Handler初步学习
最后更新于：2022-04-01 14:43:12
前面介绍了异步任务的处理，这里来学一下Handler和Message机制吧。下面这个代码和之前的一样，都是从网上下载图片的。
~~~
package com.example.myasynctask;
import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.InputStream;
import org.apache.http.HttpEntity;
import org.apache.http.HttpRequest;
import org.apache.http.HttpResponse;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;
import org.apache.http.util.EntityUtils;
import android.os.AsyncTask;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.app.Activity;
import android.app.ProgressDialog;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.ImageView;
public class MainActivity extends Activity {
private Button button;
private ImageView iv;
private String path = "http://cdn.duitang.com/uploads/item/201310/17/20131017194234_eZfFU.thumb.600_0.png";
private Handler handler = new Handler() {
@Override
public void handleMessage(Message msg) {
Bitmap bm = null;
byte[] data = (byte[])msg.obj;
bm = BitmapFactory.decodeByteArray(data, 0, data.length);
iv.setImageBitmap(bm);
};
};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
button = (Button) this.findViewById(R.id.bt);
iv = (ImageView) this.findViewById(R.id.iv);
button.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
new Thread(new myThread()).start();
}
});
}
private class myThread implements Runnable {
@Override
public void run() {
HttpClient httpClient = new DefaultHttpClient();
HttpGet httpGet = new HttpGet(path);
InputStream inputStream = null;
try {
HttpResponse httpResponse = httpClient.execute(httpGet);
if (httpResponse.getStatusLine().getStatusCode() == 200 ) {
HttpEntity httpEntity = httpResponse.getEntity();
byte[] data = EntityUtils.toByteArray(httpEntity);
Message message = Message.obtain();
message.obj = data;
handler.sendMessage(message);
}
} catch (Exception e) {
// TODO: handle exception
e.printStackTrace();
}
}
}
}
~~~
~~~
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~~~
Looper维护一个消息队列（在loop()函数里面）
Message.obtain可回收的获取Message对象（这样就不用多次new对象）
Handler通过sendMessage来把Message进行消息队列的入队，并且把Message的target（也就是对应的Handler绑定为自己）
当消息队列循环到相应的Message的时候，会调用Message绑定的Handler的dispatchMessage函数，该函数会调用handleMessage函数，也就是用户实现的handleMessage函数。Handler可以绑定多个Message，通过Message.what来识别身份。
--------源码所得，2014/ 9/ 9 修改