Office Hours — Today, February 12

← Yesterday, February 11

Feb 12
3:55 PM
Mark M.
has entered the room
Mark M.
turned on guest access
Yaniv S.
has entered the room
Mark M.
hello, Yaniv!
how can I help you today?
Yaniv S.
Hi Mark
two days ago you advised me on a problem , with the service to communicate with the client
Mark M.
yes
Yaniv S.
I couldn't apply the example from the book to my code :(
can you look at my code ?..
my service receives messages
and when a message is received I want a client app to receive it
(just a reminder)
Mark M.
considering that your code is spread across multiple apps, and since I am probably not authorized to see that code in the first place, then I cannot really look at your code
4:00 PM
Mark M.
step #1 would be to get standard service binding going, with AIDL used by all apps, to expose your service and allow the clients to bind to it
step #2 would be to augment the AIDL with some sort of callback/listener interface, where the clients pass an instance of that listener to the service, which uses it to let them know about incoming messages
step #3 is usually beer, though other beverages can be substituted as appropriate :-)
Yaniv S.
I'll just paste relevant snippets
is that ok?
Mark M.
it's up to you
Yaniv S.
ok ..
Mark M.
either paste them here, or use your favorite pastebin/gist sort of service and post the links here
the advantage of the latter is that, if the service lets you, you can pull the snippets down later
Yaniv S.
I don't know how..ill just paste here :)
so , in the service, this is my thread that receives messages
View paste (11 more lines) 
	class ReceiveThread implements Runnable {
		int numCharsToRead = 2;
		Message msg;

		@Override
		public void run() {
			try {
				serialComm.Open();
				do {
					msg = serialComm.Receive(numCharsToRead);
					if (msg != null) {
						incomingMessagesHandler.sendMessage(msg);
					}
				} while (true);

...
it's an infinite loop, and serialComm.Receive
is on a different class where I'm receiving the messages
4:05 PM
Yaniv S.
the return is the message which I pass to a handler
now my AIDL file is this :
View paste 
interface IExtMessage{
	Message receive();
}
the onBind:
Mark M.
that presumably is the callback/listener interface, where the client is the implementer of the IExtMessage.Stub
Yaniv S.
View paste 
    public IBinder onBind(Intent intent) {
        // TODO Auto-generated method stub
        return (new MessageBinder());
    }
and the messageBinder
View paste 
    private static class MessageBinder extends IExtMessage.Stub{


