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asyncTask详解
阅读量:479 次
发布时间:2019-03-06

本文共 14603 字,大约阅读时间需要 48 分钟。

api

AsyncTask是个抽象类,需要子类继承,然后调用execute()方法,继承需要设定Params,Progress,Result类型

- Params是指调用execute()方法时传入的参数类型和doInBackgound()的参数类型
- Progress是指更新进度时传递的参数类型,即publishProgress()和onProgressUpdate()的参数类型
- Result是指doInBackground()的返回值类型

public class MyActivity extends Activity{
private Button btn; private TextView tv; @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); btn = (Button) findViewById(R.id.start_btn); tv = (TextView) findViewById(R.id.content); btn.setOnClickListener(new Button.OnClickListener(){ public void onClick(View v) { update(); } }); } private void update(){ UpdateTextTask updateTextTask = new UpdateTextTask(this); updateTextTask.execute(); } class UpdateTextTask extends AsyncTask
{ private Context context; UpdateTextTask(Context context) { this.context = context; } /** * 运行在UI线程中,在调用doInBackground()之前执行 */ @Override protected void onPreExecute() { Toast.makeText(context,"开始执行",Toast.LENGTH_SHORT).show(); } /** * 后台运行的方法,可以运行非UI线程,可以执行耗时的方法 */ @Override protected Integer doInBackground(Void... params) { int i=0; while(i<10){ i++; publishProgress(i); try { Thread.sleep(1000); } catch (InterruptedException e) { } } return null; } /** * 运行在ui线程中,在doInBackground()执行完毕后执行 */ @Override protected void onPostExecute(Integer integer) { Toast.makeText(context,"执行完毕",Toast.LENGTH_SHORT).show(); } /** * 在publishProgress()被调用以后执行,publishProgress()用于更新进度 */ @Override protected void onProgressUpdate(Integer... values) { tv.setText(""+values[0]); } }}

再来个例子

public class MainActivity extends Activity{
private Button btn; private TextView tv; private ImageView imageView; private ProgressDialog progressDialog; @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); btn = (Button) findViewById(R.id.btn); imageView = findViewById(R.id.iv); // 弹出要给ProgressDialog progressDialog = new ProgressDialog(MainActivity.this); progressDialog.setTitle("提示信息"); progressDialog.setMessage("正在下载中,请稍后......"); // 设置setCancelable(false); 表示我们不能取消这个弹出框,等下载完成之后再让弹出框消失 progressDialog.setCancelable(false); // 设置ProgressDialog样式为圆圈的形式 progressDialog.setProgressStyle(ProgressDialog.STYLE_SPINNER); btn.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View view) { update(); } }); } private void update(){ MyasyncTast myasyncTast = new MyasyncTast(); myasyncTast.execute("http://developer.android.com/images/home/kk-hero.jpg"); } /** * 定义一个类,让其继承AsyncTask这个类 * Params: String类型,表示传递给异步任务的参数类型是String,通常指定的是URL路径 * Progress: Integer类型,进度条的单位通常都是Integer类型 * Result:byte[]类型,表示我们下载好的图片以字节数组返回 * @author xiaoluo * */ class MyasyncTast extends AsyncTask
{ @Override protected void onPreExecute() { super.onPreExecute(); progressDialog.show(); } @Override protected byte[] doInBackground(String... strings) { // 通过Apache的HttpClient来访问请求网络中的一张图片 HttpClient httpClient = new DefaultHttpClient(); HttpGet httpGet = new HttpGet(strings[0]); byte[] image = new byte[]{}; try { HttpResponse httpResponse = httpClient.execute(httpGet); HttpEntity httpEntity = httpResponse.getEntity(); if(httpEntity != null && httpResponse.getStatusLine().getStatusCode() == HttpStatus.SC_OK) { image = EntityUtils.toByteArray(httpEntity); } } catch (Exception e) { e.printStackTrace(); } finally { httpClient.getConnectionManager().shutdown(); } return image; } @Override protected void onPostExecute(byte[] bytes) { super.onPostExecute(bytes); // 将doInBackground方法返回的byte[]解码成要给Bitmap Bitmap bitmap = BitmapFactory.decodeByteArray(bytes, 0, bytes.length); // 更新我们的ImageView控件 imageView.setImageBitmap(bitmap); // 使ProgressDialog框消失 progressDialog.dismiss(); } }}

缺陷:

这里写图片描述

优点:简单,快捷过程可控 缺点:在使用多个异步操作和并需要进行Ui变更时,就变得复杂起来.

