CODEWITHSUNDEEP
.netwpfattached-properties
ragnarius
ragnarius

Reputation: 5813

Why are attached properties stored as class variables?

In WPF, to set the vertical location of an ellipse, for instance, one uses the method Canvas.SetTop(ellipseObj, y). That is to say, the location is stored in the class Canvas and not in the canvas object that contains the ellipse!

I like to understand why! So, why are attached properties stored as class variables?

Upvotes: 1

Views: 155

Answers (2)

McGarnagle
McGarnagle

Reputation: 102793

Attached properties are implemented that way so that any framework element (say a Line, or a TextBox) can be given a relationship to another object (say a Canvas, or a Grid), that it knows nothing about. Properties like Left and Top correctly belong to the container control, which is responsible for arranging its child element.

You can think of it like a visitor pattern -- the Canvas container is able to effectively modify the behavior of any child elements, without actually changing the structure of those elements.

Here's how the MSDN article on attached properties explains it:

One purpose of an attached property is to allow different child elements to specify unique values for a property that is actually defined in a parent element.

Upvotes: 1

Euphoric
Euphoric

Reputation: 12849

The point of WPF is, that UI Panels decide where their child elements are drawn. So, controls themselves have no say about where they can be positioned. In measure pass of UI layout, only control's size is considered.

In your case, you are using Canvas as example, which is quite short-sighted. Canvas is only one of the few possible ways to layout child controls and only it was designed to allow precise positioning of it's child elements. So it makes sense to be able to set Top and Left values for each child. And just to note, Canvas is least-advised way to layout your UI.

But in Grid, no such thing is possible. Here, you set which Row, Column and what RowSpan and ColSpan should child have. And in DockPanel, you have different ways to dock the child elements.

And other panels like StackPanel and WrapPanel have no such options.

To solve this, you have 2 options:

  1. Save all of those in each element. This will result in many unused properties AND lack of extensibility. What if someone wants to implement their own Panel, that layouts children differently and needs some other kind of property for each child?
  2. Save those properties in container itself. This gives you space saving AND option to extend the whole thing.

Also, Attached Properties are useful for many other things, than panel layout.

Upvotes: 2

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