| Description |
Asynchronous operations take place independent of the current process. After BeginWrite is called, control immediately returns to the current process thread of execution.
You specify the Count of bytes that will be written to the current stream from the Bytes array from StartIndex.
You specify the procedure that is called when the write is completed. It must have the following form :
Procedure Name (AsyncResult : IAsyncResult);
You pass one of your objects WriteControl to the write to allow control of multiple threads. This gets returned in the IAsyncResult callback method parameter.
You need not rely on the return method - you can instead wait for the write to finish by calling EndWrite. But this tends to negate the meaning of the asynchronous process in the first place.
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| | Notes | The example program seems to be suffering from a timing problem - the expected stream contents are not displayed. The author does not understand why this is the case.
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| Microsoft MSDN Links |
System.IO
System.IO.FileStream
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| A simple example |
program Project1;
{$APPTYPE CONSOLE}
uses
System.IO,
System.Runtime.Remoting.Messaging;
var
Stream : System.IO.FileStream;
ByteArray : Array of Byte;
ByteAsInt : Integer;
AsyncRes : IAsyncResult;
Index : Integer;
procedure WriteDone(AsyncResult : IAsyncResult);
var
Index : Integer;
begin
Console.WriteLine('Write is finished :');
Console.WriteLine;
// Display the stream contents
Stream.Seek(0, SeekOrigin.Begin);
ByteAsInt := Stream.ReadByte;
while ByteAsInt > 0 do
begin
Console.WriteLine(Char(ByteAsInt));
ByteAsInt := Stream.ReadByte;
end;
end;
begin
// Create our File Stream
Stream := System.IO.FileStream.Create('C:DelphiBasics.txt', FileMode.Create);
// Create our byte array
SetLength(ByteArray, 10);
// And fill it
ByteArray[0] := 65; // Ord(65) = 'A'
ByteArray[1] := 66;
ByteArray[2] := 67;
ByteArray[3] := 68;
ByteArray[4] := 69;
ByteArray[5] := 70;
ByteArray[6] := 71;
ByteArray[7] := 72;
ByteArray[8] := 73;
ByteArray[9] := 74;
// Write to the stream from the array asynchronously
AsyncRes := Stream.BeginWrite(ByteArray, 0, 10, WriteDone, TObject.Create);
// Close the file stream
Stream.Close;
Console.Readline;
end.
| | Show full unit code | The output of this program was expected to be the following, but only the first line was returned.
Write is finished :
A
B
C
D
E
F
G
H
I
J
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