源码分析

*  * Copyright (C) 2008 The Android Open Source Project  *  * Licensed under the Apache License, Version 2.0 (the "License");  * you may not use this file except in compliance with the License.  * You may obtain a copy of the License at  *  *      http://www.apache.org/licenses/LICENSE-2.0  *  * Unless required by applicable law or agreed to in writing, software  * distributed under the License is distributed on an "AS IS" BASIS,  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  * See the License for the specific language governing permissions and  * limitations under the License.  */  package android.os;  import java.util.ArrayDeque;  import java.util.concurrent.BlockingQueue;  import java.util.concurrent.Callable;  import java.util.concurrent.CancellationException;  import java.util.concurrent.Executor;  import java.util.concurrent.ExecutionException;  import java.util.concurrent.FutureTask;  import java.util.concurrent.LinkedBlockingQueue;  import java.util.concurrent.ThreadFactory;  import java.util.concurrent.ThreadPoolExecutor;  import java.util.concurrent.TimeUnit;  import java.util.concurrent.TimeoutException;  import java.util.concurrent.atomic.AtomicBoolean;  import java.util.concurrent.atomic.AtomicInteger;  public abstract class AsyncTask
{
private static final String LOG_TAG = "AsyncTask"; //获取当前的cpu核心数 private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors(); //线程池核心容量 private static final int CORE_POOL_SIZE = CPU_COUNT + 1; //线程池最大容量 private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1; //过剩的空闲线程的存活时间 private static final int KEEP_ALIVE = 1; //ThreadFactory 线程工厂,通过工厂方法newThread来获取新线程 private static final ThreadFactory sThreadFactory = new ThreadFactory() { //原子整数,可以在超高并发下正常工作 private final AtomicInteger mCount = new AtomicInteger(1); public Thread newThread(Runnable r) { return new Thread(r, "AsyncTask #" + mCount.getAndIncrement()); } }; //静态阻塞式队列,用来存放待执行的任务,初始容量:128个 private static final BlockingQueue
sPoolWorkQueue = new LinkedBlockingQueue
(128); /** * 静态并发线程池,可以用来并行执行任务,尽管从3.0开始,AsyncTask默认是串行执行任务 * 但是我们仍然能构造出并行的AsyncTask */ public static final Executor THREAD_POOL_EXECUTOR = new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE, TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory); /** * 静态串行任务执行器,其内部实现了串行控制, * 循环的取出一个个任务交给上述的并发线程池去执行 */ public static final Executor SERIAL_EXECUTOR = new SerialExecutor(); //消息类型:发送结果 private static final int MESSAGE_POST_RESULT = 0x1; //消息类型:更新进度 private static final int MESSAGE_POST_PROGRESS = 0x2; /**静态Handler,用来发送上述两种通知,采用UI线程的Looper来处理消息 * 这就是为什么AsyncTask必须在UI线程调用,因为子线程 * 默认没有Looper无法创建下面的Handler,程序会直接Crash */ private static final InternalHandler sHandler = new InternalHandler(); //默认任务执行器,被赋值为串行任务执行器,就是它,AsyncTask变成串行的了 private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR; //如下两个变量我们先不要深究,不影响我们对整体逻辑的理解 private final WorkerRunnable
mWorker; private final FutureTask
mFuture; //任务的状态 默认为挂起,即等待执行,其类型标识为易变的(volatile) private volatile Status mStatus = Status.PENDING; //原子布尔型,支持高并发访问,标识任务是否被取消 private final AtomicBoolean mCancelled = new AtomicBoolean(); //原子布尔型,支持高并发访问,标识任务是否被执行过 private final AtomicBoolean mTaskInvoked = new AtomicBoolean(); /*串行执行器的实现,我们要好好看看,它是怎么把并行转为串行的 *目前我们需要知道,asyncTask.execute(Params ...)实际上会调用 *SerialExecutor的execute方法,这一点后面再说明。也就是说:当你的asyncTask执行的时候, *首先你的task会被加入到任务队列,然后排队,一个个执行 */ private static class SerialExecutor implements Executor {
//线性双向队列,用来存储所有的AsyncTask任务 final ArrayDeque
mTasks = new ArrayDeque
(); //当前正在执行的AsyncTask任务 Runnable mActive; public synchronized void execute(final Runnable r) { //将新的AsyncTask任务加入到双向队列中 mTasks.offer(new Runnable() { public void run() { try { //执行AsyncTask任务 r.run(); } finally { //当前AsyncTask任务执行完毕后,进行下一轮执行,如果还有未执行任务的话 //这一点很明显体现了AsyncTask是串行执行任务的,总是一个任务执行完毕才会执行下一个任务 scheduleNext(); } } }); //如果当前没有任务在执行,直接进入执行逻辑 if (mActive == null) { scheduleNext(); } } protected synchronized void scheduleNext() { //从任务队列中取出队列头部的任务,如果有就交给并发线程池去执行 if ((mActive = mTasks.poll()) != null) { THREAD_POOL_EXECUTOR.execute(mActive); } } } /** * 任务的三种状态 */ public enum Status { /** * 任务等待执行 */ PENDING, /** * 任务正在执行 */ RUNNING, /** * 任务已经执行结束 */ FINISHED, } /** 隐藏API:在UI线程中调用,用来初始化Handler */ public static void init() { sHandler.getLooper(); } /** 隐藏API:为AsyncTask设置默认执行器 */ public static void setDefaultExecutor(Executor exec) { sDefaultExecutor = exec; } /** * Creates a new asynchronous task. This constructor must be invoked on the UI thread. */ public AsyncTask() { mWorker = new WorkerRunnable
