Android Question Problem with Wait For and asynchronous code in library?

RJB · Apr 25, 2024

Using something like the code below the 'following code' in the 'CallingSub' is never executed and (of course) other code continues before the asynchronous call completes.

I really need to 'wait for' the asynchronous code to complete before continuing with the 'following code'

How do I do that?

Thanks

B4X:

Sub Class_Globals
    Dim WiFi As MLwifi
End Sub

Sub CallingSub
    Try
        'Preceding code
        'Call 'CalledSub' and wait for result
        wait for (CalledSub) complete (Result As Boolean)
        'returns with 'Result' not set?
        'so following code never executes
        Log("Following code")
        Return Result

    Catch
        Log(LastException)
'e.g.: (NullPointerException) java.lang.NullPointerException
'etc

    End Try
End Sub

Sub CalledSub As ResumableSub
    'Call e.g.asynchronous code in a Library and wait for result
    WiFi.reconnectWifiAP("SSID", 10000)
    wait for Wifi_ConnectionResult(Success As Boolean) 'Returns immediately without return value set?
    Return Success
End Sub

Erel · Apr 25, 2024

RJB said:
wait for Wifi_ConnectionResult(Success As Boolean) 'Returns immediately without return value set?

This means that the ConnectionResult event was raised immediately.

RJB · Apr 25, 2024

Sorry but I don't understand:
If the ConnectionResult was raised then wouldn't the following line return true/ false, rather than the NullPointerException being reported in the calling sub?
I assumed that the Wait For is causing the calling sub to continue, whilst it waits?
Sorry if my description was not clear.

aeric · Apr 25, 2024

Maybe you don't need to use Wait For.

Erel · Apr 25, 2024

RJB said:
rather than the NullPointerException being reported in the calling sub?

We can't help you if you don't provide all the relevant information. I don't see any mention of NPE in the first post.

For me to help - start a new thread and post all relevant information.

Erel · Apr 25, 2024

Now I see it in the comments.

Remove the try / catch block.
Run the program in release mode and post the logs.

RJB · Apr 25, 2024

aeric said:
Maybe you don't need to use Wait For.

I've have already modified about 10 other subs to remove the Wait Fors and Sleeps but I have one left that gets complicated by the time you have a _ConnectionResult sub to handle the async result and then try to return a value to the original calling sub. If Waitfor and sleep can be used it's so much easier and more maintainable.

RJB · Apr 25, 2024

Erel said:
Now I see it in the comments.

Remove the try / catch block.
Run the program in release mode and post the logs.

This is the log from running the full/ original code in release mode:

B4X:

(NullPointerException) java.lang.NullPointerException
asyncfunctions$ResumableSub_RemoveSendresume (java line: 2648)
java.lang.NullPointerException
    at anywheresoftware.b4a.keywords.Common.WaitFor(Common.java:1781)
    at com.AdzDisplay.AdzMicroControl.asyncfunctions$ResumableSub_RemoveSend.resume(asyncfunctions.java:2648)
    at com.AdzDisplay.AdzMicroControl.asyncfunctions._vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv3(asyncfunctions.java:2567)
    at com.AdzDisplay.AdzMicroControl.tools$ResumableSub_RemoveSensor.resume(tools.java:2083)
    at anywheresoftware.b4a.BA.checkAndRunWaitForEvent(BA.java:275)
    at anywheresoftware.b4a.BA.raiseEvent2(BA.java:215)
    at anywheresoftware.b4a.BA.raiseEvent(BA.java:201)
    at anywheresoftware.b4a.keywords.Common$1.onClick(Common.java:490)
    at com.android.internal.app.AlertController$ButtonHandler.handleMessage(AlertController.java:166)
    at android.os.Handler.dispatchMessage(Handler.java:110)
    at android.os.Looper.loop(Looper.java:193)
    at android.app.ActivityThread.main(ActivityThread.java:5292)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:515)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:824)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:640)
    at dalvik.system.NativeStart.main(Native Method)

RJB · Apr 25, 2024

Sorry I think that still had the try/ catch. I'll re-run it without

Erel · Apr 25, 2024

Don't run it in obfuscated mode.

And add some log messages so we can see where it fails.

