Android Tutorial Android Serial tutorial

Erel

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Staff member
Licensed User
The code in this tutorial should not be used for new projects.
New tutorial:
https://www.b4x.com/android/forum/threads/android-bluetooth-bluetoothadmin-tutorial.14768/#content

This tutorial covers the Serial library. This library allows you to connect with other Bluetooth devices using virtual serial ports (RFCOMM).

The Serial library requires Android OS 2.0 or above (API level 5 or above).

We will build a simple chat example which allows two connected devices to send text messages.




We created a process global object named Serial1 of type Serial.
Usually it is a good idea to initialize process objects in Sub Activity_Create when FirstTime is True. This way the objects will be initialized exactly once.

B4X:
Sub Activity_Create(FirstTime As Boolean)
    If FirstTime Then
        Serial1.Initialize("Serial1")
        Timer1.Initialize("Timer1", 200)
    End If
    Activity.LoadLayout("1")
    Activity.AddMenuItem("Connect", "mnuConnect")
    Activity.AddMenuItem("Disconnect", "mnuDisconnect")
End Sub
The next Sub running is Activity_Resume:
B4X:
Sub Activity_Resume
    If Serial1.IsEnabled = False Then
        Msgbox("Please enable Bluetooth.", "")
    Else
        Serial1.Listen 'listen for incoming connections
    End If
End Sub
Here we are checking if the Bluetooth device is enabled. If it is not enabled we ask the user to enable it. Note that by putting this code in Activity_Resume (and not Activity_Create) this test will happen every time the activity is resumed. So if the user goes to the settings screen, enables the device and then returns to our application we will now know that the Bluetooth is enabled.

If the Bluetooth is enabled we start listening for incoming connections. This allows other devices to connect with our device. This is not required if you connect to a device that listens for connections (like external GPS for example).
Note that calling Listen more than once doesn't do anything. So we are safe calling it this way.

When the user presses on the Connect menu item we show the user the list of known paired devices. When the user clicks on a device name we fetch its MAC address from the map and connect to it:



B4X:
Sub mnuConnect_Click
    Dim PairedDevices As Map
    PairedDevices = Serial1.GetPairedDevices
    Dim l As List
    l.Initialize
    For i = 0 To PairedDevices.Size - 1
        l.Add(PairedDevices.GetKeyAt(i)) 'add the friendly name to the list
    Next
    Dim res As Int
    res = InputList(l, "Choose device", -1) 'show list with paired devices
    If res <> DialogResponse.CANCEL Then
        Serial1.Connect(PairedDevices.Get(l.Get(res))) 'convert the name to mac address
    End If
End Sub
The connection is not established immediately. It will happen in the background. When the connection is established, the Connected event will be raised. A 'Success' parameter tells us if the connection is successful.
The Connected event can also be raised if an incoming connection is established.

B4X:
Sub Serial1_Connected (Success As Boolean)
    If Success Then
        ToastMessageShow("Connected successfully", False)
        TextReader1.Initialize(Serial1.InputStream)
        TextWriter1.Initialize(Serial1.OutputStream)
        timer1.Enabled = True
        connected = True
    Else
        connected = False
        Timer1.Enabled = False
        Msgbox(LastException.Message, "Error connecting.")
    End If
End Sub
If the connection was established successfully we can start the data transfer.
TextReader1 and TextWriter1 are process global object. We now initialize them using the serial stream. This will allow us to send and receive text over our newly created connection.
Timer1 is used to test whether there is incoming data (and read this data). Now we enable it and start listening.

If the connection is not successful we retrieve the exception and show it.

Sending messages - When the user presses on btnSend we send the text:
B4X:
Sub btnSend_Click
    If connected Then
        TextWriter1.WriteLine(txtSend.Text)
        TextWriter1.Flush
        txtSend.Text = ""
    End If
End Sub
'connected' is a variable that we use to know if we are currently connected.
Note that we call Flush after writing the text. This way we make sure that TextWriter doesn't buffer the text and sends it right away.

Receiving messages - Whenever the timer ticks we check if there is any data waiting to be read. If there is, we read it and add it to the large EditText:
B4X:
Sub Timer1_Tick
    If connected Then
        If TextReader1.Ready Then 'check if there is any data waiting to be read
            txtLog.Text = txtLog.Text & TextReader1.ReadLine & CRLF
            txtLog.SelectionStart = txtLog.Text.Length
        End If
    End If
End Sub
TextReader.ReadLine is a blocking call. It waits till there is at least a single character to be read. Therefore we need to test TextReader.Ready if we don't want to block our application.

This application can also be used to connect with non-Android devices.
An external GPS for example:


The external GPS continuously sends its data as text.
I actually had quite an interesting conversation with the external GPS...

The program is attached.

Edit: It is recommended to use the new AsnycStreams object instead of polling the available bytes parameter with a timer. Using AsyncStreams is simpler and more reliable.
 

Attachments

Last edited:

schimanski

Well-Known Member
Licensed User
Hello Erel! I think, that it is a very useful lib,thanks for that.

