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??Handler經(jīng)常被我們用來在主線程和子線程之間傳遞消息，因此這里打算從源碼的角度分析下它的使用，也算是進(jìn)軍源碼的一個(gè)開端！為了使整個(gè)分析過程比較的有條理，便從一個(gè)最簡單的使用場景（代碼如下：）著手，順著源碼來看Message的傳遞流程。
public class MainActivity extends AppCompatActivity {
public static final String MSG = "msg";
// 注意內(nèi)存泄露的問題
Handler handler = new Handler(){
@Override
public void handleMessage(Message msg) {
Toast.makeText(MainActivity.this, msg.getData().getString(MSG), Toast.LENGTH_SHORT).show();
}
};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
new Thread(new Runnable() {
@Override
public void run() {
SystemClock.sleep(2000);
Message msg = Message.obtain();
Bundle bundle= new Bundle();
bundle.putString(MSG,"子線程發(fā)送的消息！");
msg.setData(bundle);
handler.sendMessage(msg);
}
}).start();
}
}
Handler中的消息處理機(jī)制主要包含四個(gè)類：
- Message消息對象
- Handler消息處理器
- MessageQueue消息隊(duì)列
- Looper消息輪詢器
Handler消息機(jī)制
一、消息機(jī)制的初始化
??在啟動(dòng)UI主線程ActivityThread（\frameworks\base\core\java\android\app\ActivityThread.java）會(huì)執(zhí)行下面的main方法：
public static void main(String[] args) {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
// CloseGuard defaults to true and can be quite spammy. We
// disable it here, but selectively enable it later (via
// StrictMode) on debug builds, but using DropBox, not logs.
CloseGuard.setEnabled(false);
Environment.initForCurrentUser();
// Set the reporter for event logging in libcore
EventLogger.setReporter(new EventLoggingReporter());
// Make sure TrustedCertificateStore looks in the right place for CA certificates
final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
TrustedCertificateStore.setDefaultUserDirectory(configDir);
Process.setArgV0("<pre-initialized>");
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
// 創(chuàng)建H extends Handler對象，H類中定義了許多跟Activity生命周期相關(guān)的常量。
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
1.1，Looper.prepareMainLooper()
在main方法中調(diào)用了Looper.prepareMainLooper()，從下面的源碼中可以看出最終調(diào)用的還是prepare()方法，并且用ThreadLocal保存了Looper的對象，這樣能夠保證每個(gè)Looper對象都與線程對應(yīng)，并且從拋出的異常信息中可以知道消息機(jī)制的一個(gè)重要特點(diǎn)：每個(gè)線程中只能創(chuàng)建一個(gè)Looper對象。
public static void prepare() {
prepare(true);
}
private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper(quitAllowed));
}
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
Looper的構(gòu)造方法中創(chuàng)建了MessageQueue的實(shí)例
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
Looper.prepare()方法實(shí)例化了Looper和MessageQueue對象。
1.2，Looper.loop()
在main方法的最后執(zhí)行Looper.loop()進(jìn)入死循環(huán)，其中queue.next()是一個(gè)阻塞式方法，直到消息隊(duì)列中獲取到Message對象才會(huì)執(zhí)行后面的dispatchMessage(msg)來處理消息。
/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/
public static void loop() {
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
final MessageQueue queue = me.mQueue;
......
for (;;) {
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
......
try {
msg.target.dispatchMessage(msg);
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
......
// 將msg回收到對象池中，便于下次復(fù)用
msg.recycleUnchecked();
}
}
二、Handler的創(chuàng)建
Handler的創(chuàng)建除了示例中的匿名內(nèi)部類方式，還可以通過實(shí)現(xiàn)Callback的方式來創(chuàng)建。這兩種方法本質(zhì)上是一樣的，只不過一提到接口就想到多繼承擴(kuò)展啥的。另外，在消息的處理上略有不同，在后面會(huì)講到。
public Handler() {
this(null, false);
}
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());
}
}
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
三、Message的創(chuàng)建
3.1，Message的數(shù)據(jù)結(jié)構(gòu)
先看Message類的定義，Message中除了包括傳遞數(shù)據(jù)所用的變量what、data、when外，還定義了一個(gè)next的引用，這說明Message是一個(gè)鏈表的數(shù)據(jù)結(jié)構(gòu)，而MessageQueue雖說叫一個(gè)消息隊(duì)列，實(shí)際上是個(gè)鏈表。另外，Messge中為了避免重復(fù)的創(chuàng)建Message對象造成內(nèi)存的抖動(dòng)，也提供了一個(gè)復(fù)用的對象池，最多容納50個(gè)對象。
public final class Message implements Parcelable {
public int what;
......
