学习 Message(7): OnMessage 只相应消息队列中的消息

Perform、SendMessage 会直接发送消息到窗体过程;

PostMessage 是把消息放入消息队列.

因为 Application.OnMessage 只接收队列中的消息,

所以 Perform、SendMessage 发送的消息, OnMessage 收不到.

测试如下:
代码文件:

unit Unit1;

interface

uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, AppEvnts, StdCtrls;

type
TForm1 = class(TForm)
ApplicationEvents1: TApplicationEvents;
Button1: TButton;
Button2: TButton;
Button3: TButton;
procedure ApplicationEvents1Message(var Msg: tagMSG; var Handled: Boolean);
procedure Button1Click(Sender: TObject);
procedure Button2Click(Sender: TObject);
procedure Button3Click(Sender: TObject);
end;

var
Form1: TForm1;

implementation

{$R *.dfm}

{通过 ApplicationEvents1.OnMessage 接受鼠标双击窗体的消息; 可以双击窗体一试}
procedure TForm1.ApplicationEvents1Message(var Msg: tagMSG;
var Handled: Boolean);
begin
if Msg.message = WM_LBUTTONDBLCLK then
begin
ShowMessage('WM_LBUTTONDBLCLK');
Handled := True;
end;
end;

{通过 Perform 向窗体发送 WM_LBUTTONDBLCLK 消息; OnMessage 收不到}
procedure TForm1.Button1Click(Sender: TObject);
begin
Self.Perform(WM_LBUTTONDBLCLK, 0, 0);
end;

{通过 SendMessage 向窗体发送 WM_LBUTTONDBLCLK 消息; OnMessage 收不到}
procedure TForm1.Button2Click(Sender: TObject);
begin
SendMessage(Self.Handle, WM_LBUTTONDBLCLK, 0, 0);
end;

{通过 PostMessage 向窗体发送 WM_LBUTTONDBLCLK 消息; OnMessage 可以收到}
procedure TForm1.Button3Click(Sender: TObject);
begin
PostMessage(Self.Handle, WM_LBUTTONDBLCLK, 0, 0);
end;

end.

窗体文件:

object Form1: TForm1
Left = 0
Top = 0
Caption = 'Form1'
ClientHeight = 145
ClientWidth = 255
Color = clBtnFace
Font.Charset = DEFAULT_CHARSET
Font.Color = clWindowText
Font.Height = -11
Font.Name = 'Tahoma'
Font.Style = []
OldCreateOrder = False
PixelsPerInch = 96
TextHeight = 13
object Button1: TButton
Left = 8
Top = 97
Width = 75
Height = 25
Caption = 'Button1'
TabOrder = 0
OnClick = Button1Click
end
object Button2: TButton
Left = 89
Top = 97
Width = 75
Height = 25
Caption = 'Button2'
TabOrder = 1
OnClick = Button2Click
end
object Button3: TButton
Left = 170
Top = 97
Width = 75
Height = 25
Caption = 'Button3'
TabOrder = 2
OnClick = Button3Click
end
object ApplicationEvents1: TApplicationEvents
OnMessage = ApplicationEvents1Message
Left = 128
Top = 24
end
end