本文有借鑒過(guò)網(wǎng)絡(luò)上優(yōu)秀的文章缺亮，加上自己的總結(jié)。
為什么要使用Handler？
為了保證Android的UI操作是線程安全的涵叮，Android規(guī)定只允許UI線程修改UI組件惭蹂。
但在實(shí)際開發(fā)中，必然會(huì)遇到多個(gè)線程并發(fā)操作UI組件的時(shí)候割粮，這將導(dǎo)致UI操作的線程不安全盾碗。
問(wèn)題在于，如何同時(shí)滿足：
- 保證線程安全
-
使多個(gè)線程并發(fā)操作UI組件
Handler消息機(jī)制可以解決這個(gè)問(wèn)題舀瓢。
是否熟悉以下相關(guān)概念廷雅？YES的話本段可略過(guò)！
-
主線程
定義：程序第一次啟動(dòng)時(shí)京髓，Android會(huì)同時(shí)啟動(dòng)一條主線程（MainThread）
作用：主線程主要負(fù)責(zé)處理與UI相關(guān)的事件航缀，所以主線程又叫UI線程 -
Message
定義：Handler接收和處理的消息對(duì)象，可以理解為封裝了某些數(shù)據(jù)的對(duì)象
使用：后臺(tái)線程處理完數(shù)據(jù)后需要更新UI堰怨，可發(fā)送一條包含更新信息的Message給UI線程 -
Message Queue
定義：消息隊(duì)列
作用：用來(lái)存放通過(guò)Handler發(fā)送的消息芥玉，按照先進(jìn)先出的順序排列 -
Handler
定義：Handler是Message的主要處理者
作用：
1.負(fù)責(zé)將Message添加到消息隊(duì)列中
2.處理Looper分派過(guò)來(lái)的Message -
Looper
定義：循環(huán)器，不斷的取出MessageQueue中的消息并傳遞給Handler
作用：循環(huán)取出MessageQueue的Message备图，將取出的Message交付給相應(yīng)的Handler
PS：每個(gè)線程只能擁有一個(gè)Looper灿巧，但一個(gè)Looper可以和多個(gè)線程的Handler綁定，也就是說(shuō)多個(gè)線程可以往一個(gè)Looper所持有的MessageQueue中發(fā)送消息揽涮，這給我們提供了線程之間通信的可能抠藕。
Handler工作流程
- 異步通信的準(zhǔn)備
1.Looper對(duì)象的創(chuàng)建和實(shí)例化
Looper.prepare()
Looper.loop()
2.MessageQueue隊(duì)列的創(chuàng)建
Looper.prepare()->new Looper()
一個(gè)線程只會(huì)有一個(gè)Looper實(shí)例，同時(shí)一個(gè)Looper實(shí)例也只有會(huì)創(chuàng)建一個(gè)MessageQueue蒋困。
3.Handler實(shí)例化
Handler是和線程綁定在一起的盾似，初始化Handler的時(shí)候一般通過(guò)指定Looper對(duì)象從而綁定相應(yīng)線程，即給Handler指定Looper對(duì)象相當(dāng)于綁定到了Looper對(duì)象所在的線程中家破。Handler的消息處理回調(diào)會(huì)在那個(gè)線程中執(zhí)行颜说。
實(shí)例化Handler的方法:
1） 通過(guò)Looper.mylooper()得到當(dāng)前線程的Looper對(duì)象，或通過(guò)Loop.getMainLooper()獲得當(dāng)前進(jìn)程的主線程的Looper對(duì)象汰聋。
2）不指定Looper對(duì)象门粪，這個(gè)Handler綁定到創(chuàng)建這個(gè)線程的線程上，消息處理回調(diào)也就在在創(chuàng)建線程中執(zhí)行烹困。
當(dāng)Handler初始化時(shí)玄妈，可通過(guò)構(gòu)造方法自動(dòng)關(guān)聯(lián)Looper和相應(yīng)的MessageQueue。
消息發(fā)送
Handler將消息發(fā)送到消息隊(duì)列中消息循環(huán)
Looper執(zhí)行Looper.loop()進(jìn)入消息循環(huán)髓梅，在循環(huán)過(guò)程中不斷從該MessageQueue取出消息拟蜻，并將取出的消息派發(fā)給創(chuàng)建該消息的Handler...消息處理
通過(guò)Handler的dispatchMessage(msg)方法，即回調(diào)handleMessage(msg)處理消息枯饿。如果時(shí)Handler的post方法發(fā)送的消息酝锅，則會(huì)在對(duì)應(yīng)的run()方法中處理回調(diào)。
源碼分析
Looper.prepare()是在做什么奢方？
/** Initialize the current thread as a looper.
