本文深入解析了Android系统中Touch事件的传递机制，包括Activity、ViewGroup及View如何处理触摸事件，详细介绍了dispatchTouchEvent、onInterceptTouchEvent等方法的作用及其执行流程。
当一个Touch事件发生，系统首先把事件传递给当前的Activity，由Activity的dispatchTouchEvent分发事件，源码如下：
public boolean dispatchTouchEvent(MotionEvent ev) {
if (ev.getAction() == MotionEvent.ACTION_DOWN) {
onUserInteraction();
}
if (getWindow().superDispatchTouchEvent(ev)) {
return true;
}
return onTouchEvent(ev);
}getWindow().superDispatchTouchEvent(ev)调用Window的superDispatchTouchEvent方法分发事件，而Window的实现是PhoneWindow，所以是调用PhoneWindow的superDispatchTouchEvent派发事件，在这里面就是将事件从最外层向里逐级派发。如果返回false，也就是事件没有消耗，会执行下面的onTouchEvent
下面就看看ViewGroup里面的事件派发
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(ev, 1);
}
// If the event targets the accessibility focused view and this is it, start
// normal event dispatch. Maybe a descendant is what will handle the click.
if (ev.isTargetAccessibilityFocus() && isAccessibilityFocusedViewOrHost()) {
ev.setTargetAccessibilityFocus(false);
}
boolean handled = false;
if (onFilterTouchEventForSecurity(ev)) {
final int action = ev.getAction();
final int actionMasked = action & MotionEvent.ACTION_MASK;
// Handle an initial down.
if (actionMasked == MotionEvent.ACTION_DOWN) {
// Throw away all previous state when starting a new touch gesture.
// The framework may have dropped the up or cancel event for the previous gesture
// due to an app switch, ANR, or some other state change.
cancelAndClearTouchTargets(ev);
resetTouchState();
}
// Check for interception.
final boolean intercepted;
if (actionMasked == MotionEvent.ACTION_DOWN
|| mFirstTouchTarget != null) {
final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
if (!disallowIntercept) {
intercepted = onInterceptTouchEvent(ev);
ev.setAction(action); // restore action in case it was changed
} else {
intercepted = false;
}
} else {
// There are no touch targets and this action is not an initial down
// so this view group continues to intercept touches.
intercepted = true;
}
// If intercepted, start normal event dispatch. Also if there is already
// a view that is handling the gesture, do normal event dispatch.
if (intercepted || mFirstTouchTarget != null) {
ev.setTargetAccessibilityFocus(false);
}
// Check for cancelation.
final boolean canceled = resetCancelNextUpFlag(this)
|| actionMasked == MotionEvent.ACTION_CANCEL;
// Update list of touch targets for pointer down, if needed.
final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;
TouchTarget newTouchTarget = null;
boolean alreadyDispatchedToNewTouchTarget = false;
if (!canceled && !intercepted) {
// If the event is targeting accessiiblity focus we give it to the
// view that has accessibility focus and if it does not handle it
// we clear the flag and dispatch the event to all children as usual.
// We are looking up the accessibility focused host to avoid keeping
// state since these events are very rare.
View childWithAccessibilityFocus = ev.isTargetAccessibilityFocus()
? findChildWithAccessibilityFocus() : null;
if (actionMasked == MotionEvent.ACTION_DOWN
|| (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
|| actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
final int actionIndex = ev.getActionIndex(); // always 0 for down
final int idBitsToAssign = split ? 1 << ev.getPointerId(actionIndex)
: TouchTarget.ALL_POINTER_IDS;
// Clean up earlier touch targets for this pointer id in case they
// have become out of sync.
removePointersFromTouchTargets(idBitsToAssign);
final int childrenCount = mChildrenCount;
if (newTouchTarget == null && childrenCount != 0) {
final float x = ev.getX(actionIndex);
final float y = ev.getY(actionIndex);
// Find a child that can receive the event.
