在看progressbar的API文档时有如下例子，在新线程中更新progress值，然后post到handler一个runnable来更新UI，这是为什么呢？
mHandler.post(new Runnable() {
public void run() {
mProgress.setProgress(mProgressStatus);
}
});
Android UI操作并不是线程安全的并且这些操作必须在UI线程中执行。常用的方法是利用Handler来实现UI线程的更新的
public class MyActivity extends Activity {
private static final int PROGRESS = 0x1;
private ProgressBar mProgress;
private int mProgressStatus = 0;
private Handler mHandler = new Handler();
protected void onCreate(Bundle icicle) {
super.onCreate(icicle);
setContentView(R.layout.progressbar_activity);
mProgress = (ProgressBar) findViewById(R.id.progress_bar);
// Start lengthy operation in a background thread
new Thread(new Runnable() {
public void run() {
while (mProgressStatus < 100) {
mProgressStatus = doWork();
// Update the progress bar
mHandler.post(new Runnable() {
public void run() {
mProgress.setProgress(mProgressStatus);
}
});
}
}
}).start();
}
}