* This gives you a chance to create handlers that then reference
* this looper, before actually starting the loop. Be sure to call
* {@link #loop()} after calling this method, and end it by calling
* {@link #quit()}.
*/
public static void prepare() {
prepare(true);
}
private static void prepare(boolean quitAllowed) {
//一個(gè)線程只能持有一個(gè)Looper實(shí)例搔扁，sThreadLocal保存線程持有的looper對(duì)象
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
//sThreadLocal是一個(gè)ThreadLocal變量爸舒，用于在一個(gè)線程中存儲(chǔ)變量，這里L(fēng)ooper變量就存放在ThreadLocal里
sThreadLocal.set(new Looper(quitAllowed));
}
Looper的構(gòu)造方法在做什么稿蹲？
private Looper(boolean quitAllowed) {
//創(chuàng)建Looper時(shí)扭勉，會(huì)自動(dòng)創(chuàng)建一個(gè)與之匹配的MessageQueue
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
Looper.loop()方法作用是什么？
/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/
public static void loop() {
//myLooper()方法是返回sThreadLocal存儲(chǔ)的Looper實(shí)例
final Looper me = myLooper();
//me==null就會(huì)拋出異常苛聘，說(shuō)明looper對(duì)象沒有被創(chuàng)建涂炎，
//也就是說(shuō)loop方法必須在prepare方法之后運(yùn)行，消息循環(huán)必須要先在線程中創(chuàng)建Looper實(shí)例
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
//獲取Looper實(shí)例中的消息隊(duì)列mQueue
final MessageQueue queue = me.mQueue;
// Make sure the identity of this thread is that of the local process,
// and keep track of what that identity token actually is.
Binder.clearCallingIdentity();
final long ident = Binder.clearCallingIdentity();
for (;;) {
//next()方法用于取出消息隊(duì)列中的消息设哗，如果取出的消息為空唱捣，則線程阻塞
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
// This must be in a local variable, in case a UI event sets the logger
Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
//把消息派發(fā)給msg的target屬性，然后用dispatchMessage方法去處理
msg.target.dispatchMessage(msg);
if (logging != null) {
logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
}
// Make sure that during the course of dispatching the
// identity of the thread wasn't corrupted.
final long newIdent = Binder.clearCallingIdentity();
if (ident != newIdent) {
Log.wtf(TAG, "Thread identity changed from 0x"
+ Long.toHexString(ident) + " to 0x"
+ Long.toHexString(newIdent) + " while dispatching to "
+ msg.target.getClass().getName() + " "
+ msg.callback + " what=" + msg.what);
}
msg.recycleUnchecked();
}
}
綜上熬拒，可以看出Looper的作用是：
1.實(shí)例化Looper對(duì)象本身（prepare()方法）爷光，創(chuàng)建與之對(duì)應(yīng)的MessageQueue（looper()構(gòu)造方法）
2.loop()方法不斷從MessageQueue中取消息，派發(fā)給Handler澎粟，然后調(diào)用相應(yīng)Handler的dispatchMessage()方法進(jìn)行消息處理蛀序。
那么，Handler是如何和Looper綁定且從MessageQueue中獲取消息執(zhí)行的呢活烙？
來(lái)看Handler的構(gòu)造方法：
public Handler(Callback callback, boolean async) {
if (FIND_POTENTIAL_LEAKS) {
final Class<? extends Handler> klass = getClass();
if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
(klass.getModifiers() & Modifier.STATIC) == 0) {
Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
klass.getCanonicalName());
}
}
//Looper.myLooper()獲得了當(dāng)前線程保存的Looper實(shí)例
mLooper = Looper.myLooper();
//如果沒有l(wèi)ooper實(shí)例就會(huì)拋出異常徐裸，這說(shuō)明一個(gè)沒有創(chuàng)建looper的線程中是無(wú)法創(chuàng)建一個(gè)Handler對(duì)象的；
//子線程中創(chuàng)建一個(gè)Handler時(shí)需要?jiǎng)?chuàng)建Looper啸盏，且開啟循環(huán)才能使用這個(gè)Handler
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
//獲取looper實(shí)例的MessageQueue重贺，保證handler實(shí)例與looper實(shí)例的MessageQueue關(guān)聯(lián)
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
以上，當(dāng)Handler構(gòu)造函數(shù)初始化時(shí)回懦，自動(dòng)關(guān)聯(lián)looper和對(duì)應(yīng)的MessageQueue气笙。
Handler向MessageQueue發(fā)送消息的代碼sendMessage執(zhí)行后，是發(fā)生了什么怯晕？
