Android 原始碼解析View的touch事件分發機制
概述
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,主要採用案例結合原始碼的方式來進行分析。
前言
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事件型別都被封裝在物件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。
案例分析
為了能夠清楚的監視事件的分發過程,我們採用自定義View的形式,檢視內部的方法執行過程。
上程式碼:
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佈局中引用該控制元件,非常簡單。
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.yuminfeng.myviewpager.FirstActivity" >
<com.yuminfeng.touch.MyButton
android:id="@+id/mybutton"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/hello_world" />
</RelativeLayout>
以上程式碼都非常簡單,沒有什麼邏輯,就是重寫Button的dispatchTouchEvent和onTouchEvent的方法,然後引用該控制元件即可。
然後執行程式碼,檢視日誌列印,如下:
由此看到,當點選控制元件時,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(預設為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設定一個setOnTouchListener監聽事件,關鍵程式碼如下:
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,日誌列印如下:
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,恢復預設返回值false,然後在button上設定一個監聽點選事件,程式碼如下:
mybutton.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
Log.i("yumf", "Activity=====onClick");
}
});
執行後,檢視日誌列印資訊,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,因此我們在分析原始碼時,可以從該方法入手。
原始碼閱讀
我們當前的MyButton是繼承自Button,而Button又是繼承自TextView,TextView繼承自View,逐步往上檢視,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;
}
如上程式碼中,我們來逐一進行分析,首先是
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(event, 0);
}
通過檢視mInputEventConsistencyVerifier,得知這段程式碼主要是用來除錯的,可以不用關注。接著繼續檢視下一段程式碼
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.
*
* <p>Calling this method when a nested scroll is not currently in progress is harmless.</p>
*
* @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。下面的一段程式碼是關鍵的程式碼,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;
}
}
上面的程式碼中,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物件。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,能看到該物件的引用:
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的物件設定監聽器時,會將對應的監聽器物件返回賦值給mListenerInfo物件,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的物件設定監聽器時,通過回撥的方式,最後都會賦值到mListenerInfo物件中。mListenerInfo類裡面包含了許多的監聽器型別,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<OnLayoutChangeListener> mOnLayoutChangeListeners;
/**
* Listeners for attach events.
*/
private CopyOnWriteArrayList<OnAttachStateChangeListener> 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;
}
完成了監聽器型別的賦值後,我們分析繼續下面的程式碼邏輯:
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中設定的setOnTouchListener中,重寫的onTouch方法。當返回為true時,result = true,這時便不執行下面程式碼中的onTouchEvent(event)方法。result 為false時,才執行onTouchEvent(event)方法。這段關鍵性的程式碼中,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;
}
首先看這個方法的說明,實現這個方法來處理觸控式螢幕幕的動作事件。如果這個方法被用來檢測點選動作,它是建議執行和呼叫的操作。如果這個事件被處理,返回true,否則返回false。
現在我們來看逐一程式碼,第一個if語句塊中,判斷View的狀態是否可用,如果不可用則設定為不可按壓,否則為設定為可點選和可長按。然後下面在可點選和可長按的條件下,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;
設定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,初始化一個例項。新增到訊息佇列中,如果新增失敗則直接執行performClick()方法,否則在PerformClick物件的run中執行performClick()。檢視一下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設定了mOnClickListener的監聽介面,在這裡我們會回撥mOnClickListener中的onClick方法。(原來點選事件的onClick方法是在ACTION_UP時,執行的)
接著,看到建立UnsetPressedState物件,然後執行UnsetPressedState物件中的run方法,我們進入這個方法檢視,
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標誌,然後重新整理背景。
ACTION_UP最後一步,removeTapCallback() 移除訊息佇列中的之前加入的所有回撥操作。
接著分析MotionEvent.ACTION_DOWN中內部程式碼:
首先mHasPerformedLongPress = false,設定長按操作為false。
接著判斷View是否處在可滑動的容器中,如果為false,則設定View的PRESSED狀態和檢查長按動作。
接著分析MotionEvent.ACTION_CANCEL的事件:
程式碼非常簡單,設定PRESSED為false,移除所有的回撥,移除長按的回撥。
最後來分析MotionEvent.ACTION_MOVE的事件:
判斷觸控點是否移出View的範圍,如果移出了則執行removeTapCallback(),取消所有的回撥。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的點選事件。在如果在點選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。