編輯:關於Android編程
本篇主要分析的是touch事件的分發機制,網上關於這個知識點的分析文章非常多。但是還是想通過結合自身的總結,來加深自己的理解。對於事件分發機制,我將使用兩篇文章對其進行分析,一篇是針對View的事件分發機制解析,一篇是針對ViewGroup的事件分發機制解析。本片是對View的事件分發機制進行解析,主要采用案例結合源碼的方式來進行分析。
在分析事件分發機制之前,我們先來學習一下基本的知識點,以便後面的理解。
View中有兩個關鍵方法參與到Touch事件分發
dispatchTouchEvent(MotionEvent event) 和 onTouchEvent(MotionEvent event)
所有Touch事件類型都被封裝在對象MotionEvent中,包括ACTION_DOWN,ACTION_MOVE,ACTION_UP等等。
每個執行動作必須執行完一個完整的流程,再繼續進行下一個動作。比如:ACTION_DOWN事件發生時,必須等這個事件的分發流程執行完(包括該事件被提前消費),才會繼續執行ACTION_MOVE或者ACTION_UP的事件。
為了能夠清楚的監視事件的分發過程,我們采用自定義View的形式,查看內部的方法執行過程。
上代碼:
package com.yuminfeng.touch;
import android.content.Context;
import android.util.AttributeSet;
import android.util.Log;
import android.view.MotionEvent;
import android.widget.Button;
public class MyButton extends Button {
public MyButton(Context context, AttributeSet attrs) {
super(context, attrs);
}
@Override
public boolean dispatchTouchEvent(MotionEvent event) {
switch (event.getAction()) {
case MotionEvent.ACTION_DOWN:
Log.i("yumf", "MyButton=====dispatchTouchEvent ACTION_DOWN");
break;
case MotionEvent.ACTION_UP:
Log.i("yumf", "MyButton=====dispatchTouchEvent ACTION_UP");
break;
}
return super.dispatchTouchEvent(event);
}
@Override
public boolean onTouchEvent(MotionEvent event) {
switch (event.getAction()) {
case MotionEvent.ACTION_DOWN:
Log.i("yumf", "MyButton=====onTouchEvent ACTION_DOWN");
break;
case MotionEvent.ACTION_UP:
Log.i("yumf", "MyButton=====onTouchEvent ACTION_UP");
break;
}
return super.onTouchEvent(event);
}
}
在XML布局中引用該控件,非常簡單。
以上代碼都非常簡單,沒有什麼邏輯,就是重寫Button的dispatchTouchEvent和onTouchEvent的方法,然後引用該控件即可。
然後執行代碼,查看日志打印,如下:
由此看到,當點擊控件時,首先執行的是dispatchTouchEvent方法,然後再執行onTouchEvent的方法。
如果此時我們修改dispatchTouchEvent的返回值為true時(默認為false),那麼onTouchEvent方法便不再執行,如下:
流程示意圖如下:
接著我們恢復之前的返回值false,繼續讓mybutton設置一個setOnTouchListener監聽事件,關鍵代碼如下:<喎?/kf/ware/vc/" target="_blank" class="keylink">vcD4NCjxwcmUgY2xhc3M9"brush:java;">
mybutton = (Button) findViewById(R.id.mybutton);
mybutton.setOnTouchListener(new OnTouchListener() {
@Override
public boolean onTouch(View v, MotionEvent event) {
switch (event.getAction()) {
case MotionEvent.ACTION_DOWN:
Log.i("yumf", "Activity=====onTouch ACTION_DOWN");
break;
case MotionEvent.ACTION_UP:
Log.i("yumf", "Activity=====onTouch ACTION_UP");
break;
}
return false;
}
});
執行後,日志打印如下:
由此我們可以看到,首先執行方法dispatchTouchEvent,然後再執行OnTouchListener中onTouch方法,最後執行onTouchEvent方法。
同上,如果我們繼續修改dispatchTouchEvent的返回值為true時,那麼後面的方法onTouch,onTouchEvent均不執行。
如果我們修改onTouch的返回值為true,那麼後面的onTouchEvent事件就不會執行了。
流程示意圖如下:
如上,恢復默認返回值false,然後在button上設置一個監聽點擊事件,代碼如下:
mybutton.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
Log.i("yumf", "Activity=====onClick");
}
});
執行後,查看日志打印信息,如下:
由此我們可以知道,在完整的事件結束之後(從ACTION_DOWN開始,到ACTION_UP結束),這時才會去執行button的onClick方法。
綜合以上所述,View在處理Touch事件時,都是從dispatchTouchEvent方法開始的,因此我們在分析源碼時,可以從該方法入手。
我們當前的MyButton是繼承自Button,而Button又是繼承自TextView,TextView繼承自View,逐步往上查看,可以發現父類的dispatchTouchEvent方法,就是View的dispatchTouchEvent方法。如下:
/**
* Pass the touch screen motion event down to the target view, or this
* view if it is the target.
