面试常客Handler详细解析（handler基础）(一)

Handler是什么？

handler是Android给我们提供用来更新UI的一套机制，也是一套消息处理的机制，我们可以用它发送消息，也可以通过它处理消息。

联系framework可以详细看到。生命周期的改变都是通过handler消息改变的。

为什么要用Handler？

不用handler更新UI是不行的，Android在设计的时候，就封装了一套消息创建、传递、处理机制，如果不遵循这样的机制，就没有办法更新UI信息的，就会抛出异常信息（不能在非UI线程中更新UI）。

子线程真的不能更新UI吗？

答案是否定的，下面看贴下自己已经运行通过的代码，用事实例证：

MainActivity.java:

package com.example.handler;

import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;

public class MainActivity extends Activity {

private TextView tv;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
tv=(TextView) findViewById(R.id.tv);

new Thread(){
@Override
public void run() {
tv.setText("hi");
}
}.start();;

}
}

对应的activity_main.xml:

<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"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.example.handler.MainActivity" >

<TextView
android:id="@+id/tv"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
/>

</RelativeLayout>

运行结果：



事实证明，子线程是可以更新UI的，但是如果子线程先休眠一段时间再去更新就会失败，这个还得从ViewRootImp的创建说起，ViewRootImp是在onResume()中创建的，没有休眠的子线程不会经历onResume()阶段，只有休眠过的子线程才会经历onResume()阶段，才会创建ViewRootImp，才会不能让子线程更新UI。但是我们不建议这么做，我们最好不要在子线程中更新UI。

Handler怎么用？

handler更新UI：

既然不能在子线程中更新UI，那我们就只能使用handler来更新UI了，下面配合代码实现：

MainActivity.java:

package com.example.handler;

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.widget.TextView;

public class MainActivity extends Activity {

private TextView tv;
private Handler handler = new Handler();

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
tv=(TextView) findViewById(R.id.tv);

new Thread(){
@Override
public void run() {
try {
Thread.sleep(1000);
//                  tv.setText("hi");
handler.post(new Runnable() {

@Override
public void run() {
tv.setText("update...");

}
});
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}

}
}.start();

}

}

activity_main.xml:

<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"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.example.handler.MainActivity" >

<TextView
android:id="@+id/tv"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
/>

</RelativeLayout>

通过runnable轮换显示三张照片：

MainActivity.java:

package com.example.handler;

import android.app.Activity;

import android.os.Bundle;

import android.os.Handler;

import android.widget.ImageView;

import android.widget.TextView;

public class MainActivity extends Activity {

private TextView tv;
private Handler handler = new Handler();
private ImageView imageView;
private int[] images = {R.drawable.a,R.drawable.b,R.drawable.c};
private int index = 0;

private MyRunnable  runnable =  new MyRunnable();

class MyRunnable implements Runnable{
@Override
public void run() {
index++;
index=index%3;
imageView.setImageResource(images[index]);
handler.postDelayed(runnable, 1000);

}
}

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
tv = (TextView) findViewById(R.id.tv);
imageView = (ImageView) findViewById(R.id.imageView1);

new Thread(runnable).start();//这两句话的作用是一样的
//  handler.postDelayed(runnable, 1000);//这两句话的作用是一样的，用哪句都一样

// new Thread(){

// @Override

// public void run() {

// try {

// Thread.sleep(1000);

//// tv.setText(“hi”);

// handler.post(new Runnable() {

//

// @Override

// public void run() {

// tv.setText(“update…”);

//

// }// });

// } catch (InterruptedException e) {

// // TODO Auto-generated catch block

// e.printStackTrace();

// }//

// }// }.start();

}

}对应的activity_main.xml:

<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"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.example.handler.MainActivity" >

<TextView
android:id="@+id/tv"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="@+id/imageView1"
android:layout_below="@+id/imageView1"
android:layout_marginTop="25dp"
android:text="hello" />

<ImageView
android:id="@+id/imageView1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:layout_centerHorizontal="true"
android:layout_marginTop="164dp"
android:src="@drawable/ic_launcher" />

</RelativeLayout>

发送处理消息：

MainActivity.java:

package com.example.handler;

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.widget.ImageView;
import android.widget.TextView;

public class MainActivity extends Activity {

private TextView tv;
private Handler handler = new Handler(){
public void handleMessage(android.os.Message msg) {
tv.setText(""+msg.arg1);
};
};
private ImageView imageView;
private int[] images = {R.drawable.a,R.drawable.b,R.drawable.c};
private int index = 0;

private MyRunnable  runnable =  new MyRunnable();

class MyRunnable implements Runnable{
@Override
public void run() {
index++;
index=index%3;
imageView.setImageResource(images[index]);
handler.postDelayed(runnable, 1000);

