引
只要把这个算法移植到RecyclerView就可以了！但是有一个新的问题：如何在RecyclerView中检测到单击事件？ 当然可以通过综合判断ACTION_DOWN和ACTION_UP来实现，但这略复杂，Andriod 提供的GestureDetector能帮我们处理这个需求：
public class BaseRecyclerView extends RecyclerView {
//'持有GestureDetector'
private GestureDetector gestureDetector;
public BaseRecyclerView(Context context) {
super(context);
init();
}
private void init() {
//'新建GestureDetector'
gestureDetector = new GestureDetector(getContext(), new GestureListener());
}
@Override
public boolean onTouchEvent(MotionEvent e) {
//'让触摸事件经由GestureDetector处理'
gestureDetector.onTouchEvent(e);
//'一定要调super.onTouchEvent()否则列表就不会滚动了'
return super.onTouchEvent(e);
}
private class GestureListener implements GestureDetector.OnGestureListener {
@Override
public boolean onDown(MotionEvent e) { return false;}
@Override
public void onShowPress(MotionEvent e) {}
@Override
public boolean onSingleTapUp(MotionEvent e) { return false; }
@Override
public boolean onScroll(MotionEvent e1, MotionEvent e2, float distanceX, float distanceY) { return false; }
@Override
public void onLongPress(MotionEvent e) { }
@Override
public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX, float velocityY) { return false; }
}
}
这样BaseRecyclerView就具有检测单击事件的能力了，下一步就是将AbsListView.pointToPosition()复制过来，重写onSingleTapUp()：
public class BaseRecyclerView extends RecyclerView {
...
private class GestureListener implements GestureDetector.OnGestureListener {
private static final int INVALID_POSITION = -1;
private Rect mTouchFrame;
@Override
public boolean onDown(MotionEvent e) { return false; }
@Override
public void onShowPress(MotionEvent e) {}
@Override
public boolean onSingleTapUp(MotionEvent e) {
//'获取单击坐标'
int x = (int) e.getX();
int y = (int) e.getY();
//'获得单击坐标对应的表项索引'
int position = pointToPosition(x, y);
if (position != INVALID_POSITION) {
try {
//'获取索引位置的表项，通过接口传递出去'
View child = getChildAt(position);
if (onItemClickListener != null) {
onItemClickListener.onItemClick(child, getChildAdapterPosition(child), getAdapter());
}
} catch (Exception e1) {
}
}
return false;
}
@Override
public boolean onScroll(MotionEvent e1, MotionEvent e2, float distanceX, float distanceY) { return false; }
@Override
public void onLongPress(MotionEvent e) {}
@Override
public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX, float velocityY) { return false; }
/**
* convert pointer to the layout position in RecyclerView
*/
public int pointToPosition(int x, int y) {
Rect frame = mTouchFrame;
if (frame == null) {
mTouchFrame = new Rect();
frame = mTouchFrame;
}
final int count = getChildCount();
for (int i = count - 1; i >= 0; i--) {
final View child = getChildAt(i);
if (child.getVisibility() == View.VISIBLE) {
child.getHitRect(frame);
if (frame.contains(x, y)) {
return i;
}
}
}
return INVALID_POSITION;
}
}
//'将表项单击事件传递出去的接口'
public interface OnItemClickListener {
//'将表项view，表项适配器位置，适配器传递出去'
void onItemClick(View item, int adapterPosition, Adapter adapter);
}
private OnItemClickListener onItemClickListener;
public void setOnItemClickListener(OnItemClickListener onItemClickListener) {
this.onItemClickListener = onItemClickListener;
}
}
大功告成！，现在就可以像这样监听RecyclerView的点击事件了
public class MainActivity extends AppCompatActivity {
public static final String[] DATA = {"item1", "item2", "item3", "item4"};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
MyAdapter myAdapter = new MyAdapter(Arrays.asList(DATA));
BaseRecyclerView rv = (BaseRecyclerView) findViewById(R.id.rv);
rv.setAdapter(myAdapter);
rv.setLayoutManager(new LinearLayoutManager(this));
//'为RecyclerView设置单个表项点击事件监听器'
rv.setOnItemClickListener(new BaseRecyclerView.OnItemClickListener() {
@Override
public void onItemClick(View item, int adapterPosition, RecyclerView.Adapter adapter) {
Toast.makeText(MainActivity.this, ((MyAdapter) adapter).getData().get(adapterPosition), Toast.LENGTH_SHORT).show();
}
});
}
}
更简约的 Kotlin 版本
感谢 HitenDev 的评论，所以就用了下面这个更加简洁 Kotlin 版本：
//'为 RecyclerView 扩展表项点击监听器'
fun RecyclerView.setOnItemClickListener(listener: (View, Int) -> Unit) {
//'为 RecyclerView 子控件设置触摸监听器'
addOnItemTouchListener(object : RecyclerView.OnItemTouchListener {
//'构造手势探测器，用于解析单击事件'
val gestureDetector = GestureDetector(context, object : GestureDetector.OnGestureListener {
override fun onShowPress(e: MotionEvent?) {
}
override fun onSingleTapUp(e: MotionEvent?): Boolean {
//'当单击事件发生时，寻找单击坐标下的子控件，并回调监听器'
e?.let {
findChildViewUnder(it.x, it.y)?.let { child ->
listener(child, getChildAdapterPosition(child))
}
}
return false
}
override fun onDown(e: MotionEvent?): Boolean {
return false
}
override fun onFling(e1: MotionEvent?, e2: MotionEvent?, velocityX: Float, velocityY: Float): Boolean {
return false
}
override fun onScroll(e1: MotionEvent?, e2: MotionEvent?, distanceX: Float, distanceY: Float): Boolean {
return false
}
override fun onLongPress(e: MotionEvent?) {
}
})
override fun onTouchEvent(rv: RecyclerView, e: MotionEvent) {
}
//'在拦截触摸事件时，解析触摸事件'
override fun onInterceptTouchEvent(rv: RecyclerView, e: MotionEvent): Boolean {
gestureDetector.onTouchEvent(e)
return false
}
override fun onRequestDisallowInterceptTouchEvent(disallowIntercept: Boolean) {
}
})
}
然后就可以像这样监听 RecyclerView 表项点击事件了：
recyclerView.setOnItemClickListener { view, pos ->
// view 是表项根视图，pos是表项在adapter中的位置
}
下一篇会继续深入，讲解如何扩展这套方案以处理 RecyclerView 表项子控件的点击事件。
talk is cheap, show me the code
推荐阅读
RecyclerView 系列文章目录如下:
