public class MainActivity extends Activity {private TextView tv;@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);tv = (TextView) findViewById(R.id.tv);}private Handler handler = new Handler() {@Overridepublic void handleMessage(Message msg) {super.handleMessage(msg);switch (msg.what) {case 100:tv.setText("下载完成");break;}}};public void downloadClick(View view) {//使用线程模拟下载操作new Thread(new Runnable() {@Overridepublic void run() {while (true) {try {Thread.sleep(3000);} catch (InterruptedException e) {e.printStackTrace();}break;}handler.sendEmptyMessage(100);}}).start();}
}

                //发送一个空消息，即obj为空，标记为100handler.sendEmptyMessage(100);//获取一个消息对象，返回一个Msg对象Message msg = handler.obtainMessage();msg.what = 100;msg.obj = "要存的信息";//任意类型handler.sendMessage(msg);//发送消息//在制定时间后发送消息handler.sendEmptyMessageAtTime(200, System.currentTimeMillis() + 3000);//延迟2s后发送消息handler.sendEmptyMessageDelayed(300, 2000);

/*** Handler机制* 1、Message 消息对象，内部使用链表数据结构实现一个消息池，用于重复利用，避免大量创建消息对象，造成内存浪费* 2、Handler 消息处理者，通过该对象把消息存入消息队列，并最后通过HandlerMessage方法处理消息* 3、MessageQueue 消息队列，用于存储Message对象的数据结构，先进先出* 4、Looper 消息队列的处理者，用于循环检查消息队列，从消息队列中一个一个的取出消息对象，传入HandlerMessage方法*/

/*** Handler的内存泄露问题* 1、定义一个内部类时，会默认拥有外部类对象的引用，所以建议使用内部类时，最好定义为一个静态内部类* 2、引用的强弱，强引用->软引用 ->弱引用*/public class HandlerMemoryActivity extends Activity {private MyHandler handler;@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_handler_memory);//使用Handler延迟执行一个Runnable（10分钟）handler.postDelayed(new Runnable() {@Overridepublic void run() {System.out.println("!!!!!!run");}}, 1000 * 60 * 10);//关闭当前的Activity
        finish();}private static class MyHandler extends Handler {WeakReference<HandlerMemoryActivity> weakReference;public MyHandler(HandlerMemoryActivity activity) {weakReference = new WeakReference<HandlerMemoryActivity>(activity);}@Overridepublic void handleMessage(Message msg) {super.handleMessage(msg);HandlerMemoryActivity activity = weakReference.get();if (activity != null) {//做处理
            }}}
}

public class MainActivity extends Activity {private TextView tv;private ProgressBar progressBar;@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);tv = (TextView) findViewById(R.id.tv);progressBar = (ProgressBar) findViewById(R.id.progressBar);}public void downloadClick(View view) {new MyAsyncTask(MainActivity.this).execute("http://a.hiphotos.baidu.com/image/pic/item/d50735fae6cd7b8926b326c20c2442a7d8330e97.jpg");}/*** 通过AsyncTask实现一个异步任务*/private static class MyAsyncTask extends AsyncTask<String, Integer, Integer> {private MainActivity activity;public MyAsyncTask(MainActivity activity) {this.activity = activity;}//执行任务之前触发的事件，可以在该方法中做一些初始化动作，例如显示一个dialog//这个是在主线程中
        @Overrideprotected void onPreExecute() {super.onPreExecute();activity.progressBar.setProgress(0);}//在子线程中//执行后台任务的方法
        @Overrideprotected Integer doInBackground(String... params) {String s = params[0];try {URL url = new URL(s);HttpURLConnection urlConnection = (HttpURLConnection) url.openConnection();//获取文件的大小int size = urlConnection.getContentLength();//0是一个标记，表示需要更新的最大进度值，1表示更新当下下载的进度值publishProgress(0, size);byte[] bytes = new byte[100];int len = -1;FileInputStream in = (FileInputStream) urlConnection.getInputStream();FileOutputStream out = new FileOutputStream("/sdcard/" + System.currentTimeMillis() + ".jpg");while ((len = in.read(bytes)) != -1) {out.write(bytes, 0, len);//更新进度publishProgress(1, len);out.flush();}out.close();in.close();} catch (Exception e) {e.printStackTrace();}return 200;}//更新进度
        @Overrideprotected void onProgressUpdate(Integer... values) {super.onProgressUpdate(values);switch (values[0]) {case 0:activity.progressBar.setMax(values[1]);break;case 1:activity.progressBar.incrementProgressBy(values[1]);break;}}@Overrideprotected void onPostExecute(Integer integer) {super.onPostExecute(integer);if (integer == 200) {activity.tv.setText("下载完成");}}}
}