Android Question Return value from a sub with a call to Wait For

nirmelamoud · Dec 6, 2022

Hi,

I have a sub that use Wait For in order to call rest interface, I want to make it a sync call and return value, can I do that ?
or do I have to use call chain at the end and call other function with the 'return' value

Erel · Dec 7, 2022

Yes: [B4X] Resumable subs that return values (ResumableSub)

nirmelamoud · Dec 8, 2022

Thanks, but not what I was asking,

Im asking if there is a way to do this
funca (1)
...
ret = funcb()
... do something with ret
...

funcb()
....
http call in a sync way (wait for....)
....
return job.getText

like in Java for example, it sound like B4A automatically run funcb in a separate thread so you cant get value back in funca

Erel · Dec 8, 2022

nirmelamoud said:
like in Java for example

There is no such feature in Java. Resumable subs are similar (but not exactly the same) to C# async / await feature.

nirmelamoud said:
it sound like B4A automatically run funcb in a separate thread so you cant get value back in funca

No additional threads are created for resumable subs.

The way to implement the logic that you are describing is with subs that return ResuableSub + Wait For.

B4X:

Sub FuncA
 Wait For (FuncB) Complete (Result As Something)
 Log(Result)
End Sub

Sub FuncB As ResumableSub
 Dim j As HttpJob
 ...
 Dim res As String = j.GetString
 j.Release
 Return res
End Sub

nirmelamoud · Dec 8, 2022

of course I can do that in Java, but regardless, what you are saying is that I can't do that in basic and I need to use callback-like functionality or with double wait for - ok
thanks

Erel · Dec 8, 2022

nirmelamoud said:
of course I can do that in Java,

No, you can't. There is no equivalent feature in Java.

Even trivial code such as this will be much more complicated in Java:

B4X:

For i = 1 To 10
 btn.SetVisibleAnimated(100 * i, i Mod 2 = 0)
 Sleep(100 * i)
Next

And of course that you can't hold the main thread or touch the UI from a background thread.

nirmelamoud · Dec 8, 2022

of course, not meant to hold the UI thread,

5 ways to make HTTP requests in Java

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Java 11 HttpClient

More than twenty years after HttpURLConnection we had Black Panther in the cinemas and a new HTTP client added to Java 11: java.net.http.HttpClient. This has a much more logical API and can handle HTTP/2, and Websockets. It also has the op

Erel · Dec 8, 2022

I was talking about the resumable subs feature. This is not specific to http requests. It is a language feature which allows you to deal with event-driven asynchronous code in almost the same way that you will do with synchronous code.

nirmelamoud · Dec 8, 2022

I agree resumable is a nice feature , when I was talking about the java feature is to implement what I was asking about which is sync call to HTTP get

OliverA · Dec 9, 2022

nirmelamoud said:
or with double wait for

Even a double Wait For does not block. That's the beauty of Wait For, you don't have to worry about grinding the event queue to a halt. Looking at the doc for the Java 11 implementation, both HttpClient.send and CompletableFuture.get are fully blocking, therefore forcing you to use threads in cases where the return may take a while (since at that point, the event queue would be blocked). Now, if you are looking at a full blocking HTTP request that stalls out the event queue, then you'll have to come up with something else.

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