Handler深入淺出
摘要：
Handler 組成部分
Message：訊息物件
MessageQueue：訊息佇列
Looper：訊息輪詢器
Handler 工作原理
Message：用於記錄訊息攜帶的資訊
MessageQueue：存取 Messag...
Handler 組成部分
-
Message：訊息物件
-
MessageQueue：訊息佇列
-
Looper：訊息輪詢器
Handler 工作原理
-
Message：用於記錄訊息攜帶的資訊
-
MessageQueue：存取 Message 的佇列集合
-
Looper：不斷獲取是否有新的 Message 需要執行
Message 物件介紹
建立 Message 的兩種方式
有什麼不一樣？接下來檢視一下 Message.obtain 這個靜態方法做了什麼操作
先翻譯一下 obtain 的方法的註釋文件
Return a new Message instance from the global pool. Allows us to avoid allocating new objects in many cases.
從全域性池返回一個新的訊息例項。允許我們在許多情況下避免分配新物件。
看到這裡大家心裡應該有底了，就是在複用之前用過的 Message 物件，這裡實際上是用到了一種享元設計模式，這種設計模式最大的特點就是複用物件，避免重複建立導致的記憶體浪費
再介紹一下 Message 物件的一些特殊的屬性，待會我們會用得到
Handler.sendMessage 解析
public final boolean sendMessage(Message msg) {
return sendMessageDelayed(msg, 0);
}
public final boolean sendEmptyMessage(int what) {
return sendEmptyMessageDelayed(what, 0);
}
public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageDelayed(msg, delayMillis);
}
public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageAtTime(msg, uptimeMillis);
}
public final boolean sendMessageDelayed(Message msg, long delayMillis) {
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
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);
}
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);
}
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
簡單過一遍，發現一個問題，sendXXX 這些方式最終還是會呼叫到 enqueueMessage 這個方法上來，所以讓我們重點看一下這個方法
就在剛剛給大家看了一下 Handler 的特殊屬性，target 其實就是一個 Handler 型別的物件，現在給它賦值為當前的 Handler 物件，其實這樣我們已經不難斷定，它最後肯定會這樣回撥 Handler 的 handleMessage 的方法了
msg.target.handleMessage(msg);
MessageQueue.enqueueMessage 解析
這裡只是設想，接下來繼續看 queue.enqueueMessage 的方法，發現這裡標紅點不進去，我們可以直接點選 MessageQueue 物件進去，由於 enqueueMessage 程式碼太長，沒法放截圖，就直接放程式碼了
boolean enqueueMessage(Message msg, long when) {
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
if (msg.isInUse()) {
throw new IllegalStateException(msg + " This message is already in use.");
}
synchronized (this) {
if (mQuitting) {
IllegalStateException e = new IllegalStateException(
msg.target + " sending message to a Handler on a dead thread");
Log.w(TAG, e.getMessage(), e);
msg.recycle();
return false;
}
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 {
// Inserted within the middle of the queue.Usually we don't have to wake
// up the event queue unless there is a barrier at the head of the queue
// and the message is the earliest asynchronous message in the queue.
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;
}
這裡我們先講一個細節的問題，MessageQueue 類中的幾乎所有的方法裡面都有 synchronized 關鍵字，證明這個類已經處理過執行緒安全的問題了
剛剛的原始碼你只需要簡單過一遍，接下來我們挑重點的講，如果對連結串列不熟悉的先去百度瞭解一下（簡單點的來說就是物件自己巢狀自己），這裡用的是單向連結串列，我已經把註釋打上去了，要集中精力看
// 標記這個 Message 已經被使用
msg.markInUse();
msg.when = when;
// mMessages 是一個 Message 物件
Message p = mMessages;
boolean needWake;
// 如果這個是第一個訊息，如果這個訊息需要馬上執行，如果這個訊息執行的時間要比之前的訊息要提前的話
if (p == null || when == 0 || when < p.when) {
// 把這個 Message 物件放置在連結串列第一個位置
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
needWake = mBlocked && p.target == null && msg.isAsynchronous();
// 這塊比較難理解了，要注意集中精力，不要腦子被轉暈了
// 記錄跳出迴圈前最後的一個 Message 物件
Message prev;
// 不斷迴圈，根據執行時間進行對連結串列進行排序
for (;;) {
// 你沒有看錯，這個物件就只是記錄而已，迴圈裡面沒有用到
prev = p;
// 獲取連結串列的下一個
p = p.next;
// 如果這個是連結串列的最後一個，如果這個訊息執行時間要比連結串列的下一個要提前的話
if (p == null || when < p.when) {
// 跳出迴圈
break;
}
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
// 將剛剛符合要求的物件 p 排在 msg 後面
msg.next = p;
// 再將 msg 排在 prev 的後面（溫馨提醒：prev 和 p 是不一樣的，p 其實等於 prev.next，不信你回去看原始碼）
prev.next = msg;
// 排序前：prev ---> p
// 排序後：prev ---> msg ---> p
}
Message（訊息） 物件已經在 MessageQueue（訊息佇列）中排序好了，那麼問題來了，MessageQueue.enqueueMessage 方法壓根沒呼叫 Handler.handleMessage 方法？你讓我情何以堪？
糾正一個剛剛的設想
Handler.handleMessage 到底被誰呼叫了？請看下圖
handleMessage 原來是被 Handler.dispatchMessage 回撥的，那麼我們之前那種設想還不太對
// 剛剛的設想 msg.target.handleMessage(msg); // 錯誤
// 現在的設想 msg.target.dispatchMessage(msg); // 正確
Handler 和 Looper 的關係
讓我們先來看一下 Handler 建構函式
public class Handler {
