Android Handler消息传递机制
Android Handler的消息传递机制
Handler类的只要作用有两个:
1. 在新启动的线程中发送消息;2. 在主线程中获取、处理消息。
看上去,Handler的机制很简单。读者不免会问:新启动的线程何时发送消息?主线程何时去获取并处理消息?
为了能让主线程“适时”地处理新启动的线程所发送的消息,显然只能通过回调的方式来实现:开发者只要重写Handler类的处理消息的方法,这样,在新启动的线程发送消息时Handler类中处理消息的方法被自动调用。
Handler类包含如下方法用于发送、处理消息。
1. void handleMessage(Message msg):处理消息方法,该方法通常用于被重写;
2. final boolean hasMessages(int what):检查消息队列中是否包含what属性为指定值的消息;
3. final boolean hasMessage(int what, Object object):检查消息队列中是否包含what属性为指定值且Object属性为指定对象的消息;
4. Message obtainMessage():获取消息;
5. sendEmptyMessage(int what):发送空消息;
6. final boolean sendEmptyMessageDelayed(int what, long delayMillis):指定多少毫秒后发送空消息:
7. final boolean sendMessage(Message msg):立即发送消息;
8. final boolean sendMessageDelayed(Message msg, long delayMillis):指定多少毫秒之后发送消。
HandlerDemo:
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=".MainActivity" > <TextView android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="@string/hello_world" /> <ImageView android:id="@+id/imgv" android:layout_width="fill_parent" android:layout_height="fill_parent" /> </RelativeLayout>
Java代码:MainActivity.java
package com.zsl.handlerdemo; import java.util.Timer; import java.util.TimerTask; import android.os.Bundle; import android.os.Handler; import android.os.Message; import android.app.Activity; import android.view.Menu; import android.widget.ImageView; public class MainActivity extends Activity { private static int[] pic = new int [] { R.drawable.jay1, R.drawable.jay2, R.drawable.jay3, R.drawable.jay4 }; private static int curPicId = 0; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); final ImageView show = (ImageView)findViewById(R.id.imgv); final Handler handler = new Handler() { @Override public void handleMessage(Message msg) { // TODO Auto-generated method stub super.handleMessage(msg); if(msg.what == 13) { show.setImageResource(pic[curPicId]); curPicId = (curPicId + 1) % pic.length; } } }; new Timer().schedule(new TimerTask() { @Override public void run() { // TODO Auto-generated method stub Message msg = new Message(); msg.what = 13; handler.sendMessage(msg); } }, 0, 1000); } @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; } }