我们在初学Android都用过AsyncTask 一个很方便用来请求网络改变UI控件的类。面试官不多不少也会问到AsyncTask的内容,下面来分析一下这个类内部的逻辑。
AsyncTask内部封装了Handler和Thread分别原来改变ui线程和在子线程做耗时操作。同时AsyncTask是一个抽象的泛型类
public abstract class AsyncTask<Params,Progress,Result>{}
AsyncTask 4个核心方法
ps: 当doInBackground的线程被取消就会回调给onCancelled这个方法,此方法被调用onPostExecute就不会调用了。
总结:
AsyncTask开始执行的方法是在new AsyncTask().execute(url1),execute又调用了executeOnExetutor方法 传入一个串行的线程池SDefaultExecutor和url参数。以下代码:
@MainThread
public final AsyncTask<Params, Progress, Result> execute(Params... params) {
return executeOnExecutor(sDefaultExecutor, params);
}
上面的sDefaultExecute就是new一个SerialExecute类
public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
进入SerialExecutor类,我们看到这个类有个mTask 里面的Runnable会排队执行。并且判断执行完开始执行下一个任务。
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);
}
}
}
接来下,回到executeOnExetutor 这个方法里面 我们可以看到一个mStatus来记录当前任务的状态,期间不断改变状态。
每个AsyncTask最先执行onPreExecute方法,然后线程池exec.execute(mFuture);开始执行。这里的mFuture就是一个Runnable
@MainThread
public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,
Params... params) {
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)");
}
}
mStatus = Status.RUNNING;
onPreExecute();
mWorker.mParams = params;
exec.execute(mFuture);
return this;
}
接着将mFuture传入SerialExecutor的execute方法执行,将之前的mFuture对象插入队列mTask里面,判断当前有没有任务在进行,没活动就调用schedulNext方法执行下一个asynctask任务。
Asynctask里面有2个线程池(一个用于排队的serialexecutor和用于真正执行任务的THREAD_POOL_EXECUTOR )和(负责线程调度的)internalHandler
然后在Asynctask的构造方法 mWork会调用call方法将mTaskInvoked设置为true,再调用doinbackground 得到返回值再调用onpostexecute方法,
在postResult方法里面会发送message到sHandle来执行finish task还是更新进度条,这里的finish有2种情况分别是调用onCancelled 和onPostexecute.
public AsyncTask() {
mWorker = new WorkerRunnable<Params, Result>() {
public Result call() throws Exception {
mTaskInvoked.set(true);
Result result = null;
try {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
//noinspection unchecked
result = doInBackground(mParams);
Binder.flushPendingCommands();
} catch (Throwable tr) {
mCancelled.set(true);
throw tr;
} finally {
postResult(result);
}
return result;
}
};