Android - Broadcast Receivers

2021-12-07 14:05 Android开发

<blockquote > Broadcast Receiverssimply respond to broadcast messages from other applications or from the system itself. These messages are sometime called events or intents. For example, applications can also initiate broadcasts to let other applications know that some data has been downloaded to the device and is available for them to use, so this is broadcast receiver who will intercept this communication and will initiate appropriate action. </blockquote> There are following two important steps to make BroadcastReceiver works for the system broadcasted intents − <ul class="list" > <li > Creating the Broadcast Receiver. </li> <li > Registering Broadcast Receiver </li> </ul> There is one additional steps in case you are going to implement your custom intents then you will have to create and broadcast those intents. <h2 >Creating the Broadcast Receiver</h2> A broadcast receiver is implemented as a subclass ofBroadcastReceiverclass and overriding the onReceive() method where each message is received as aIntentobject parameter. <pre class="prettyprint notranslte prettyprinted" > public class MyReceiver extends BroadcastReceiver { @Override public void onReceive(Context context, Intent intent) { Toast.makeText(context, "Intent Detected.", Toast.LENGTH_LONG).show(); } }</pre> <h2 >Registering Broadcast Receiver</h2> An application listens for specific broadcast intents by registering a broadcast receiver inAndroidManifest.xmlfile. Consider we are going to registerMyReceiverfor system generated event ACTION_BOOT_COMPLETED which is fired by the system once the Android system has completed the boot process. <img alt="broadcast" src="http://www.tutorialspoint.com/android/images/broadcast.jpg" /> <h4 align="center" >BROADCAST-RECEIVER</h4> <pre class="prettyprint notranslte prettyprinted" > <application android:icon="@drawable/ic_launcher" android:label="@string/app_name" android:theme="@style/AppTheme" > <receiver android:name="MyReceiver"> <intent-filter> <action android:name="android.intent.action.BOOT_COMPLETED"> </action> </intent-filter> </receiver> </application></pre> Now whenever your Android device gets booted, it will be intercepted by BroadcastReceiverMyReceiverand implemented logic insideonReceive()will be executed. There are several system generated events defined as final static fields in theIntentclass. The following table lists a few important system events. <table class="table table-bordered" > <tbody > <tr > <th >Event Constant</th> <th >Description</th> </tr> <tr > <td >android.intent.action.BATTERY_CHANGED</td> <td >Sticky broadcast containing the charging state, level, and other information about the battery.</td> </tr> <tr > <td >android.intent.action.BATTERY_LOW</td> <td >Indicates low battery condition on the device.</td> </tr> <tr > <td >android.intent.action.BATTERY_OKAY</td> <td >Indicates the battery is now okay after being low.</td> </tr> <tr > <td >android.intent.action.BOOT_COMPLETED</td> <td >This is broadcast once, after the system has finished booting.</td> </tr> <tr > <td >android.intent.action.BUG_REPORT</td> <td >Show activity for reporting a bug.</td> </tr> <tr > <td >android.intent.action.CALL</td> <td >Perform a call to someone specified by the data.</td> </tr> <tr > <td >android.intent.action.CALL_BUTTON</td> <td >The user pressed the "call" button to go to the dialer or other appropriate UI for placing a call.</td> </tr> <tr > <td >android.intent.action.DATE_CHANGED</td> <td >The date has changed.</td> </tr> <tr > <td >android.intent.action.REBOOT</td> <td >Have the device reboot.</td> </tr> </tbody> </table> <h2 >Broadcasting Custom Intents</h2> If you want your application itself should generate and send custom intents then you will have to create and send those intents by using thesendBroadcast()method inside your activity class. If you use thesendStickyBroadcast(Intent)method, the Intent issticky, meaning theIntentyou are sending stays around after the broadcast is complete. <pre class="prettyprint notranslte prettyprinted" > public void broadcastIntent(View view) { Intent intent = new Intent(); intent.setAction("com.tutorialspoint.CUSTOM_INTENT"); sendBroadcast(intent); }</pre> This intentcom.tutorialspoint.CUSTOM_INTENTcan also be registered in similar way as we have regsitered system generated intent. <pre class="prettyprint notranslte prettyprinted" > <application android:icon="@drawable/ic_launcher" android:label="@string/app_name" android:theme="@style/AppTheme" > <receiver android:name="MyReceiver"> <intent-filter> <action android:name="com.tutorialspoint.CUSTOM_INTENT"> </action> </intent-filter> </receiver> </application></pre> <h2 >Example</h2> This example will explain you how to createBroadcastReceiverto intercept custom intent. Once you are familiar with custom intent, then you can program your application to intercept system generated intents. So let's follow the following steps to modify the Android application we created inHello World Examplechapter − <table class="table table-bordered" > <tbody > <tr > <th class="fivepct" >Step</th> <th >Description</th> </tr> <tr > <td >1</td> <td >You will use Android studio to create an Android application and name it asMy Applicationunder a packagecom.example.My Applicationas explained in theHello World Examplechapter.</td> </tr> <tr > <td >2</td> <td >Modify main activity fileMainActivity.javato addbroadcastIntent()method.</td> </tr> <tr > <td >3</td> <td >Create a new java file calledMyReceiver.javaunder the packagecom.example.My Applicationto define a BroadcastReceiver.