和活動、服務及ContentProvider一樣BroadcastReceiver也是Android組件之一,它是可以對客戶端發送的廣播消息作出響應。消息本身是一個Android廣播Intent,廣播消息可以被多個接收程序接收。
在Android系統中,廣播體現在方方面面,例如當開機完成後系統會產生一條廣播,接收到這條廣播就能實現開機啟動服務的功能;當網絡狀態改變時系統會產生一條廣播,接收到這條廣播就能及時地做出提示和保存數據等操作;當電池電量改變時,系統會產生一條廣播,接收到這條廣播就能在電量低時告知用戶及時保存進度,等等。
下面我們就對BroadcastReceiver進行全面的介紹,以瞭解和掌握它的各種功能和用法。
首先,我們來看一下如何發送廣播:
[java]
private void testSendBroadcast(Activity activity){
//create an intent with an action
String uniqueActionString = "com.test.broadcast";
Intent intent = new Intent(uniqueActionString);
intent.putExtra("message","HelloWoreld!");
//send Broadcast
activity.sendBroadcast(intent);
}
在上面的代碼中,我們創建瞭一個唯一、特定操作的Intet,並向其中添加瞭一個extra消息,然後調用sendBroadcast()方法,發送瞭一條廣播。
那麼如何接收廣播呢?看代碼
[java]
public class TestReceiver extends BroadcastReceiver{
private static final String tag = "TestReceiver";
public void onReceive(Context context,Intent intent){
Log.i(tag,"intent" + intent);
String message = intent.getStringExtra("message");
Log.i(tag,message);
}
}
創建廣播接收程序非常簡單,隻需擴展BroadcastReceiver類並改寫onReceive()方法。我們可以在接收程序中通過Intent取得廣播發送的具體消息內容。
最後,我們還必須在描述文件中註冊我們編寫的接收程序,否則你將無法收到廣播。
[java]
<receiver android:name=".TestReceiver">
<intent-filter>
<action android:name="com.test.broadcast"/>
<category android:name="android.intent.category.DEFAULT" />
</intent-filter>
</receiver>
這種在描述文件中註冊的方式我們稱之為:靜態方式。這種方式的註冊是常駐型的,也就是說當應用關閉後,如果有廣播信息傳來,TestReceiver也會被系統調用而自動運行,從而接收到廣播消息。
還有一種代碼中實現的動態註冊方式,具體代碼為:
[java]
TestReceiver receiver = new TestReceiver();
IntentFilter filter = new IntentFilter();
filter.addAction("com.test.broadcast");
registerReceiver(receiver, filter);
registerReceiver是android.content.ContextWrapper類中的方法,Activity和Service都繼承瞭ContextWrapper,所以可以直接調用。在實際應用中,我們在Activity或Service中註冊瞭一個BroadcastReceiver,當這個Activity或Service被銷毀時如果沒有解除註冊,系統會報一個異常,提示我們是否忘記解除註冊瞭。可以通過在onDestory()方法中解除註冊來解決這個問題:
[java]
protected void onDestroy() {
super.onDestroy();
unregisterReceiver(receiver);
}
動態註冊與靜態註冊不同的是:它不是常駐的,一旦程序結束,廣播接收也將結束。
下面我們看一個發送接收廣播的完整例子:
首先來看主活動類:
[java]
public class MainActivity extends Activity
{
private static final String TAG = "MainActivity";
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
}
//創建菜單
@Override
public boolean onCreateOptionsMenu(Menu menu)
{
super.onCreateOptionsMenu(menu);
MenuInflater inflater = getMenuInflater();
inflater.inflate(R.menu.main_menu, menu);
return true;
}
//綁定菜單事件
@Override
public boolean onOptionsItemSelected(MenuItem item)
{
appendMenuItemText(item);
//清空textview
if (item.getItemId() == R.id.menu_clear)
{
this.emptyText();
return true;
}
//發送廣播
if (item.getItemId() == R.id.menu_menu_send_broadcast)
{
this.testSendBroadcast();
return true;
}
return true;
}
private TextView getTextView()
{
return (TextView)findViewById(R.id.text1);
}
private void appendMenuItemText(MenuItem item)
{
String title = item.getTitle().toString();
TextView tv = getTextView();
tv.setText(tv.getText() + "\n" + title);
}
private void emptyText()
{
TextView tv = getTextView();
tv.setText("");
}
private void testSendBroadcast()
{
//create an intent with an action
String uniqueActionString = "com.test.intents.broadcast";
Intent intent = new Intent(uniqueActionString);