}}@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
tv = (TextView) findViewById(R.id.tv);
imageView = (ImageView) findViewById(R.id.imageView1);

new Thread(){
@Override
public void run() {
// TODO Auto-generated method stub
super.run();
try {
Thread.sleep(2000);
Message message = new Message();
message.arg1 = 88;
handler.sendMessage(message);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}}}.start();
//
//      new Thread(runnable).start();

//      new Thread(){
//          @Override
//          public void run() {
//              try {
//                  Thread.sleep(1000);
////                    tv.setText("hi");
//                  handler.post(new Runnable() {
//
//                      @Override
//                      public void run() {
//                          tv.setText("update...");
//
//                      }//                  });
//              } catch (InterruptedException e) {
//                  // TODO Auto-generated catch block
//                  e.printStackTrace();
//              }//
//          }//      }.start();

}}

activity_main.xml:

<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"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.example.handler.MainActivity" >

<TextView
android:id="@+id/tv"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="@+id/imageView1"
android:layout_below="@+id/imageView1"
android:layout_marginTop="25dp"
/>

<ImageView
android:id="@+id/imageView1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:layout_centerHorizontal="true"
android:layout_marginTop="164dp"
android:src="@drawable/ic_launcher" />

</RelativeLayout>

传递复杂数据，比如自定义类，通过obj传递：

MainActivity.java:

package com.example.handler;

import android.app.Activity;

import android.os.Bundle;

import android.os.Handler;

import android.os.Message;

import android.widget.ImageView;

import android.widget.TextView;

public class MainActivity extends Activity {

private TextView tv;
private Handler handler = new Handler(){
public void handleMessage(android.os.Message msg) {
tv.setText(""+msg.obj);
};
};
private ImageView imageView;
private int[] images = {R.drawable.a,R.drawable.b,R.drawable.c};
private int index = 0;

private MyRunnable  runnable =  new MyRunnable();

class MyRunnable implements Runnable{
@Override
public void run() {
index++;
index=index%3;
imageView.setImageResource(images[index]);
handler.postDelayed(runnable, 1000);

}}class Person{
private int age;
private String name;
@Override
public String toString() {
// TODO Auto-generated method stub
return "name="+name+" age="+age;
}}@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
tv = (TextView) findViewById(R.id.tv);
imageView = (ImageView) findViewById(R.id.imageView1);

new Thread(){
@Override
public void run() {
// TODO Auto-generated method stub
super.run();
try {
Thread.sleep(2000);
Message message = new Message();
message.arg1 = 88;
message.arg2 = 100;
Person person = new Person();
person.age = 23;
person.name = "Joe";
message.obj= person;
handler.sendMessage(message);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}}}.start();

//

// handler.postDelayed(runnable, 1000);

// new Thread(){

// @Override

// public void run() {

// try {

// Thread.sleep(1000);

//// tv.setText(“hi”);

// handler.post(new Runnable() {

//

// @Override

// public void run() {

// tv.setText(“update…”);

//

// }// });

// } catch (InterruptedException e) {

// // TODO Auto-generated catch block

// e.printStackTrace();

// }//

// }// }.start();

}

}对应的布局文件activity_main.xml:

<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"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.example.handler.MainActivity" >

<TextView
android:id="@+id/tv"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="@+id/imageView1"
android:layout_below="@+id/imageView1"
android:layout_marginTop="25dp"
/>

<ImageView
android:id="@+id/imageView1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:layout_centerHorizontal="true"
android:layout_marginTop="164dp"
android:src="@drawable/ic_launcher" />

</RelativeLayout>

消息的移除，当button被按下后，轮询图片的runnable被移除：

MainActivity.java:
package com.example.handler;

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.TextView;

public class MainActivity extends Activity implements OnClickListener {

private TextView tv;
private Button button;
private Handler handler = new Handler(){
public void handleMessage(android.os.Message msg) {
tv.setText(""+msg.obj);
};
};

private ImageView imageView;
private int[] images = {R.drawable.a,R.drawable.b,R.drawable.c};
private int index = 0;

private MyRunnable  runnable =  new MyRunnable();

//轮询显示三张图片
class MyRunnable implements Runnable{
@Override
public void run() {
index++;
index=index%3;
imageView.setImageResource(images[index]);
handler.postDelayed(runnable, 1000);

}}//自定义类，传递复杂数据类型使用
class Person{
private int age;
private String name;
@Override
public String toString() {
// TODO Auto-generated method stub
return "name="+name+" age="+age;
}}@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
button = (Button) findViewById(R.id.button1);
button.setOnClickListener(this);
tv = (TextView) findViewById(R.id.tv);
imageView = (ImageView) findViewById(R.id.imageView1);

new Thread(){
@Override
public void run() {
// TODO Auto-generated method stub
super.run();
try {
Thread.sleep(2000);
//                  Message message = new Message();
//                  message.arg1 = 88;
//                  message.arg2 = 100;
Message message = handler.obtainMessage();
Person person = new Person();
person.age = 23;
person.name = "Joe";
message.obj= person;
//                  handler.sendMessage(message);
message.sendToTarget();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}}}.start();
//
handler.postDelayed(runnable, 1000);

