WPF Undo Redo Property System to highlight in red color if value has changed

Question

I have a following requirement for a very complex UI. (Complex here means there are lot of controls in the form [approximately 100]). I am using MVVM (if my problem requires it to slightly go away from MVVM I am ok with it)

My question is for Editable ComboBox and TextBox. But I would say I like to hear a common algorithm which will fit all controls.

Requirement 1 : The user edits the content and goes to next control, the color of the control/text should become red.

Requirement 2 : When the user comes back to the previously edited control and enters the value which was initially present, the color of the control/text should become back to black.

I know the requirement is tough and I have been breaking my head to design a generic algorithm using which I can store the previous value and call a function to change the color of control.

To just give you all an idea,
--> I tried storing 2 properties for every TextBox like Default_Text and Text. But since the number of properties are huge, the memory footprint is very huge. Also maintaining so many properties is very tough.
--> I tried adding a Dictionary to every ViewModel to store what values have got changed. But here the problem I faced was giving unique keys to all the controls in my application, which is not very helpful
--> I had even thought and tried about subclassing controls like TextBox, ComboBox and overriding some methods to suit my requirement, but sadly I failed miserabley when I started adding validations and all.

So here I am stuck with designing a generic WPF property system/algorithm to handle all undo redo functionality, changing styles of controls,etc!!!

It will be really great if you experts can guide me in right direction and also help me in developing such an algorithm/system. A sample illustration will be nice though!!!

Answer 1

I found an answer to the above problem. I used attached behavior for this. More details on this link Function call from XAML from StackOverFlow.

When I databind, I store the initial value of the DataBound variable in the Tag property by using Binding=OneWay. Then I have written a attached behaviour for LostFocus event.

Whenever the user enters a control and then goes to other control, it fires LostFocus event and calls my attached behaviour. In this, I check whether the value is equal to the value in Tag. If it is same, I display in black else I display in red.

Attached Behaviour rocks in WPF. I can achieve anything from that cleanly without code cluttering!!!!