intent.putExtra("message","廣播消息發送!");
//send Broadcast
this.sendBroadcast(intent);
}
}
廣播接收器類:
[java]
public class TestReceiver extends BroadcastReceiver{
private static final String tag = "TestReceiver";
public void onReceive(Context context,Intent intent){
Log.i(tag,"intent" + intent);
String message = intent.getStringExtra("message");
Log.i(tag,message);
}
}
下來是佈局文件:
layout/main.xml
[java]
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
>
<TextView
android:id="@+id/text1"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:text="Your debug will appear here"
/>
</LinearLayout>
菜單資源文件:
menu/main_menu.xml
[java]
<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android">
<group android:id="@+id/menuGroup_Main">
<item android:id="@+id/menu_clear" android:title="clear"/>
<item android:id="@+id/menu_menu_send_broadcast" android:title="broadcast"/>
</group>
</menu>
描述文件註冊廣播:
[java]
<application android:icon="@drawable/icon" android:label="@string/app_name">
<activity android:name=".MainActivity"
android:label="@string/app_name">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<receiver android:name=".TestReceiver">
<intent-filter>
<action android:name="com.test.intents.broadcast"></action>
</intent-filter>
</receiver>
</application>
運行程序,點擊模擬器menu菜單,結果如圖:
點擊broadcast,clear結果如圖:
廣播的概念意味著可能有多個接收程序。我們將再建兩個BroadcastReceiver,並在描述文件中註冊,以測試多個接收者情況。
public class TestReceiver2 extends BroadcastReceiver{
private static final String tag = "TestReceiver2";
public void onReceive(Context context,Intent intent){
Log.i(tag,"intent" + intent);
String message = intent.getStringExtra("message");
Log.i(tag,message);
}
}
同時修改描述文件,註冊關播:
[java]
<receiver android:name=".TestReceiver2">
<intent-filter>
<action android:name="com.test.intents.broadcast"></action>
</intent-filter>
</receiver>
運行程序,打印日志如下:
[java]
04-27 15:03:24.028: INFO/TestReceiver1(1317): intentIntent { act=com.test.intents.broadcast cmp=com.test.broadcast/.TestReceiver (has extras) }
04-27 15:03:24.038: INFO/TestReceiver1(1317): 廣播消息發送!
04-27 15:03:24.088: INFO/TestReceiver2(1317): intentIntent { act=com.test.intents.broadcast cmp=com.test.broadcast/.TestReceiver2 (has extras) }
04-27 15:03:24.088: INFO/TestReceiver2(1317): 廣播消息發送!
可以知道多個接收器都接收到瞭廣播,並且是依次接收,我們還可以通過在描述文件中修改,從而改變接收順序:
[java]
<receiver android:name=".TestReceiver">
<intent-filter android:priority="999">
<action android:name="com.test.intents.broadcast"></action>
</intent-filter>
</receiver>
<receiver android:name=".TestReceiver2">
<intent-filter android:priority="1000">
<action android:name="com.test.intents.broadcast"></action>
</intent-filter>
</receiver>
運行,打印日志如下:
[java]
04-27 15:08:09.778: INFO/TestReceiver2(1470): intentIntent { act=com.test.intents.broadcast cmp=com.test.broadcast/.TestReceiver2 (has extras) }
04-27 15:08:09.778: INFO/TestReceiver2(1470): 廣播消息發送!
04-27 15:08:09.888: INFO/TestReceiver1(1470): intentIntent { act=com.test.intents.broadcast cmp=com.test.broadcast/.TestReceiver (has extras) }
04-27 15:08:09.898: INFO/TestReceiver1(1470): 廣播消息發送!
可知receiver2先接收,receiver1後接收,這是因為priority值越大,優先級越高。priority值在-1000到1000之
好瞭,關於廣播接收器就暫時介紹到這。
摘自 fwwdn的專欄