() { public Result call() throws Exception { mTaskInvoked.set(true); Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND); //noinspection unchecked return postResult(doInBackground(mParams)); } }; 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 occured while executing doInBackground()", e.getCause()); } catch (CancellationException e) { postResultIfNotInvoked(null); } } }; } private void postResultIfNotInvoked(Result result) { final boolean wasTaskInvoked = mTaskInvoked.get(); if (!wasTaskInvoked) { postResult(result); } } //doInBackground执行完毕,发送消息 private Result postResult(Result result) { @SuppressWarnings("unchecked") Message message = sHandler.obtainMessage(MESSAGE_POST_RESULT, new AsyncTaskResult
(this, result)); message.sendToTarget(); return result; } /** * 返回任务的状态 */ public final Status getStatus() { return mStatus; } /** * 这个方法是我们必须要重写的,用来做后台计算 * 所在线程:后台线程 */ protected abstract Result doInBackground(Params... params); /** * 在doInBackground之前调用,用来做初始化工作 * 所在线程:UI线程 */ protected void onPreExecute() { } /** * 在doInBackground之后调用,用来接受后台计算结果更新UI * 所在线程:UI线程 */ protected void onPostExecute(Result result) { } /** * Runs on the UI thread after {@link #publishProgress} is invoked. /** * 在publishProgress之后调用,用来更新计算进度 * 所在线程:UI线程 */ protected void onProgressUpdate(Progress... values) { } /** * cancel被调用并且doInBackground执行结束,会调用onCancelled,表示任务被取消 * 这个时候onPostExecute不会再被调用,二者是互斥的,分别表示任务取消和任务执行完成 * 所在线程:UI线程 */ @SuppressWarnings({ "UnusedParameters"}) protected void onCancelled(Result result) { onCancelled(); } protected void onCancelled() { } public final boolean isCancelled() { return mCancelled.get(); } public final boolean cancel(boolean mayInterruptIfRunning) { mCancelled.set(true); return mFuture.cancel(mayInterruptIfRunning); } public final Result get() throws InterruptedException, ExecutionException { return mFuture.get(); } public final Result get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException { return mFuture.get(timeout, unit); } /** * 这个方法如何执行和系统版本有关,在AsyncTask的使用规则里已经说明,如果你真的想使用并行AsyncTask, * 也是可以的,只要稍作修改 * 必须在UI线程调用此方法 */ public final AsyncTask
execute(Params... params) { //串行执行 return executeOnExecutor(sDefaultExecutor, params); //如果我们想并行执行,这样改就行了,当然这个方法我们没法改 //return executeOnExecutor(THREAD_POOL_EXECUTOR, params); } /** * 通过这个方法我们可以自定义AsyncTask的执行方式,串行or并行,甚至可以采用自己的Executor * 为了实现并行,我们可以在外部这么用AsyncTask: * asyncTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, Params... params); * 必须在UI线程调用此方法 */ public final AsyncTask
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; //这里#onPreExecute会最先执行 onPreExecute(); mWorker.mParams = params; //然后后台计算#doInBackground才真正开始 exec.execute(mFuture); //接着会有#onProgressUpdate被调用,最后是#onPostExecute return this; } /** * 这是AsyncTask提供的一个静态方法,方便我们直接执行一个runnable */ public static void execute(Runnable runnable) { sDefaultExecutor.execute(runnable); } /** * 打印后台计算进度,onProgressUpdate会被调用 */ protected final void publishProgress(Progress... values) { if (!isCancelled()) { sHandler.obtainMessage(MESSAGE_POST_PROGRESS, new AsyncTaskResult
(this, values)).sendToTarget(); } } //任务结束的时候会进行判断,如果任务没有被取消,则onPostExecute会被调用 private void finish(Result result) { if (isCancelled()) { onCancelled(result); } else { onPostExecute(result); } mStatus = Status.FINISHED; } //AsyncTask内部Handler,用来发送后台计算进度更新消息和计算完成消息 private static class InternalHandler extends Handler { @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; } } } private static abstract class WorkerRunnable
implements Callable
{ Params[] mParams; } @SuppressWarnings({ "RawUseOfParameterizedType"}) private static class AsyncTaskResult
{ final AsyncTask mTask; final Data[] mData; AsyncTaskResult(AsyncTask task, Data... data) { mTask = task; mData = data; } } }
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