MVVM设计模式

Model-View-ViewModel is a design paradigm.

It must be adhered to for SwiftUI to work.

Model

• UI Independent
• Data + Logic

View

• The reflection of the Model
• Stateless
• Declared(只有“var body”决定了view是如何绘制的)
• Reactive (Always reacting efficiently to the change on the model)
• Automatically observes publications from ViewModel( or subscribe what they interested at from the ViewModel) .Pulls data and rebuild itself.

ViewModel

• Binds View to Model(so the change on the model cause the view to react and get rebuilt)
• Interpreter (between Model and View). Help View code stay clean and neat.
• Gatekeeper.
• Constantly noticing changes in the Model
• Publish a message globally once any change in the Model is noticed (avoid have any connection to any of the View that using it to access the Model)
• Processing User Intent(Change the Model based on the events occurs in the View)

Rules in MVVM

• The View must always get data from the Model by asking it from the ViewModel.
• The ViewModel never stores the data for the Model in side of itself.

@State

View is Read-Only, So “let/constant properties” or “computed properties” make much sense for a View. The EXCEPTION is property wrappers like @ObservedObject which must be marked as a “var”