The problem
When running an application written in C, that uses some dll's written in Delphi XE7, I run into an access violation in the following code, which is in the vcl.forms.pas of the vcl library.
procedure TCustomForm.CMAppSysCommand(var Message: TMessage);
{$IF NOT DEFINED(CLR)}
type
PWMSysCommand = ^TWMSysCommand;
{$ENDIF}
begin
Message.Result := 0;
if (csDesigning in ComponentState) or (FormStyle = fsMDIChild) or
(Menu = nil) or Menu.AutoMerge then
{$IF DEFINED(CLR)}
with TWMSysCommand.Create(Message) do
{$ELSE}
with PWMSysCommand(Message.lParam)^ do
{$ENDIF}
begin
SendCancelMode(nil);
if SendAppMessage(CM_APPSYSCOMMAND, CmdType, Key) <> 0 then //Here the debugger shows the access violation
Message.Result := 1;
end;
end;
The access violation occurs on the line with SendAppMessage, and seems to be caused by the fact that the Message.LParam is 0. The message is a WM_SYSCOMMAND message. Is there a way to track where this message originated? In the call stack, all functions are part of the VCL or system files.
This answer suggest that in general it is hard to trace the sender of a windows message. However, since in my case everything is within the same application, I hope that might make it easier.
What have I tried?
Overruling the vcl source
Previously, this same bug appeared in the forms.pas and was fixed by adding a copy of that file to the project and then checking that LParam <> 0 in this function.
I have tried doing the same thing with the vcl.forms.pas that is now used, but this leads to compilation errors. Even with answers as here I was not able to build it. However, many google hits also suggested that it is in general a bad idea to change things in the vcl, so I try to avoid that option.
Other questions on StackOverFlow
This article gave me good information about the underlying system and how it might have occured that the Message.LParam is 0. However, I did not know how to find the source of the message or what class I should be looking for that generated it.
The solution
As described in Remy's accepted answer below, the immediate problem could be solved by having the class provide a CMAppSysCommand function to guard against LParam = 0.
What you describe should not be possible under normal conditions.
There are only two places in the entire VCL where CM_APPSYSCOMMAND is sent from:
TWinControl.WMSysCommand(), which is called when a UI control receives a WM_SYSCOMMAND message. The LParam of the CM_APPSYSCOMMAND message is never set to 0, it is set to a pointer to the TMessage record of the original WM_SYSCOMMAND message:
Form := GetParentForm(Self);
if (Form <> nil) and
(Form.Perform(CM_APPSYSCOMMAND, 0, Winapi.Windows.LPARAM(#Message)) <> 0) then
Exit;
TCustomForm.CMAppSysCommand(), which is called when a Form receives a CM_APPSYSCOMMAND message. It forwards the message to the TApplication window (using SendAppMessage(), which just calls SendMessage(Application.Handle, ...) with the provided parameters):
with PWMSysCommand(Message.lParam)^ do
begin
...
if SendAppMessage(CM_APPSYSCOMMAND, CmdType, Key) <> 0 then
Message.Result := 1;
end;
The other question you mention explains how CM_APPSYSCOMMAND is used by the VCL, but does not say anything that would suggest how its LParam could ever be 0 in TCustomForm.CMAppSysCommand(), because it can't ever be 0 under normal circumstances. It can be 0 in TApplication.WndProc(), but that is perfectly OK.
The only possibility I can think of would be if someone is manually sending a fake CM_APPSYSCOMMAND message (which is CM_BASE + 23 = $B017, aka WM_APP + $3017) directly to your TForm window. Only TWinControl should ever be doing that. And since TWinControl uses Perform() instead of SendMessage() for that send, you should be seeing TWinControl.WMSysCommand() on the call stack of TCustomForm.CMAppSysCommand(). If you do not, then the message is fake. And if it is being sent using SendMessage() instead of Perform(), there is no way to know where the message is coming from.
However, in any case, this is very easy to guard against, without altering any VCL source code. Simply have your DLL's TForm class provide its own message handler for CM_APPSYSCOMMAND, either using the message directive, or by overriding the virtual WndProc() method. Either way, you can discard the message if the LParam is 0, eg:
type
TMyForm = class(TForm)
...
private
procedure CMAppSysCommand(var Message: TMessage); message CM_APPSYSCOMMAND;
...
end;
procedure TMyForm.CMAppSysCommand(var Message: TMessage);
begin
if Message.LParam = 0 then
Message.Result := 0
else
inherited;
end;
type
TMyForm = class(TForm)
...
protected
procedure WndProc(var Message: TMessage); override;
...
end;
procedure TMyForm.WndProc(var Message: TMessage);
begin
if (Message.Msg = CM_APPSYSCOMMAND) and (Message.LParam = 0) then
Message.Result := 0
else
inherited;
end;
As far as I understand and know the method of the TThread Class, if you synchronize your code, it actually get's executed in the main Application Thread (just like a timer/buttonclick/etc.)