RJB · Apr 25, 2024

As requested:

B4X:

Remove: A1
13:30:58: ASNR Connect to: 192.168.1.87
13:30:58: ASNR 1
13:30:58 CTS 1
13:30:58: CTS 2
asyncfunctions$ResumableSub_AsyncSend_NoReturnresume (java line: 390)
java.lang.NullPointerException
    at anywheresoftware.b4a.keywords.Common.WaitFor(Common.java:1781)
    at com.AdzDisplay.AdzMicroControl.asyncfunctions$ResumableSub_AsyncSend_NoReturn.resume(asyncfunctions.java:390)
    at com.AdzDisplay.AdzMicroControl.asyncfunctions._asyncsend_noreturn(asyncfunctions.java:350)
    at com.AdzDisplay.AdzMicroControl.asyncfunctions$ResumableSub_RemoveSend.resume(asyncfunctions.java:2545)
    at com.AdzDisplay.AdzMicroControl.asyncfunctions._removesend(asyncfunctions.java:2464)
    at com.AdzDisplay.AdzMicroControl.tools$ResumableSub_RemoveSensor.resume(tools.java:1862)
    at anywheresoftware.b4a.BA.checkAndRunWaitForEvent(BA.java:275)
    at anywheresoftware.b4a.BA.raiseEvent2(BA.java:215)
    at anywheresoftware.b4a.BA.raiseEvent(BA.java:201)
    at anywheresoftware.b4a.keywords.Common$1.onClick(Common.java:490)
    at com.android.internal.app.AlertController$ButtonHandler.handleMessage(AlertController.java:166)
    at android.os.Handler.dispatchMessage(Handler.java:110)
    at android.os.Looper.loop(Looper.java:193)
    at android.app.ActivityThread.main(ActivityThread.java:5292)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:515)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:824)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:640)
    at dalvik.system.NativeStart.main(Native Method)

Below is the actual code with unrelated code removed:

B4X:

Public Sub AsyncSend_NoReturn(IP As String, Count As Long, SendType As Int, Info As Long, ExtraInfo As String, SendData() As Byte) As ResumableSub
    Log(DateTime.Time(DateTime.Now) & ": ASNR Connect to: " & IP)

        Log(DateTime.Time(DateTime.Now) & ": ASNR 1")
    
        wait for (ConnectToServer(IP, 5)) complete (ASNRSuccess As Boolean)
        
        Log(DateTime.Time(DateTime.Now) & ": ASNR 2")
        If ASNRSuccess Then
            Log(DateTime.Time(DateTime.Now) & ": ASNR 3")
            Dim DataBytes() As Byte
            DataBytes = EncryptDataBytes(Ser.ConvertArrayToBytes(Array(Count, SendType, Info, ExtraInfo, SendData)))
            Dim Result As Boolean = SendAstream.Write(DataBytes)
            Log(DateTime.Time(DateTime.Now) & ": ASNR 4")
            Return Result
        Else
            Log(DateTime.Time(DateTime.Now) & ": ASNR 5")
            Return False
        End If

End Sub

Public Sub ConnectToServer(IP As String, NumberOfTries As Int) As ResumableSub
    Log(DateTime.Time(DateTime.Now) & " CTS 1")

    Dim SendClient As Socket
        
    For i = 0 To NumberOfTries - 1
        SendClient.Initialize("SendClient")
        Dim Y As Int = AsyncSendPortWaitTimes.Length - 1
        If i < AsyncSendPortWaitTimes.Length - 1 Then Y = i
        SendClient.Connect(IP, AsyncSendPort, AsyncSendPortWaitTimes(Y))
        Log(DateTime.Time(DateTime.Now) & ": CTS 2")
        Wait For SendClient_Connected (Successful As Boolean)
        Log(DateTime.Time(DateTime.Now) & ": CTS 3")
        If Successful Then
            Log(DateTime.Time(DateTime.Now) & " CTS 4")
            SendAstream.InitializePrefix(SendClient.InputStream, False, SendClient.OutputStream, "SendAstream")
            Log(DateTime.Time(DateTime.Now) & " CTS 5")
            Return True
        End If
    Next
    LogColor(DateTime.Time(DateTime.Now) & " Failed to connect: " & IP, Colors.red)
    Return False
End Sub

Erel · Apr 25, 2024

Are you creating any background threads? Maybe with the Threading library?

If not, can you send me the project to erel@basic4ppc.com?

RJB · Apr 25, 2024

No, no threads.. Unless a library does, MLwifi400?

There's quite a lot in the project so I'll try to put something together using only the relevant modules. It'll take a little while though!
I've spent quite a while over the last week trying to work out what's going on and the nearest I got was that the Wait For (or Sleep) caused a return to the calling sub whilst it waited, but that it hadn't initialised the return (resumablesub) value. I couldn't locate a fault in the calling or called sub, it seemed to be somewhere in between if that makes any sense.