        @Override
        public Message receive() throws RemoteException {
            return message;
        }
    }
Mark M.
that does not make much sense to me
that would be a polling interface
Yaniv S.
ok..so what am I doing weong here?
Mark M.
bad bad bad bad bad
Yaniv S.
wrong
tinoper
has entered the room
Mark M.
use a push system
tinoper
hi
Mark M.
(BTW, hello, tinoper -- I will be with you shortly)
Yaniv S.
whats a push system?
Mark M.
listeners, callbacks, etc.
with a Button, we do not have a busy loop asking "were you just clicked?"
instead, we register a listener, and we are told when the button is clicked
similarly, your clients should not be polling the service asking "did I get a message?"
instead, your clients should be supplying a listener to the service, and the service will tell the client when it receives a message
this is what I pointed out to you in the book, with the Binding/Callback sample app
Yaniv S.
so how do I supply that listener?
Mark M.
via two AIDL interfaces
as is shown in the Binding/Callback sample app from the book that I pointed out to you last time
4:10 PM
Yaniv S.
Iv'e seen it , but did not understand how to apply it to my case
Mark M.
one is the service's AIDL, exposing some sort of registerClientListener(IExtMessageListener l) method
the other is the client's AIDL, defining IExtMessageListener
Yaniv S.
how should I write the aidl files then?
Mark M.
in my sample, the listener is per-operation ("here is the listener to use for this download")
Yaniv S.
when you say listener you mean IDownloadCallBack?
Mark M.
yes, that was the one from the Binding/Callback sample app
"listener" and "callback" tend to be used interchangeably as terms
peek again at the "Servicing the Service" section in the book that I mentioned last time, while I take a question from tinoper, and I will be back with you shortly
Yaniv S.
ok, so which methods I need to code in my listener?
Mark M.
tinoper: your turn! do you have a question?
tinoper
hi Mark
4:15 PM
tinoper
MediaPlayer class can play RTSP streaming?
Mark M.
it is supposed to be able to handle RTSP, last time I checked
as I do not have an RTSP server, I have not tried it
tinoper
ok
Mark M.
http://developer.android.com/guide/appendix/med...
tinoper
Where can I check that?
Mark M.
that page lists the supported network protocols for Android's media framework
"RTSP (RTP, SDP)" is listed
tinoper
I only found local files examples but not with rtsp streaming and when I try to do it fails.
Mark M.
well, I know other audio players use RTSP, as I have seen where you can enter in RTSP URLs into them (though I haven't done this in ages)
bear in mind that RTSP only supports a handful of codecs
AFAIK, stuff like Shoutcast go beyond the RTSP spec
tinoper
Instead of this I try with vitamio library and it works.
Mark M.
and, at least back in the early days of Android, Android's support for RTSP was limited to the spec
so, IIRC, RTSP does not support MP3, but Shoutcast added it
but, at least early versions of Android did not support MP3 over RTSP, as that was out of spec
I haven't looked at RTSP support in Android in years, and so I do not know if compatibility with popular extensions to the spec has improved
4:20 PM
Mark M.
third-party libraries give you greater flexibility, if licensing and the bigger APK size are not impediments
tinoper
Do you suggest anyone?
Mark M.
I have never used a third-party media library, so I have no recommendations
tinoper
ups
what about Exoplayer?
Mark M.
I have heard the name, but I don't know much about it
tinoper
Well,I will try something more with them.
Right now,only Vitamio is working for me.
Mark M.
sorry I could not help you much on this -- media (outside of the camera) is not one of my deeper areas in Android
let me swing back to Yaniv for a bit, and I will come back to you shortly
Yaniv: your turn!
tinoper
No problem.Indeed,it's an answer too!
have a nice day
Mark M.
Yaniv: "so which methods I need to code in my listener?" -- that is up to you, but presumably it would be something like onMessageReceived()
tinoper: you too!
Yaniv S.
and it returns void or a message?
sorry i'm really having trouble with this aidl stuff
Mark M.
it takes a message as a parameter, from the service
what it returns is up to you
usually, you return void, simply because you do not want to tie things up with blocking calls
but if there's some simple rapid response you want to return, that is OK too
Yaniv S.
but I need to return a message to the client no
?
Mark M.
correct
4:25 PM
Yaniv S.
so if it's void.. how does the client receives it?
Mark M.
as a parameter to the method on the listener
let's review how this is done in the Binding/Callback sample
https://github.com/commonsguy/cw-omnibus/blob/m...
Yaniv S.
so something like this?
Mark M.
this is the AIDL exposed by the service
the download() method takes the URL to download, plus an IDownloadCallback object
IDownloadCallback is another AIDL interface: https://github.com/commonsguy/cw-omnibus/blob/m...
