Sunday, September 8, 2013

AttachedProperty - clarifying questions

Attached Property is a tricky construct. When confused, a beginner (like me) could ask a few key questions

Q: Which class (not "object") defines the AProp?
Q: Which object's (not "class") hashtable holds the property value?

The holder object is the visual/screen object "decorated" by the property value. Just like Opacity, "Dock.Left" describes some aspect of a visual. The "provider" is a class not an object. The AProp provider class is like a category or namespace, and tells us what category this property falls into. Is it a docking property or a text formatting property?

Any AProp (or any DProp) data can be seen as a key-value pair. In the Dock example, an OKButton's hashtable holds the value of "Left" under the key DockPanel.DockProperty. Note the property is "specified" or "defined" not by Button class but by the DockPanel class.

After the OKButton saves the "Left" value, the natural question is who reads it? Answer -- The container. In this case, the user of this property happens to be an instance of the provider class -- DockPanel. In some cases, the user is unrelated to the AProp provider. You can even attach a nonsensical property to a nonsensical object, so the property value is never used.

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