*
* @param event The motion event to be dispatched.
* @return True if the event was handled by the view, false otherwise.
*/
public boolean dispatchTouchEvent(MotionEvent event) {
boolean result = false;
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(event, 0);
}
final int actionMasked = event.getActionMasked();
if (actionMasked == MotionEvent.ACTION_DOWN) {
// Defensive cleanup for new gesture
stopNestedScroll();
}
if (onFilterTouchEventForSecurity(event)) {
//noinspection SimplifiableIfStatement
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnTouchListener != null
&& (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) {
result = true;
}
if (!result && onTouchEvent(event)) {
result = true;
}
}
if (!result && mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
}
// Clean up after nested scrolls if this is the end of a gesture;
// also cancel it if we tried an ACTION_DOWN but we didn't want the rest
// of the gesture.
if (actionMasked == MotionEvent.ACTION_UP ||
actionMasked == MotionEvent.ACTION_CANCEL ||
(actionMasked == MotionEvent.ACTION_DOWN && !result)) {
stopNestedScroll();
}
return result;
}
如上代碼中,我們來逐一進行分析,首先是
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(event, 0);
}
通過查看mInputEventConsistencyVerifier,得知這段代碼主要是用來調試的,可以不用關注。接著繼續查看下一段代碼
final int actionMasked = event.getActionMasked();
if (actionMasked == MotionEvent.ACTION_DOWN) {
// Defensive cleanup for new gesture
stopNestedScroll();
}
當執行ACTION_DOWN事件時,進入方法stopNestedScroll()中,進入該方法中
/**
* Stop a nested scroll in progress.
*
*
Calling this method when a nested scroll is not currently in progress is harmless.
* * @see #startNestedScroll(int) */ public void stopNestedScroll() { if (mNestedScrollingParent != null) { mNestedScrollingParent.onStopNestedScroll(this); mNestedScrollingParent = null; } }
該方法主要是用來停止View的滑動,當一個滾動的view不是當前進行接收事件的View時不會受到影響。下面的一段代碼是關鍵的代碼,我們來看看
if (onFilterTouchEventForSecurity(event)) {
//noinspection SimplifiableIfStatement
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnTouchListener != null
&& (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) {
result = true;
}
if (!result && onTouchEvent(event)) {
result = true;
}
}
上面的代碼中,首先根據安全策略過濾event,來確定是否響應這個事件,返回true表示響應。響應該事件後,將mListenerInfo賦值給ListenerInfo對象。那麼這個mListenerInfo到底是什麼呢,我們現在來分析一下mListenerInfo的初始化
首先,我們可以在View的屬性中,能看到該對象的引用:
ListenerInfo mListenerInfo;
接著,在getListenerInfo()方法中初始化:
ListenerInfo getListenerInfo() {
if (mListenerInfo != null) {
return mListenerInfo;
}
mListenerInfo = new ListenerInfo();
return mListenerInfo;
}
最後,在為該View的對象設置監聽器時,會將對應的監聽器對象返回賦值給mListenerInfo對象,如下:
/**
* Register a callback to be invoked when focus of this view changed.
*
* @param l The callback that will run.
*/
public void setOnFocusChangeListener(OnFocusChangeListener l) {
getListenerInfo().mOnFocusChangeListener = l;
}
/**
* Returns the focus-change callback registered for this view.
*
* @return The callback, or null if one is not registered.
*/
public OnFocusChangeListener getOnFocusChangeListener() {
ListenerInfo li = mListenerInfo;
return li != null ? li.mOnFocusChangeListener : null;
}
/**
* Register a callback to be invoked when this view is clicked. If this view is not
* clickable, it becomes clickable.