I've been playing around and noticed that a MessageBox DOES NOT block the main application, however sleep does just as expected. Why is that?
type
TTestThread = class(TThread)
private
procedure SynchThread;
protected
procedure Execute; override;
public
constructor Create(CreateSuspended: Boolean);
end;
procedure TTestThread.SynchThread;
begin
MessageBoxA (0, 'Hello', 'Test', 0);
end;
procedure TTestThread.Execute;
begin
Synchronize (SynchThread)
end;
constructor TTestThread.Create(CreateSuspended: Boolean);
begin
inherited;
FreeOnTerminate := True;
end;
procedure StartThread;
var
TestThread : TTestThread;
begin
TestThread := TTestThread.Create (FALSE);
end;
There are two parts to this answer.
Part 1 is nicely explained in If MessageBox()/related are synchronous, why doesn't my message loop freeze?. The MessageBox function is not blocking, it merely creates a dialog box with its own message loop.
Part 2 is explained in the MessageBox documentation.
hWnd: A handle to the owner window of the message box to be created. If this
parameter is NULL, the message box has no owner window.
When you display a modal dialog, Windows disables its owner, but if you pass 0 for the first parameter, there is no owner and nothing to disable. Therefore, your program will continue to process messages (and react to them) while the message box is displayed.
To change this behaviour, pass form's handle as a first parameter. For example:
procedure TTestThread.SynchThread;
begin
MessageBoxA (Form1.Handle, 'Hello', 'Test', 0);
end;
I suspect that the question boils down to what you mean when you say:
A message box does not block the main application.
What I take this to mean is that when you show the message box, your VCL form can still be interacted with. The issue here is unrelated to threads and I suggest we remove them from the equation. Your understanding of what Synchronize does is sound.
The issue is entirely related to the concept of a window's owner, and how modal dialog windows behave with respect to their owners. Note that by owner, I don't mean the Delphi property TComponent.Owner, but I mean the Windows API meaning of owner.
Create a VCL app and drop two buttons on the form. Add the following OnClick handlers.
procedure TForm1.Button1Click(Sender: TObject);
begin
MessageBox(0, 'Not owned', nil, MB_OK);
end;
procedure TForm1.Button2Click(Sender: TObject);
begin
MessageBox(Handle, 'Owned by the VCL form', nil, MB_OK);
end;
Now observe what happens when you click on Button1. The message box shows, but you can still click on the VCL form. And compare with Button2. When it shows the message box, the VCL form cannot be interacted with.
When a modal dialog window is shown, the dialog window disables its owner. In the case of Button2, the owner is the VCL form. And once the form is disabled, you cannot interact with it. In the case of Button1, there is no owner and so the modal dialog window does not disable any other window. That's why the VCL form can be interacted with.
Raymond Chen has a long series on modality at his Old New Thing blog:
Modality, part 1: UI-modality vs code-modality
Modality, part 2: Code-modality vs UI-modality
Modality, part 3: The WM_QUIT message
Modality, part 4: The importance of setting the correct owner for modal UI
Modality, part 5: Setting the correct owner for modal UI
Modality, part 6: Interacting with a program that has gone modal
Modality, part 7: A timed MessageBox, the cheap version
Modality, part 8: A timed MessageBox, the better version
Modality, part 9: Setting the correct owner for modal UI, practical exam
Synchronize will execute the code in the Mainthread.
