Only the original thread that created a view hierarchy can touch its views——Handler的使用

今天写了一个更新UI的小例子，没想到出了log打印了这样一个错误：Only the original thread that created a view hierarchy can touch its views。goolgle了一下找到了原因。

原来android中相关的view和控件不是线程安全的，我们必须单独做处理。这里借此引出Handler的使用。

Handler的官方描述：

A Handler allows you to send and process

Message

and
Runnable objects associated with a thread's

MessageQueue

.
Each Handler instance is associated with a single thread and that thread's message queue. When you create a new Handler, it is bound to the thread / message queue of the thread that is creating it -- from that point on, it will deliver messages and runnables
to that message queue and execute them as they come out of the message queue

.Handler的使用场合：

1、 to schedule messages and runnables to be executed as some point in the future;

安排messages和runnables在将来的某个时间点执行。

2、 to enqueue an action to be performed on a different thread than your own.

将action入队以备在一个不同的线程中执行。即可以实现线程间通信。比如当你创建子线程时，你可以再你的子线程中拿到父线程中创建的Handler对象，就可以通过该对象向父线程的消息队列发送消息了。由于Android要求在UI线程中更新界面，因此，可以通过该方法在其它线程中更新界面。

通过Handler更新UI实例：

步骤：

1、创建Handler对象（此处创建于主线程中便于更新UI）。

2、构建Runnable对象，在Runnable中更新界面。

3、在子线程的run方法中向UI线程post，runnable对象来更新UI。

详细代码如下：

[java]
view plaincopy

package djx.android;

import djx.downLoad.DownFiles;

import android.app.Activity;

import android.os.Bundle;

import android.os.Handler;

import android.view.View;

import android.view.View.OnClickListener;

import android.widget.Button;

import android.widget.TextView;

public class downLoadPractice extends Activity {

private Button button_submit=null;

private TextView textView=null;

private String content=null;

private Handler handler=null;

/** Called when the activity is first created. */

@Override

public void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.main);

//创建属于主线程的handler

handler=new Handler();

button_submit=(Button)findViewById(R.id.button_submit);

textView=(TextView)findViewById(R.id.textView);

button_submit.setOnClickListener(new submitOnClieckListener());

}

//为按钮添加监听器

class submitOnClieckListener implements OnClickListener{

@Override

public void onClick(View v) {

//本地机器部署为服务器，从本地下载a.txt文件内容在textView上显示

final DownFiles df=new DownFiles("http://192.168.75.1:8080/downLoadServer/a.txt");

textView.setText("正在加载......");

new Thread(){

public void run(){

content=df.downLoadFiles();

handler.post(runnableUi);

}

}.start();

}

}

// 构建Runnable对象，在runnable中更新界面

Runnable runnableUi=new Runnable(){

@Override

public void run() {

//更新界面

textView.setText("the Content is:"+content);

}

};

}

*java线程的详细讲解链接