*
* @param l The callback that will run
*
* @see #setClickable(boolean)
*/
public void setOnClickListener(OnClickListener l) {
if (!isClickable()) {
setClickable(true);
}
getListenerInfo().mOnClickListener = l;
}
/**
* Register a callback to be invoked when this view is clicked and held. If this view is not
* long clickable, it becomes long clickable.
*
* @param l The callback that will run
*
* @see #setLongClickable(boolean)
*/
public void setOnLongClickListener(OnLongClickListener l) {
if (!isLongClickable()) {
setLongClickable(true);
}
getListenerInfo().mOnLongClickListener = l;
}
/**
* Register a callback to be invoked when a touch event is sent to this view.
* @param l the touch listener to attach to this view
*/
public void setOnTouchListener(OnTouchListener l) {
getListenerInfo().mOnTouchListener = l;
}
如上,其實裡面涉及的方法非常多,我只抽出了幾個常見的方法,如:setOnClickListener,setOnTouchListener等。
所以說當我們給View的對象設置監聽器時,通過回調的方式,最後都會賦值到mListenerInfo對象中。mListenerInfo類裡面包含了許多的監聽器類型,如下:
static class ListenerInfo {
/**
* Listener used to dispatch focus change events.
* This field should be made private, so it is hidden from the SDK.
* {@hide}
*/
protected OnFocusChangeListener mOnFocusChangeListener;
/**
* Listeners for layout change events.
*/
private ArrayList mOnLayoutChangeListeners;
/**
* Listeners for attach events.
*/
private CopyOnWriteArrayList mOnAttachStateChangeListeners;
/**
* Listener used to dispatch click events.
* This field should be made private, so it is hidden from the SDK.
* {@hide}
*/
public OnClickListener mOnClickListener;
/**
* Listener used to dispatch long click events.
* This field should be made private, so it is hidden from the SDK.
* {@hide}
*/
protected OnLongClickListener mOnLongClickListener;
/**
* Listener used to build the context menu.
* This field should be made private, so it is hidden from the SDK.
* {@hide}
*/
protected OnCreateContextMenuListener mOnCreateContextMenuListener;
private OnKeyListener mOnKeyListener;
private OnTouchListener mOnTouchListener;
private OnHoverListener mOnHoverListener;
private OnGenericMotionListener mOnGenericMotionListener;
private OnDragListener mOnDragListener;
private OnSystemUiVisibilityChangeListener mOnSystemUiVisibilityChangeListener;
OnApplyWindowInsetsListener mOnApplyWindowInsetsListener;
}
完成了監聽器類型的賦值後,我們分析繼續下面的代碼邏輯:
if (li != null && li.mOnTouchListener != null
&& (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) {
result = true;
}
if (!result && onTouchEvent(event)) {
result = true;
}
這裡在if條件裡面我們看到了一個屬性的方法li.mOnTouchListener.onTouch(this, event),這就是我們在Activity中設置的setOnTouchListener中,重寫的onTouch方法。當返回為true時,result = true,這時便不執行下面代碼中的onTouchEvent(event)方法。result 為false時,才執行onTouchEvent(event)方法。這段關鍵性的代碼中,對應了我之前所做的實驗結果。
下面,我們繼續分析方法View的onTouchEvent(MotionEvent event)的內部執行。
/** * Implement this method to handle touch screen motion events. * <p> * If this method is used to detect click actions, it is recommended that * the actions be performed by implementing and calling * {@link #performClick()}. This will ensure consistent system behavior, * including: * <ul> * <li>obeying click sound preferences * <li>dispatching OnClickListener calls * <li>handling {@link AccessibilityNodeInfo#ACTION_CLICK ACTION_CLICK} when * accessibility features are enabled * </ul> * * @param event The motion event. * @return True if the event was handled, false otherwise. */ public boolean onTouchEvent(MotionEvent event) { final float x = event.getX(); final float y = event.getY(); 