Erel · Apr 25, 2024

RJB said:
Unless a library does, MLwifi400?

Might be.

Handle the event in a separate sub and check the output of this:

B4X:

Dim thread As JavaObject
thread = thread.InitializeStatic("java.lang.Thread").RunMethod("currentThread", Null)
Log(thread.RunMethod("getName", Null))

It should show main

RJB · Apr 25, 2024

Yes that shows main

RJB · Apr 28, 2024

I'm having a little trouble putting together a small example of the problem!
In the meantime I've noticed that if I exit the app then restart it the code works without failing. (is new question: https://www.b4x.com/android/forum/t...sub-that-can-be-called-multiple-times.160802/ related?)
Is there any way of finding out what exactly is causing the NullPointerException?

Erel · Apr 28, 2024

RJB said:
Is there any way of finding out what exactly is causing the NullPointerException?

Not really.

I would still bet on a threading issue. Maybe CallingSub is called on non-main thread somehow.

DonManfred · Apr 28, 2024

I found this in the MLWifi Sources (i found a copy somewhere in the forum).
Maybe related. Never used the lib though.

B4X:

public void connectWifiAP(BA ba, String ssid, int security, String password, int timeout) {
        if (debug)
            BA.Log("SDK: " + Build.VERSION.SDK_INT);
        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.Q) {
            tmr = new Timer();
            tmr.schedule(new TimeoutTask(ba), timeout);
            connectWifiAP2(ba, ssid, "", security, password);
        } else {
            connectWifiAPQ(ba, ssid, security, password, timeout);
        }
    }

static private void connectToWifi(BA ba, WifiManager wm, int NetId, String ssid) {
    try {
        connectionSuccess = false;
        if (debug)
            BA.Log("[MLwifi] Connecting to " + ssid);


        if (receiverRegistered) {
            unregisterReceiver(broadcastReceiver);
        }

        if (broadcastReceiver == null) {
            final IntentFilter intentFilter = new IntentFilter(WifiManager.NETWORK_STATE_CHANGED_ACTION);
            broadcastReceiver = new BroadcastReceiver() {
                @Override
                public void onReceive(Context context, Intent intent) {
                    if (!intent.getAction().equals(WifiManager.NETWORK_STATE_CHANGED_ACTION)) {
                        if (debug)
                            BA.Log("BroadcastReceiver Intent Action: " + intent.getAction());
                    }
                    // Post connection
                    @SuppressLint("MissingPermission") NetworkInfo networkInfo = intent.getParcelableExtra(WifiManager.EXTRA_NETWORK_INFO);
                    if (networkInfo != null) {
                        if (debug)
                            BA.Log(networkInfo.toString());
                        if (networkInfo.isConnected()) { //&& networkInfo.getExtraInfo().equals(ssid)) {
                            WifiManager wifiManager = (WifiManager) BA.applicationContext.getSystemService(Context.WIFI_SERVICE);
                            WifiInfo info = wifiManager.getConnectionInfo();
                           // BA.Log(info.getSSID() + " : " + ssid);
                            if (info.getSSID().equals(ssid)) {
                                if (ba.subExists("wifi_connectionresult") && networkInfo.getState() == NetworkInfo.State.CONNECTED && !connectionSuccess) {
                                    connectionSuccess = true;
                                    if (debug)
                                        BA.Log("[MLwifi] Connected to " + info.getSSID());
                                    tmr.cancel();
                                    ScheduledThreadPoolExecutor exec = new ScheduledThreadPoolExecutor(1);
                                    exec.schedule(new Runnable() {
                                        public void run() {
                                            ba.raiseEvent(this, "wifi_connectionresult", true);
                                        }
                                    }, 1, TimeUnit.SECONDS);

                                }
//                                } else {
//                                    if (ba.subExists("wifi_connectionresult")) {
//                                        ba.raiseEvent(this, "wifi_connectionresult", false);
//                                    }
                            }
                        } else if (!networkInfo.isAvailable() && networkInfo.getExtraInfo().equals(ssid)) {
                            tmr.cancel();
                            if (debug)
                                BA.Log("[MLwifi] " + ssid + " not available");
                            if (ba.subExists("wifi_connectionresult")) {
                                ba.raiseEvent(this, "wifi_connectionresult", false);
                            }
                        }
                    }
                }
            };
            BA.applicationContext.registerReceiver(broadcastReceiver, intentFilter);
            receiverRegistered = true;
        }
        wm.enableNetwork(NetId, true);