/*package*/ long when;
/*package*/ Bundle data;
/*package*/ Handler target;
/*package*/ Runnable callback;
// sometimes we store linked lists of these things
/*package*/ Message next;
private static Message sPool;
private static int sPoolSize = 0;
private static final int MAX_POOL_SIZE = 50;
......
}
3.2，Message.obtain()
該方法中會(huì)先判斷對象池中是否有可復(fù)用的Message對象，如果有就從鏈表頭部取出一個(gè)Message對象返回，否則就會(huì)重新創(chuàng)建一個(gè)Message對象。除了這個(gè)無參的方法，還有7個(gè)重載方法，這些重載方法無非就是加了一些初始化參數(shù)并且都調(diào)用了obtain()方法。
/**
* Return a new Message instance from the global pool. Allows us to
* avoid allocating new objects in many cases.
*/
public static Message obtain() {
synchronized (sPoolSync) {
if (sPool != null) {
Message m = sPool;
sPool = m.next;
m.next = null;
m.flags = 0; // clear in-use flag
sPoolSize--;
return m;
}
}
return new Message();
}
四、Message的發(fā)送
4.1，sendMessage()
消息創(chuàng)建完后就可以通過handler將消息發(fā)送出去，主要有sendXxx(),sendEmptyXxx(),postXxx()三類方法，sendXxx方法是在調(diào)用之前就初始化好了Message對象，后面兩類方法是在執(zhí)行過程中封裝的Message對象。
- sendXxx()：該系列方法最終調(diào)用的都是
sendMessageAtTime(msg,uptimeMills)。從源代碼中可以知道延遲執(zhí)行的時(shí)間不能小于0，并且要求MessageQueue必須先實(shí)例化。
// 發(fā)送立即處理的消息
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
// 發(fā)送延遲執(zhí)行的消息
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
// 發(fā)送在制定時(shí)間點(diǎn)執(zhí)行的消息
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);
}
// 特殊情況下使用，該方法僅僅是將uptimeMills置為0
public final boolean sendMessageAtFrontOfQueue(Message msg) {
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, 0);
}
- sendEmptyXxx()和postXxx()：該系列的方法基本上都有與之對應(yīng)的sendXxx()方法。
public final boolean sendEmptyMessage(int what);
public final boolean sendEmptyMessageDelayed(int what, long delayMillis);
public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis);
// Runnable對象會(huì)賦值給callback
public final boolean post(Runnable r);
public final boolean postAtTime(Runnable r, long uptimeMillis);
public final boolean postAtTime(Runnable r, Object token, long uptimeMillis);
public final boolean postDelayed(Runnable r, long delayMillis);
public final boolean postAtFrontOfQueue(Runnable r);
這里以post(Runnable r)方法為例，其它的都差不多。msg中的callback指向了該Runnable對象，后面在處理的時(shí)候會(huì)直接調(diào)用里面的run方法。
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
4.2，enqueueMessage()
從上面的的sendMessageAtTime()得知會(huì)調(diào)用enqueueMessage()，源碼如下。在這里將當(dāng)前Handler的引用this賦值給了msg.target（其實(shí)這是為了避免在創(chuàng)建多個(gè)Handler對象的情況下消息的沖突）。
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
MessageQueue中enqueueMessage()需要遍歷單鏈表將msg插入到鏈表中，并且還要判斷是否該喚醒主線程，即next()方法。
boolean enqueueMessage(Message msg, long when) {
......
synchronized (this) {
.....
msg.markInUse();
msg.when = when;
Message p = mMessages;
boolean needWake;
if (p == null || when == 0 || when < p.when) {
// New head, wake up the event queue if blocked.