// Scan children from front to back.
final ArrayList<View> preorderedList = buildOrderedChildList();
final boolean customOrder = preorderedList == null
&& isChildrenDrawingOrderEnabled();
final View[] children = mChildren;
for (int i = childrenCount - 1; i >= 0; i--) {
final int childIndex = customOrder
? getChildDrawingOrder(childrenCount, i) : i;
final View child = (preorderedList == null)
? children[childIndex] : preorderedList.get(childIndex);
// If there is a view that has accessibility focus we want it
// to get the event first and if not handled we will perform a
// normal dispatch. We may do a double iteration but this is
// safer given the timeframe.
if (childWithAccessibilityFocus != null) {
if (childWithAccessibilityFocus != child) {
continue;
}
childWithAccessibilityFocus = null;
i = childrenCount - 1;
}
if (!canViewReceivePointerEvents(child)
|| !isTransformedTouchPointInView(x, y, child, null)) {
ev.setTargetAccessibilityFocus(false);
continue;
}
newTouchTarget = getTouchTarget(child);
if (newTouchTarget != null) {
// Child is already receiving touch within its bounds.
// Give it the new pointer in addition to the ones it is handling.
newTouchTarget.pointerIdBits |= idBitsToAssign;
break;
}
resetCancelNextUpFlag(child);
if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
// Child wants to receive touch within its bounds.
mLastTouchDownTime = ev.getDownTime();
if (preorderedList != null) {
// childIndex points into presorted list, find original index
for (int j = 0; j < childrenCount; j++) {
if (children[childIndex] == mChildren[j]) {
mLastTouchDownIndex = j;
break;
}
}
} else {
mLastTouchDownIndex = childIndex;
}
mLastTouchDownX = ev.getX();
mLastTouchDownY = ev.getY();
newTouchTarget = addTouchTarget(child, idBitsToAssign);
alreadyDispatchedToNewTouchTarget = true;
break;
}
// The accessibility focus didn't handle the event, so clear
// the flag and do a normal dispatch to all children.
ev.setTargetAccessibilityFocus(false);
}
if (preorderedList != null) preorderedList.clear();
}
if (newTouchTarget == null && mFirstTouchTarget != null) {
// Did not find a child to receive the event.
// Assign the pointer to the least recently added target.
newTouchTarget = mFirstTouchTarget;
while (newTouchTarget.next != null) {
newTouchTarget = newTouchTarget.next;
}
newTouchTarget.pointerIdBits |= idBitsToAssign;
}
}
}
// Dispatch to touch targets.
if (mFirstTouchTarget == null) {
// No touch targets so treat this as an ordinary view.
handled = dispatchTransformedTouchEvent(ev, canceled, null,
TouchTarget.ALL_POINTER_IDS);
} else {
// Dispatch to touch targets, excluding the new touch target if we already
// dispatched to it. Cancel touch targets if necessary.
TouchTarget predecessor = null;
TouchTarget target = mFirstTouchTarget;
while (target != null) {
final TouchTarget next = target.next;
if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
handled = true;
} else {
final boolean cancelChild = resetCancelNextUpFlag(target.child)
|| intercepted;
if (dispatchTransformedTouchEvent(ev, cancelChild,
target.child, target.pointerIdBits)) {
handled = true;
}
if (cancelChild) {
if (predecessor == null) {
mFirstTouchTarget = next;
} else {
predecessor.next = next;
}
target.recycle();
target = next;
continue;
}
}
predecessor = target;
target = next;
}
}
// Update list of touch targets for pointer up or cancel, if needed.
if (canceled
|| actionMasked == MotionEvent.ACTION_UP
|| actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
resetTouchState();
} else if (split && actionMasked == MotionEvent.ACTION_POINTER_UP) {
final int actionIndex = ev.getActionIndex();
final int idBitsToRemove = 1 << ev.getPointerId(actionIndex);
removePointersFromTouchTargets(idBitsToRemove);
}
}
if (!handled && mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onUnhandledEvent(ev, 1);
}
return handled;
}2.到28行，是判断是否拦截事件，看看29~42这段代码
if (actionMasked == MotionEvent.ACTION_DOWN
|| mFirstTouchTarget != null) {
final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
if (!disallowIntercept) {
intercepted = onInterceptTouchEvent(ev);
ev.setAction(action); // restore action in case it was changed
} else {
intercepted = false;
}
} else {
// There are no touch targets and this action is not an initial down
// so this view group continues to intercept touches.