IDownloadCallback has onSuccess() and onFailure() methods, for the service to send results back
in the case of your app, IDownload would be replaced by something like IExtMessageSource
it would have a method, something like registerMessageClient()
registerMessageClient() would take an AIDL-defined listener as a parameter
just like how download() takes an AIDL-defined listener as a parameter
Yaniv S.
ok.. with you so far
Mark M.
so, we say that registerMessageClient() takes an IExtMessageListener parameter
IExtMessageListener is another AIDL-defined interface
it defines an onMessageReceived() method
4:30 PM
Mark M.
the service, in its implementation of registerMessageClient(), takes the IExtMessageListener and hangs onto it somewhere
when a message comes in that is appropriate for that client, it calls onMessageReceived() on the IExtMessageListener object
the client has the implementation of onMessageReceived(), and it receives the message as a parameter
what the message itself is, I don't know -- String, Bundle, another AIDL-defined interface, whatever
Yaniv S.
umm
ok
wait
let me tell you where im confused
in your example
in the onbind method
View paste 
  public IBinder onBind(Intent intent) {
    return(new DownloadBinder());
  }
you return a new downloadBinder
where it is a class that from there you start the thread
Mark M.
correct
Yaniv S.
but in my case
the thread starts automatically when the service stars
starts
Mark M.
so?
the listener is a Java object
Yaniv S.
what do I return in y onBind?
my
Mark M.
you return the IExtMessageSource.Stub, based on the names I used above
and note that you are looking at the wrong sample
oh, no, you're right
DownloadBinder is a subclass of IDownload.Stub
my apologies
so, you would return your subclass of IExtMessageSource.Stub
4:35 PM
Yaniv S.
what is this subclass use for?!
Mark M.
where you have your implementation of registerMessageClient()
Yaniv S.
why do i need it?
Mark M.
you are implementing an API
you need to get the listener from the client
Yaniv S.
how do I use the listener?
in your case you passed it to the thread
Mark M.
you call methods on it when the messages come in
Yaniv S.
no sure I follow
View paste 
  private static class DownloadBinder extends IDownload.Stub {
    @Override
    public void download(String url, IDownloadCallback cb) {
      new DownloadThread(url, cb).start();
    }
  }
Mark M.
then I am afraid that I will be of little use to you
let me take a question from tinoper, and I will be back with you in a bit
tinoper: your turn! do you have another question?
4:40 PM
tinoper
has left the room
Mark M.
OK, Yaniv, back to you
Yaniv S.
so, in my case, the download method is a registerMessage method
it passes the listener
Mark M.
registerMessageListener()
the client isn't registering a message, it is registering a listener to receive messages
Yaniv S.
ok
so it takes the listener as parameter
Mark M.
right
Yaniv S.
and..how do I use it?
you start a thread
what do I do ?
Mark M.
store it somewhere
your service is listening for messages on something
when it gets a message, and it determines that it needs to pass the message to the client, it calls the method on the listener
so, if the listener has an onMessageReceived() method, your service calls onMessageReceived()
since I don't know your object model, I cannot tell you exactly what should hold onto the listeners, other than it is tied into your service somehow
4:45 PM
Yaniv S.
and and when onMessageReceived is called
it's passed to the client?
Mark M.
onMessageReceived() would take the message as a parameter
Yaniv S.
oooohhhh
Mark M.
the client would have the listener's .Stub subclass, with the implementation of onMessageReceived()
Yaniv S.
now i get it
Mark M.
and now the client has the message
Yaniv S.
ok, I think I got it
thank you very much Mark
Mark M.
sorry that was so tough to get through
Yaniv S.
that's ok ,I'm a beginner not only in android but in programming in general
Mark M.
this is a tough project for a beginner
Yaniv S.
so this is all really confusing
I know, thank god you are here to help
one more question:
can I start as many threads as I want, in a service?
Mark M.
well, don't go crazy :-)
but there's no specific limit
4:50 PM
Yaniv S.
3-4?
Mark M.
that's reasonable
particularly if they are blocking waiting on I/O a lot
Yaniv S.
blocking waiting is what I've used with the message receiveing?
Mark M.
I do not know, as I do not know how your service is getting the messages
if your serialComm object really means serial I/O, then presumably it is blocking in there somewhere
Yaniv S.
what do you mean by blocking?
Mark M.
that's a little complicated for a chat
http://stackoverflow.com/questions/3982501/mute...
Yaniv S.
thanks, I'll read it there
so thanks again Mark ,
bye bye
Yaniv S.
has left the room
5:00 PM
Mark M.
turned off guest access

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People in this transcript

  • Mark Murphy
  • tinoper
  • Yaniv Sheinfeld