B4J Tutorial [B4X] Resumable Subs - Sleep / Wait For

New video tutorial:


Resumable subs is a new feature added in B4J v5.50 / B4i v4.00 / B4A v7.00. It dramatically simplifies the handling of asynchronous tasks.
(This feature is a variant of stackless coroutines.)

The special feature of resumable subs is that they can be paused, without pausing the executing thread, and later be resumed.
The program doesn't wait for the resumable sub to be continued. Other events will be raised as usual.

Any sub with one or more calls to Sleep or Wait For is a resumable subs. The IDE shows an indicator next to the sub declaration:

Sleep

Using Sleep is simple:

Log(1)
Sleep(1000)
Log(2)

The sub will be paused for 1000 milliseconds and then be resumed.

You can call Sleep(0) for the shortest pause. This can be used to allow the UI to be refreshed. It is a good alternative to DoEvents (which doesn't exist in B4J and B4i and should be avoided in B4A).

Sub VeryBusySub
   For i = 1 To 10000000
     'do something
     If i Mod 1000 = 0 Then Sleep(0) 'allow the UI to refresh every 1000 iterations.
   Next
   Log("finished!")
End Sub

As demonstrated in the following example, each call to a resumable sub creates a different instance which is not affected by other calls:

Sub btn_Action
   Dim b As Button = Sender
   For i = 10 To 0 Step - 1
     b.Text = i
     Sleep(100)
   Next
   b.Text = "Takeoff!"
End Sub

Wait For

B4X programming language is event driven. Asynchronous tasks run in the background and raise an event when the task completes.
With the new Wait For keyword you can handle the event inside the current sub.

For example, this code will wait for the GoogleMap Ready event:

Sub AppStart (Form1 As Form, Args() As String)
   MainForm = Form1
   MainForm.RootPane.LoadLayout("1") 'Load the layout file.
   gmap.Initialize("gmap")
   Pane1.AddNode(gmap.AsPane, 0, 0, Pane1.Width, Pane1.Height)
   MainForm.Show
   Wait For gmap_Ready '<----------------
   gmap.AddMarker(10, 10, "Marker")
End Sub

A bit more complicated example with FTP:
Listing all files in a remote folder and then downloading all the files:

Sub DownloadFolder (ServerFolder As String)
   FTP.List(ServerFolder)
   Wait For FTP_ListCompleted (ServerPath As String, Success As Boolean, Folders() As FTPEntry, Files() As FTPEntry) '<----
   If Success Then
     For Each f As FTPEntry In Files
         FTP.DownloadFile(ServerPath & f.Name, False, File.DirApp, f.Name)
         Wait For FTP_DownloadCompleted (ServerPath2 As String, Success As Boolean) '<-----
         Log($"File ${ServerPath2} downloaded. Success = ${Success}"$)
     Next
   End If
   Log("Finish")
End Sub

When the Wait For keyword is called, the sub is paused and the internal events dispatcher takes care to resume it when the event is raised. If the event is never raised then the sub will never be resumed. The program will still be completely responsive.
If Wait For is later called with the same event then the new sub instance will replace the previous one.

Lets say that we want to create a sub that downloads an image and sets it to an ImageView:

'Bad example. Don't use.
Sub DownloadImage(Link As String, iv As ImageView)
   Dim job As HttpJob
   job.Initialize("", Me) 'note that the name parameter is no longer needed.
   job.Download(Link)
   Wait For JobDone(job As HttpJob)
   If job.Success Then
     iv.SetImage (job.GetBitmap) 'replace with iv.Bitmap = job.GetBitmap in B4A / B4i
   End If
   job.Release
End Sub

It will work properly if we call it once (more correctly, if we don't call it again before the previous call completes).
If we call it like this:

DownloadImage("https://www.b4x.com/images3/android.png", ImageView1)
DownloadImage("https://www.b4x.com/images3/apple.png", ImageView2)

Then only the second image will show because the second call to Wait For JobDone will overwrite the previous one.
This brings us to the second variant of Wait For.
To solve this issue Wait For can distinguish between events based on the event sender.
This is done with an optional parameter:

Wait For (<sender>) <event signature>

Example:

'Good example. Use.
Sub DownloadImage(Link As String, iv As ImageView)
   Dim job As HttpJob
   job.Initialize("", Me) 'note that the name parameter is no longer needed.
   job.Download(Link)
   Wait For (job) JobDone(job As HttpJob)
   If job.Success Then
     iv.SetImage (job.GetBitmap) 'replace with iv.Bitmap = job.GetBitmap in B4A / B4i
   End If
   job.Release
End Sub

With the above code, each resumable sub instance will wait for a different event and will not be affected by other calls.

Code Flow

Sub S1
Log("S1: A")
S2
Log("S1: B")
End Sub

Sub S2
Log("S2: A")
Sleep(0)
Log("S2: B")
End Sub

The output is:
S1: A
S2: A
S1: B
S2: B

Whenever Sleep or Wait For are called, the current sub is paused. This is equivalent to calling Return.

Wait for a resumable sub to complete and return values from a resumable sub:

https://www.b4x.com/android/forum/threads/b4x-resumable-subs-that-return-values-resumablesub.82670/

Notes & Tips

- Resumable subs cannot return a value.
- The performance overhead of resumable subs in release mode should be insignificant in most cases. The overhead can be larger in debug mode. (If this becomes an issue then take the slow parts of the code and move them to other subs that are called from the resumable sub.)
- Wait For events handlers precede the regular event handlers.
- Resumable subs do not create additional threads. The code is executed by the main thread, or the handler thread in server solutions.

 

[Erel]

Please start a new thread in the questions forum.

 

[Dave O]

How does a resumable sub using Sleep interact with the activity pausing and resuming?

For example, I'm working on a poker app that uses sleep(250) to pause between each player's bet. Works great while the app is running.

However, if I switch out of my app while it's stepping through the players, then return to the app, the sub doesn't appear to resume.

(If I was using the old method of timers, I would pause/resume the timer when my app was paused/resumed.)

Thanks!

 

[LucaMs]

Interesting question (although, as always, it should have been done in a new thread, now I realize this is useful).

Obviously, nobody forbids you to use timers again in situations like these, but, waiting for an Erel's answer, it would be useful to perform some tests: where does the code continue after... [read the question above]?

P.S. I think it will be from a line - unfortunately random - of the code at a higher level than the resumable routine.

 

[Erel]

Please start a new thread for this question.

 

[LucaMs]

Interesting question (although, as always, it should have been done in a new thread

Please start a new thread for this question.

Given that for the moment a new thread has not been opened, I add only this post, about this topic, because I did a little test, I created a loop with "Sleep" inside, paused the app, resumed it and the loop continued in background.

 

[Dave O]

Please start a new thread for this question.

New thread about Sleep vs. Pause/Resume is here.

 

[wle1036]

wait for have time out ?

for example if after 5 second don't receive anything then exit waitfor

 

[Erel]

There is no timeout. However nothing bad happens if a sub is waiting for an event that is never raised.

You can implement a timeout feature in your code. Start a new thread for further discussion.