intercepted = true;
} 当Action_Down时，如果disallowIntercept为true，intercepted 为false，不拦截；disallowIntercept为false，执行onInterceptTouchEvent(ev)方法，返回值决定intercepted
当不是Action_Down时，如果mFirstTouchTarget ==null，intercepted 为true，拦截事件；mFirstTouchTarget !=null，执行和上面一样。onInterceptTouchEvent(ev)默认返回时false，可以在ViewGroup里面重写，修改返回是否拦截。
ViewGroup中的requestDisallowInterceptTouchEvent( )方法可以用来禁止或允许ViewGroup拦截Touch事件，但是它对于ACTION_DOWN是无效的。 因为在ACTION_DOWN时会调用resetTouchState()重置了FLAG_DISALLOW_INTERCEPT，所以总会调用onInterceptTouchEvent判读是否拦截。
3.28~158，是对down事件的处理
检查是否cancel，然后进行事件的分发。58行，事件没有被cancel或者拦截会进入下面判断
68~70行判断事件是否是ACTION_DOWN，ACTION_POINTER_DOWN（多点触控），ACTION_HOVER_MOVE（悬停），如果是执行内部的语句。
89行的for循环是查找接收touch事件的view，找到点击位置的子view，调用dispatchTransformedTouchEvent()，把touch事件派发给子view，dispatchTransformedTouchEvent()的返回值为true，代表子view消费了事件，并且添加此子view到触摸链表，放到表头。mFirstTouchTarget 也就不为空了，否则返回false，mFirstTouchTarget 还是空。
newTouchTarget = addTouchTarget(child, idBitsToAssign);
当mFirstTouchTarget为空，表示ViewGroup拦截了事件，或者子view没有消费事件，也就直接调用dispatchTransformedTouchEvent()
当mFirstTouchTarget不为空，174~175，alreadyDispatchedToNewTouchTarget为true表示已经处理了down事件
move和up事件没有处理，调用dispatchTransformedTouchEvent()处理
下面写个demo测试
主界面
public class MainActivity extends AppCompatActivity {
private static final String TAG = "lzy";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
@Override
public boolean dispatchTouchEvent(MotionEvent event) {
switch (event.getAction()) {
case MotionEvent.ACTION_DOWN:
Log.i(TAG, "Activity-------------dispatchTouchEvent: ACTION_DOWN");
break;
case MotionEvent.ACTION_MOVE:
Log.i(TAG, "Activity-------------dispatchTouchEvent: ACTION_MOVE");
break;
case MotionEvent.ACTION_UP:
Log.i(TAG, "Activity-------------dispatchTouchEvent: ACTION_UP");
break;
}
return super.dispatchTouchEvent(event);
}
@Override
public boolean onTouchEvent(MotionEvent event) {
switch (event.getAction()) {
case MotionEvent.ACTION_DOWN:
Log.i(TAG, "Activity-------------onTouchEvent: ACTION_DOWN");
break;
case MotionEvent.ACTION_MOVE:
Log.i(TAG, "Activity-------------onTouchEvent: ACTION_MOVE");
break;
case MotionEvent.ACTION_UP:
Log.i(TAG, "Activity-------------onTouchEvent: ACTION_UP");
break;
}
return super.onTouchEvent(event);
}
}public class MyView extends View {
private static final String TAG = "lzy";
public MyView(Context context) {
super(context);
}
public MyView(Context context, AttributeSet attrs) {
super(context, attrs);
}
public MyView(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
}
@Override
public boolean dispatchTouchEvent(MotionEvent event) {
switch (event.getAction()) {
case MotionEvent.ACTION_DOWN:
Log.i(TAG, "MyView-------------dispatchTouchEvent: ACTION_DOWN");
break;
case MotionEvent.ACTION_MOVE:
Log.i(TAG, "MyView-------------dispatchTouchEvent: ACTION_MOVE");
break;
case MotionEvent.ACTION_UP:
Log.i(TAG, "MyView-------------dispatchTouchEvent: ACTION_UP");
