Question

android中为什么Handler 能更新UI

Answer 1

public class MainActivity extends Activity {
private Button mButton;
private TextView mText;

@SuppressLint("HandlerLeak")
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);

mButton = (Button)this.findViewById(R.id.button);
mText = (TextView)this.findViewById(R.id.text);

final Handler handler = new Handler(){
@Override
public void handleMessage(Message msg){
super.handleMessage(msg);
if(msg.what == 1){
mText.setText("更新后");
}
}
};

mText.setText("更新前");
final Thread thread = new Thread(new Runnable(){

@Override
public void run() {
Message message = new Message();
message.what = 1;
handler.sendMessage(message);
}

});
mButton.setOnClickListener(new OnClickListener() {

@Override
public void onClick(View v) {
thread.start();
}
});
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}

}

Answer 2

只有在ui主线程创建的Handler才能更新UI。
Handler实现了消息队列，循环消息，更新UI。