B4R Code Snippet Uploading data to ThingSpeak using MQTT

As I was not able to find a complete example for doing this, just snippets of code, I offer this to everyone who needs help. You will need to add your own WiFi credentials and API Keys of course.

Libraries required: rCore, rESP8266WiFi, rMQTT

Upload is based on the timer tick, not on a sleep mode!

#Region Project Attributes
    #AutoFlushLogs: True
    #CheckArrayBounds: True
    #StackBufferSize: 300
#End Region

'Ctrl+Click to open the C code folder: ide://run?File=%WINDIR%\System32\explorer.exe&Args=%PROJECT%\Objects\Src

'Thingspeak_test by Mark Read 2021
'Hardware:  Wemos D1 Mini


Sub Process_Globals
    Public Serial1 As Serial
    Public WiFi As ESP8266WiFi
    
    Public MQTTApiKey As String="xxxxxxxxxxxx"                'Get from MyProfile in Thingspeak
    Public WriteAPIKey As String="xxxxxxxxxxxx"                'Same here
    Public UserName As String="xxxxxxxxxxxx"                'obvious
    Public WifiSSID As String="xxxxxxxxxxxx"                'your WiFi name
    Public WifiPass As String="xxxxxxxxxxxx"                'and password
    Public ThinkspeakServer As String="mqtt.thingspeak.com"
    Private ChannelID As String="xxxxxxxxxxxx"                'The destination channel number
    Private ClientID As String="MrW"                        'can be anything you like
    
    Public WiFiClient As WiFiSocket
    Private MQTT As MqttClient
    Private MQTTOptions As MqttConnectOptions
   
    Public TEMPERATURE, PRESSURE, HUMIDITY As Double
    Public HeatIndex, DewPoint, Altitude As Double
            
    Public DelayTimer As Timer
    
End Sub

Private Sub AppStart
    Serial1.Initialize(115200)
    Log("AppStart")
        
    DelayTimer.Initialize("DelayTimer_Tick", 1*60*1000)    '1 mins
    
    MQTT.Initialize2(WiFiClient.Stream, ThinkspeakServer,1883, ClientID,"MQTT_MessageArrived", "MQTT_Disconnected")
    MQTTOptions.Initialize(UserName, MQTTApiKey)

    DelayTimer_Tick        'call the tick sub once or we will have to wait for the timer!
    
    DelayTimer.Enabled=True
End Sub

Sub DelayTimer_Tick
    If WiFi.IsConnected=False Then
        ConnectToNetwork
    End If
    
    'Generate some data
    TEMPERATURE=Rnd(0,40)
    PRESSURE=Rnd(925,975)
    HUMIDITY=Rnd(0,30)
    HeatIndex=Rnd(0,40)
    DewPoint=Rnd(10,15)
    Altitude=Rnd(525,275)
    
    UploadThinkgspeak   
    MQTT_Disconnected
    Log("Going to sleep ...")
End Sub

Sub UploadThinkgspeak
    If MQTT.Connect2(MQTTOptions) Then
        Log("MQTT Connected")
        Dim PayLoad, Topic As String
        PayLoad=JoinStrings(Array As String("field1=",TEMPERATURE,"&field2=",PRESSURE,"&field3=",HUMIDITY,"&field4=",HeatIndex,"&field5=",DewPoint,"&field6=",Altitude))
        Log("Sending data: ",PayLoad)
        
        Topic=JoinStrings(Array As String("channels/", ChannelID, "/publish/", WriteAPIKey))
        
        Log(MQTT.Publish(Topic,PayLoad))
        
    Else
        Log("Error - Not connected")   
    End If
        
End Sub

Sub MQTT_MessageArrived (Topic As String, Payload() As Byte)
    Log("Message arrived. Topic=", Topic, " payload: ", Payload)
End Sub

Sub MQTT_Disconnected
    Log("MQTT Disconnected")
    MQTT.Close
End Sub

Sub ConnectToNetwork
    If    WiFi.IsConnected Then Return
    If WiFi.Connect2(WifiSSID, WifiPass) Then
        Log("Connected successfully to: ", WifiSSID)
        Log("With IP: ",WiFi.LocalIp)
        
    Else
        Log("Failed to connect.")
    End If
End Sub