break;
}
return super.dispatchTouchEvent(event);
}
@Override
public boolean onTouchEvent(MotionEvent event) {
switch (event.getAction()) {
case MotionEvent.ACTION_DOWN:
Log.i(TAG, "MyView-------------onTouchEvent: ACTION_DOWN");
break;
case MotionEvent.ACTION_MOVE:
Log.i(TAG, "MyView-------------onTouchEvent: ACTION_MOVE");
break;
case MotionEvent.ACTION_UP:
Log.i(TAG, "MyView-------------onTouchEvent: ACTION_UP");
break;
}
return false;
}
}public class MyViewGroup extends ViewGroup {
private static final String TAG = "lzy";
public MyViewGroup(Context context) {
super(context);
}
public MyViewGroup(Context context, AttributeSet attrs) {
super(context, attrs);
}
public MyViewGroup(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(measureSize(widthMeasureSpec), measureSize(heightMeasureSpec));
//测量每个children
measureChildren(widthMeasureSpec, heightMeasureSpec);
}
private int measureSize(int measureSpec) {
int result = 0;
int specMode = MeasureSpec.getMode(measureSpec);
int specSize = MeasureSpec.getSize(measureSpec);
if (specMode == MeasureSpec.EXACTLY) {
result = specSize;
} else {
result = 300;
if (specMode == MeasureSpec.AT_MOST) {
result = Math.min(result, specSize);
}
}
return result;
}
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
MyView view = (MyView) findViewById(R.id.view);
view.layout(0, 0, view.getMeasuredWidth(), view.getMeasuredHeight());
}
@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
switch (ev.getAction()) {
case MotionEvent.ACTION_DOWN:
Log.i(TAG, "MyViewGroup-------------onInterceptTouchEvent: ACTION_DOWN");
break;
case MotionEvent.ACTION_MOVE:
Log.i(TAG, "MyViewGroup-------------onInterceptTouchEvent: ACTION_MOVE");
break;
case MotionEvent.ACTION_UP:
Log.i(TAG, "MyViewGroup-------------onInterceptTouchEvent: ACTION_UP");
break;
}
return super.onInterceptTouchEvent(ev);
}
@Override
public boolean dispatchTouchEvent(MotionEvent event) {
switch (event.getAction()) {
case MotionEvent.ACTION_DOWN:
Log.i(TAG, "MyViewGroup-------------dispatchTouchEvent: ACTION_DOWN");
break;
case MotionEvent.ACTION_MOVE:
Log.i(TAG, "MyViewGroup-------------dispatchTouchEvent: ACTION_MOVE");
break;
case MotionEvent.ACTION_UP:
Log.i(TAG, "MyViewGroup-------------dispatchTouchEvent: ACTION_UP");
break;
}
return super.dispatchTouchEvent(event);
}
@Override
public boolean onTouchEvent(MotionEvent event) {
switch (event.getAction()) {
case MotionEvent.ACTION_DOWN:
Log.i(TAG, "MyViewGroup-------------onTouchEvent: ACTION_DOWN");
break;
case MotionEvent.ACTION_MOVE:
Log.i(TAG, "MyViewGroup-------------onTouchEvent: ACTION_MOVE");
break;
case MotionEvent.ACTION_UP:
Log.i(TAG, "MyViewGroup-------------onTouchEvent: ACTION_UP");
break;
}
return super.onTouchEvent(event);
}
}
2.把View的onTouchEvent返回true，打印日志如下
3.把ViewGroup的onInterceptTouchEvent返回true，View收不到touch事件
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