/*
* Set this flag to true to detect anonymous, local or member classes
* that extend this Handler class and that are not static. These kind
* of classes can potentially create leaks.
* 將此標志設置為true以檢測此 Handler 類不是靜態(tài)的匿名，本地或成員類匪凉。這些類
* 可能會產(chǎn)生泄漏。
*/
private static final boolean FIND_POTENTIAL_LEAKS = false;
/**
* Callback interface you can use when instantiating a Handler to avoid
* having to implement your own subclass of Handler.
* 你可以在你實例化一個 Handler 的時候使用 Callback 接口（就是我們說的回調(diào)接口）
* 避免必須實現(xiàn)一個 Handler 的子類捺檬。
*
* @param msg A {@link android.os.Message Message} object
* @return True if no further handling is desired
*/
public interface Callback {
public boolean handleMessage(Message msg);
}
/**
* Subclasses must implement this to receive messages.
* 子類必須實現(xiàn)這個方法來接受 Message 對象
*/
public void handleMessage(Message msg) {
}
/**
* Handle system messages here.
* 該方法用于處理系統(tǒng)的 Message
*/
public void dispatchMessage(Message msg) {
// 如果傳入的 Message 的回調(diào)函數(shù)不為空
if (msg.callback != null) {
// 調(diào)用 handleCallback 方法
handleCallback(msg);
} else { // 不為空
if (mCallback != null) { // 如果 Handler 的 Callback 不為空
if (mCallback.handleMessage(msg)) { // 返回值為 true 調(diào)用 return
return;
}
}
handleMessage(msg);// 調(diào)用 handleMessage() 上面有介紹
}
}
/**
* Default constructor associates this handler with the {@link Looper} for the
* current thread.
*
* If this thread does not have a looper, this handler won't be able to receive messages
* so an exception is thrown.
* 默認的構(gòu)造函數(shù)再层，通過當前線程的 Lopper 對象關(guān)聯(lián)當前 Handler。
* 如果這個線程沒有 Lopper堡纬，那么該 Handler 將無法接受 Message聂受，因此會拋出一個異常。
*/
public Handler() {
this(null, false);
}
/**
* Constructor associates this handler with the {@link Looper} for the
* current thread and takes a callback interface in which you can handle
* messages.
*
* If this thread does not have a looper, this handler won't be able to receive messages
* so an exception is thrown.
*
* @param callback The callback interface in which to handle messages, or null.
* 處理 Message 的回調(diào)接口烤镐，或者傳 null
* 相對于默認構(gòu)造函數(shù)蛋济，添加了一個 Callback 參數(shù)，表示會通過該 Callback 對象處理 Message
*/
public Handler(Callback callback) {
this(callback, false);
}
/**
* Use the provided {@link Looper} instead of the default one.
*
* @param looper The looper, must not be null.
* Looper 對象炮叶，不能傳入 null
* 使用傳入的 Looper 代替默認的 Looper
*/
public Handler(Looper looper) {
this(looper, null, false);
}
/**
* Use the provided {@link Looper} instead of the default one and take a callback
* interface in which to handle messages.
*
* @param looper The looper, must not be null.
* @param callback The callback interface in which to handle messages, or null.
* 使用傳入的 Looper 代替默認的碗旅，并且傳入 Callback 用于處理 Message
*/
public Handler(Looper looper, Callback callback) {
this(looper, callback, false);
}
/**
* Use the {@link Looper} for the current thread
* and set whether the handler should be asynchronous.
*
* Handlers are synchronous by default unless this constructor is used to make
* one that is strictly asynchronous.
*
* Asynchronous messages represent interrupts or events that do not require global ordering
* with respect to synchronous messages. Asynchronous messages are not subject to
* the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
*
* @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
* each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
*
* @hide
* 使用當前線程默認的 Looper 并且設置該 Handler 是否需要是異步的。
* Handler 是默認同步的镜悉，除非你調(diào)用該構(gòu)造函數(shù)設置表示使用異步扛芽。
* 異步消息相對于同步消息，不需要對消息進行同步排序积瞒，并且不受 MessageQueue 的 enqueueSyncBarrier(long) 所帶來的同步障礙影響川尖。
*/
public Handler(boolean async) {
this(null, async);
}
前面所有的構(gòu)造函數(shù)本質(zhì)上都是調(diào)用了下面兩個構(gòu)造函數(shù)，我們來看看構(gòu)造函數(shù)到底做了什么茫孔？
/**
* Use the {@link Looper} for the current thread with the specified callback interface
* and set whether the handler should be asynchronous.