I have tow additional questions about that:

- The free market GPS-Software connects normally with the internal GPS- Chip. But what's happen, if I have an external GPS. Is in premium navigation-apps like copilot or navgear (I don't have one yet) an option for choosing the gps-serial-port?

- Can we only init a bluetooth-port with the serial-lib or is it possible to create two virtual ports on one device to send data from the first to the second like a virtual null modem emulator?

Rgds

schimanski
 

Erel

Administrator
Staff member
Licensed User
But what's happen, if I have an external GPS. Is in premium navigation-apps like copilot or navgear (I don't have one yet) an option for choosing the gps-serial-port?
I don't know.

Can we only init a bluetooth-port with the serial-lib or is it possible to create two virtual ports on one device to send data from the first to the second like a virtual null modem emulator?
You cannot use it this way.
 

Cor

Active Member
Licensed User
Error: discovery failed java error

When trying to connect to a device which is not available but showed in the list as a device

i get an java error on device:
java.io.IOException:Service discovery failed
because it's trying to connect in background

try catch will not help

How to catch the exception?

B4X:
Try
  Serial1.Connect(PairedDevices.Get(l.Get(res))) 'convert the name to mac address
Catch
Msgbox(LastException.Message, "Error connecting.")
End Try
 

sinktoma

New Member
Licensed User
Terminal sample works not good

Hi, I tested your Serial terminal.
Program finds bluetooth devices and connects to them.
The Hyper terminal on other computer shows received data, but
when i sending data to HTC, the "Serial termilal" software are a halts.
Yes sometimes some data are received, but after time apears timeout Error.

Firs time i instaled test software it was workinq fast about 3 minutes, but later halted.

where is the problem?
The paltform:
HTC desire, Android 2.2

Thank you
 

Erel

Administrator
Staff member
Licensed User
B4X:
Sub Timer1_Tick
    If connected Then
        If TextReader1.Ready Then 'check if there is any data waiting to be read
            txtLog.Text = txtLog.Text & TextReader1.ReadLine & CRLF
            txtLog.SelectionStart = txtLog.Text.Length
        End If
    End If
End Sub
This code reads all text until it finds an end of line character (chr(10)). Maybe you are sending a string without an end of line character?
 

andreas

Member
Licensed User
Unicode characters in TextReader/TextWriter

Hi,

I'm trying to send/recieve Greek characters (Unicode characters) with Serial1 object via TextReader1 and TextWriter1 but the app stop working on phone.

I used the Erel's example taken from this thread.
I have made the following changes in "Sub Serial1_Connected()"
From :
TextReader1.Initialize(Serial1.InputStream)
TextWriter1.Initialize(Serial1.OutputStream)
To:
TextReader1.Initialize2(Serial1.InputStream ,"UNICODE")
TextWriter1.Initialize2(Serial1.OutputStream, "UNICODE")

B4X:
Sub Serial1_Connected (Success As Boolean)
   If Success Then
      ToastMessageShow("Connected successfully", False)
      TextReader1.Initialize2(Serial1.InputStream ,"UNICODE")
      TextWriter1.Initialize2(Serial1.OutputStream, "UNICODE")
      timer1.Enabled = True
      connected = True
   Else
      connected = False
      Timer1.Enabled = False
      Msgbox(LastException.Message, "Error connecting.")
   End If
End Sub
At the Encoding parametr of Initialize2, I tested values like UTF8,UTF7 and ANSI and all goes ok. But when i change to "UNICODE" the app halts (the previus Toastmessage remaines infinitely).

The only I want is to send Unicode characters via BlueTooth.
Any idea?
 

andreas

Member
Licensed User
You should probably use UTF8. It is a Unicode encoding. I don't think that "UNICODE" is a valid value.
Thank you Erel for your respone. You are right.
The "TextReader1.Initialize(Serial1.InputStream)" is ok.
The property was wrong from the computer side.
Thank you
 

marcode

New Member
Licensed User
When trying to connect to a device which is not available but showed in the list as a device

i get an java error on device:
java.io.IOException:Service discovery failed
because it's trying to connect in background

try catch will not help

How to catch the exception?

B4X:
Try
  Serial1.Connect(PairedDevices.Get(l.Get(res))) 'convert the name to mac address
Catch
Msgbox(LastException.Message, "Error connecting.")
End Try
hi, new basic4android customer.

trying this sample and getting this exact same error. how exactly was this resolved?

thanks in advance....
 

cmartins

Member
Licensed User
Erel,

By using the serial example it works fine. then I disconnect and make the connection again and it works well, but if I try again the same procedure I get the message "java.io.IOException:Service discovery failed", or if the application loses focus.

When I get this error. I try to disable and enable the BT service, but it freeze. only restarting the cell phone to connect again.

I tried to use connect and connect3.

How can I solve it?

thanks in advance
 
Last edited:

defillo

Member
Licensed User
Unable to start Service Discover

Always get this error

java.io.IOException: unable to start service discovery

devices paired, service SPP installed over pc, phone is an HTC Desire Z running android 2.2.1

any suggestion?
 

defillo

Member
Licensed User
better

looks better, but unable to exchange data with a pc, spp service recognized on com4 (in this case)
maybe it is a pc s.o. problem, I'll check..
 
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