Property State

When dealing with change tracking systems it is important to know the state of the graph from 2 different point of views
System point of view
User point of view
The Radical ChangeTrackingService allows to gather both point of views with property level granularity.
Given a Person entity with a Name property we can basically be in 2 major scenarios:
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var person = new Person(name: "Mauro");
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var memento = new ChangeTrackingService();
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memento.Attach(person);
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​
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person.Name = "Giovanni"
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Or something a bit more problematic:
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var person = new Person(name: "Mauro");
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​
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var memento = new ChangeTrackingService();
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memento.Attach(person);
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​
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person.Name = "Giovanni"
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​
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//some other changes
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​
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person.Name = "Mauro"
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In both cases from the ChangeTrackingService perspective the Person instance is changed, however from the user perspective maybe it's not since the Name property has the exact same value as when it was instantiated.
The GetPropertyState(entity, property) method of ChangeTrackingService, given a tracked entity and a property name, returns the current state of the property as seen by the tracking service.
The state can be:
None : The property has no tracked states, thus is not changed either;
Changed: The property has tracked states, thus has changes; This means that in some way the property changed over time, this state does not take into account the actual value of the property;
ValueChanged: The property has tracked states and the actual value is different from the original one;
Using as a sample the above Person related changes:
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var state = memento.GetPropertyState(person, p => p.Name);
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The value of state is Changed & ValueChanged. On the contrary the second one is Changed only, because the memento service has tracked some changes but the current value is the same as the original one.
Extensions
In order to simply the query there is also an extension method for IMemento entities:
IsPropertyValueChanged(propertyName): Is an extension method to IMemento entities that acts like a shortcut to get if a property value changed over time or not;
Sample usage
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var person = new Person(name: "Mauro");
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​
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var memento = new ChangeTrackingService();
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memento.Attach(person);
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​
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person.Name = "Giovanni"
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var isNameValueChanged = person.IsPropertyValueChanged(p => p.Name); //will be true
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