关于View的dispatchEvent onTouch onTouchEvent

首先先贴出View类中dispatchEvent 方法的源码。

public boolean dispatchTouchEvent(MotionEvent event) {
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(event, 0);
}

if (onFilterTouchEventForSecurity(event)) {
//noinspection SimplifiableIfStatement
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) {
return true;
}

if (onTouchEvent(event)) {
return true;
}
}

if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
}
return false;
}

在android系统中当控件，例如：Button、ImageView等被触碰时，会调用其dispatchTouchEvent方法,它们的dispatchTouchEvent方法都继承自View的dispatchTouchEvent方法。该方法的返回值决定着对应控件是否会接受接下来的事件。

下面这段代码决定着dispatchTouchEvent的返回值。

若返回true则意味着控件会继续会接收到TOUCH事件后续的UP、MOVE、CANCLE事件。

if (li != null && li.mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) {
return true;
}

if (onTouchEvent(event)) {
return true;
}

而第一个if判断语句中的

mViewFlags & ENABLED_MASK

则是判断控件的ENABLE属性的状态，该状态决定着控件是否响应onTouch(？) ，若为false则直接进入到下一个if中判断onTouchEvent的返回值。下面时onTouchEvent的源码

public boolean onTouchEvent(MotionEvent event) {
final int viewFlags = mViewFlags;

if ((viewFlags & ENABLED_MASK) == DISABLED) {
if (event.getAction() == MotionEvent.ACTION_UP && (mPrivateFlags & PFLAG_PRESSED) != 0) {
setPressed(false);
}
// A disabled view that is clickable still consumes the touch
// events, it just doesn't respond to them.
return (((viewFlags & CLICKABLE) == CLICKABLE ||
(viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE));
}

if (mTouchDelegate != null) {
if (mTouchDelegate.onTouchEvent(event)) {
return true;
}
}

if (((viewFlags & CLICKABLE) == CLICKABLE ||
(viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)) {
switch (event.getAction()) {
case MotionEvent.ACTION_UP:
boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0;
if ((mPrivateFlags & PFLAG_PRESSED) != 0 || prepressed) {
// take focus if we don't have it already and we should in
// touch mode.
boolean focusTaken = false;
if (isFocusable() && isFocusableInTouchMode() && !isFocused()) {
focusTaken = requestFocus();
}

if (prepressed) {
// The button is being released before we actually
// showed it as pressed.  Make it show the pressed
// state now (before scheduling the click) to ensure
// the user sees it.
setPressed(true);
}

if (!mHasPerformedLongPress) {
// This is a tap, so remove the longpress check
removeLongPressCallback();

// Only perform take click actions if we were in the pressed state
if (!focusTaken) {
// Use a Runnable and post this rather than calling
// performClick directly. This lets other visual state
// of the view update before click actions start.
if (mPerformClick == null) {
mPerformClick = new PerformClick();
}
if (!post(mPerformClick)) {
performClick();
}
}
}

if (mUnsetPressedState == null) {
mUnsetPressedState = new UnsetPressedState();
}

if (prepressed) {
postDelayed(mUnsetPressedState,
ViewConfiguration.getPressedStateDuration());
} else if (!post(mUnsetPressedState)) {
// If the post failed, unpress right now
mUnsetPressedState.run();
}
removeTapCallback();
}
break;

case MotionEvent.ACTION_DOWN:
mHasPerformedLongPress = false;

if (performButtonActionOnTouchDown(event)) {
break;
}

// Walk up the hierarchy to determine if we're inside a scrolling container.
boolean isInScrollingContainer = isInScrollingContainer();

// For views inside a scrolling container, delay the pressed feedback for
// a short period in case this is a scroll.
if (isInScrollingContainer) {
mPrivateFlags |= PFLAG_PREPRESSED;
if (mPendingCheckForTap == null) {
mPendingCheckForTap = new CheckForTap();
}
postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());
} else {
// Not inside a scrolling container, so show the feedback right away
setPressed(true);
checkForLongClick(0);
}
break;

case MotionEvent.ACTION_CANCEL:
setPressed(false);
removeTapCallback();
removeLongPressCallback();
break;

case MotionEvent.ACTION_MOVE:
final int x = (int) event.getX();
final int y = (int) event.getY();

// Be lenient about moving outside of buttons
if (!pointInView(x, y, mTouchSlop)) {
// Outside button
removeTapCallback();
if ((mPrivateFlags & PFLAG_PRESSED) != 0) {
// Remove any future long press/tap checks
removeLongPressCallback();

setPressed(false);
}
}
break;
}
return true;
}

return false;
}

而当控件是textview 或 buton是它们的方法复写了View的dispatchEvent方法，从代码中也可以看见它会调用到View的onTouchEvent方法。所以即使在onTouch中返回false时它也能接收到接下来的UP MOVE事件因为它在onTouchEvent中会可能?返回true,从而接收到接下来的事件.

public boolean onTouchEvent(MotionEvent event) {
final int action = event.getActionMasked();

if (mEditor != null) mEditor.onTouchEvent(event);

final boolean superResult = super.onTouchEvent(event);

/*
* Don't handle the release after a long press, because it will
* move the selection away from whatever the menu action was
* trying to affect.
*/
if (mEditor != null && mEditor.mDiscardNextActionUp && action == MotionEvent.ACTION_UP) {
mEditor.mDiscardNextActionUp = false;
return superResult;
}

final boolean touchIsFinished = (action == MotionEvent.ACTION_UP) &&
(mEditor == null || !mEditor.mIgnoreActionUpEvent) && isFocused();

if ((mMovement != null || onCheckIsTextEditor()) && isEnabled()
&& mText instanceof Spannable && mLayout != null) {
boolean handled = false;

if (mMovement != null) {
handled |= mMovement.onTouchEvent(this, (Spannable) mText, event);
}

final boolean textIsSelectable = isTextSelectable();
if (touchIsFinished && mLinksClickable && mAutoLinkMask != 0 && textIsSelectable) {
// The LinkMovementMethod which should handle taps on links has not been installed
// on non editable text that support text selection.
// We reproduce its behavior here to open links for these.
ClickableSpan[] links = ((Spannable) mText).getSpans(getSelectionStart(),
getSelectionEnd(), ClickableSpan.class);

if (links.length > 0) {
links[0].onClick(this);
handled = true;
}
}

if (touchIsFinished && (isTextEditable() || textIsSelectable)) {
// Show the IME, except when selecting in read-only text.
final InputMethodManager imm = InputMethodManager.peekInstance();
viewClicked(imm);
if (!textIsSelectable && mEditor.mShowSoftInputOnFocus) {
handled |= imm != null && imm.showSoftInput(this, 0);
}

// The above condition ensures that the mEditor is not null
mEditor.onTouchUpEvent(event);

handled = true;
}

if (handled) {
return true;
}
}

return superResult;
}