</td> </tr> <tr > <td >4</td> <td >An application can handle one or more custom and system intents without any restrictions. Every indent you want to intercept must be registered in yourAndroidManifest.xmlfile using <receiver.../> tag</td> </tr> <tr > <td >5</td> <td >Modify the default content ofres/layout/activity_main.xmlfile to include a button to broadcast intent.</td> </tr> <tr > <td >6</td> <td >No need to modify the string file, Android studio take care of string.xml file.</td> </tr> <tr > <td >7</td> <td >Run the application to launch Android emulator and verify the result of the changes done in the application.</td> </tr> </tbody> </table> Following is the content of the modified main activity filesrc/com.example.My Application/MainActivity.java. This file can include each of the fundamental life cycle methods. We have addedbroadcastIntent()method to broadcast a custom intent. <pre class="prettyprint notranslte prettyprinted" > package com.example.My Application; import android.os.Bundle; import android.app.Activity; import android.view.Menu; import android.content.Intent; import android.view.View; public class MainActivity extends Activity { @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); } @Override public boolean onCreateOptionsMenu(Menu menu) { getMenuInflater().inflate(R.menu.activity_main, menu); return true; } // broadcast a custom intent. public void broadcastIntent(View view){ Intent intent = new Intent(); intent.setAction("com.tutorialspoint.CUSTOM_INTENT"); sendBroadcast(intent); } }</pre> Following is the content ofsrc/com.example.My Application/MyReceiver.java: <pre class="prettyprint notranslte prettyprinted" > package com.example.My Application; import android.content.BroadcastReceiver; import android.content.Context; import android.content.Intent; import android.widget.Toast; public class MyReceiver extends BroadcastReceiver { @Override public void onReceive(Context context, Intent intent) { Toast.makeText(context, "Intent Detected.", Toast.LENGTH_LONG).show(); } }</pre> Following will the modified content ofAndroidManifest.xmlfile. Here we have added <service.../> tag to include our service: <pre class="prettyprint notranslte prettyprinted" > <manifest xmlns:android="http://schemas.android.com/apk/res/android" package="com.example.My Application" android:versionCode="1" android:versionName="1.0" > <uses-sdk android:minSdkVersion="8" android:targetSdkVersion="22" /> <application android:icon="@drawable/ic_launcher" android:label="@string/app_name" android:theme="@style/AppTheme" > <activity android:name=".MainActivity" android:label="@string/title_activity_main" > <intent-filter> <action android:name="android.intent.action.MAIN" /> <category android:name="android.intent.category.LAUNCHER"/> </intent-filter> </activity> <receiver android:name="MyReceiver"> <intent-filter> <action android:name="com.tutorialspoint.CUSTOM_INTENT"> </action> </intent-filter> </receiver> </application> </manifest></pre> Following will be the content ofres/layout/activity_main.xmlfile to include a button to broadcast our custom intent − <pre class="prettyprint notranslte prettyprinted" > <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:paddingLeft="@dimen/activity_horizontal_margin" android:paddingRight="@dimen/activity_horizontal_margin" android:paddingTop="@dimen/activity_vertical_margin" android:paddingBottom="@dimen/activity_vertical_margin" tools:context=".MainActivity"> <TextView android:id="@+id/textView1" android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="Example of Broadcast" android:layout_alignParentTop="true" android:layout_centerHorizontal="true" android:textSize="30dp" /> <TextView android:id="@+id/textView2" android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="Tutorials point " android:textColor="#ff87ff09" android:textSize="30dp" android:layout_above="@+id/imageButton" android:layout_centerHorizontal="true" android:layout_marginBottom="40dp" /> <ImageButton android:layout_width="wrap_content" android:layout_height="wrap_content" android:id="@+id/imageButton" android:src="@drawable/abc" android:layout_centerVertical="true" android:layout_centerHorizontal="true" /> <Button android:layout_width="wrap_content" android:layout_height="wrap_content" android:id="@+id/button2" android:text="Broadcast Intent" android:onClick="broadcastIntent" android:layout_below="@+id/imageButton" android:layout_centerHorizontal="true" /> </RelativeLayout></pre> Following will be the content ofres/values/strings.xmlto define two new constants − <pre class="prettyprint notranslte prettyprinted" > <resources> <string name="menu_settings">Settings</string> <string name="title_activity_main">My Application</string> </resources></pre> Let's try to run our modifiedHello World!application we just modified. I assume you had created yourAVDwhile doing environment set-up. To run the app from Android studio, open one of your project's activity files and click Run<img alt="Eclipse Run Icon" class="inline" src="http://www.tutorialspoint.com/android/images/eclipse_run.jpg" />icon from the tool bar. Android Studio installs the app on your AVD and starts it and if everything is fine with your set-up and application, it will display following Emulator window − <img alt="Android Broadcast Demo" src="http://www.tutorialspoint.com/android/images/broadcast1.jpg" /> Now to broadcast our custom intent, let's click onBroadcast Intentbutton, this will broadcast our custom intent"com.tutorialspoint.CUSTOM_INTENT"which will be intercepted by our registered BroadcastReceiver i.e. MyReceiver and as per our implemented logic a toast will appear on the bottom of the the simulator as follows − <img alt="Android Broadcast Intent" src="http://www.tutorialspoint.com/android/images/broadcast2.jpg" /> You can try implementing other BroadcastReceiver to intercept system generated intents like system boot up, date changed, low battery etc.