/*
* 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(biāo)志設(shè)置為true以檢測(cè)此 Handler 類(lèi)不是靜態(tài)的匿名，本地或成員類(lèi)。這些類(lèi)
* 可能會(huì)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(shí)例化一個(gè) Handler 的時(shí)候使用 Callback 接口（就是我們說(shuō)的回調(diào)接口）
* 避免必須實(shí)現(xiàn)一個(gè) Handler 的子類(lèi)。
*
* @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(lèi)必須實(shí)現(xiàn)這個(gè)方法來(lái)接受 Message 對(duì)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(rèn)的構(gòu)造函數(shù)，通過(guò)當(dāng)前線程的 Lopper 對(duì)象關(guān)聯(lián)當(dāng)前 Handler。
* 如果這個(gè)線程沒(méi)有 Lopper，那么該 Handler 將無(wú)法接受 Message，因此會(huì)拋出一個(gè)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，或者傳 null
* 相對(duì)于默認(rèn)構(gòu)造函數(shù)，添加了一個(gè) Callback 參數(shù)，表示會(huì)通過(guò)該 Callback 對(duì)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(duì)象，不能傳入 null
* 使用傳入的 Looper 代替默認(rèn)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(rèn)的，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(dāng)前線程默認(rèn)的 Looper 并且設(shè)置該 Handler 是否需要是異步的。
* Handler 是默認(rèn)同步的，除非你調(diào)用該構(gòu)造函數(shù)設(shè)置表示使用異步。
* 異步消息相對(duì)于同步消息，不需要對(duì)消息進(jìn)行同步排序，并且不受 MessageQueue 的 enqueueSyncBarrier(long) 所帶來(lái)的同步障礙影響。
*/
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(gè)構(gòu)造函數(shù)，我們來(lái)看看構(gòu)造函數(shù)到底做了什么？
/**
* 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(dāng)前默認(rèn) Looper 和傳入的 Callback ，傳入 boolean 來(lái)設(shè)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(xiǎn)
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(yīng)該為 static ，否則可能會(huì)導(dǎo)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(wú)法在線程內(nèi)部創(chuàng)建沒(méi)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，其余跟上一個(gè)一樣。
*/
public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
下面是各個(gè)方法的解讀
getMessageName(Message message):
該方法返回一個(gè) Message 的指定名稱(chēng)（大概就是說(shuō)返回一個(gè)唯一標(biāo)識(shí)）。默認(rèn)返回Message 的 Callback 的類(lèi)名，如果 message.callback 為空，返回 Message.what 的十六進(jìn)制數(shù)。obtainMessage():
該方法從全局的 Message Pool（消息池）返回一個(gè) Message 對(duì)象。這樣的效率比重新創(chuàng)建一個(gè)實(shí)例要高。并設(shè)置 Message.target == this。如果你不想用這種方法，可以直接調(diào)用 Message.obtain()。obtainMessage(int what):
和 obtainMessage() 一樣，但是給返回的 Message 對(duì)象設(shè)置了 what 。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，但是設(shè)置了 whta 和 obj 兩個(gè)屬性。public final boolean post(Runnable r):
添加 Runnable 對(duì)象到 MessageQueue 中。該 Runnable 對(duì)象將會(huì)在該 Handler bain綁定到的 Thread 上運(yùn)行。public final boolean postAtTime(Runnable r, long uptimeMillis)：
添加 Runnable 對(duì)象到 MessageQueue 中。該 Runnable 對(duì)象將會(huì)在給定的時(shí)間點(diǎn)運(yùn)行。返回：true 表示該 Runnable 成功添加到 MessageQueue 中，否則返回 false，通常是因?yàn)?Looper 處理 MessageQueue 正在退出。注意，返回 true 并不意味著該 Runnable 一定會(huì)被執(zhí)行 -- 如果 Looper 在 Message 送達(dá)時(shí)間之前 quit，那么 Message 將會(huì)丟失。