A good explanation can be found here Synchronization in Delphi TThread class
You just will have to prevent user from interacting with the forms of your application, eg. by
procedure TTestThread.SynchThread;
begin
MessageBoxA (0, 'Hello', 'Test', MB_TASKMODAL);
end;
using MessageBoxA as you did, won't prevent the Mainthread from reacting on those events triggerd by ueser interaction with your forms, just try
procedure TForm4.Button2Click(Sender: TObject);
begin
MessageBoxA (0, 'Hello', 'Test', 0);
// vs
// MessageBoxA (0, 'Hello', 'Test', MB_TASKMODAL);
end;
MessageBoxA
that synchronize will be executed in the main thread can be shown (IMHO) by
type
TTestThread = class(TThread)
private
FSync:Boolean;
FCalled:TDateTime;
procedure SynchThread;
protected
procedure Execute; override;
public
constructor Create(CreateSuspended: Boolean;sync:Boolean);
end;
procedure TTestThread.SynchThread;
begin
MessageBox (0,PChar(DateTimeToStr(FCalled)+#13#10+DateTimeToStr(Now)),'Hello' , 0);
end;
procedure TTestThread.Execute;
begin
sleep(100); // give Caller Time to fell asleep
if Fsync then Synchronize (SynchThread) else SynchThread;
end;
constructor TTestThread.Create(CreateSuspended: Boolean;sync:Boolean);
begin
inherited Create(CreateSuspended);
FSync := Sync;
FCalled :=Now;
FreeOnTerminate := True;
end;
procedure StartThread(sync:Boolean);
var
TestThread : TTestThread;
begin
TestThread := TTestThread.Create (FALSE,sync);
end;
procedure TForm4.RunUnsynchronizedClick(Sender: TObject);
begin
StartThread(false);// no sync
Sleep(5000); // Stop messageloop
end;
procedure TForm4.RunSynchronizedClick(Sender: TObject);
begin
StartThread(true); // sync
Sleep(5000); // Stop messageloop
end;
I use a number of scrolling controls: TTreeViews, TListViews, DevExpress cxGrids and cxTreeLists, etc. When the mouse wheel is spun, the control with focus receives the input no matter what control the mouse cursor is over.
How do you direct the mouse wheel input to whatever control the mouse cursor is over? The Delphi IDE works very nicely in this regard.
Scrolling origins
An action with the mouse wheel results in a WM_MOUSEWHEEL message being sent:
Sent to the focus window when the mouse wheel is rotated. The DefWindowProc function propagates the message to the window's parent. There should be no internal forwarding of the message, since DefWindowProc propagates it up the parent chain until it finds a window that processes it.
A mouse wheel's odyssey 1)
The user scrolls the mouse wheel.
The system places a WM_MOUSEWHEEL message into the foreground window’s thread’s message queue.
The thread’s message loop fetches the message from the queue (Application.ProcessMessage). This message is of type TMsg which has a hwnd member designating the window handle the message is ment for.
The Application.OnMessage event is fired.
Setting the Handled parameter True stops further processing of the message (except for the next to steps).
The Application.IsPreProcessMessage method is called.
If no control has captured the mouse, the focused control's PreProcessMessage method is called, which does nothing by default. No control in the VCL has overriden this method.
The Application.IsHintMsg method is called.
The active hint window handles the message in an overriden IsHintMsg method. Preventing the message from further processing is not possible.
DispatchMessage is called.
The TWinControl.WndProc method of the focused window receives the message. This message is of type TMessage which lacks the window (because that is the instance this method is called upon).
The TWinControl.IsControlMouseMsg method is called to check whether the mouse message should be directed to one of its non-windowed child controls.
If there is a child control that has captured the mouse or is at the current mouse position2), then the message is sent to the child control's WndProc method, see step 10. (2) This will never happen, because WM_MOUSEWHEEL contains its mouse position in screen coordinates and IsControlMouseMsg assumes a mouse position in client coordinates (XE2).)
The inherited TControl.WndProc method receives the message.
When the system does not natively supports mouse wheel (< Win98 or < WinNT4.0), the message is converted to a CM_MOUSEWHEEL message and is send to TControl.MouseWheelHandler, see step 13.
Otherwise the message is dispatched to the appropriate message handler.
The TControl.WMMouseWheel method receives the message.
The WM_MOUSEWHEEL window message (meaningful to the system and often to the VCL too) is converted to a CM_MOUSEWHEEL control message (meaningful only to the VCL) which provides for the convenient VCL's ShiftState information instead of the system's keys data.
The control's MouseWheelHandler method is called.
If the control is a TCustomForm, then the TCustomForm.MouseWheelHandler method is called.
If there is a focused control on it, then CM_MOUSEWHEEL is sent to the focused control, see step 14.
Otherwise the inherited method is called, see step 13.2.
Otherwise the TControl.MouseWheelHandler method is called.
If there is a control that has captured the mouse and has no parent3), then the message is sent to that control, see step 8 or 10, depending on the type of the control. (3) This will never happen, because Capture is gotten with GetCaptureControl, which checks for Parent <> nil (XE2).)
If the control is on a form, then the control's form's MouseWheelHandler is called, see step 13.1.
Otherwise, or if the control ís the form, then CM_MOUSEWHEEL is sent to the control, see step 14.
The TControl.CMMouseWheel method receives the message.
The TControl.DoMouseWheel method is called.
The OnMouseWheel event is fired.
If not handled, then TControl.DoMouseWheelDown or TControl.DoMouseWheelUp is called, depending on the scroll direction.
