編輯：關於Android編程
當一個Touch事件發生，系統首先把事件傳遞給當前的Activity，由Activity的dispatchTouchEvent分發事件，源碼如下：
public boolean dispatchTouchEvent(MotionEvent ev) {
if (ev.getAction() == MotionEvent.ACTION_DOWN) {
onUserInteraction();
}
if (getWindow().superDispatchTouchEvent(ev)) {
return true;
}
return onTouchEvent(ev);
}
Down事件發生時會進入一個onUserInteraction()方法，點進去可以看到這是一個空方法，我們可以根據需求重寫該方法。
getWindow().superDispatchTouchEvent(ev)調用Window的superDispatchTouchEvent方法分發事件，而Window的實現是PhoneWindow，所以是調用PhoneWindow的superDispatchTouchEvent派發事件，在這裡面就是將事件從最外層向裡逐級派發。如果返回false，也就是事件沒有消耗，會執行下面的onTouchEvent
下面就看看ViewGroup裡面的事件派發
@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 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;
}
1.在19~25行，是處理down事件，清除之前的狀態，cancelAndClearTouchTargets(ev)中把mFirstTouchTarget設置為空。
2.到28行，是判斷是否攔截事件，看看29~42這段代碼
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;
}
disallowIntercept是禁止攔截
當Action_Down時，如果disallowIntercept為true，intercepted 為false，不攔截；disallowIntercept為false，執行onInterceptTouchEvent(ev)方法，返回值決定intercepted
當不是Action_Down時，如果mFirstTouchTarget ==null，intercepted 為true，攔截事件；mFirstTouchTarget !=null，執行和上面一樣。onInterceptTouchEvent(ev)默認返回時false，可以在ViewGroup裡面重寫，修改返回是否攔截。
ViewGroup中的requestDisallowInterceptTouchEvent( )方法可以用來禁止或允許ViewGroup攔截Touch事件，但是它對於ACTION_DOWN是無效的。因為在ACTION_DOWN時會調用resetTouchState()重置了FLAG_DISALLOW_INTERCEPT，所以總會調用onInterceptTouchEvent判讀是否攔截。
3.28~158，是對down事件的處理
檢查是否cancel，然後進行事件的分發。58行，事件沒有被cancel或者攔截會進入下面判斷
68~70行判斷事件是否是ACTION_DOWN，ACTION_POINTER_DOWN（多點觸控），ACTION_HOVER_MOVE（懸停），如果是執行內部的語句。
89行的for循環是查找接收touch事件的view，找到點擊位置的子view，調用dispatchTransformedTouchEvent()，把touch事件派發給子view，dispatchTransformedTouchEvent()的返回值為true，代表子view消費了事件，並且添加此子view到觸摸鏈表，放到表頭。mFirstTouchTarget 也就不為空了，否則返回false，mFirstTouchTarget 還是空。
newTouchTarget = addTouchTarget(child, idBitsToAssign);
163~197，繼續分發事件，down，move，up都會進入。注意這裡是mFirstTouchTarget不為空才進入的，也就是說view沒有處理down事件，那麼接下來的move，up事件也不會處理
當mFirstTouchTarget為空，表示ViewGroup攔截了事件，或者子view沒有消費事件，也就直接調用dispatchTransformedTouchEvent()
當mFirstTouchTarget不為空，174~175，alreadyDispatchedToNewTouchTarget為true表示已經處理了down事件
move和up事件沒有處理，調用dispatchTransformedTouchEvent()處理
下面寫個demo測試
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);
}
}
自定義View
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;
}
}
自定義ViewGroup
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));
//測量每個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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