What is the difference between Owner and Parent of a control?

Question

I am bit curious about the two properties of a Delphi VCL control.

Each component has 2 properties as Owner and Parent at runtime. Can anyone help me to understand the difference between both? And how are they used by windows for displaying control or form?

Answer 1

Owner

Owner is a property introduced in TComponent, and Owner itself has type TComponent. The Owner is used primarily to manage the lifetime of designed components. That is, components that you place on the form designer (or indeed other design surfaces) and whose lifetime is managed entirely by the framework. The documentation says:

Indicates the component that is responsible for streaming and freeing this component.

When a form is created, the streaming framework parses the .dfm file and instantiates the components that are listed within. These components are typically created with Owner specified to be the form.

At the other end of a component's life is destruction. When a component is destroyed, it also destroys all of the components which it owns. Consider, for the sake of a concrete example, a TButton that sits on a TForm, placed there at design time. The streaming framework creates the button setting its Owner to be the form. The form, as a descendent of TComponent maintains a list of all the components that it owns. When the form is destroyed it walks over that list of owned components and destroys them. The button is destroyed in this manner.

There are many nuances to this:

• Components don't need to be owned by the form. For instance, components created at run time are owned by whatever component you pass to the constructor.
• Components need not have an owner, you can pass nil to the constructor. In this case, the programmer remains responsible for destroying the component.
• Ownership can be changed during the lifetime of a component.

Because the streaming framework instantiates components, the constructor of TComponent is declared virtual:

constructor Create(AOwner: TComponent); virtual;

If you derived from a TComponent descendent, and you wish for that derived component to be placed on a design surface, then you must respect this virtual constructor. If you define a constructor in your TComponent descendent then it must override this virtual constructor.

It is worth pointing out that Win32 has a completely different concept of a window's owner, that should not be confused with the VCL concept of the same name. The Windows documentation says:

Owned Windows

An overlapped or pop-up window can be owned by another overlapped or pop-up window. Being owned places several constraints on a window. - An owned window is always above its owner in the z-order. - The system automatically destroys an owned window when its owner is destroyed. - An owned window is hidden when its owner is minimized.

Only an overlapped or pop-up window can be an owner window; a child window cannot be an owner window.

In VCL terms, this concept is exposed by the PopupParent property. That property was introduced after Delphi 7 so will not be available to you. In Delphi 7, the framework sets the window owner, and gives no easy mechanism to override the framework's choice. If you do need to influence window ownership then you must override CreateParams and set Params.WndParent. Unfortunately, there are a number of problems with the VCL handling of ownership in Delphi 7 and it is sometimes necessary to grub around in these somewhat gory details.

To show how easy it is to get confused, the VCL documentation says:

WndParent: The window handle of the parent window. This is the same as the Handle property of the parent control.

This is simply wrong. For a top-level window, this is the owner rather than the parent.

Parent

Parent is a property defined in TControl and has type TWinControl. This property is used, broadly, to expose the Win32 concept of parent and child controls. The Windows documentation says:

A window can have a parent window. A window that has a parent is called a child window. The parent window provides the coordinate system used for positioning a child window. Having a parent window affects aspects of a window's appearance; for example, a child window is clipped so that no part of the child window can appear outside the borders of its parent window. A window that has no parent, or whose parent is the desktop window, is called a top-level window.

Essentially, the VCL Parent property maps directly onto the Win32 parent concept.

Note however, that Parent is defined in TControl. Now, TControl is not windowed, so a TControl is not a child control in the Win32 sense, because a Win32 window's children are themselves windows. So, a TControl with a defined Parent is a child in the VCL sense, known as a non-windowed child control. These non-windowed controls paint themselves as part of their parent's paint handlers. The canonical example of such a control is TLabel.

Note that when a windowed control, that is a TWinControl descendent, is destroyed, it also destroys all of its children.

A TWinControl descendent can enumerate its children using the ControlCount and Controls[] properties. These enumerate both windowed and non-windowed children.