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Destructive Property

Indicates whether the stream is in destructive mode or not.

Syntax

Public Property Get Destructive() As Boolean
Public Property Let Destructive(ByVal bMode As Boolean)

Remarks

The Destructive property indicates whether the stream is in destructive mode or not. This mode deletes blocks as data are read, freeing storage automatically. This feature is only available on memory blocks located in the dynamic heap. If the StreamMemory object holds a block in the storage heap when you set this property to True, it is first copied to the dynamic heap.

In normal read mode, calling the Read function increments the stream pointer by the amount of bytes read, while leaving the stream size unchanged. On the other hand, in destructive mode, calling the Read statement leaves the stream pointer unchanged, while decrementing the stream size by the amount of data read. The following illustrates this behavior:

This feature is typically used to implement a pseudo-pipe, which may prove useful to process incoming frames of data. For example, suppose an external device sends frames over the serial port, each frame being separated by a CR-LF sequence. Depending on the control flow and other hardware considerations, your program may receive incomplete frames, or several frames in one read operation. A pseudo-pipe can be used to ensure that your program will only have to deal with complete frames, as shown below:

Private Data  As New StreamSerial
Private Pipe  As New StreamMemory
Private Tim   As New Timer

Private Sub Button1_Click()
  Pipe.Clear
  Pipe.Destructive=True
  Tim.Interval=500
  Tim.Enabled=True
  Data.Open hbPortSerial, 9600
End Sub

Private Sub Tim_Timer()
  Dim k As Long

  k=Data.Ready(hbStreamRead)
  If k>0 Then
    Pipe.SeekToEnd
    Pipe.Write Data, k
    Process
  End If
End Sub

Private Sub Process()
  Dim k As Long, frame As String

  Do
    k=Pipe.FindByte(0,Asc("\n"))
    If k<0 Then Exit Sub

    Pipe.SeekToBegin
    Pipe.Read frame, k+1

    ' Here goes the code that process a frame

  Loop
End Sub

You can switch destructive mode on and off whenever needed.

System requirements