The OnMouseWheelDown or the OnMouseWheelUp event is fired.
If not handled, then CM_MOUSEWHEEL is sent to the parent control, see step 14. (I believe this is against the advice given by MSDN in the quote above, but that undoubtedly is a thoughtful decision made by the developers. Possibly because that would start this very chain al over.)
Remarks, observations and considerations
At almost every step in this chain of processing the message can be ignored by doing nothing, altered by changing the message parameters, handled by acting on it, and canceled by setting Handled := True or setting Message.Result to non-zero.
Only when some control has focus, this message is received by the application. But even when Screen.ActiveCustomForm.ActiveControl is forcefully set to nil, the VCL ensures a focused control with TCustomForm.SetWindowFocus, which defaults to the previously active form. (With Windows.SetFocus(0), indeed the message is never sent.)
Due to the bug in IsControlMouseMsg 2), a TControl can only receive the WM_MOUSEWHEEL message if it has captured the mouse. This can manually be achieved by setting Control.MouseCapture := True, but you have to take special care of releasing that capture expeditiously, otherwise it will have unwanted side effects like the need for an unnecessary extra click to get something done. Besides, mouse capture typically only takes place between a mouse down and a mouse up event, but this restriction does not necessarily have to be applied. But even when the message reaches the control, it is sent to its MouseWheelHandler method which just sends it back to either the form or the active control. Thus non-windowed VCL controls can never act on the message by default. I believe this is another bug, otherwise why would all wheel handling have been implemented in TControl? Component writers may have implemented their own MouseWheelHandler method for this very purpose, and whatever solution comes to this question, there has to be taken care of not breaking this kind of existing customization.
Native controls which are capable of scrolling with the wheel, like TMemo, TListBox, TDateTimePicker, TComboBox, TTreeView, TListView, etc. are scrolled by the system itself. Sending CM_MOUSEWHEEL to such a control has no effect by default. These subclassed controls scroll as a result of the WM_MOUSEWHEEL message sent to the with the subclass associated API window procedure with CallWindowProc, which the VCL takes care of in TWinControl.DefaultHandler. Oddly enough, this routine does not check Message.Result before calling CallWindowProc, and once the message is sent, scrolling cannot be prevented. The message comes back with its Result set depending on whether the control normally is capable of scrolling or on the type of control. (E.g. a TMemo returns <> 0, and TEdit returns 0.) Whether it actually scrolled has no influence on the message result.
VCL controls rely on the default handling as implemented in TControl and TWinControl, as layed out above. They act on wheel events in DoMouseWheel, DoMouseWheelDown or DoMouseWheelUp. For as far I know, no control in the VCL has overriden MouseWheelHandler in order to handle wheel events.
Looking at different applications, there seems to be no conformity on which wheel scroll behaviour is the standard. For example: MS Word scrolls the page that is hovered, MS Excel scrolls the workbook that is focused, Windows Eplorer scrolls the focused pane, websites implement scroll behaviour each very differently, Evernote scrolls the window that is hovered, etc... And Delphi's own IDE tops everything by scrolling the focused window as well as the hovered window, except when hovering the code editor, then the code editor steals focus when you scroll (XE2).
Luckily Microsoft offers at least user experience guidelines for Windows-based desktop applications:
Make the mouse wheel affect the control, pane, or window that the pointer is currently over. Doing so avoids unintended results.
Make the mouse wheel take effect without clicking or having input focus. Hovering is sufficient.
Make the mouse wheel affect the object with the most specific scope. For example, if the pointer is over a scrollable list box control in a scrollable pane within a scrollable window, the mouse wheel affects the list box control.
Don't change the input focus when using the mouse wheel.
So the question's requirement to only scroll the hovered control has enough grounds, but Delphi's developers haven't made it easy to implement it.
Conclusion and solution
The preferred solution is one without subclassing windows or multiple implementations for different forms or controls.
To prevent the focused control from scrolling, the control may not receive the CM_MOUSEWHEEL message. Therefore, MouseWheelHandler of any control may not be called. Therefore, WM_MOUSEWHEEL may not be send to any control. Thus the only place left for intervention is TApplication.OnMessage. Furthermore, the message may not escape from it, so all handling should take place in that event handler and when all default VCL wheel handling is bypassed, every possible condition is to be taken care of.