//      new Thread(){
//          @Override
//          public void run() {
//              try {
//                  Thread.sleep(1000);
////                    tv.setText("hi");
//                  handler.post(new Runnable() {
//
//                      @Override
//                      public void run() {
//                          tv.setText("update...");
//
//                      }//                  });
//              } catch (InterruptedException e) {
//                  // TODO Auto-generated catch block
//                  e.printStackTrace();
//              }//
//          }//      }.start();

}@Override
public void onClick(View v) {
// TODO Auto-generated method stub
handler.removeCallbacks(runnable);

}}

对应的布局文件：

<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"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.example.handler.MainActivity" >

<TextView
android:id="@+id/tv"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="@+id/imageView1"
android:layout_below="@+id/imageView1"
android:layout_marginTop="25dp"
/>

<ImageView
android:id="@+id/imageView1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:layout_centerHorizontal="true"
android:layout_marginTop="164dp"
android:src="@drawable/ic_launcher" />

<Button
android:id="@+id/button1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:layout_alignRight="@+id/imageView1"
android:text="Button" />

</RelativeLayout>

消息的截获：

通过生成handler时，带有一个callback接口，如果返回false，则不截获数据，如果返回true，则将数据截获，看下面的代码，查看按钮按下的toast信息就知道了：

MainActivity.java:

package com.example.handler;

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Handler.Callback;
import android.os.Message;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.TextView;
import android.widget.Toast;

public class MainActivity extends Activity implements OnClickListener {

private TextView tv;
private Button button;
private Handler handler = new Handler(new Callback() {

@Override
public boolean handleMessage(Message msg) {
// TODO Auto-generated method stub
Toast.makeText(getApplicationContext(), ""+1, 1).show();
return true;
}}){
@Override
public void handleMessage(Message msg) {
// TODO Auto-generated method stub
Toast.makeText(getApplicationContext(), ""+2, 1).show();
}};

private ImageView imageView;
private int[] images = {R.drawable.a,R.drawable.b,R.drawable.c};
private int index = 0;

private MyRunnable  runnable =  new MyRunnable();

//轮询显示三张图片
class MyRunnable implements Runnable{
@Override
public void run() {
index++;
index=index%3;
imageView.setImageResource(images[index]);
handler.postDelayed(runnable, 1000);

}}//自定义类，传递复杂数据类型使用
class Person{
private int age;
private String name;
@Override
public String toString() {
// TODO Auto-generated method stub
return "name="+name+" age="+age;
}}@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
button = (Button) findViewById(R.id.button1);
button.setOnClickListener(this);
tv = (TextView) findViewById(R.id.tv);
imageView = (ImageView) findViewById(R.id.imageView1);

//      new Thread(){
//          @Override
//          public void run() {
//              // TODO Auto-generated method stub
//              super.run();
//              try {
//                  Thread.sleep(2000);
////                    Message message = new Message();
////                    message.arg1 = 88;
////                    message.arg2 = 100;
//                  Message message = handler.obtainMessage();
//                  Person person = new Person();
//                  person.age = 23;
//                  person.name = "Joe";
//                  message.obj= person;
////                    handler.sendMessage(message);
//                  message.sendToTarget();
//              } catch (InterruptedException e) {
//                  // TODO Auto-generated catch block
//                  e.printStackTrace();
//              }//          }//
//      }.start();
//
//      handler.postDelayed(runnable, 1000);

//      new Thread(){
//          @Override
//          public void run() {
//              try {
//                  Thread.sleep(1000);
////                    tv.setText("hi");
//                  handler.post(new Runnable() {
//
//                      @Override
//                      public void run() {
//                          tv.setText("update...");
//
//                      }//                  });
//              } catch (InterruptedException e) {
//                  // TODO Auto-generated catch block
//                  e.printStackTrace();
//              }//
//          }//      }.start();

}@Override
public void onClick(View v) {
// TODO Auto-generated method stub
//      handler.removeCallbacks(runnable);
handler.sendEmptyMessage(1);

}}

对应的布局文件activity_main.xml:

<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"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.example.handler.MainActivity" >

<TextView
android:id="@+id/tv"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="@+id/imageView1"
android:layout_below="@+id/imageView1"
android:layout_marginTop="25dp"
/>

<ImageView
android:id="@+id/imageView1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:layout_centerHorizontal="true"
android:layout_marginTop="164dp"
android:src="@drawable/ic_launcher" />

<Button
android:id="@+id/button1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:layout_alignRight="@+id/imageView1"
android:text="Button" />

</RelativeLayout>