public Handler() {
this(null, false);
}
public Handler(Callback callback) {
this(callback, false);
}
public Handler(Looper looper) {
this(looper, null, false);
}
public Handler(Looper looper, Callback callback) {
this(looper, callback, false);
}
public Handler(boolean async) {
this(null, async);
}
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 " + Thread.currentThread()
+ " that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
}
我們先來看一下兩句重點程式碼
mLooper = looper; mQueue = looper.mQueue;
你會發現，Handler 和 Looper 有很大關係，就連 MessageQueue 也是 Looper 裡面的物件，看來還真的不簡單
Looper.loop
既然如此，我上去一頓搜尋，Looper 類中只有一個地方呼叫了 Handler.dispatchMessage 方法
由於這個方法太長，我們把這個方法原始碼單獨拎出來，簡單過一遍就好
/**
* 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;
// 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();
// Allow overriding a threshold with a system prop. e.g.
// adb shell 'setprop log.looper.1000.main.slow 1 && stop && start'
final int thresholdOverride =
SystemProperties.getInt("log.looper."
+ Process.myUid() + "."
+ Thread.currentThread().getName()
+ ".slow", 0);
boolean slowDeliveryDetected = false;
for (;;) {
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
final Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
final long traceTag = me.mTraceTag;
long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
long slowDeliveryThresholdMs = me.mSlowDeliveryThresholdMs;
if (thresholdOverride > 0) {
slowDispatchThresholdMs = thresholdOverride;
slowDeliveryThresholdMs = thresholdOverride;
}
final boolean logSlowDelivery = (slowDeliveryThresholdMs > 0) && (msg.when > 0);
final boolean logSlowDispatch = (slowDispatchThresholdMs > 0);
final boolean needStartTime = logSlowDelivery || logSlowDispatch;
final boolean needEndTime = logSlowDispatch;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
final long dispatchStart = needStartTime ? SystemClock.uptimeMillis() : 0;
final long dispatchEnd;
try {
msg.target.dispatchMessage(msg);
dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
if (logSlowDelivery) {
if (slowDeliveryDetected) {
if ((dispatchStart - msg.when) <= 10) {
Slog.w(TAG, "Drained");
slowDeliveryDetected = false;
}
} else {
if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery",
msg)) {
// Once we write a slow delivery log, suppress until the queue drains.
slowDeliveryDetected = true;
}
}
}
if (logSlowDispatch) {
showSlowLog(slowDispatchThresholdMs, dispatchStart, dispatchEnd, "dispatch", 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();
}
}
我們先翻譯一下這個 Looper.loop 方法的註釋
Run the message queue in this thread. Be sure to call{@link #quit()} to end the loop.
在這個執行緒中執行訊息佇列。確保呼叫{@link #quit()}來結束迴圈。
看完這個翻譯你是不是頓悟了，原來 MessageQueue 訊息佇列最後是在這個方法執行的，接下來我們分析一下里面比較重點的原始碼
// 不斷迴圈
for (;;) {
// 取 MessageQueue 中的 Message 物件，具體方法就不帶大家看了
Message msg = queue.next();
if (msg == null) {
// 直到訊息佇列沒有 Message 物件了就跳出迴圈和退出方法
return;
}
// This must be in a local variable, in case a UI event sets the logger
final Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
final long traceTag = me.mTraceTag;
long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
long slowDeliveryThresholdMs = me.mSlowDeliveryThresholdMs;
if (thresholdOverride > 0) {
slowDispatchThresholdMs = thresholdOverride;
slowDeliveryThresholdMs = thresholdOverride;
}
final boolean logSlowDelivery = (slowDeliveryThresholdMs > 0) && (msg.when > 0);
final boolean logSlowDispatch = (slowDispatchThresholdMs > 0);
final boolean needStartTime = logSlowDelivery || logSlowDispatch;
final boolean needEndTime = logSlowDispatch;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
final long dispatchStart = needStartTime ? SystemClock.uptimeMillis() : 0;
final long dispatchEnd;
try {
// msg.target 之前說過了，在 sendMessage 的時候已經賦值自身給這個欄位了
msg.target.dispatchMessage(msg);
dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
}
看完原始碼後總結
-
Message：訊息
-
MessageQueue：訊息集合
-
Looper：執行訊息