View 作为 Android UI 控件的 基础控件里面包含了重要的渲染逻辑 点击事件传递逻辑等，因为是基础控件，代码依赖比较少阅读起来也比较简单。
1. findViewById的真实实现逻辑
protected <T extends View> T findViewTraversal(@IdRes int id) {
if (id == mID) {
return (T) this;
}
return null;
}
@Nullable
public final <T extends View> T findViewById(@IdRes int id) {
if (id == NO_ID) {
return null;
}
return findViewTraversal(id);
}
@NonNull
public final <T extends View> T requireViewById(@IdRes int id) {
T view = findViewById(id);
if (view == null) {
throw new IllegalArgumentException("ID does not reference a View inside this View");
}
return view;
}
总结：
在 View 类中 findViewById() 方法直接调用了 findViewTraversal() 方法，在此方法里匹配入参 id 值，一旦匹配直接返回此 View 示例自己，至此完成了 findViewById() 方法。
2. 点击事件的实现逻辑
public void setOnClickListener(@Nullable OnClickListener l) {
if (!isClickable()) {
setClickable(true);
}
getListenerInfo().mOnClickListener = l;
}
public interface OnClickListener {
void onClick(View v);
}
public boolean onTouchEvent(MotionEvent event) {
final float x = event.getX();
final float y = event.getY();
final int viewFlags = mViewFlags;
final int action = event.getAction();
final boolean clickable = ((viewFlags & CLICKABLE) == CLICKABLE
|| (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)
|| (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE;
if ((viewFlags & ENABLED_MASK) == DISABLED) {
if (action == MotionEvent.ACTION_UP && (mPrivateFlags & PFLAG_PRESSED) != 0) {
setPressed(false);
}
mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
return clickable;
}
// 整个View类的点击时间传递在这5行代码里清晰可见。
if (mTouchDelegate != null) {
if (mTouchDelegate.onTouchEvent(event)) {
return true;
}
}
if (clickable || (viewFlags & TOOLTIP) == TOOLTIP) {
switch (action) {
case MotionEvent.ACTION_UP:
mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
if ((viewFlags & TOOLTIP) == TOOLTIP) {
handleTooltipUp();
}
if (!clickable) {
removeTapCallback();
removeLongPressCallback();
mInContextButtonPress = false;
mHasPerformedLongPress = false;
mIgnoreNextUpEvent = false;
break;
}
boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0;
if ((mPrivateFlags & PFLAG_PRESSED) != 0 || prepressed) {
boolean focusTaken = false;
if (isFocusable() && isFocusableInTouchMode() && !isFocused()) {
focusTaken = requestFocus();
}
if (prepressed) {
setPressed(true, x, y);
}
if (!mHasPerformedLongPress && !mIgnoreNextUpEvent) {
removeLongPressCallback();
if (!focusTaken) {
if (mPerformClick == null) {
mPerformClick = new PerformClick();
}
if (!post(mPerformClick)) {
performClickInternal(); // 重点代码
}
}
}
if (mUnsetPressedState == null) {
mUnsetPressedState = new UnsetPressedState();
}
if (prepressed) {
postDelayed(mUnsetPressedState,
ViewConfiguration.getPressedStateDuration());
} else if (!post(mUnsetPressedState)) {
mUnsetPressedState.run();
}
removeTapCallback();
}
mIgnoreNextUpEvent = false;
break;
case MotionEvent.ACTION_DOWN:
if (event.getSource() == InputDevice.SOURCE_TOUCHSCREEN) {
mPrivateFlags3 |= PFLAG3_FINGER_DOWN;
}
mHasPerformedLongPress = false;
if (!clickable) {
checkForLongClick(0, x, y);
break;
}
if (performButtonActionOnTouchDown(event)) {
break;
}
boolean isInScrollingContainer = isInScrollingContainer();
if (isInScrollingContainer) {
mPrivateFlags |= PFLAG_PREPRESSED;
if (mPendingCheckForTap == null) {
mPendingCheckForTap = new CheckForTap();
}
mPendingCheckForTap.x = event.getX();
mPendingCheckForTap.y = event.getY();
postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());
} else {
setPressed(true, x, y);
checkForLongClick(0, x, y);
}
break;
case MotionEvent.ACTION_CANCEL:
if (clickable) {
setPressed(false);
}
removeTapCallback();
removeLongPressCallback();
mInContextButtonPress = false;
mHasPerformedLongPress = false;
mIgnoreNextUpEvent = false;
mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
break;
case MotionEvent.ACTION_MOVE:
if (clickable) {
drawableHotspotChanged(x, y);
}
if (!pointInView(x, y, mTouchSlop)) {
removeTapCallback();
removeLongPressCallback();
if ((mPrivateFlags & PFLAG_PRESSED) != 0) {
setPressed(false);
}
mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
}
break;
}
return true;
}
return false;
}
private boolean performClickInternal() {
notifyAutofillManagerOnClick();
return performClick();
}
public boolean performClick() {
notifyAutofillManagerOnClick();
final boolean result;
final ListenerInfo li = mListenerInfo;
if (li != null && li.mOnClickListener != null) {
playSoundEffect(SoundEffectConstants.CLICK);
li.mOnClickListener.onClick(this);
result = true;
} else {
