Tuesday, June 25, 2013

OneWayToSource, briefly

Logically, there are 2 sides to a (logical) binding -- Source property and Target property. Both must be properties (not fields) of some class. Target must be a dependency property, not a plain property.

Behind the scene, there is a Binding object, with a Source field (not to be confused with the "source" property) and a Path field (Better rename to "PropertyNameInSourceObject"). You inject Binding object into the Target visual control. Once injected, the target visual is wired up to the source property, via the binding.

I feel target is usually a visual. Source property often isn't a visual -- it could be a plain property in a POCO object. When source property gets edited, WPF engine would somehow update the visual. That's the plain vanilla OneWay update mode.

When used as a Target visual, some visuals default to 2-way update, like TextBox. Other visuals default to one-way update i.e. source to target.

OneWayToSource is a special mode. One reason you may need it -- when exactly one of the 2 sides is a dependency property. By WPF policy that side must be the Target, whereas the non-dependency side will be Source. If you need to update the non-dependency side, you can use OneWayToSource. Imagine the non-dependency side could be a POCO object with a plain property, but you need to update it when the textbox gets edits. In summary, system automatically updates the non-dependency property from a visual control on screen.

When both source Obj and target Obj are visuals, we can often make do without data binding -- use event instead.

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