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
// 將msg插入到隊(duì)列中
needWake = mBlocked && p.target == null && msg.isAsynchronous();
Message prev;
for (;;) {
prev = p;
p = p.next;
if (p == null || when < p.when) {
break;
}
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
msg.next = p; // invariant: p == prev.next
prev.next = msg;
}
// We can assume mPtr != 0 because mQuitting is false.
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}
五、Message的處理
5.1，next()
當(dāng)執(zhí)行nativeWake(mPtr)方法喚醒了主線程，MessageQueue的next()方法（該方法是在Looper.loop()中調(diào)用的，前面有相關(guān)源碼）就會(huì)從鏈表中取出消息。
Message next() {
......
nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) {
// Try to retrieve the next message. Return if found.
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;
if (msg != null && msg.target == null) {
// Stalled by a barrier. Find the next asynchronous message in the queue.
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
if (msg != null) {
if (now < msg.when) {
// Next message is not ready. Set a timeout to wake up when it is ready.
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
// Got a message.
mBlocked = false;
if (prevMsg != null) {
prevMsg.next = msg.next;
} else {
mMessages = msg.next;
}
msg.next = null;
if (DEBUG) Log.v(TAG, "Returning message: " + msg);
msg.markInUse();
return msg;
}
......
}
}
5.2，dispatchMessage()
在Looper.loop()方法中會(huì)調(diào)用dispatchMessage()，具體的源碼如下。并且這個(gè)方法是在主線程中調(diào)用的，因此消息就從子線程傳遞到了UI線程中啦。
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
分為三種情況來處理消息：
- post一個(gè)Runnable對象：該方式會(huì)執(zhí)行
handleCallback()，即執(zhí)行Runnable中run方法。
private static void handleCallback(Message message) {
message.callback.run();
}
- 實(shí)現(xiàn)Callback接口：mCallback是在Handler創(chuàng)建的時(shí)候初始化的，需要自己去實(shí)現(xiàn)這個(gè)內(nèi)部接口。需要注意的是：這個(gè)接口中的方法比Handler中的空方法
void handleMessage(Message msg){}多了個(gè)boolean類型的返回值。
public interface Callback {
public boolean handleMessage(Message msg);
}
- 發(fā)送普通的Message對象：該方式直接調(diào)用自己實(shí)現(xiàn)的
handleMessage()方法。當(dāng)然，這個(gè)方法的執(zhí)行與否還與前面的兩種情況有關(guān)。
5.3，recycleUnchecked()
在msg消息處理完成之后，就會(huì)執(zhí)行recycleUnchecked()方法完成的這個(gè)msg的復(fù)用。該方法會(huì)將msg對象中成員變量還原，并采用頭插法將msg插入到對象池（Message.sPool）中。
void recycleUnchecked() {
// Mark the message as in use while it remains in the recycled object pool.
// Clear out all other details.
flags = FLAG_IN_USE;
what = 0;
arg1 = 0;
arg2 = 0;
obj = null;
replyTo = null;
sendingUid = -1;
when = 0;
target = null;
callback = null;
data = null;
synchronized (sPoolSync) {
if (sPoolSize < MAX_POOL_SIZE) {
next = sPool;
sPool = this;
sPoolSize++;
}
}
}
六、總結(jié)
??Handler不僅僅只有上面一種使用場景，還可以用于子線程和子線程，子線程和主線程的通信，只是在使用時(shí)要注意以下幾點(diǎn)：
- 在子線程使用時(shí)中需調(diào)用
Looper.prepare()方法來初始化Looper和MessageQueue； - 在子線程使用時(shí)中需調(diào)用
Looper.loop()方法來輪詢消息隊(duì)列； - 一個(gè)線程中只能有一個(gè)Looper對象；
- 一個(gè)線程中可以創(chuàng)建多個(gè)Handler對象；
- 第五部分中消息處理的集中情況（本來想畫個(gè)流程圖但是源碼中已經(jīng)很清晰就懶得畫了）
參考資料
Android7.0源碼
