AsyncTask enables proper and easy use of the UI thread. This class allows you
* to perform background operations and publish results on the UI thread without
* having to manipulate threads and/or handlers.
inner class TestAsyncTask : AsyncTask() {
override fun doInBackground(vararg params: Int?): String {
Log.e("method:", "doInBackground()")
Log.e("Thread:", Thread.currentThread().name)
val stringBuilder = StringBuilder()
for (i in params[0]!!..params[0]!! + 10) { // 输出信息
stringBuilder.append(i)
publishProgress(i) // 更新当前进度
Thread.sleep(500)
}
return stringBuilder.toString()
}
override fun onPreExecute() {
super.onPreExecute()
Log.e("method:", "onPreExecute()")
Log.e("Thread:", Thread.currentThread().name)
}
override fun onProgressUpdate(vararg i: Int?) {
super.onProgressUpdate(*i)
Log.e("method:", "onProgressUpdate()")
Log.e("Thread:", Thread.currentThread().name)
Log.e("Progress:", i.toString())
}
override fun onPostExecute(result: String?) {
super.onPostExecute(result)
Log.e("method:", "onPostExecute()")
Log.e("Thread:", Thread.currentThread().name)
}
}
TestAsyncTask().execute(0)
相信对于AsyncTask 的使用没什么难度,系统封装好了处理数据和线程的功能,只需按照步骤把要执行的逻辑放进去就好了,现在来看看AsyncTask 是如何处理了复杂逻辑而却给我们提供简单的使用方法,我们知道AsyncTask 的执行是从调用AsyncTask().execute()开始的,一起来看看他的源码吧:
@MainThread
public final AsyncTask execute(Params... params) {
return executeOnExecutor(sDefaultExecutor, params);
}
@MainThread
public final AsyncTask executeOnExecutor(Executor exec,
Params... params) {
if (mStatus != Status.PENDING) { //判断此时的AsyncTask对象的状态
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; //标记AsyncTask对象为运行状态
onPreExecute();
mWorker.mParams = params; //设置任务参数
exec.execute(mFuture);
return this;
}
从源码中可以看出execute()的方法执行过程:
上述所述的执行过程外,方法中还出现了三个变量:sDefaultExecutor 、mWorker、mFuture,从变量的名称和执行的操作我们姑且可以猜测一下,sDefaultExecutor 应该为任务的直接执行者(即线程、线程池)或间接调用线程执行,mWork对执行参数的封装,mFuture是在exec.execute(mFuture)中执行的,所以应该Runnable类的实例,三个对象实现了任务的保存、获取、执行者、如何执行;那如果我们搞懂了这几个类的工作过程也就搞懂了AsyncTask的实现。
3.1 sDefaultExecutor
public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
private static class SerialExecutor implements Executor {
final ArrayDeque mTasks = new ArrayDeque();
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);
}
}
}
private static final BlockingQueue sPoolWorkQueue =
new LinkedBlockingQueue(128); // 任务队列
private static final ThreadFactory sThreadFactory = new ThreadFactory() { // 生成命名的线程
private final AtomicInteger mCount = new AtomicInteger(1);
public Thread newThread(Runnable r) {
return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());
}
};
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;
}
上述为手动创建线程池的基本过程,还记得前面的Log中执行onBackground()的线程 AsyncTask #1吗;就是在这个地方创建的,也间接说明了真正执行任务的是线程池
忽略参数细节处理的话,到此Async的执行流程基本出来了,首先调用execute()方法 --> 判断状态是否为PENDING -->调用prepare准备数据 -->一系列的参数操作 -->在SerialExecutor中的队列中保存任务 -> 取出头部任务 --> 线程池执行取出的任务;
3.2 mWorker
从上面的赋值可以看出,mWork保存了AsyncTask传入的参数params,下面看看它是如何执行的:
private static abstract class WorkerRunnable implements Callable {
Params[] mParams;
}
mWorker = new WorkerRunnable() {
public Result call() throws Exception {
mTaskInvoked.set(true);
Result result = null;
try {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
result = doInBackground(mParams);//执行任务
Binder.flushPendingCommands();
} catch (Throwable tr) {
mCancelled.set(true);
throw tr;
} finally {
postResult(result);
}
return result;
}
};
private Result postResult(Result result) {
@SuppressWarnings("unchecked")
Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult(this, result));
message.sendToTarget();
return result;
}
3.2 mFuture
mFuture = new FutureTask(mWorker) {
@Override
protected void done() {
try {
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);
}
}
};
还记的前面的判断吗?mFuture 为Runnable的对象,在AsyncTask中同样也实例化了mFuture的对象,实现done()方法,调用 postResultIfNotInvoked()传递结果:
private void postResultIfNotInvoked(Result result) {
final boolean wasTaskInvoked = mTaskInvoked.get();
if (!wasTaskInvoked) {
postResult(result);
}
}
3.4 线程池执行mFuture
public void run() {
if (state != NEW ||
!U.compareAndSwapObject(this, RUNNER, null, Thread.currentThread()))
return;
try {
Callable c = callable;
if (c != null && state == NEW) {
V result;
boolean ran;
try {
result = c.call();//调用call()
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex);
}
if (ran)
set(result);
}
} finally {
runner = null;
int s = state;
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
}
到此AsyncTask的参数处理、任务执行和结果的传递都介绍完毕了,相信大家对此过程应该有所了解了,所剩部分就是AsyncTask的线程切换了,前面也提到了是利用Handler实现的,一起看看吧
3.5 线程的切换
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:
result.mTask.onProgressUpdate(result.mData);
break;
}
}
}
protected final void publishProgress(Progress... values) {
if (!isCancelled()) {
getHandler().obtainMessage(MESSAGE_POST_PROGRESS,
new AsyncTaskResult
好了,大功告成,所有的内容讲完了,有没有感觉代码封装的奇妙之处呢?有没想自己尝试一下写一个简单的AsyncTask呢?心动不如行动,一起实现一个简单的AsyncTask类吧
代码如下:
/**
* @author : Alex
* @date : 2018/08/25
* @version : V 2.0.0
*/
const val PROGRESS = 100
const val RESULT = 101
abstract class SimpleAsyncTask {
val handler = object : Handler(Looper.getMainLooper()) {
override fun handleMessage(msg: Message?) {
super.handleMessage(msg)
when (msg?.what) {
PROGRESS -> onProgressUpdate(msg.obj as Progress)
RESULT -> onPostExecute(msg.obj as Result)
}
}
}
open fun onPreExecute() {
}
open abstract fun doInBackground(params: Array): Result
open fun onProgressUpdate( progress: Progress) {
}
open fun onPostExecute(result: Result) {
}
fun postResult(result: Result) {
val msg = handler.obtainMessage(RESULT, result)
handler.sendMessage(msg)
}
fun publishProgress(progress: Progress) {
val msg = handler.obtainMessage(PROGRESS, progress)
handler.sendMessage(msg)
}
fun execute(params: Array) {
onPreExecute()
Thread {
postResult(doInBackground(params))
}.start()
}
}
将上面例子中改为继承SimpleAsyncTask,执行结果与AsyncTask执行结果一样:
例子非常简单也体现了执行的过程和逻辑,好了本次到此结束。