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, x, y); } 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(); } mPendingCheckForTap.x = event.getX(); mPendingCheckForTap.y = event.getY(); postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout()); } else { // Not inside a scrolling container, so show the feedback right away setPressed(true, x, y); checkForLongClick(0); } break; case MotionEvent.ACTION_CANCEL: setPressed(false); removeTapCallback(); removeLongPressCallback(); break; case MotionEvent.ACTION_MOVE: drawableHotspotChanged(x, y); // 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; } 1
首先看這個方法的說明,實現這個方法來處理觸摸屏幕的動作事件。如果這個方法被用來檢測點擊動作,它是建議執行和調用的操作。如果這個事件被處理,返回true,否則返回false。
現在我們來看逐一代碼,第一個if語句塊中,判斷View的狀態是否可用,如果不可用則設置為不可按壓,否則為設置為可點擊和可長按。然後下面在可點擊和可長按的條件下,進行touch事件的邏輯處理。在這個if語句內部有switch條件判斷,將分別對不同的事件進行處理,如MotionEvent.ACTION_UP,MotionEvent.ACTION_DOWN,MotionEvent.ACTION_CANCEL 和MotionEvent.ACTION_MOVE幾個不同的事件。下面我們將逐一對其進行分析。
首先是MotionEvent.ACTION_UP中:
判斷prepressed為true後,進入執行體;
設置setPressed(true, x, y);
判斷mHasPerformedLongPress是否執行長按操作,如果mOnLongClickListener.onLongClick 返回true時,mHasPerformedLongPress = true,這時便不會執行performClick()方法。否則繼續執行如下,判斷mPerformClick為空,初始化一個實例。添加到消息隊列中,如果添加失敗則直接執行performClick()方法,否則在PerformClick對象的run中執行performClick()。查看一下performClick()方法,如下:
/**
* Call this view's OnClickListener, if it is defined. Performs all normal
* actions associated with clicking: reporting accessibility event, playing
* a sound, etc.
*
* @return True there was an assigned OnClickListener that was called, false
* otherwise is returned.
*/
public boolean performClick() {
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);
return result;
}
如上,我們可以看到一個非常熟悉的方法onClick。在if中判斷,如果我們給View設置了mOnClickListener的監聽接口,在這裡我們會回調mOnClickListener中的onClick方法。(原來點擊事件的onClick方法是在ACTION_UP時,執行的)
接著,看到創建UnsetPressedState對象,然後執行UnsetPressedState對象中的run方法,我們進入這個方法查看,
private final class UnsetPressedState implements Runnable {
@Override
public void run() {
setPressed(false);
}
}
/**
* Sets the pressed state for this view.
*
* @see #isClickable()
* @see #setClickable(boolean)
*
* @param pressed Pass true to set the View's internal state to "pressed", or false to reverts
* the View's internal state from a previously set "pressed" state.
*/
public void setPressed(boolean pressed) {
final boolean needsRefresh = pressed != ((mPrivateFlags & PFLAG_PRESSED) == PFLAG_PRESSED);
if (pressed) {
mPrivateFlags |= PFLAG_PRESSED;
} else {
mPrivateFlags &= ~PFLAG_PRESSED;
}
if (needsRefresh) {
refreshDrawableState();
}
dispatchSetPressed(pressed);
}
可以看到,這裡面是用來取消mPrivateFlags 中的PFLAG_PRESSED標志,然後刷新背景。
ACTION_UP最後一步,removeTapCallback() 移除消息隊列中的之前加入的所有回調操作。
接著分析MotionEvent.ACTION_DOWN中內部代碼:
首先mHasPerformedLongPress = false,設置長按操作為false。
接著判斷View是否處在可滑動的容器中,如果為false,則設置View的PRESSED狀態和檢查長按動作。
接著分析MotionEvent.ACTION_CANCEL的事件:
代碼非常簡單,設置PRESSED為false,移除所有的回調,移除長按的回調。
最後來分析MotionEvent.ACTION_MOVE的事件:
判斷觸摸點是否移出View的范圍,如果移出了則執行removeTapCallback(),取消所有的回調。接著判斷是否包含PRESSED標識,如果包含則執行方法removeLongPressCallback() 和 setPressed(false);
到這裡我們可以知道,onTouchEvent方法中處理Touch事件的具體操作,並控制了View的點擊事件。在如果在點擊View時,想要長按和短按都產生效果,即setOnLongClickListener和setOnClickListener都能夠執行的話,只需要在setOnLongClickListener的onLongClick方法中返回false,這時兩個方法便都能執行。
至此關於View的Touch事件分發流程已經分析完成,下一篇將介紹ViewGroup的分發機制。
obeying click sound preferences *
dispatching OnClickListener calls *
handling {@link AccessibilityNodeInfo#ACTION_CLICK ACTION_CLICK} when * accessibility features are enabled *
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