ViewModel or Model: Who Should Implement INotifyPropertyChanged in MVVM?

Should the ViewModel or Model Implement INotifyPropertyChanged in MVVM?

Within the realm of MVVM, the choice between the ViewModel or Model implementing INotifyPropertyChanged has sparked debate. Traditional MVVM implementations often assign this responsibility to the Model, while Josh Smith's CommandSink example deviates by having the ViewModel implement it.

Model as INotifyPropertyChanged

Arguments:

• Clear separation of concerns: The Model encapsulates business logic and data, while the ViewModel manages user interactions. INotifyPropertyChanged aligns with the Model's data-centric nature.
• Efficient change propagation: Binding to changes in the Model directly ensures that UI updates occur immediately and accurately.

Counterarguments:

• UI dependency in the Model: INotifyPropertyChanged is essentially a UI-related interface, introducing a dependency in the supposedly data-agnostic Model.

ViewModel as INotifyPropertyChanged

Arguments:

• Simplified data binding: Implementing INotifyPropertyChanged in the ViewModel eliminates the need for complex event-handling mechanisms between the Model and ViewModel.
• Centralized change tracking: The ViewModel serves as a single point of contact for tracking and propagating changes to the UI, avoiding the risk of multiple sources of notifications.

Counterarguments:

• Duplication of effort: If the Model also implements INotifyPropertyChanged, it may lead to redundant change notifications.

Conclusion

Ultimately, the choice between the Model or ViewModel implementing INotifyPropertyChanged depends on the specific context and application requirements. Consider the following factors:

• If the Model is highly data-centric and requires independent change notification, it may be appropriate for it to implement INotifyPropertyChanged.
• If the ViewModel plays a central role in managing UI interactions and simplifying data binding, it could benefit from being the INotifyPropertyChanged implementer.
• Avoid redundancy by implementing the interface only where necessary.

Remember, MVVM is a flexible framework with diverse approaches, and the best solution may vary based on project considerations.

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