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
//向下調(diào)用
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
//向下調(diào)用
public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
MessageQueue queue = mQueue;
if (queue == null) {
RuntimeException e = new RuntimeException(
this + " sendMessageAtTime() called with no mQueue");
Log.w("Looper", e.getMessage(), e);
return false;
}
return enqueueMessage(queue, msg, uptimeMillis);
}
//sendMessage最后的最后調(diào)用到了enqueueMessage方法
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
// msg.target = this潜圃，也就是把當(dāng)前handler作為msg的target屬性
// 在Looper的loop()方法中會(huì)取出msg，然后執(zhí)行msg.target.dispatchMessage(msg)去處理消息
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
//handler發(fā)出的消息最終會(huì)保存到消息隊(duì)列中
return queue.enqueueMessage(msg, uptimeMillis);
}
Handler的post()和sendMessage()有什么不同舟茶？
public final boolean post(Runnable r) {
return sendMessageDelayed(getPostMessage(r), 0);
}
private static Message getPostMessage(Runnable r) {
//使用obtain方法創(chuàng)建一個(gè)Message對(duì)象谭期，因?yàn)镸essage內(nèi)部維護(hù)了一個(gè)Message池用于Message復(fù)用，避免使用new重新分配內(nèi)存
Message m = Message.obtain();
//將創(chuàng)建的Runnable對(duì)象作為callback屬性吧凉，賦值給message
m.callback = r;
//返回一個(gè)message對(duì)象
return m;
}
繼續(xù)向下調(diào)用：
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
//向下調(diào)用
public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
MessageQueue queue = mQueue;
if (queue == null) {
RuntimeException e = new RuntimeException(
this + " sendMessageAtTime() called with no mQueue");
Log.w("Looper", e.getMessage(), e);
return false;
}
return enqueueMessage(queue, msg, uptimeMillis);
}
由上可知隧出，Handler的post()方法和sendmessage()方法一樣，最終都調(diào)用了sendMessageAtTime阀捅，然后調(diào)用了enqueueMessage方法胀瞪，將msg.target賦值為handler，然后將Handler加入到MessageQueue中饲鄙。
But赏廓，在使用post方法時(shí)涵紊，將創(chuàng)建的Runnable對(duì)象作為callback屬性賦值給了message，那么該如何執(zhí)行handler的回調(diào)方法呢幔摸？請(qǐng)看如下代碼：
/**
* Handle system messages here.
*/
public void dispatchMessage(Message msg) {
//如果msg.callback不為null，則執(zhí)行handleCallback回調(diào)颤练，也就是我們的Runnable里的回調(diào)
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
//Runnable的run()方法中執(zhí)行回調(diào)函數(shù)
private static void handleCallback(Message message) {
message.callback.run();
}
/**
* Subclasses must implement this to receive messages.
*/
public void handleMessage(Message msg) {
}
可以看到dispatchMessage()方法中調(diào)用了handleMessage()方法既忆，但是是一個(gè)空方法，在創(chuàng)建Handler時(shí)通過(guò)復(fù)寫handleMessage()方法來(lái)實(shí)現(xiàn)我們需要的消息處理方式嗦玖，根據(jù)msg.what標(biāo)識(shí)進(jìn)行區(qū)分處理患雇。
為什么在UI線程中使用Handler時(shí)不需要?jiǎng)?chuàng)建Looper？
當(dāng)一個(gè)Android應(yīng)用程序啟動(dòng)時(shí)宇挫，會(huì)創(chuàng)建一個(gè)主線程ActivityThread苛吱，在ActivityThread中有一個(gè)靜態(tài)的main()方法，也是應(yīng)用程序的入口點(diǎn)器瘪。
public static void main(String[] args) {
...
// 通過(guò)prepareMainLooper方法為主線程創(chuàng)建一個(gè)looper
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
...
// 開啟消息循環(huán)
Looper.loop();
...
}
/**
* Initialize the current thread as a looper, marking it as an
* application's main looper. The main looper for your application
* is created by the Android environment, so you should never need
* to call this function yourself. See also: {@link #prepare()}
*/
public static void prepareMainLooper() {
//prepare()方法用于創(chuàng)建一個(gè)looper對(duì)象
//主線程的消息循環(huán)是不允許退出的
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
舉幾個(gè)例子來(lái)加強(qiáng)理解
子線程和主線程通信翠储，使用sendMessage()更新UI
/**
* Created by Kathy on 17-2-15.