    } catch (Exception e) {
        BA.LogError(e.getLocalizedMessage());
    }
}

static class TimeoutTask extends TimerTask {
    BA _ba;
    public TimeoutTask(BA ba) {
        _ba = ba;
    }
    public void run() {
        tmr.cancel(); //Terminate the timer thread
        if (_ba.subExists("wifi_connectionresult")) {
            _ba.raiseEvent(this, "wifi_connectionresult", false);
        }
        if (receiverRegistered) {
            unregisterReceiver(broadcastReceiver);
        }
    }
}

RJB · Apr 29, 2024

Thanks DonManfred. I found the source at: https://www.b4x.com/android/forum/threads/mlwifi-library-updated-to-v4-00.125051/. I'll take a look and see if it helps.

I've done some more testing with the following results - I don't know if it helps at all!

The code is something like the following:

B4X:

Sub CallingSub1()    'Note: no return type
     CallingSub2(AList)
End Sub


Public Sub CallingSub2(BList As List) As ResumableSub

.......
Try
    wait for (CallingSub3(.........)) complete (Result As Boolean)
Catch
    Log("CS2: " & LastException)
End Try

..........

Return True    'Note: Returns a fixed value

End Sub


Public Sub CallingSub3(............) As ResumableSub

........
    Try
        wait for (.............) complete (ASNRSuccess As Boolean)
.......
    Catch
        LogColor("CS3: " & LastException, Colors.red)
    End Try
    Return False    'Note: Returns a fixed value
End Sub

Public Sub ConnectToServer(............) As ResumableSub

...........

        SendClient.Initialize("SendClient")
        SendClient.Connect(IP, AsyncSendPort, AsyncSendPortWaitTime)

        Wait For SendClient_Connected (Successful As Boolean)

        Log(DateTime.Time(DateTime.Now) & ": CTS 3")
        If Successful Then
            SendAstream.InitializePrefix(SendClient.InputStream, False, SendClient.OutputStream, "SendAstream")
            Return True
        End If

    Return False    'Note: Returns a fixed value

End Sub

where CallingSub1 calls CallingSub2, which calls CallingSub3, which calls ConnectToServer.
In this case both CallingSub3 and CallingSub2 log (NullPointerException) java.lang.NullPointerException

If I change the code to:

B4X:

Sub CallingSub1()    'Note: no return type
     CallingSub2(AList)
End Sub


Public Sub CallingSub2(BList As List) As ResumableSub

.......
Try
    wait for (CallingSub3(.........)) complete ' Don't expect a value (Result As Boolean)
Catch
    Log("CS2: " & LastException)
End Try

..........

Return True    'Note: Return a fixed value

End Sub


Public Sub CallingSub3(............) ' Don't return a value As ResumableSub

........
    Try
        wait for (.............) complete (ASNRSuccess As Boolean)
.......
    Catch
        LogColor("CS3: " & LastException, Colors.red)
    End Try
    ' Don't return a value Return False    'Note: Return a fixed value
End Sub

Public Sub ConnectToServer(............) As ResumableSub

...........

        SendClient.Initialize("SendClient")
        SendClient.Connect(IP, AsyncSendPort, AsyncSendPortWaitTime)

        Wait For SendClient_Connected (Successful As Boolean)

        Log(DateTime.Time(DateTime.Now) & ": CTS 3")
        If Successful Then
            SendAstream.InitializePrefix(SendClient.InputStream, False, SendClient.OutputStream, "SendAstream")
            Return True
        End If

    Return False    'Note: Return a fixed value

End Sub

so that CallingSub3 doesn't return a value to CallingSub2 then only CallingSub3 logs the (NullPointerException) java.lang.NullPointerException.

As I say, don't know if that helps?

RJB · May 2, 2024

I THINK I found what caused this i.e. I've changed the code and it now works!

What it seemed to be is as follows:

I called a sub (using Wait For(CallSub2(TMod, TSub, True)) Complete) which sent some information to an ESP32 and then returned.
However the response was received and the _NewData sub took action before the above return completed.
The return never completed (or at least not till a long time later) so the code that should have run didn't.

Wait for is very useful until you do something wrong. Then it can be very difficult to find the problem!!

Android Question Problem with Wait For and asynchronous code in library?

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