安卓UI线程与异步消息处理机制

1，事实上安卓的UI线程是不安全的，所以如果想要更新应用程序的UI元素，必须在主线程中更新

先看一个例子

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.example.androidthreadtest.MainActivity" >

    <Button
        android:id="@+id/change_text"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Change Text" />

    <TextView
        android:id="@+id/text"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerInParent="true"
        android:text="Hello world"
        android:textSize="20sp" />

</RelativeLayout>

我们想要的效果是点击上面的按钮，然后改变下面的文本操作

所以假定我们有如下的代码

public class MainActivity extends Activity implements OnClickListener {
	private static final int UPDATE_TEXT=1;
private TextView text;
private Button changeText; 
	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		
		text = (TextView) findViewById(R.id.text);
		changeText = (Button) findViewById(R.id.change_text);
		changeText.setOnClickListener(this);
	}
	@Override
	public void onClick(View v) {
		// TODO Auto-generated method stub
		switch (v.getId()) {
		case R.id.change_text:
			new Thread(new Runnable() {
				
				@Override
				public void run() {
					// TODO Auto-generated method stub
			    text.setText("内容都已经改变咯");
				}
			}).start();
			break;

		default:
			break;
		}
	}
}

所以改进后的代码如下：

public class MainActivity extends Activity implements OnClickListener {
	private static final int UPDATE_TEXT=1;
private TextView text;
private Button changeText; 

private Handler handler=new Handler(){
	public void handleMessage(Message msg){
		switch (msg.what) {
		case UPDATE_TEXT:
			//在此处进行UI操作
			text.setText("内容在这里进行改变");
			break;
		default:
			break;
		}
	}
};

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		
		text = (TextView) findViewById(R.id.text);
		changeText = (Button) findViewById(R.id.change_text);
		changeText.setOnClickListener(this);
	}
	@Override
	public void onClick(View v) {
		// TODO Auto-generated method stub
		switch (v.getId()) {
		case R.id.change_text:
			new Thread(new Runnable() {
				
				@Override
				public void run() {
					// TODO Auto-generated method stub
			//不能在这里直接改变
			/*	text.setText("内容都已经改变咯");	*/
					Message message=new Message();
					message.what=UPDATE_TEXT;
					//将message对象发送出去
					handler.sendMessage(message);
				}
			}).start();
			break;

		default:
			break;
		}
	}
}