提示：在閱讀本文章以前，請確保您對Touch事件的分發機制有必定的瞭解，若是您不瞭解請參閱：Touch事件的分發機制。java
在Android的學習過程當中常常會聽到或者見到「回調」這個詞，那麼什麼是回調呢？所謂的回調函數就是：在A類中定義了一個方法，這個方法中用到了一個接口和該接口中的抽象方法，可是抽象方法沒有具體的實現，須要B類去實現，B類實現該方法後，它自己不會去調用該方法，而是傳遞給A類，供A類去調用，這種機制就稱爲回調。ide
下面咱們拿具體的Button的點擊事件進行模擬分析：函數
首先，在View類中咱們能找到setOnClickListener(OnClickListener l)方法：post
public void setOnClickListener(OnClickListener l) {
if (!isClickable()) {
setClickable(true);
}
getListenerInfo().mOnClickListener = l;
}
能夠看到，在該方法中將OnClickListener賦值給了mOnClickListener，那麼咱們繼續向下找，會看到在performClick()方法中執行了咱們實現的onClick()方法。學習
public boolean performClick() {
sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnClickListener != null) {
playSoundEffect(SoundEffectConstants.CLICK);
li.mOnClickListener.onClick(this);
return true;
}
return false;
}
由此咱們能夠清楚的看不到，在父類中咱們要用到onClick()方法，可是父類卻沒有去實現該方法，而是定義了一個方法setOnClickListener(OnClickListener l)，若是子類想要本身可以響應點擊事件，則它就必須重寫父類的該方法，實現OnClickListener接口和它的onClick()方法。在子類實現該接口和方法後，將其經過參數傳遞給父類，在父類中執行onClick()方法。this
那麼，爲何會在父類中執行到該方法呢，這就要說到Android中的另外一個重要的機制——觸摸事件的傳遞機制。spa
咱們知道，只要咱們的手指觸摸到手機屏幕，就必定會執行dispatchTouchEvent(MotionEvent event)方法，接下來咱們就看一下dispatchTouchEvent方法中都有哪些內容：.net
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;
}
這裏咱們不細講Touch事件的分發機制，由於網上有哥們已經講的很清楚了。請參看篇首提供的連接。code
咱們看一下第17行，因爲咱們沒有實現OnTouchListener接口，而onTouch()方法的默認返回值爲false，因此第一個if語句中的代碼不會被執行到，進入第二個if語句中，執行了onTouchEvent()方法。那麼咱們再來看一下該方法：orm
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;
}
代碼太長，咱們只看重點，在ACTION_UP這個case當中，咱們找到了關鍵的代碼（第109行）：performClick()。
至此，咱們已經基本搞清楚了回調機制的整個過程。
原創文章，能夠自由傳播，但轉載請註明出處。