Let's start simple. The enabled window which currently is hovered is gotten with WindowFromPoint.
procedure TForm1.ApplicationEvents1Message(var Msg: tagMSG;
var Handled: Boolean);
var
Window: HWND;
begin
if Msg.message = WM_MOUSEWHEEL then
begin
Window := WindowFromPoint(Msg.pt);
if Window <> 0 then
begin
Handled := True;
end;
end;
end;
With FindControl we get a reference to the VCL control. If the result is nil, then the hovered window does not belong to the application's process, or it is a window not known to the VCL (e.g. a dropped down TDateTimePicker). In that case the message needs to be forwarded back to the API, and its result we are not interested in.
WinControl: TWinControl;
WndProc: NativeInt;
WinControl := FindControl(Window);
if WinControl = nil then
begin
WndProc := GetWindowLongPtr(Window, GWL_WNDPROC);
CallWindowProc(Pointer(WndProc), Window, Msg.message, Msg.wParam,
Msg.lParam);
end
else
begin
end;
When the window ís a VCL control, multiple message handlers are to be considered calling, in a specific order. When there is an enabled non-windowed control (of type TControl or descendant) on the mouse position, it first should get a CM_MOUSEWHEEL message because that control is definitely the foreground control. The message is to be constructed from the WM_MOUSEWHEEL message and translated into its VCL equivalent. Secondly, the WM_MOUSEWHEEL message has to be send to the control's DefaultHandler method to allow handling for native controls. And at last, again the CM_MOUSEWHEEL message has to be send to the control when no previous handler took care of the message. These last two steps cannot take place in reversed order because e.g. a memo on a scroll box must be able to scroll too.
Point: TPoint;
Message: TMessage;
Point := WinControl.ScreenToClient(Msg.pt);
Message.WParam := Msg.wParam;
Message.LParam := Msg.lParam;
TCMMouseWheel(Message).ShiftState :=
KeysToShiftState(TWMMouseWheel(Message).Keys);
Message.Result := WinControl.ControlAtPos(Point, False).Perform(
CM_MOUSEWHEEL, Message.WParam, Message.LParam);
if Message.Result = 0 then
begin
Message.Msg := Msg.message;
Message.WParam := Msg.wParam;
Message.LParam := Msg.lParam;
WinControl.DefaultHandler(Message);
end;
if Message.Result = 0 then
begin
Message.WParam := Msg.wParam;
Message.LParam := Msg.lParam;
TCMMouseWheel(Message).ShiftState :=
KeysToShiftState(TWMMouseWheel(Message).Keys);
Message.Result := WinControl.Perform(CM_MOUSEWHEEL, Message.WParam,
Message.LParam);
end;
When a window has captured the mouse, all wheel messages should be sent to it. The window retrieved by GetCapture is ensured to be a window of the current process, but it does not have to be a VCL control. E.g. during a drag operation, a temporary window is created (see TDragObject.DragHandle) that receives mouse messages. All messages? Noooo, WM_MOUSEWHEEL is not sent to the capturing window, so we have to redirect it. Furthermore, when the capturing window does not handle the message, all other previously covered processing should take place. This is a feature which is missing in the VCL: on wheeling during a drag operation, Form.OnMouseWheel indeed is called, but the focused or hovered control does not receive the message. This means for example that a text cannot be dragged into a memo's content on a location that is beyond the visible part of the memo.
Window := GetCapture;
if Window <> 0 then
begin
Message.Result := GetCaptureControl.Perform(CM_MOUSEWHEEL, Message.WParam,
Message.LParam);
if Message.Result = 0 then
Message.Result := SendMessage(Window, Msg.message, Msg.wParam,
Msg.lParam);
end;
This essentially does the job, and it was the basis for the unit presented below. To get it working, just add the unit name to one of the uses clauses in your project. It has the following additional features:
The possibility to preview a wheel action in the main form, the active form, or the active control.
Registration of control classes for which their MouseWheelHandler method has to be called.
The possibility to bring this TApplicationEvents object in front of all others.
The possibility to cancel dispatching the OnMessage event to all other TApplicationEvents objects.
The possibility to still allow for default VCL handling afterwards for analytical or testing purposes.
