BroadcastReceiver

1, Definition

• BroadcastReceiver is one of the four components of Android

  android broadcast is divided into two roles: Broadcast sender and broadcast receiver

2, Function

• It is used to monitor / receive the broadcast information sent by the application and respond

• Application scenario

  a. Communication between different components (in application and different applications)

  b. Communicate with Android system components

  c. Multithreaded communication

3, Implementation principle

• The broadcast in Android uses the observer mode in the design mode: message based subscription and publishing event model.

• Three roles in the model:

  1. Message subscriber (broadcast receiver)

  2. Message publisher (broadcast sender)

  3. activity manager service

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• Principle description:

  The broadcast receiver registers in AMS through Binder mechanism

  Broadcast sender sends broadcast to AMS through Binder mechanism

  AMS searches for a suitable broadcast receiver in the registered list according to the requirements of the broadcast sender

  AMS sends the broadcast to the corresponding message loop queue of the appropriate broadcast receiver

  The broadcast receiver gets the broadcast through the message loop and calls back onReceive()

  Special note: the execution of the broadcast sender and the broadcast receiver is asynchronous, and the broadcast sent will not care about the problem

  There is no receiver and it is uncertain when the receiver will receive it

4, Specific use

4.1 user defined broadcast receiver

• Inherit BroadcastReceivre

• Abstract method onReceive() must be replicated

  When the corresponding receiver of the broadcast is called, recon1. Will automatically receive the broadcast

  2. Generally, onRecevice involves the interaction between components, such as sending notification and starting service

  3. By default, it runs in the UI thread, so onrevise cannot perform time-consuming operations

• Code demonstration

  public class mBroadcastReceiver extends BroadcastReceiver {
  //This method is called automatically after receiving the broadcast
  @Override
  public void onReceive(Context context, Intent intent) {
  //Write operation after receiving broadcast
  } }
  

4.2 broadcast receiver registration

There are two ways to register: one is static registration and the other is dynamic registration.

4.2.1 static registration

• Label declaration in Android manifest

• Attribute description

  <receiver
  android:enabled=["true" | "false"] 
  //Can this broadcastReceiver receive broadcasts from other apps 
  //The default value is determined by whether there is an intent filter in the receiver: if there is an intent filter, the default value is true, otherwise it is false
  android:exported=["true" | "false"]
  android:icon="drawable resource"
  android:label="string resource" 
  //Class name inheriting the subclass of BroadcastReceiver 
  android:name=".mBroadcastReceiver" 
  //Only the broadcast sent by the broadcast sender with corresponding permission can be received by this BroadcastReceiver; 
  android:permission="string"
  //The process in which BroadcastReceiver runs 
  //The default is app process. You can specify an independent process 
  //Note: the four basic components of Android can specify their own independent processes through this attribute 
  android:process="string" >
    //Specifies the type of broadcast this broadcast receiver will receive 
    //In this example, it is used to receive the broadcast when the network state changes 
    <intent-filter>
      <action android:name="android.net.conn.CONNECTIVITY_CHANGE"/>
    </intent-filter>
  </receiver>
  

• Registration example

  <receiver 
  //This broadcast receiver class is mBroadcastReceiver 
    android:name=".mBroadcastReceiver" > 
  //Used to receive the broadcast when the network state changes 
    <intent-filter>
        <action android:name="android.net.conn.CONNECTIVITY_CHANGE" />
    </intent-filter>
  </receiver>
  

  When this App is started for the first time, the system will automatically instantiate the mbaroadcastreceiver class and register it with the system

  Tongzhong.

4.2.2 dynamic registration

In the code, dynamically register BroadcastReceiver by calling the registerReceiver() method of Context. The specific code is as follows:

@Override
protected void onResume() {
    super.onResume();
//Instantiate the subclass of BroadcastReceiver & intentfilter
    mBroadcastReceiver mBroadcastReceiver = new mBroadcastReceiver();
    IntentFilter intentFilter = new IntentFilter();
//Set the type of broadcast received
    intentFilter.addAction(android.net.conn.CONNECTIVITY_CHANGE);
//Call the registerReceiver() method of Context for dynamic registration
    registerReceiver(mBroadcastReceiver, intentFilter);
}
//After registering the broadcast, remember to destroy the broadcast in the corresponding position
//That is, unregisterReceiver(mBroadcastReceiver) in onPause() 
//When this Activity is instantiated, MyBroadcastReceiver will be dynamically registered in the system 
//When this Activity is destroyed, the dynamically registered MyBroadcastReceiver will no longer receive the corresponding broadcast.
@Override
protected void onPause() {
super.onPause(); 
//Destroy the broadcast in the onResume() method 
unregisterReceiver(mBroadcastReceiver);
}

• Particular attention

  Dynamic broadcasting is best registered in onResume() and cancelled in onPause() of Activity.

  Reason: for dynamic broadcasting, if there is registration, there must be logout, otherwise it will lead to memory leakage

4.2.3 differences between two registrations

4.3 broadcast sender sends broadcast to AMS

4.3.1 transmission of broadcast

• Broadcasting is identified by "intention"
• Define the essence of Broadcasting: define the "intention" of broadcasting
• Broadcast sending: the broadcast sender sends the "intention" of this broadcast through * * sendBroadcast() * * method

4.3.2 type of broadcast

• Normal broadcast

  That is, the developer's own definition intent Broadcast of(Most commonly used). Send broadcast using the following:
   
  Intent intent = new Intent(); 
  //action corresponding to intentFilter in BroadcastReceiver 
  intent.setAction(BROADCAST_ACTION); 
  //Send broadcast
  sendBroadcast(intent);
  //If the action of intentFilter matches the above when registering in the registered broadcast receiver, the broadcast will be received (i.e. callback onReceive()).
  //The following mBroadcastReceiver will receive the above broadcast
  <receiver 
    //This broadcast receiver class is mBroadcastReceiver 
    android:name=".mBroadcastReceiver" > 
    //Used to receive the broadcast when the network state changes 
    <intent-filter>
          <action android:name="BROADCAST_ACTION" />
    </intent-filter>
  </receiver>
            
  If the sending broadcast has corresponding permission, the broadcast receiver also needs corresponding permission
  

• System broadcast

  • Android has built-in multiple system broadcasts: as long as it involves the basic operation of the mobile phone (such as power on, network state change, taking photos, etc.), it will send out corresponding broadcasts

  • Each broadcast has a specific intent - filter (including specific actions). The broadcast actions of Android system are as follows:

  

• Ordered broadcast

  • Defines that the broadcast sent is received by the broadcast receiver in sequence

  • Sequence rules for broadcast recipients to receive broadcasts (for both static and dynamic registered broadcast recipients)

  • characteristic

    1. Receive broadcasts in sequence

    2. The receiver of the first received broadcast can cut off the broadcast, that is, the receiver of the later received broadcast will no longer receive the broadcast

       Receive this broadcast;

    3. The first received broadcast receiver can modify the broadcast, and the later received broadcast receiver will receive the broadcast

       Receive modified broadcast

• Sticky broadcast

  Because in Android 5 0 & API 21 has been invalidated, so it is not recommended to use it, and there is no excessive summary here

  Knot.

• App local broadcast

  • Background the broadcast in Android can communicate directly across apps (exported for the case with intent filter)

    (the default value is true)