android.content.BroadcastReceier.
public abstract class
extends Object
Base class for code that will receive intents sent by sendBroadcast(). You can either dynamically register an instance of this class with Context.registerReceiver()
or statically publish an implementation through the
tag in your AndroidManifest.xml
. Note: If registering a receiver in your Activity.onResume()
implementation, you should unregister it in Activity.onPause()
. (You won't receive intents when paused, and this will cut down on unnecessary system overhead). Do not unregister in Activity.onSaveInstanceState()
, because this won't be called if the user moves back in the history stack.
作用:接收由sendBroadcast()发送的Intent。
There are two major classes of broadcasts that can be received:
Context.sendBroadcast
) are completely asynchronous. All receivers of the broadcast are run in an undefined order, often at the same time. This is more efficient, but means that receivers cannot use the result or abort APIs included here. Context.sendOrderedBroadcast
) are delivered to one receiver at a time. As each receiver executes in turn, it can propagate a result to the next receiver, or it can completely abort the broadcast so that it won't be passed to other receivers. The order receivers run in can be controlled with the android:priority
attribute of the matching intent-filter; receivers with the same priority will be run in an arbitrary order. 这里:Normal broadcasts是不按顺序发送广播消息的。而Ordered broadcasts是一次只发送一个广播消息给一个接收者。
Even in the case of normal broadcasts, the system may in some situations revert to 恢复delivering the broadcast one receiver at a time. In particular, for receivers that may require the creation of a process, only one will be run at a time to avoid overloading the system with new processes. In this situation, however, the non-ordered semantics hold: these receivers still cannot return results or abort their broadcast.
Note that, although the Intent class is used for sending and receiving these broadcasts, the Intent broadcast mechanism here is completely separate from Intents that are used to start Activities with Context.startActivity()
. There is no way for a BroadcastReceiver to see or capture Intents used with startActivity(); likewise, when you broadcast an Intent, you will never find or start an Activity. These two operations are semantically very different: starting an Activity with an Intent is a foreground operation that modifies what the user is currently interacting with; broadcasting an Intent is a background operation that the user is not normally aware of.
注意:通过Context.startActivity启动Intent和使用BroadcastReceiver广播Intent在运行方式中是不同的,前者是处于前台的,而后者处于后台的。
The BroadcastReceiver class (when launched as a component through a manifest's
tag) is an important part of an application's overall lifecycle.
Topics covered here:
A BroadcastReceiver object is only valid for the duration of the call to onReceive(Context, Intent)
. Once your code returns from this function, the system considers the object to be finished and no longer active.
This has important repercussions不良后果 to what you can do in an onReceive(Context, Intent)
implementation: anything that requires asynchronous operation is not available, because you will need to return from the function to handle the asynchronous operation, but at that point the BroadcastReceiver is no longer active and thus the system is free to kill its process before the asynchronous operation completes.
In particular, you may not show a dialog or bind to a service from within a BroadcastReceiver. For the former, you should instead use the NotificationManager
API. For the latter, you can use Context.startService()
to send a command to the service.
BroadcastReceiver的生命期只处于调用onReceive期间,所以在onReceive函数执行过程中,不能调用异步操作。也不能显示对话框或和某个服务进行绑定。
Access permissions can be enforced by either the sender or receiver of an Intent.
To enforce a permission when sending, you supply a non-null permission argument to sendBroadcast(Intent, String)
or sendOrderedBroadcast(Intent, String, BroadcastReceiver, android.os.Handler, int, String, Bundle)
. Only receivers who have been granted this permission (by requesting it with the
tag in their AndroidManifest.xml
) will be able to receive the broadcast.
To enforce a permission when receiving, you supply a non-null permission when registering your receiver -- either when calling registerReceiver(BroadcastReceiver, IntentFilter, String, android.os.Handler)
or in the static
tag in your AndroidManifest.xml
. Only broadcasters who have been granted this permission (by requesting it with the
tag in their AndroidManifest.xml
) will be able to send an Intent to the receiver.
See the Security and Permissions document for more information on permissions and security in general.
A process that is currently executing a BroadcastReceiver (that is, currently running the code in its onReceive(Context, Intent)
method) is considered to be a foreground process and will be kept running by the system except under cases of extreme memory pressure.
Once you return from onReceive(), the BroadcastReceiver is no longer active, and its hosting process is only as important as any other application components that are running in it. This is especially important because if that process was only hosting the BroadcastReceiver (a common case for applications that the user has never or not recently interacted with), then upon returning from onReceive() the system will consider its process to be empty and aggressively kill it so that resources are available for other more important processes.
This means that for longer-running operations you will often use a Service
in conjunction with a BroadcastReceiver to keep the containing process active for the entire time of your operation.
需要特别注意的是:如果hosting process中只有Broadcast Receiver的话,一旦Broadcast Receiver的onReceive()函数执行完后,就会被系统认为是空进程,而被杀掉。所以一般要使Broadcast Receiver长期在操作过程中有效的话,就和Service一起绑定使用。
下面是一些常用的成员函数:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~·
Retrieve the current result data, as set by the previous receiver. Often this is null.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~·
This method is called when the BroadcastReceiver is receiving an Intent broadcast. During this time you can use the other methods on BroadcastReceiver to view/modify the current result values. The function is normally called within the main thread of its process, so you should never perform long-running operations in it (there is a timeout of 10 seconds that the system allows before considering the receiver to be blocked and a candidate to be killed). You cannot launch a popup dialog in your implementation of onReceive().
If this BroadcastReceiver was launched through a startService(Intent)
instead of bindService(Intent, ServiceConnection, int)
. If you wish to interact with a service that is already running, you can use peekService(Context, Intent)
.
The Intent filters used in registerReceiver(BroadcastReceiver, IntentFilter)
and in application manifests are not guaranteed to be exclusive. They are hints to the operating system about how to find suitable recipients. It is possible for senders to force delivery to specific recipients, bypassing filter resolution. For this reason, onReceive()
implementations should respond only to known actions, ignoring any unexpected Intents that they may receive.
context | The Context in which the receiver is running. |
---|---|
intent | The Intent being received. |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Change all of the result data returned from this broadcasts; only works with broadcasts sent through Context.sendOrderedBroadcast
. All current result data is replaced by the value given to this method.
This method does not work with non-ordered broadcasts such as those sent with Context.sendBroadcast
code | The new result code. Often uses the Activity RESULT_CANCELED and RESULT_OK constants, though the actual meaning of this value is ultimately up to the broadcaster. |
---|---|
data | The new result data. This is an arbitrary string whose interpretation is up to the broadcaster; may be null. |
extras | The new extra data map. This is a Bundle holding arbitrary data, whose interpretation is up to the broadcaster. Can be set to null. This completely replaces the current map (if any). |