ScrollAnywhere.pas
unit ScrollAnywhere;
interface
uses
System.Classes, System.Types, System.Contnrs, Winapi.Windows, Winapi.Messages,
Vcl.Controls, Vcl.Forms, Vcl.AppEvnts;
type
TWheelMsgSettings = record
MainFormPreview: Boolean;
ActiveFormPreview: Boolean;
ActiveControlPreview: Boolean;
VclHandlingAfterHandled: Boolean;
VclHandlingAfterUnhandled: Boolean;
CancelApplicationEvents: Boolean;
procedure RegisterMouseWheelHandler(ControlClass: TControlClass);
end;
TMouseHelper = class helper for TMouse
public
class var WheelMsgSettings: TWheelMsgSettings;
end;
procedure Activate;
implementation
type
TWheelInterceptor = class(TCustomApplicationEvents)
private
procedure ApplicationMessage(var Msg: tagMSG; var Handled: Boolean);
public
constructor Create(AOwner: TComponent); override;
end;
var
WheelInterceptor: TWheelInterceptor;
ControlClassList: TClassList;
procedure TWheelInterceptor.ApplicationMessage(var Msg: tagMSG;
var Handled: Boolean);
var
Window: HWND;
WinControl: TWinControl;
WndProc: NativeInt;
Message: TMessage;
OwningProcess: DWORD;
procedure WinWParamNeeded;
begin
Message.WParam := Msg.wParam;
end;
procedure VclWParamNeeded;
begin
TCMMouseWheel(Message).ShiftState :=
KeysToShiftState(TWMMouseWheel(Message).Keys);
end;
procedure ProcessControl(AControl: TControl;
CallRegisteredMouseWheelHandler: Boolean);
begin
if (Message.Result = 0) and CallRegisteredMouseWheelHandler and
(AControl <> nil) and
(ControlClassList.IndexOf(AControl.ClassType) <> -1) then
begin
AControl.MouseWheelHandler(Message);
end;
if Message.Result = 0 then
Message.Result := AControl.Perform(CM_MOUSEWHEEL, Message.WParam,
Message.LParam);
end;
begin
if Msg.message <> WM_MOUSEWHEEL then
Exit;
with Mouse.WheelMsgSettings do
begin
Message.Msg := Msg.message;
Message.WParam := Msg.wParam;
Message.LParam := Msg.lParam;
Message.Result := LRESULT(Handled);
// Allow controls for which preview is set to handle the message
VclWParamNeeded;
if MainFormPreview then
ProcessControl(Application.MainForm, False);
if ActiveFormPreview then
ProcessControl(Screen.ActiveCustomForm, False);
if ActiveControlPreview then
ProcessControl(Screen.ActiveControl, False);
// Allow capturing control to handle the message
Window := GetCapture;
if (Window <> 0) and (Message.Result = 0) then
begin
ProcessControl(GetCaptureControl, True);
if Message.Result = 0 then
Message.Result := SendMessage(Window, Msg.message, Msg.wParam,
Msg.lParam);
end;
// Allow hovered control to handle the message
Window := WindowFromPoint(Msg.pt);
if (Window <> 0) and (Message.Result = 0) then
begin
WinControl := FindControl(Window);
if WinControl = nil then
begin
// Window is a non-VCL window (e.g. a dropped down TDateTimePicker), or
// the window doesn't belong to this process
WndProc := GetWindowLongPtr(Window, GWL_WNDPROC);
Message.Result := CallWindowProc(Pointer(WndProc), Window,
Msg.message, Msg.wParam, Msg.lParam);
end
else
begin
// Window is a VCL control
// Allow non-windowed child controls to handle the message
ProcessControl(WinControl.ControlAtPos(
WinControl.ScreenToClient(Msg.pt), False), True);
// Allow native controls to handle the message
if Message.Result = 0 then
begin
WinWParamNeeded;
WinControl.DefaultHandler(Message);
end;
// Allow windowed VCL controls to handle the message
if not ((MainFormPreview and (WinControl = Application.MainForm)) or
(ActiveFormPreview and (WinControl = Screen.ActiveCustomForm)) or
(ActiveControlPreview and (WinControl = Screen.ActiveControl))) then
begin
VclWParamNeeded;
ProcessControl(WinControl, True);
end;
end;
end;
// Bypass default VCL wheel handling?
Handled := ((Message.Result <> 0) and not VclHandlingAfterHandled) or
((Message.Result = 0) and not VclHandlingAfterUnhandled);
// Modify message destination for current process
if (not Handled) and (Window <> 0) and
(GetWindowThreadProcessID(Window, OwningProcess) <> 0) and
(OwningProcess = GetCurrentProcessId) then
begin
Msg.hwnd := Window;
end;
if CancelApplicationEvents then
CancelDispatch;
end;
end;
constructor TWheelInterceptor.Create(AOwner: TComponent);
begin
inherited Create(AOwner);
OnMessage := ApplicationMessage;
end;
procedure Activate;
begin
WheelInterceptor.Activate;
end;
{ TWheelMsgSettings }
procedure TWheelMsgSettings.RegisterMouseWheelHandler(
ControlClass: TControlClass);
begin
ControlClassList.Add(ControlClass);
end;
initialization
ControlClassList := TClassList.Create;
WheelInterceptor := TWheelInterceptor.Create(Application);
finalization
ControlClassList.Free;
end.