result = false;
}
sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);
notifyEnterOrExitForAutoFillIfNeeded(true);
return result;
}
总结：
View.java 通过 ListenerInfo 保存了几乎所有的监听者，其中就有内部接口 OnClickListener定义的 OnClickListener，通过 setOnClickListener 设置了监听者。在 OnTouchEvent 方法里 performClickInternal() 被调用最后调用到 performClick() 至此完成了 onClick() 方法的回调。
3. findViewById() 的延伸发现
有新的发现：
final <T extends View> T findViewByAccessibilityId(int accessibilityId) {
if (accessibilityId < 0) {
return null;
}
T view = findViewByAccessibilityIdTraversal(accessibilityId);
if (view != null) {
return view.includeForAccessibility() ? view : null;
}
return null;
}
public <T extends View> T findViewByAccessibilityIdTraversal(int accessibilityId) {
if (getAccessibilityViewId() == accessibilityId) {
return (T) this;
}
return null;
}
public <T extends View> T findViewByAutofillIdTraversal(int autofillId) {
if (getAutofillViewId() == autofillId) {
return (T) this;
}
return null;
}
public final <T extends View> T findViewByPredicate(Predicate<View> predicate) {
return findViewByPredicateTraversal(predicate, null);
}
protected <T extends View> T findViewByPredicateTraversal(Predicate<View> predicate,
View childToSkip) {
if (predicate.test(this)) {
return (T) this;
}
return null;
}
public final <T extends View> T findViewByPredicateInsideOut(
View start, Predicate<View> predicate) {
View childToSkip = null;
for (;;) {
T view = start.findViewByPredicateTraversal(predicate, childToSkip);
if (view != null || start == this) {
return view;
}
ViewParent parent = start.getParent();
if (parent == null || !(parent instanceof View)) {
return null;
}
childToSkip = start;
start = (View) parent;
}
}
总结：
找 View 的方法就这里几个，通过 id, Pridicate, accessbilityId， autoFillId。
猜想 Pridicate是测试用的，accessibility适用于辅助功能方面，autoFillId应该是系统自动分配的id。
4. View 类点击事件传递的关键代码被发现
接下来我们看一下 mTouchDelegate 是什么东西，猜想是点击事件代理者。
public void setTouchDelegate(TouchDelegate delegate) {
mTouchDelegate = delegate;
}
public TouchDelegate getTouchDelegate() {
return mTouchDelegate;
}
在Android源码里给的注释如下：
我们先来看看 postDelayed() 方法的全部内容.
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这里 getRunQueue() 方法 new 了一个新的 HandlerActionQueue, 这个类和 View.java 在同一目录下所以不用导包.
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那我们现在看一下 HandlerActionQueue中的 postDelayed() 方法做了什么事情吧.
package android.view;
import android.os.Handler;
import com.android.internal.util.GrowingArrayUtils;
public class HandlerActionQueue {
private HandlerAction[] mActions;
private int mCount;
public void post(Runnable action) {
postDelayed(action, 0);
}
public void postDelayed(Runnable action, long delayMillis) {
final HandlerAction handlerAction = new HandlerAction(action, delayMillis);
synchronized (this) {
if (mActions == null) {
mActions = new HandlerAction[4];
}
mActions = GrowingArrayUtils.append(mActions, mCount, handlerAction);
mCount++;
}
}
public void removeCallbacks(Runnable action) {
synchronized (this) {
final int count = mCount;
int j = 0;
final HandlerAction[] actions = mActions;
for (int i = 0; i < count; i++) {
if (actions[i].matches(action)) {
// Remove this action by overwriting it within
// this loop or nulling it out later.
continue;
}
if (j != i) {
// At least one previous entry was removed, so
// this one needs to move to the "new" list.
actions[j] = actions[i];
}
j++;
}
// The "new" list only has j entries.
mCount = j;
// Null out any remaining entries.
for (; j < count; j++) {
actions[j] = null;
}
}
}
public void executeActions(Handler handler) {
synchronized (this) {
final HandlerAction[] actions = mActions;
for (int i = 0, count = mCount; i < count; i++) {
final HandlerAction handlerAction = actions[i];
handler.postDelayed(handlerAction.action, handlerAction.delay);
}
mActions = null;
mCount = 0;
}
}
public int size() {
return mCount;
}
public Runnable getRunnable(int index) {
if (index >= mCount) {
throw new IndexOutOfBoundsException();
}
return mActions[index].action;
}
public long getDelay(int index) {
if (index >= mCount) {
throw new IndexOutOfBoundsException();
}
return mActions[index].delay;
}
//HandlerAction内部类用来封装 Runnable的action和delay的时间
private static class HandlerAction {
final Runnable action;
final long delay;
public HandlerAction(Runnable action, long delay) {
this.action = action;
this.delay = delay;
}
public boolean matches(Runnable otherAction) {
return otherAction == null && action == null
|| action != null && action.equals(otherAction);
}
}
}
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