protected override void OnObservedInstanceChanging(INotifyPropertyChanged oldInstance, INotifyPropertyChanged newInstance)
{
if (oldInstance != null)
{
oldInstance.PropertyChanged -= InvokeCallback;
}
if (newInstance != null)
{
newInstance.PropertyChanged += InvokeCallback;
}
if (_isInitialized)
{
PerformInvokeCallback();
}
if (newInstance == null)
{
_oldValue = default(T);
}
else
{
_oldValue = (T)ObservedProperty.GetValue(newInstance);
}
}
private void HandleNextPropertyChanged(object sender, PropertyChangedEventArgs args)
{
if (args.PropertyName != ObservedProperty.Name)
{
return;
}
var newValue = ObservedProperty.GetValue(ObservedInstance);
if (newValue != null && !(newValue is INotifyPropertyChanged))
{
throw new InvalidOperationException();
}
NextEntry.Reattach(newValue as INotifyPropertyChanged);
}
protected virtual void Reattach(INotifyPropertyChanged observedInstance)
{
ObservedInstance = observedInstance;
if (observedInstance == null || NextEntry == null)
{
return;
}
var nextInstance = ObservedProperty.GetValue(ObservedInstance);
if (nextInstance != null && !(nextInstance is INotifyPropertyChanged))
{
throw new InvalidOperationException();
}
NextEntry.Reattach(nextInstance as INotifyPropertyChanged);
}
private void PerformInvokeCallback()
{
var newValue = ObservedInstance != null ? (T)ObservedProperty.GetValue(ObservedInstance) : default(T);
if (_oldValue == null && newValue == null)
{
return;
}
if (_oldValue != null && _oldValue.Equals(newValue))
{
return;
}
Callback?.Invoke(_oldValue, newValue);
_oldValue = newValue;
}