Pro Android with Kotlin

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CHAPTER 5: Broadcasts
Table 
5-3
 shows some examples (one filter only; if there are several filters, the matching 
happens on an “or” basis).
Active or On-Hold Listening
Which state must an app be in to be able to receive implicit broadcasts? If we want a 
broadcast receiver to be just registered in the system and fired up only on demand when 
a matching broadcast arrives, the listener must be specified in the manifest file of the app. 
However, for implicit broadcasts, this cannot be freely done. It is only for predefined system 
broadcasts, as listed in section “System Broadcasts” of the online text companion.
Table 5-3. Data Matches
Receiver Type
Receiver URI
.* = any string
Sender
Match
text/html
type = "text/html"
Yes
text/html
text/plain
type = "text/html"
Yes
text/html
text/plain
type = "image/jpeg"
No
http://a.b.c/xyz
data = "http://a.b.c/xyz"
Yes
http://a.b.c/xyz
data = "http://a.b.c/qrs"
No
http://a.b.c/xyz/.*
data = "http://a.b.c/xyz/3"
Yes
http://.*/xyz
data = "http://a.b.c/xyz"
Yes
http://.*/xyz
data = "http://a.b.c/qrs"
No
text/html
http://a.b.c/xyz/.*
type = "text/html"
data = "http://a.b.c/xyz/1"
Yes
text/html
http://a.b.c/xyz/.*
type = "image/jpeg"
data = "http://a.b.c/xyz/1"
No
Note This restriction for implicit intent filters specified in the manifest was introduced in Android 8.0 
(API level 26). Before that, any implicit filters could be specified inside the manifest file.

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52
CHAPTER 5: Broadcasts
If, however, you start your broadcast listeners programmatically from inside an app and this 
app is running, you can define as many implicit broadcast listeners as you want, and there 
is no restriction on whether the broadcasts come from the system, your app, or other apps. 
Likewise, there is no restriction on usable action or category names.
Since listening for booting-completed events is included in the list for allowed listeners 
inside the manifest file, you are free to start apps there as activities or services, and inside 
those apps you can register any implicit listener. But that means you can legally work around 
the restrictions imposed starting with Android 8.0. Just be aware that such apps may be 
killed by the Android OS if resource shortages occur, so you have to take appropriate 
precautions.
Sending Implicit Broadcasts
To prepare to send an implicit broadcast, you specify actions, categories, data, and extra 
data as follows:
val intent = Intent()
intent.action = "de.pspaeth.myapp.DO_STH"
intent.addCategory("de.pspaeth.myapp.CATEG1")
intent.addCategory("de.pspaeth.myapp.CATEG2")
// ... more categories
intent.type = "text/html"
intent.data = Uri.parse("content://myContent")
intent.putExtra("EXTRA_KEY", "extraVal")
intent.flags = ...
Only the action is mandatory; all the others are optional. Now to send the broadcast, you’d 
write the following for a remote broadcast:
sendBroadcast(intent)
For a local broadcast, you’d write this:
LocalBroadcastManager.getInstance(this).
sendBroadcast(intent)
The this must be a Context or a subclass thereof; it will work exactly like shown here if the 
code is from inside an activity or a service class.
For remote messages, there is also a variant sending a broadcast to applicable receivers one 
at a time.
...
sendOrderedBroadcast(...)
...
This makes the receivers get the message sequentially, and each receiver may cancel, 
forwarding the message to the next receiver in the line by using BroadcastReceiver.
abortBroadcast().

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