Custom Component TImage events causing error "Does not exist" - delphi

Question

I have a custom component witht the following events
private
{ Private declarations }
...
fOnImageClick: TNotifyEvent;
fOnImageMouseUp: TMouseEvent;
fOnImageMouseDown: TMouseEvent;
fOnImageMouseMove: TMouseMoveEvent;
fOnImageMouseEnter: TNotifyEvent;
fOnImageMouseLeave: TNotifyEvent;
fOnImageSelect: TNotifyEvent;
fOnImageDblClick: TNotifyEvent;
protected
...
public
{ Public declarations }
...
published
...
property OnImageClick: TNotifyEvent read fOnImageClick write fOnImageClick;
property OnImageSelect: TNotifyEvent read fOnImageSelect write fOnImageSelect;
property OnImageDblClick: TNotifyEvent read fOnImageDblClick write fOnImageDblClick;
property OnImageMouseDown: TMouseEvent read fOnImageMouseDown write fOnImageMouseDown;
property OnImageMouseUp: TMouseEvent read fOnImageMouseUp write fOnImageMouseUp;
property OnImageMouseMove: TMouseMoveEvent read fOnImageMouseMove write fOnImageMouseMove;
property OnImageMouseLeave: TNotifyEvent read fOnImageMouseLeave write fOnImageMouseLeave;
property OnImageMouseEnter: TNotifyEvent read fOnImageMouseEnter write fOnImageMouseEnter;
end;
I assign them to a TImage whose parent is TPanel whos parent is TScrollBox
img:= TImage.Create(ThumbPnl);
img.Parent:= ThumbPnl;
img.Tag:= I;
img.Align:= alClient;
img.Stretch:= true;
img.OnClick:= fOnImageClick;
img.OnDblClick:= fOnImageDblClick;
img.OnMouseEnter:= fOnImageMouseEnter;
img.OnMouseLeave:= fOnImageMouseLeave;
img.OnMouseDown:= fOnImageMouseDown;
img.OnMouseUp:= fOnImageMouseUp;
img.OnMouseMove:= fOnImageMouseMove;
The component compiles and bulds just fine. The application with this component also compiles and runs jus fine. If I assign an OnClick event, it works. All other events, if I assign them and try to run the app, i get an error saying the event doesn't exist
Anyone know why that is?

Answer 1

You are making a fairly common mistake for new component creators. You are breaking this rule:
Component users write event handlers, component writers "fire" events
In your case what you are doing is assigning events when you as the component developer shouldn't be doing that. It's up to the component user to assign code to occur in the OnClick, OnDblClick, etc. events.
It's enough for you to declare the events as Published and as TNotifyEvent types.
property OnSomeEvent: TNotifyEvent read FOnSomeEvent write FOnSomeEvent;
That's all you need to do to create the events. Your job is to "fire" them; that is, make them happen.
That is done in your component at the appropriate moment. Usually, what you'll do is create a method DoXXXXX where XXXXX is the thing that is happening. So you'd create
procedure TMyImage.DoSomeEvent;
begin
if Assigned(FOnSomeEvent) then FOnSomeEvent(Self);
end;
Then, within your code, when the event should be fired, you simply call DoSomeEvent.
Now, if you want to enhance the functionality of the default events, then you need to override the method that fires the event.
I hate to do it, but my classic component TSmiley illustrates this simply and clearly:
http://tsmiley.svn.sourceforge.net/viewvc/tsmiley/
Take a look at that example and you should see how to create events.

Answer 2

What is an event
The event in Delphi is a method pointer. A method pointer is basically two pointers, one points to the method you assign to the event, and the other points to the live instance (object) you assign.
What I assume you're expecting
I think you expect the event in your inner object follows the events assigned to your outer object, and that will not happen automatically.
When you assign it, you're doing pointer assignments. So to take an example, your line
img.OnDblClick:= fOnImageDblClick;
Performs a pointer assignment with the value fOnImageDblClick have at the time. If it is nil, img.OnDblClick will be nil from now. If it points to Form1.MyComponentImageClick at the time, img.OnDblClick will point to the same method on the same object from now, but, if you later change where fOnImageDblClick points, the imgOnDblClick will not follow... it will remains pointing to the same old address. If you want it to change, you have to make that change by code also.
If you want that to happen, you can do that in a event setter in the outer class.
First, declare your events like this:
published
...
property OnImageDblClick: TNotifyEvent read fOnImageDblClick write SetOnImageDblClick;
...
procedure TMyClass.SetOnImageDblClick(Value: TNotifyEvent);
begin
FOnImageDblClick := Value;
//pass the new value to the inner object.
if Assigned(Img) then
Img.OnDblClick := Value;
end;
If your Img inner object exists all the time, you don't need the FOnImageDblClick variable, you can write also a getter for the property and take the value directly from the inner object, like this:
function TMyClass.GetOnImageDblClick: TNotifyEvent;
begin
Result := Img.OnDblClick;
end;