AsyncTask 封裝了新建后臺工作線程再用Handler更新UI的過程疮跑。在官方文檔也說明這個類最好用于時間短的操作食寡，如果要保持線程長連接辟汰，還是推薦使用Executor、ThreadPoolExecutor等APIs.
AsynTask有三個范型參數(shù)（Params, Progress 岛宦，Result）和四個順序調(diào)用的方法台丛。
- onPreExecute
- doInBackground 此方法是虛方法，必須實現(xiàn)的。
- onProgressUpdate
- onPostExecute
AsyncTask 一般用法如下
AsyncTask<String, Integer, String> execute = new AsyncTask<String, Integer, String>() {
@Override
protected String doInBackground(String... strings) {
Log.e(TAG, "doInBackground");
return "" + strings;
}
}.execute("leavesC");
AsyncTask構(gòu)造方法
AsyncTask有三個構(gòu)造方法：
public AsyncTask()
public AsyncTask(Handler)
public AsyncTask(Looper)
最后都會調(diào)用下面這個構(gòu)造方法挽霉，核心代碼重要思路都在這個方法里面
public AsyncTask(@Nullable Looper callbackLooper) {
//初始化mHandler
mHandler = callbackLooper == null || callbackLooper == Looper.getMainLooper()
? getMainHandler()
: new Handler(callbackLooper);
//WokerRunable 是抽象類防嗡，實現(xiàn)Callable接口。源碼如下文
mWorker = new WorkerRunnable<Params, Result>() {
public Result call() throws Exception {
//AtomicBoolean 實例 mTaskInvoked
mTaskInvoked.set(true);
Result result = null;
try {
//設(shè)置線程優(yōu)先線
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
//noinspection unchecked
//調(diào)用doInBackground并且獲得返回
result = doInBackground(mParams);
//刷新Pending命名
Binder.flushPendingCommands();
} catch (Throwable tr) {
//異常處理
//AtomicBoolean mCancelled = new AtomicBoolean();
mCancelled.set(true);
//拋出異常
throw tr;
} finally {
postResult(result);
}
return result;
}
};
//java的FutureTask 實例化
mFuture = new FutureTask<Result>(mWorker) {
@Override
protected void done() {
try {//發(fā)送結(jié)果
postResultIfNotInvoked(get());
} catch (InterruptedException e) {
android.util.Log.w(LOG_TAG, e);
} catch (ExecutionException e) {
throw new RuntimeException("An error occurred while executing doInBackground()",
e.getCause());
} catch (CancellationException e) {
postResultIfNotInvoked(null);
}
}
};
}
postResultIfNotInvoked() 方法
private void postResultIfNotInvoked(Result result) {
// private final AtomicBoolean mTaskInvoked = new AtomicBoolean();
//原子類AtomicBoolean 實例mTaskInvoked
final boolean wasTaskInvoked = mTaskInvoked.get();
if (!wasTaskInvoked) {
//發(fā)送結(jié)果
postResult(result);
}
}
//我們熟悉的handler發(fā)送消息
private Result postResult(Result result) {
@SuppressWarnings("unchecked")
Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(this, result));
message.sendToTarget();
return result;
}
AsyncTaskResult 異步結(jié)果包裝類 使用了范型
private static class AsyncTaskResult<Data> {
final AsyncTask mTask;
final Data[] mData;
AsyncTaskResult(AsyncTask task, Data... data) {
mTask = task;
mData = data;
}
}
WokerRunable 類 實現(xiàn)Callable接口至于Callable的作用筆者前面的文章有仔細說明侠坎。請自行查閱蚁趁。
private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> {
Params[] mParams;
}
execute()方法
public final AsyncTask<Params, Progress, Result> execute(Params... params) {
return executeOnExecutor(sDefaultExecutor, params);
}
private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,
Params... params) {
//異常狀態(tài)處理
if (mStatus != Status.PENDING) {
switch (mStatus) {
case RUNNING:
throw new IllegalStateException("Cannot execute task:"
+ " the task is already running.");
case FINISHED:
throw new IllegalStateException("Cannot execute task:"
+ " the task has already been executed "
+ "(a task can be executed only once)");
}
}
//設(shè)置當前狀態(tài)是running
mStatus = Status.RUNNING;
//調(diào)用onPreExecute();
onPreExecute();
mWorker.mParams = params;
exec.execute(mFuture);
return this;
}
SerialExecutor繼承了Executor,并且使用ArrayDeque來保證了線程的順序執(zhí)行
private static class SerialExecutor implements Executor {
final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();
Runnable mActive;
public synchronized void execute(final Runnable r) {
mTasks.offer(new Runnable() {
public void run() {
try {
r.run();
} finally {
scheduleNext();
}
}
});
if (mActive == null) {
scheduleNext();
}
}
protected synchronized void scheduleNext() {
if ((mActive = mTasks.poll()) != null) {
THREAD_POOL_EXECUTOR.execute(mActive);
}
}
}
//THREAD_POOL_EXECUTOR定義
public static final Executor THREAD_POOL_EXECUTOR;
static {
ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(
CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE_SECONDS, TimeUnit.SECONDS,
sPoolWorkQueue, sThreadFactory);
threadPoolExecutor.allowCoreThreadTimeOut(true);
THREAD_POOL_EXECUTOR = threadPoolExecutor;
}
InternalHandler 處理與UI線程
private static class InternalHandler extends Handler {
public InternalHandler(Looper looper) {
super(looper);
}
@SuppressWarnings({"unchecked", "RawUseOfParameterizedType"})
@Override
public void handleMessage(Message msg) {
AsyncTaskResult<?> result = (AsyncTaskResult<?>) msg.obj;
switch (msg.what) {
case MESSAGE_POST_RESULT:
// There is only one result
result.mTask.finish(result.mData[0]);
break;
case MESSAGE_POST_PROGRESS:
//更新進度
result.mTask.onProgressUpdate(result.mData);
break;
}
}
}
總結(jié)
- 1，AsyncTask 使用ThreadPoolExecutor 實現(xiàn)線程池來管理線程
- 2实胸，AsyncTask的SerialExecutor使用 ArrayDeque<Runnable>來保證順序執(zhí)行線程他嫡。
- 3，使用WorkerRunable和FutureTask來實現(xiàn)子線程和獲得子線程的執(zhí)行的結(jié)果庐完。
- 4钢属，使用Handler用于線程與主線程之間的通信和回調(diào)結(jié)果。
