Android中的消息機(jī)制其實(shí)就是指的是Handler消息機(jī)制以及附帶的Looper和MessageQueue的工作流程。
1.Android 為什么提供Handler？
- 解決子線程不能訪問
UI的問題
在ViewRootImpl中有一個checkThread()方法：
void checkThread() {
if (mThread != Thread.currentThread()) {
throw new CalledFromWrongThreadException(
"Only the original thread that created a view hierarchy can touch its views.");
}
}
2. 為什么子線程不能訪問UI呢？
-
Android的UI控件不是線程安全的，如果在多線程中并發(fā)訪問UI控件會導(dǎo)致UI控件處于不可預(yù)期的狀態(tài)
3.那為什么不給UI控件加上鎖機(jī)制呢？
- 加鎖會增加UI邏輯復(fù)雜性
- 鎖機(jī)制會降低線程訪問UI的效率
4.Handler 的工作原理
5.ThreadLocal
Looper.java
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));
}
ThreadLocal.java
class Thread{
/* ThreadLocal values pertaining to this thread. This map is maintained
* by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null;
}
threadLocals 屬于 Thread 里的一個變量
ThreadLocalMap getMap(Thread t) {
return t.threadLocals;
}
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}
public void set(T value) {
//獲取當(dāng)前的線程對象
Thread t = Thread.currentThread();
//getMap(t) 我們就知道了就是獲取當(dāng)前線程里的
//threadLocals 變量 ，
//類型是ThreadLocal.ThreadLocalMap類型
ThreadLocalMap map = getMap(t);
//判斷取出的是否為空，第一次應(yīng)該為空
if (map != null)
//第二次不為空，將key:ThreadLocal;value:Looper存入
map.set(this, value);
else
//為null時,創(chuàng)建ThreadLocalMap對象,并且將值與線程存入
createMap(t, value);
}
那么我們需要知道 ThreadLocalMap 是如何存儲的？
//ThreadLocal 靜態(tài)內(nèi)部類
static class ThreadLocalMap {
static class Entry extends WeakReference<ThreadLocal<?>> {
/** The value associated with this ThreadLocal. */
Object value;
Entry(ThreadLocal<?> k, Object v) {
super(k);
value = v;
}
}
/**
* The initial capacity -- MUST be a power of two.
*/
private static final int INITIAL_CAPACITY = 16;
/**
* The table, resized as necessary.
* table.length MUST always be a power of two.
*/
private Entry[] table;
xxx ....
ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
table = new Entry[INITIAL_CAPACITY];
int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
table[i] = new Entry(firstKey, firstValue);
size = 1;
setThreshold(INITIAL_CAPACITY);
}
/**
* Set the value associated with key.
*
* @param key the thread local object
* @param value the value to be set
*/
private void set(ThreadLocal<?> key, Object value) {
// We don't use a fast path as with get() because it is at
// least as common to use set() to create new entries as
// it is to replace existing ones, in which case, a fast
// path would fail more often than not.
Entry[] tab = table;
int len = tab.length;
//取出那個下標(biāo)
int i = key.threadLocalHashCode & (len-1);
for (Entry e = tab[i];
e != null;
e = tab[i = nextIndex(i, len)]) {
ThreadLocal<?> k = e.get();
if (k == key) {
e.value = value;
return;
}
if (k == null) {
replaceStaleEntry(key, value, i);
return;
}
}
tab[i] = new Entry(key, value);
int sz = ++size;
if (!cleanSomeSlots(i, sz) && sz >= threshold)
rehash();
}
}
6. 主線程 的 Looper 是何時創(chuàng)建的？
在 ActivityThread 的 main 方法中創(chuàng)建的，一起來看下：
public static void main(String[] args) {
Looper.prepareMainLooper();
xxx....
Looper.loop();
}
Looper.prepareMainLooper();
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
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));
}
prepare 方法中又創(chuàng)建threadLocal與Looper的關(guān)聯(lián) , 而且加了判斷，說明一個線程只能有一個Looper,而后創(chuàng)建了 Looper 的實(shí)例。
Looper 構(gòu)造方法
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
創(chuàng)建了 MessageQueue 實(shí)例，獲取當(dāng)前的線程。
而在Handler 的創(chuàng)建構(gòu)造函數(shù)中：
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;
}
直接取出當(dāng)前線程的 Looper--mLooper 和 Looper 中的 MessageQueue 對象。后面就是發(fā)消息，消息入隊(duì)，Looper取出消息，進(jìn)而調(diào)用 handler 的 handleMessage 方法。
看書，隨筆記。如有問題，可指出。
