Android开发实践：线程与异步任务

基于移动客户端的软件特别强调实时性，Android程序更是如此，任何一个程序超过5s没有响应，都会被系统强制杀掉。而且Android也不允许在UI线程中进行任何网络操作，否则就会产生NetworkOnMainThreadException 异常。因此，凡是耗时的操作，都不应该直接出现在UI线程中。今天，我通过最简单直观地示例总结下Android开发中最常用的两种处理耗时操作的方法：一个是线程，另一个是异步任务。

首先，看看示例效果，点击Download后，进度条每1秒中增加1%，直到增加到100%。

我将分别用两种方式实现这个功能。

首先，给出 XML 的布局文件：

（1）线程（Thread，Runnable）

第一种方法是通过线程的形式来实现，代码如下：

public class DownloadRunnable implements Runnable {                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          private RunnableStateListener mStateListener;    private String mURL;                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         public static interface RunnableStateListener {        public void onRunnableUpdate(int progress);        public void onRunnableComplete(boolean isSuccess);    }                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                public DownloadRunnable(RunnableStateListener listener, String url ) {        mStateListener = listener;        mURL = url;    }                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                @Override    public void run() {                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   Log.d("DownloadTask", "Begin download, the URL is " + mURL );        for( int i=1; i<=100; i++ ) {                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                mStateListener.onRunnableUpdate(i);                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 try {                Thread.sleep(500);            }            catch (InterruptedException e) {                         e.printStackTrace();                mStateListener.onRunnableComplete(false);                return;            }        }                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      mStateListener.onRunnableComplete(true);    }}

Android/Java中，线程是通过 new Thread(Runnable runnable).start(); 来创建和执行的，所以可以先定义一个类实现 Runnalbe 接口，在 run 函数中完成耗时的操作。本类中，定义RunnableStateListener ，是为了方便线程与外界（调用者）交流，将线程中的任务运行状态传递到外界。

（2）异步任务（AsyncTask）

另一种方法则是采用异步任务来实现，代码如下：

public class DownloadTask extends AsyncTask {                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              private TaskStateListener mTaskStateListener;                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            public static interface TaskStateListener {        public void onTaskUpdate(int progress);        public void onTaskComplete(boolean isSuccess);    }                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  public DownloadTask(TaskStateListener listener) {        mTaskStateListener = listener;    }                         @Override    protected Boolean doInBackground(String ... params )                                                Log.d("DownloadTask", "Begin download, the URL is " + params[0] );                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          for( int i=1; i<=100; i++ ) {                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  //会回调onProgressUpdate            super.publishProgress(i);                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  try {                Thread.sleep(500);            }            catch (InterruptedException e) {                         e.printStackTrace();                return Boolean.FALSE;            }        }                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          return Boolean.TRUE;    }                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            @Override    protected void onProgressUpdate(Integer... values)    {                 mTaskStateListener.onTaskUpdate(values[0]);         super.onProgressUpdate(values);    }                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  @Override    protected void onPostExecute(Boolean result) {        mTaskStateListener.onTaskComplete(result);    }}

异步任务与线程的实现方式有很大不同，异步任务主要通过实例化AsyncTask类来实现，该类有三个接口，doInBackground，该函数是任务的主体部分，将耗时的操作可以放在这里；onProgressUpdate是由系统回调的函数，当doInBackground主体中调用了publishProgress后，则会进入onProgressUpdate更新当前任务的状态，因此，在这里可以通过本类定义的TaskStateListener将状态传递给外界（调用者）；而onPostExecute则是在doInBackground主体任务return（结束）后由系统回调。

AsyncTask的原型定义如下：AsyncTask，有点像C++里的模板，子类可以实例化这三个参数，依次对应 doInBackground 的参数，onProgressUpdate的参数，onPostExecute的返回值。

AsyncTask 通过 new AsyncTask().execute() 来启动，其中 execute 的参数会被传递给 doInBackground 函数。AsyncTask可以通过 getStatus 来获取当前任务的执行状态，通过 cancel来取消。

（3） MainActivity 的实现

MainActivity 的代码如下，我把两种方式的代码都集成在里面了。

public class MainActivity extends Activity implements TaskStateListener,RunnableStateListener {                                                                                                                                                                                                                                                     private TextView mProgressShow;    private ProgressBar mProgressBar;                                                                                                                                                                                                                                                       @Override    protected void onCreate(Bundle savedInstanceState) {                                                                                                                                                                                                                                                                super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);                                                                                                                                                                                                                                                               mProgressShow = (TextView)findViewById(R.id.TextShow);        mProgressBar  = (ProgressBar)findViewById(R.id.ProgressBar);    }                                                                                                                                                                                                                                                       public void onClickDownLoad(View v) {                                                                                                                                                                                                                                                               //new DownloadTask(this).execute("blog.ticktick.51cto.com");                                                                                                                                                                                                                                                            new Thread(new DownloadRunnable(this,"blog.ticktick.51cto.com")).start();    }                                                                                                                                                                                                                                                           @Override    public void onTaskUpdate(int progress) {                                                                                                                                                                                                                                                          mProgressShow.setText(progress+"%");        mProgressBar.setProgress(progress);    }                                                                                                                                                                                                                                                                @Override    public void onTaskComplete(boolean isSuccess) {                                                                                                                                                                                                                                                                if( isSuccess ) {            Toast.makeText(this,"Download Complete",Toast.LENGTH_LONG).show();        }        else {            Toast.makeText(this,"Download Failed",Toast.LENGTH_LONG).show();        }    }                                                                                                                                                                                                                                                        @Override    public void onRunnableUpdate(final int progress) {                                                                                                                                                                                                                                                               this.runOnUiThread(new Runnable() {                                                                                                                                                                                                                                                                   @Override            public void run() {                      mProgressShow.setText(progress+"%");                mProgressBar.setProgress(progress);            }        });          }                                                                                                                                                                                                                                                          @Override    public void onRunnableComplete(final boolean isSuccess) {                                                                                                                                                                                                                                                                this.runOnUiThread(new Runnable() {                                                                                                                                                                                                                                                                         @Override            public void run() {                                                                                                                                                                                                                                                                                   if( isSuccess ) {                    Toast.makeText(MainActivity.this,"Download Complete",Toast.LENGTH_LONG).show();                }                else {                    Toast.makeText(MainActivity.this,"Download Failed",Toast.LENGTH_LONG).show();                }            }        });          }}

这里注意，由线程类回调的onRunnableUpdate和onRunnableComplete函数中，通过this.runOnUiThread的方式在更新UI，而由AsynTask回调的则不需要采用这种方式，因为Android不允许非UI线程改变UI元素，所以必须通过runOnUiThread的方式来更新，而AsynTask则是在内部通过handle收发消息的方式自动切换到了UI线程，所以可以直接更新UI。

关于Android开发中常用的两种耗时操作的处理方式就总结到这儿了，主要通过一个简单的示例程序示范了Runnable和AsynTask的使用方法，工程代码见文章后面的附件。有不清楚的地方，欢迎留言或者来信lujun.hust@gmail.com交流，或者关注我的新浪微博 @卢_俊获取最新的文章和资讯。

更多相关文章

随机推荐