A new object type is available in the RandomAccessFile library named AsyncStreams.
AsyncStreams allows you to read data from an InputStream and write data to an OutputStream without blocking your program. The reading and writing are done with two separate threads.
When new data is available the NewData event is raised with the data.
When you write data to the OutputStream the data is added to an internal queue and then sent in the background.
AsyncStreams is very useful when working with slow streams like network streams or Bluetooth streams.
If you work with such streams using the main thread your program may hang or block while waiting for a value.
The way we tried to deal with it in the Serial tutorial and Network tutorial is with a Timer that checks whether there are bytes waiting in the buffer. However even if there are bytes available there is a chance that not enough are available and then our program will hang or block until those are available.
Using AsyncStreams is usually simpler and safer.
AsyncStreams can work in two modes. Regular mode and "prefix mode". Both work as described above.
The following code demonstrates a simple program that sends text to a connected device or computer:
Code:
Sub Process_Globals
Dim AStreams As AsyncStreams
Dim Server As ServerSocket
Dim Socket1 As Socket
End Sub
Sub Globals
Dim EditText1 As EditText
End Sub
Sub Activity_Create(FirstTime As Boolean)
If FirstTime Then
Server.Initialize(5500, "Server")
Server.Listen
Log("MyIp = " & Server.GetMyIP)
End If
EditText1.Initialize("EditText1")
EditText1.ForceDoneButton = True
Activity.AddView(EditText1, 10dip, 10dip, 300dip, 60dip)
End Sub
Sub Server_NewConnection (Successful As Boolean, NewSocket As Socket)
If Successful Then
ToastMessageShow("Connected", False)
Socket1 = NewSocket
AStreams.InitializePrefix(Socket1.InputStream, False, Socket1.OutputStream, "AStreams")
Else
ToastMessageShow(LastException.Message, True)
End If
Server.Listen
End Sub
Sub AStreams_NewData (Buffer() As Byte)
Dim msg As String
msg = BytesToString(Buffer, 0, Buffer.Length, "UTF8")
ToastMessageShow(msg, False)
Log(msg)
End Sub
Sub AStreams_Error
ToastMessageShow(LastException.Message, True)
End Sub
'press on the Done button to send text
Sub EditText1_EnterPressed
If AStreams.IsInitialized = False Then Return
If EditText1.Text.Length > 0 Then
Dim buffer() As Byte
buffer = EditText1.Text.GetBytes("UTF8")
AStreams.Write(buffer)
EditText1.SelectAll
Log("Sending: " & EditText1.Text)
End If
End Sub
Sub Activity_Pause(UserClosed As Boolean)
If UserClosed Then
Log("closing")
AStreams.Close
Socket1.Close
End If
End Sub
Once there is a connection we initialize the AsyncStreams object:
Code:
AStreams.InitializePrefix(Socket1.InputStream, False, Socket1.OutputStream, "AStreams")
Then when there is new data available we convert the bytes to string and show it:
Code:
Sub AStreams_NewData (Buffer() As Byte)
Dim msg As String
msg = BytesToString(Buffer, 0, Buffer.Length, "UTF8")
ToastMessageShow(msg, False)
End Sub
When the user enters text in the EditText, the text is being sent:
Code:
Sub EditText1_EnterPressed
If AStreams.IsInitialized = False Then Return
If EditText1.Text.Length > 0 Then
Dim buffer() As Byte
buffer = EditText1.Text.GetBytes("UTF8")
AStreams.Write(buffer)
EditText1.SelectAll
Log("Sending: " & EditText1.Text)
End If
End Sub
Prefix mode
When the object is initialized in prefix mode the data is expected to adhere to the following protocol: every message (bytes array) should be prefixed with the bytes array length (as an Int). So if another device sends us a message made of 100 bytes. The stream is expected to include 4 bytes with a value of 100 and then the 100 bytes.
The NewData event will be raised with the 100 bytes. It will not include the 4 prefix bytes.
When you send data with Write or Write2 the bytes length will be added automatically as the prefix of this message.
If you can work in prefix mode, which is usually only possible if you are implementing both sides, it is highly recommended to do so. Under the cover AsyncStreams uses the message length prefix to make sure that the NewData event is always raised with complete messages. If for example it expects 100 bytes and only 60 bytes arrived, it will wait till the other 40 bytes arrive. In regular mode the event will be raised twice and you will need to handle the two parts of the message.
AsyncStreams also handles the case where 100 bytes are expected and more than 100 bytes arrived (which means that there are several messages).
The Error event will be raised if there is any error. You can use LastException to find the reason for the error.
AsyncStreams Tutorial
Linear Mode