Disclaimer:
This code intentionally does not scroll anything, it only prepares the message routing for VCL's OnMouseWheel* events to get the proper opportunity to get fired. This code is not tested on third-party controls. When VclHandlingAfterHandled or VclHandlingAfterUnhandled is set True, then mouse events may be fired twice. In this post I made some claims and I considered there to be three bugs in the VCL, however, that is all based on studying documentation and testing. Please do test this unit and comment on findings and bugs. I apologize for this rather long answer; I simply do not have a blog.
1) Naming cheeky taken from A Key’s Odyssey
2) See my Quality Central bug report #135258
3) See my Quality Central bug report #135305
Try overriding your form's MouseWheelHandler method like this (I have not tested this thoroughly):
procedure TMyForm.MouseWheelHandler(var Message: TMessage);
var
Control: TControl;
begin
Control := ControlAtPos(ScreenToClient(SmallPointToPoint(TWMMouseWheel(Message).Pos)), False, True, True);
if Assigned(Control) and (Control <> ActiveControl) then
begin
Message.Result := Control.Perform(CM_MOUSEWHEEL, Message.WParam, Message.LParam);
if Message.Result = 0 then
Control.DefaultHandler(Message);
end
else
inherited MouseWheelHandler(Message);
end;
Override the TApplication.OnMessage event (or create a
TApplicationEvents component) and redirect the WM_MOUSEWHEEL message in
the event handler:
procedure TMyForm.AppEventsMessage(var Msg: tagMSG;
var Handled: Boolean);
var
Pt: TPoint;
C: TWinControl;
begin
if Msg.message = WM_MOUSEWHEEL then begin
Pt.X := SmallInt(Msg.lParam);
Pt.Y := SmallInt(Msg.lParam shr 16);
C := FindVCLWindow(Pt);
if C = nil then
Handled := True
else if C.Handle <> Msg.hwnd then begin
Handled := True;
SendMessage(C.Handle, WM_MOUSEWHEEL, Msg.wParam, Msg.lParam);
end;
end;
end;
It works fine here, though you may want to add some protection to keep
it from recursing if something unexpected happens.
You might find this article useful: send a scroll down message to listbox using mousewheel, but listbox doesn't have focus [1], it is written in C#, but converting to Delphi shouldn't be too big a problem. It uses hooks to accomplish the wanted effect.
To find out which component the mouse is currently over, you can use the FindVCLWindow function, an example of this can be found in this article: Get the Control Under the Mouse in a Delphi application [2].
[1] http://social.msdn.microsoft.com/forums/en-US/winforms/thread/ec1fbfa2-137e-49f6-b444-b634e4f44f21/
[2] http://delphi.about.com/od/delphitips2008/qt/find-vcl-window.htm
This is the solution I've been using:
Add amMouseWheel to the uses clause of the implementation section of the unit of your form after the forms unit:
unit MyUnit;
interface
uses
Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms,
// Fix and util for mouse wheel
amMouseWheel;
...
Save the following code to amMouseWheel.pas:
unit amMouseWheel;
// -----------------------------------------------------------------------------
// The original author is Anders Melander, anders#melander.dk, http://melander.dk
// Copyright © 2008 Anders Melander
// -----------------------------------------------------------------------------
// License:
// Creative Commons Attribution-Share Alike 3.0 Unported
// http://creativecommons.org/licenses/by-sa/3.0/
// -----------------------------------------------------------------------------
interface
uses
Forms,
Messages,
Classes,
Controls,
Windows;
//------------------------------------------------------------------------------
//
// TForm work around for mouse wheel messages
//
//------------------------------------------------------------------------------
// The purpose of this class is to enable mouse wheel messages on controls
// that doesn't have the focus.
//
// To scroll with the mouse just hover the mouse over the target control and
// scroll the mouse wheel.
//------------------------------------------------------------------------------
type
TForm = class(Forms.TForm)
public
procedure MouseWheelHandler(var Msg: TMessage); override;
end;
//------------------------------------------------------------------------------
//
// Generic control work around for mouse wheel messages
//
//------------------------------------------------------------------------------
// Call this function from a control's (e.g. a TFrame) DoMouseWheel method like
// this:
//
// function TMyFrame.DoMouseWheel(Shift: TShiftState; WheelDelta: Integer;
// MousePos: TPoint): Boolean;
// begin
// Result := ControlDoMouseWheel(Self, Shift, WheelDelta, MousePos) or inherited;
// end;
//
//------------------------------------------------------------------------------
function ControlDoMouseWheel(Control: TControl; Shift: TShiftState;
WheelDelta: Integer; MousePos: TPoint): Boolean;
implementation
uses
Types;
procedure TForm.MouseWheelHandler(var Msg: TMessage);
var
Target: TControl;
begin
// Find the control under the mouse
Target := FindDragTarget(SmallPointToPoint(TCMMouseWheel(Msg).Pos), False);
while (Target <> nil) do
begin
// If the target control is the focused control then we abort as the focused
// control is the originator of the call to this method.
if (Target = Self) or ((Target is TWinControl) and (TWinControl(Target).Focused)) then
begin
Target := nil;
break;
end;
// Let the target control process the scroll. If the control doesn't handle
// the scroll then...
