| Description |
Asynchronous operations take place independent of the current process. After BeginRead is called, control immediately returns to the current process thread of execution.
You specify the Count of bytes that will be read into the Bytes array from StartIndex.
You specify the procedure that is called when the read is completed. It must have the following form :
Procedure Name (AsyncResult : IAsyncResult);
You pass one of your objects ReadControl to the read 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 read to finish by calling EndRead. But this tends to negate the meaning of the asynchronous process in the first place.
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| Microsoft MSDN Links |
System.IO
System.IO.FileStream
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| A simple example |
// Full Unit code.
// -------------------------------------------------------------
// Create a new WinForm application, double click the form to
// create an OnLoad event, and then replace the WinForm unit
// with this text.
unit WinForm;
interface
uses
System.Drawing, System.Collections, System.ComponentModel,
System.Windows.Forms, System.Data;
type
TWinForm = class(System.Windows.Forms.Form)
\{REGION 'Designer Managed Code'\} // Note that REGION and ENREGION should be prefixed by a dollar sign
strict private
///
/// Required designer variable.
///
Components: System.ComponentModel.Container;
///
/// Required method for Designer support - do not modify
/// the contents of this method with the code editor.
///
procedure InitializeComponent;
procedure TWinForm_Load(sender: System.Object; e: System.EventArgs);
{ENDREGION}
strict protected
///
/// Clean up any resources being used.
///
procedure Dispose(Disposing: Boolean); override;
private
{ Private Declarations }
public
constructor Create;
end;
[assembly: RuntimeRequiredAttribute(TypeOf(TWinForm))]
implementation
\{REGION 'Windows Form Designer generated code'\}
///
/// Required method for Designer support -- do not modify
/// the contents of this method with the code editor.
///
procedure TWinForm.InitializeComponent;
begin
//
// TWinForm
//
Self.AutoScaleBaseSize := System.Drawing.Size.Create(5, 13);
Self.ClientSize := System.Drawing.Size.Create(292, 266);
Self.Name := 'TWinForm';
Self.Text := 'WinForm';
Include(Self.Load, Self.TWinForm_Load);
end;
{ENDREGION}
procedure TWinForm.Dispose(Disposing: Boolean);
begin
if Disposing then
begin
if Components <> nil then
Components.Dispose();
end;
inherited Dispose(Disposing);
end;
constructor TWinForm.Create;
begin
inherited Create;
//
// Required for Windows Form Designer support
//
InitializeComponent;
//
// TODO: Add any constructor code after InitializeComponent call
//
end;
procedure TWinForm.TWinForm_Load(sender: System.Object; e: System.EventArgs);
program Project1;
{$APPTYPE CONSOLE}
uses
System.IO;
var
Stream : System.IO.FileStream;
ByteArray : Array of Byte;
procedure ReadDone(AsyncResult : IAsyncResult);
var
Index : Integer;
begin
Console.WriteLine('Read is finished :');
Console.WriteLine;
// Display our array contents
for Index := 0 to Length(ByteArray)-1 do
Console.WriteLine(ByteArray[Index].ToString);
end;
begin
// Create our File Stream
Stream := System.IO.FileStream.Create('C:\DelphiBasics.txt', FileMode.Create);
// Write to the stream
Stream.WriteByte(1);
Stream.WriteByte(2);
Stream.WriteByte(3);
Stream.WriteByte(4);
Stream.WriteByte(5);
Stream.WriteByte(6);
Stream.WriteByte(7);
Stream.WriteByte(8);
Stream.WriteByte(9);
// Move to the start of the stream
Stream.Seek(0, SeekOrigin.Begin);
// Declare our array to hold the returned data
SetLength(ByteArray, 12);
// Read from the stream asynchronously
Stream.BeginRead(ByteArray, 0, 10, ReadDone, TObject.Create);
// Close the file stream
Stream.Close;
Console.Readline;
end.
end. | | Hide full unit code | Read is finished :
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