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