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Controls do not recieve middle mouse button events occurring outside the client area.

Posted on 2011-02-18
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Last Modified: 2012-05-11
I have a TImage-derived component to which I'm attempting to add middle-click panning functionality.  It seems that middle mouse events do not get delivered to my component when the mouse cursor is outside of it's client area, like left and right mouse events do.

I've tried intercepting the WM_NCMBUTTONUP message using a message handler, and even overriding the WndProc function, and I still never see this message come through.

The component also does not seem to receive Mouse Move messages when the cursor is outside the client area and the middle button is down.  It does receive the non-client Mouse Moves when the left or right button is down.

I can provide a small demo project if what I'm asking isn't clear enough.
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Question by:ShawnCurry
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by:jimyX
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Could you provide a demo or show some code please?
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by:ShawnCurry
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I've attached a demo that I've created exploring 2 alternate methods for receiving the missing events.  All the results are essentially the same.

To Test:
Place the mouse cursor inside the client area of the component.
Click and release each mouse button individually.
 - Should receive MouseUp notification
For each mouse button, place cursor in client area, click and drag outside client area, release.
 - Should receive MouseUp notification
 * Only receive MouseUp notification for left mouse button.

MiddleMouse.zip
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by:jimyX
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That seems not catching. I can think of work around solution:
I used a Panel but you can apply this to the descendant components as well.
MiddleMouseUpdated.zip
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by:jimyX
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Here is a complete demo for the three Mouse Buttons:
MiddleMouseUpdated-2.zip
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by:jimyX
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Just noticed that will not work outside the client area.
Then you can try the global mouse hook which catches all the mouse events:
http://delphigeek.blogspot.com/2007/02/global-mouse-hook-in-delphi.html
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by:ewangoya
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Why would you expect the component to receive messages that do not belong to it. If you click on the form, thats a mouse click on the form and not on your component.

Same as when you have a button on the form, you don't expect the button to get a mousedown event when you click on the form, imagine what kind of problems we would run into.
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by:ShawnCurry
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The Mouse Hook method does look promising; but I wish I didn't have to put it into a DLL.  Do you have any idea how to figure out what thread I need to attach it to in order to allow it to live in my application?

I am not interested in listening for every single mouse event that occurs.  I'm really only interested in a very narrow set of them that I think I should be receiving now in order to make dragging behavior consistent for all mouse buttons.

Normally, a component wouldn't receive Mouse Move events when the cursor is outside it's client area.  However, they do if the left mouse button is down.  For example, try highlighting some text on this page, and drag the mouse cursor outside of the browser window.  The text selection still responds - or at least that seems normal for Windows applications.  Likewise, I still get the Mouse Up event (for the left button) so long as a Mouse Down event occurred inside the client area.

So, I'm only interested in certain events, and only under certain conditions:

Precondition: Mouse Down inside client area.
Deliver the missing non-client move events.
Deliver the missing non-client mouse up event if it occurs.
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developmentguru earned 500 total points
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Here is a small project that demonstrates capturing all mouse events to a single control, without the use of a DLL.  Place a single panel on your main form in a new project.  Tie in the events as shown in the code.  You will notice that if you move the mouse over the panel that it's caption shows the mouse coordinates for mouse move messages that happen while the mouse is over the control.  Now, hold down the middle mouse button while over the panel and move the mouse off the panel while holding down the middle mouse button.  You will see that you now get mouse move messages even when you are not on the control (until you let go of the middle mouse button).

Let me know if you need more.
unit Unit1;

interface

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

type
  TForm1 = class(TForm)
    Panel1: TPanel;
    procedure Panel1MouseDown(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
    procedure Panel1MouseMove(Sender: TObject; Shift: TShiftState; X,
      Y: Integer);
    procedure Panel1MouseUp(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.Panel1MouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  //start capture of mouse messages if the middle button just got pressed.
  if Button = mbMiddle then
    SetCaptureControl(Panel1);
end;

procedure TForm1.Panel1MouseMove(Sender: TObject; Shift: TShiftState; X,
  Y: Integer);
begin
  Panel1.Caption := IntToStr(X) + ' ' + IntToStr(Y);
end;

procedure TForm1.Panel1MouseUp(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  //end capture of mouse messages if middle button has been let go.
  if Button = mbMiddle then
    SetCaptureControl(nil);
end;

end.

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Author Closing Comment

by:ShawnCurry
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Perfect.  Exactly what I was looking for.  Short and sweet.
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