CODEWITHSUNDEEP
delphivcl
Dave
Dave

Reputation: 5634

Why Do Some Delphi Components Require "AOwner: TComponent" To Construct Them?

It seems completely irrelevant to require a TComponent as an owner to instantiate an object of some kind. Why are there so many Delphi components that require this?

For example, TXMLDocument requires a TComponent object to instantiate.

Why is this and if there's a good reason, what should I be using in there to "do the right thing"?

Upvotes: 15

Views: 6967

Answers (6)

Remy Lebeau
Remy Lebeau

Reputation: 596592

An Owner parameter is only needed for TComponent descendants, as it is a parameter of the TComponent constructor. All components that are accessible at design-time for dropping on TForm, TFrame, and TDataModule classes are TComponent descendants (which includes TXMLDocument).

Upvotes: 0

az01
az01

Reputation: 2018

You shoudl use this for two reasons: - the ownership mechanism stands also as a kind of garbage collecting system - the ownership mechanism is important in Delphi serialization process (Stream.ReadComponent/WriteComponent etc).

Upvotes: 0

user9977
user9977

Reputation: 454

The owner component is supposed to manage all its owned components. The owned components gets destroyed automatically when the owner is destroyed.

This helps the developer who just drags components from the tool-palette, drops them on the form and just hooks up the events to get his work done without worrying about managing the lifetime of the components.

The form is the owner of all components dropped on it. The Application object is owner of the form. When the app is closed the Application object is destroyed which in turn destroys the forms and all the components.

But the owner is not really necessary when a components is created. If you pass Nil to the parameter, the component will be created without an owner and in this case it will be your responsibility to manage the component's lifetime.

Upvotes: 38

Cesar Romero
Cesar Romero

Reputation: 4027

All TComponent descendents require Owner, it is defined in TComponent constructor. The Owner component is responsible to destroy all the Owned components.

if you want to control the life time, you can pass nil as parameter.

Upvotes: 9

Aikislave
Aikislave

Reputation: 646

There is also something else to be aware of. I have used more than a couple of third party components which rely on an Owner component being passed in the Constructor Create, and if you pass in Nil will throw an exception/AV.

The net result is that these components will work fine when you use the visually within the IDE but cause problems when they are created at run-time.

The cause of these problems is, in a sense, bad design. Nothing in the rules state you cannot/should not pass in NIL as the aOwner parameter.

Upvotes: 3

Toon Krijthe
Toon Krijthe

Reputation: 53366

Just to add some extra information.

Each control also has a parent. (A TWinControl). Where the owner takes care of the lifetime, the parent takes care of showing the object.

For example a form has a panel and the panel has a button. In that case, the form owns the panel and the button. But the form is the parent of the panel and the panel is the parent of the button.

Upvotes: 3

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