* 子線程與主線程通信
*/
public class MainActivity extends Activity {
private static final int INT_ONE = 1;
//在主線程里創(chuàng)建一個(gè)mHandler實(shí)例
private Handler mHandler = new Handler(new Handler.Callback() {
@Override
public boolean handleMessage(Message msg) {
switch (msg.what) {
case INT_ONE:
String text = (String) msg.obj;
Toast.makeText(MainActivity.this, text,Toast.LENGTH_SHORT).show();
break;
default:
break;
}
return false;
}
});
class ThreadOne extends Thread{
@Override
public void run() {
//創(chuàng)建需要發(fā)送的消息，注意使用的是主線程的mHandler橡疼，所以也在主線程的handleMessage()中回調(diào)
Message msg = Message.obtain();
msg.what = INT_ONE; //標(biāo)識(shí)消息
msg.obj = "One"; //存放消息
mHandler.sendMessage(msg);
}
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
//啟動(dòng)子線程
new ThreadOne().start();
}
}
啟動(dòng)這個(gè)Activity援所，執(zhí)行子線程，在子線程中通過(guò)UI線程的mHandler發(fā)送消息欣除，會(huì)發(fā)現(xiàn)最終會(huì)執(zhí)行到handleMessage()方法住拭，彈出Toast。
使用Handler.post()更新UI
/**
* Created by Kathy on 17-2-16.
* 使用Handler.post()通信
*/
public class MainActivityTwo extends Activity {
private Handler mHandler;
//使用mHandler.post()方法只需要在run()方法中寫回調(diào)內(nèi)容即可
class ThreadTwo extends Thread {
@Override
public void run() {
mHandler.post(new Runnable() {
@Override
public void run() {
Toast.makeText(MainActivityTwo.this, "Two", Toast.LENGTH_SHORT).show();
}
});
}
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// 實(shí)例化Handler历帚，無(wú)需指定looper滔岳，mHandler自動(dòng)綁定當(dāng)前線程（UI線程）的Looper和MessageQueue
mHandler = new Handler();
new ThreadTwo().start();
}
}
主線程如何向子線程中發(fā)送消息的呢?
/**
* Created by Kathy on 17-2-16.
*/
public class MainActivityThree extends Activity {
private Handler mChildHandlerOne;
private Handler mChildHandlerTwo;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Log.d("Kathy", "mainLooper = " + getMainLooper());
new Thread(new Runnable() {
@Override
public void run() {
Looper.prepare();
//子線程中實(shí)例化mChildHandlerOne
mChildHandlerOne = new Handler(new Handler.Callback() {
@Override
public boolean handleMessage(Message msg) {
Log.d("Kathy", "mChildHandlerOne received Message!");
// 在此線程中，用mChildHandlerTwo發(fā)送消息
// 消息被加入到mChildHandlerTwo的消息隊(duì)列中挽牢，進(jìn)而會(huì)執(zhí)行到mChildHandlerTwo的回調(diào)方法
// 這就完成了子線程向子線程之前發(fā)消息的可能
Message msg2 = Message.obtain();
msg2.what = 1;
mChildHandlerTwo.sendMessage(msg2);
return false;
}
});
Log.d("Kathy", "mChildHandlerOne = " + mChildHandlerOne.getLooper());
Looper.loop();
}
}).start();
new Thread(new Runnable() {
@Override
public void run() {
Looper.prepare();
//子線程中實(shí)例化mChildHandlerTwo
mChildHandlerTwo = new Handler(new Handler.Callback() {
@Override
public boolean handleMessage(Message msg) {
Log.d("Kathy", "mChildHandlerTwo received Message!");
return false;
}
});
Log.d("Kathy", "mChildHandlerTwo = " + mChildHandlerTwo.getLooper());
Looper.loop();
}
}).start();
// 點(diǎn)擊按鈕谱煤，如果兩個(gè)子線程中的handler都被實(shí)例化后，在主線程中卓研，用mChildHandlerOne發(fā)送消息
// 消息會(huì)加入到mChildHandlerOne的消息隊(duì)列中趴俘，進(jìn)而會(huì)執(zhí)行到mChildHandlerOne的回調(diào)方法
// 這就完成了在主線程中向子線程發(fā)送消息的可能
findViewById(R.id.btn).setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
if (null != mChildHandlerOne && null != mChildHandlerTwo) {
Message msg1 = Message.obtain();
msg1.what = 1;
mChildHandlerOne.sendMessage(msg1);
}
}
});
}
}
Log輸出如下：
02-17 15:41:14.998 22131-22131/com.demo.kathy.demo D/Kathy: mainLooper = Looper (main, tid 1) {56801a8}
02-17 15:41:15.007 22131-22170/com.demo.kathy.demo D/Kathy: mChildHandlerOne = Looper (Thread-5565, tid 5565) {dba14c1}
02-17 15:41:15.009 22131-22171/com.demo.kathy.demo D/Kathy: mChildHandlerTwo = Looper (Thread-5566, tid 5566) {8d3d466}
02-17 15:41:28.490 22131-22170/com.demo.kathy.demo D/Kathy: mChildHandlerOne received Message!
02-17 15:41:28.490 22131-22171/com.demo.kathy.demo D/Kathy: mChildHandlerTwo received Message!
若有錯(cuò)誤，請(qǐng)及時(shí)指出奏赘，感謝閱讀!