Msg.Result := Target.Perform(CM_MOUSEWHEEL, Msg.WParam, Msg.LParam);
if (Msg.Result <> 0) then
break;
// ...let the target's parent give it a go instead.
Target := Target.Parent;
end;
// Fall back to the default processing if none of the controls under the mouse
// could handle the scroll.
if (Target = nil) then
inherited;
end;
type
TControlCracker = class(TControl);
function ControlDoMouseWheel(Control: TControl; Shift: TShiftState;
WheelDelta: Integer; MousePos: TPoint): Boolean;
var
Target: TControl;
begin
(*
** The purpose of this method is to enable mouse wheel messages on controls
** that doesn't have the focus.
**
** To scroll with the mouse just hover the mouse over the target control and
** scroll the mouse wheel.
*)
Result := False;
// Find the control under the mouse
Target := FindDragTarget(MousePos, False);
while (not Result) and (Target <> nil) do
begin
// If the target control is the focused control then we abort as the focused
// control is the originator of the call to this method.
if (Target = Control) or ((Target is TWinControl) and (TWinControl(Target).Focused)) then
break;
// Let the target control process the scroll. If the control doesn't handle
// the scroll then...
Result := TControlCracker(Target).DoMouseWheel(Shift, WheelDelta, MousePos);
// ...let the target's parent give it a go instead.
Target := Target.Parent;
end;
end;
end.
I had the same problem and solved it with some little hack, but it works.
I didn't want to mess around with messages and decided just to call DoMouseWheel method to control I need. Hack is that DoMouseWheel is protected method and therefore not accessible from form unit file, that's why I defined my class in form unit:
TControlHack = class(TControl)
end; //just to call DoMouseWheel
Then I wrote TForm1.onMouseWheel event handler:
procedure TForm1.FormMouseWheel(Sender: TObject; Shift: TShiftState;
WheelDelta: Integer; MousePos: TPoint; var Handled: Boolean);
var i: Integer;
c: TControlHack;
begin
for i:=0 to ComponentCount-1 do
if Components[i] is TControl then begin
c:=TControlHack(Components[i]);
if PtInRect(c.ClientRect,c.ScreenToClient(MousePos)) then
begin
Handled:=c.DoMouseWheel(shift,WheelDelta,MousePos);
if Handled then break;
end;
end;
end;
As you see, it search for all the controls on form, not only immediate children, and turns out to search from parents to children. It would be better (but more code) to make recursive search at children, but code above works just fine.
To make only one control respond to mousewheel event, you should always set Handled:=true when it's implemented. If for example you have listbox inside panel, then panel will execute DoMouseWheel first, and if it didn't handle event, listbox.DoMouseWheel will execute. If no control under mouse cursor handled DoMouseWheel, the focused control will, it seems rather adequate behavior.
Only for using with DevExpress controls
It works on XE3. It was not tested on other versions.
procedure TMainForm.DoApplicationMessage(var AMsg: TMsg; var AHandled: Boolean);
var
LControl: TWinControl;
LMessage: TMessage;
begin
if AMsg.message <> WM_MOUSEWHEEL then
Exit;
LControl := FindVCLWindow(AMsg.pt);
if not Assigned(LControl) then
Exit;
LMessage.WParam := AMsg.wParam;
// see TControl.WMMouseWheel
TCMMouseWheel(LMessage).ShiftState := KeysToShiftState(TWMMouseWheel(LMessage).Keys);
LControl.Perform(CM_MOUSEWHEEL, LMessage.WParam, AMsg.lParam);
AHandled := True;
end;
if you don't use DevExpress controls, then Perform -> SendMessage
SendMessage(LControl.Handle, AMsg.message, AMsg.WParam, AMsg.lParam);
In the OnMouseEnter event for each scrollable control add a respective call to SetFocus
So for ListBox1:
procedure TForm1.ListBox1MouseEnter(Sender: TObject);
begin
ListBox1.SetFocus;
end;
Does this achieve the desired effect?