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How I make a component wiht their own events?

Posted on 2000-02-23
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Last Modified: 2010-04-04
I have to make a component that draws
lines, circles and polygons (like the line of Visual Basic). But I don`t know how to make the events for this component. What can I do?
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Question by:mariotwsh
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6 Comments
 
LVL 8

Expert Comment

by:ZifNab
ID: 2549585
what about looking to the TShape component of the extctrls unit?
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Expert Comment

by:octavioh
ID: 2549704
My recommendations:
a) Take a look at the chapters on component development in the Developer's Guide.
b) Take a look at the source code of the TShape component provided with Delphi.
c) Take a look at the example included with Delphi under
  Program Files\Delphi5\Demos\Propedit.

ThePIELIB package described there shows how to create a simple component that draws pies. This component implements several properties and an OnChange event. Furthermore, this sample shows how to build specialized property editors.

IMHO, any developer designing Delphi components should read "Developing Delphi components" by Ray Konopka. It was written for Delphi 3, but almost nothing has changed regarding component development since then.

Regards,

OH
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LVL 1

Accepted Solution

by:
AttarSoftware earned 200 total points
ID: 2550063
Set up a type, eg:

type
  tMyEvent = procedure( s : string ) of object ;

and then in the private section of your component, add :

    fMyEvent : tMyEvent ;

and then in the public or protected areas, add :

    property OnSomething : tMyEvent read fMyEvent write fMyEvent ;

and then where you want the event to be fired (in some procedure in your code), add :

  if( assigned( fMyEvent ) ) then
    fMyEvent( 'Hello World' ) ;

That will set up an event, that when triggered will send the 'Hello World' string...

  Hope this points you in the right direction,

  Good luck,

  Tim.
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Expert Comment

by:octavioh
ID: 2550327
Regarding Tim' answer:

The suggested approach is on the right direction, but using the type tMyEvent as he proposes will not allow the programmer using your components to distinguish which concrete component generated the event. For instance, if an user of your component places two 'shapes' on a form, he may want to share a common event handler (so it's fired by both components), but detecting inside that method which one of the two components actually 'launched' it.

All the VCL events use a first parameter 'Sender: TObject' in event types, and Delphi automatically pushes onto the stack the address of the component that fired the event. The programmer that uses your class can then use the Sender parameter to communicate with the concrete component for which the event was fired.

The simplest event type is called TNotifyEvent, and is already defined by Delphi:

type
  TNotifyEvent = procedure(Sender: TObject) of object;

Other, more complex event handler patterns are also defined for keyboard events, mouse events, etc.

Again, consult the manual and check out the VCL source code (the biggest source of information, actually).

Regards,

OH

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LVL 1

Expert Comment

by:AttarSoftware
ID: 2553974
Oh yeah :O)

Just me being lazy, and avoiding typing the ( sender : tObject ) bit ;O)

Sorry about that...

Tim.
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Author Comment

by:mariotwsh
ID: 2554360


   A lot of thanks!!!!!!!!!!!
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