*
* Handlers are synchronous by default unless this constructor is used to make
* one that is strictly asynchronous.
*
* Asynchronous messages represent interrupts or events that do not require global ordering
* with respect to synchronous messages. Asynchronous messages are not subject to
* the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
*
* @param callback The callback interface in which to handle messages, or null.
* @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
* each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
* 使用當前默認 Looper 和傳入的 Callback 叮喳，傳入 boolean 來設置該 Handler 是否需要是異步的。
* @hide
*/
public Handler(Callback callback, boolean async) {
// 如果有內(nèi)存泄漏危險
if (FIND_POTENTIAL_LEAKS) {
final Class<? extends Handler> klass = getClass();
if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
(klass.getModifiers() & Modifier.STATIC) == 0) {
// 打印警告 log"該 Handler 應該為 static 缰贝，否則可能會導致內(nèi)存泄漏"
Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
klass.getCanonicalName());
}
}
// 獲取 Looper
mLooper = Looper.myLooper();
if (mLooper == null) { // 獲取到為 null 拋出異常 “無法在線程內(nèi)部創(chuàng)建沒有調(diào)用
// Looper.prepare() 方法的 handler”
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
// 從 Looper 獲取 MessageQueue
mQueue = mLooper.mQueue;
// mCallback賦值
mCallback = callback;
// mAsynchronous 賦值
mAsynchronous = async;
}
/**
* Use the provided {@link Looper} instead of the default one and take a callback
* interface in which to handle messages. Also set whether the handler
* should be asynchronous.
*
* Handlers are synchronous by default unless this constructor is used to make
* one that is strictly asynchronous.
*
* Asynchronous messages represent interrupts or events that do not require global ordering
* with respect to synchronous messages. Asynchronous messages are not subject to
* the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
*
* @param looper The looper, must not be null.
* @param callback The callback interface in which to handle messages, or null.
* @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
* each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
*
* @hide
* 添加了 Looper 參數(shù)指定 Looper馍悟，其余跟上一個一樣。
*/
public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
下面是各個方法的解讀
getMessageName(Message message):
該方法返回一個 Message 的指定名稱（大概就是說返回一個唯一標識）剩晴。默認返回Message 的 Callback 的類名锣咒，如果 message.callback 為空，返回 Message.what 的十六進制數(shù)赞弥。obtainMessage():
該方法從全局的 Message Pool（消息池）返回一個 Message 對象毅整。這樣的效率比重新創(chuàng)建一個實例要高。并設置 Message.target == this绽左。如果你不想用這種方法悼嫉，可以直接調(diào)用 Message.obtain()。obtainMessage(int what):
和 obtainMessage() 一樣拼窥，但是給返回的 Message 對象設置了 what 戏蔑。obtainMessage(int what, Object obj):
obtainMessage(int what, int arg1, int arg2):
obtainMessage(int what, int arg1, int arg2, Object obj):
與 obtainMessage() 相同蹋凝，但是設置了 whta 和 obj 兩個屬性。public final boolean post(Runnable r):
添加 Runnable 對象到 MessageQueue 中总棵。該 Runnable 對象將會在該 Handler bain綁定到的 Thread 上運行鳍寂。public final boolean postAtTime(Runnable r, long uptimeMillis)：
添加 Runnable 對象到 MessageQueue 中。該 Runnable 對象將會在給定的時間點運行情龄。返回：true 表示該 Runnable 成功添加到 MessageQueue 中伐割，否則返回 false，通常是因為 Looper 處理 MessageQueue 正在退出刃唤。注意隔心，返回 true 并不意味著該 Runnable 一定會被執(zhí)行 -- 如果 Looper 在 Message 送達時間之前 quit，那么 Message 將會丟失尚胞。
