Why props-based MVVM does not suit React04 March 2018
In this post we’ll talk about a variant of the MVC patter to decouple views from the rest of the implementation of a user-facing application. We’ll first introduce MVVM (that’s model-view-viewmodel), how it relates to MVC, and why it is a good pattern. Then we’ll talk a bit about the React component update model (but will however assume general familiarity with its details), and argue that only some approaches that marry MVVM with React do not violate the semantics of the latter.
Historically, MVC has risen as a way to decouple the view and logic that controls the “state” of the view from the application model. At its core MVVM is a constrained version of MVC where the way the view and the “controller” interact is regulated by the following rules: the view invokes behavior of the view model directly, and listens for changes in the view model state via an event or subscription-based approach or similar. The view model, on the other hand, exposes methods and events for views but is not tied to any particular specification of a view, it doesn’t know what the view is. In other words, the dependency relation between the two proceeds from the view to the view model with no way backwards.
In a typical example, the user clicks a button, this results in the view calling a method on the view model triggering a change in the model and eventually alerting all registered listeners of a change in its state, for example by declaring some state as now “pending” and some other state as now “disabled”. The view receives the notification as it is one of the registered listeners and updates itself to now show some result as “pending” and the button clicked as “disabled”, for example.
This differs from some implementations of MVC where, for example, the interaction between view and view model is unconstrained and both view and view model share some mutable state they can both read and write over (hopefully from the same UI thread.) Constraints are good to the exercise of creativity. MVVM provides constraints where the view model is completely independent of the view implementation. This means, for example, that you could in principle migrate your views from React to Vue and not have to worry about re-wiring them all up to your business logic / domain model.
The React component update model in a nutshell
As mentioned in the React documentation, the rendering output of a React component is meant to be a pure function on props and state for each individual render cycle: given the same props and state the it returns the same virtual representation of a view1. A pure function is a function that is: (1) free of side effects, (2) deterministically returns the same value given a specific output. The React component model (in a nutshell) proposes a hierarchical view architecture where a view component tickles down
props to its children (or the components which compose it), these can range from state, callbacks, rendering information etc. In turn, an individual component can queue updates to its
state. This is normally used for things the component itself should know and manage internally.
Any appropriately propagated changed to props and states triggers a component re-render: React will render the component again, get a new version of it, then compare it to the previous render output and selectively dispatch changes to the underlying view engine (which may or may not be the browser’s DOM.)
You can check out several implementations of MVVM for React that are fun and likely profitable.
Legend of the immutable prop
To implement a simple MVVM pattern in React, we can define the props or state for the view as the properties the view model is intended to expose. The parent component (or rendering component) passes the properties of a new instance of the view model down to the view, where the state of the new view model is an appropriately updated version of its previous state.
This would in turn force us to make the parent component in some way aware of the properties of the view model for each of its children. We can do this by organizing our view models hierarchically like we do with our views in React: every view model is composed of its inner or child view models. The child view then reads the state of the view model and wires up its controls to its methods as appropriate. We then force a re-render of the parent any time the state of the view model updates as the model changes.
And therein lies the problem.
Because the parent view was re-rendered due to a change in the state of the view model, which is other than its state and the props, it is no longer a function on props and state. Worse yet, it is now a function on the state of the view model which makes its output not deterministic on its input.
This basically does violence to both React and MVVM: it does violence to React as it is necessary to force a re-render on the parent component that is independent of props and state. It does violence to MVVM as it requires a view different from the one the view model is intended for: the parent view of that view. Moreover, the parent view now has to basically be aware of the properties of all of its children. A
props based approach, even where these are immutably renewed whenever necessary, does not respect React component semantics.
State of the art
In order to implement MVVM in React in a way that does not directly violate the principle that a React component should be a pure function on props and state, it is necessary to use
Match a view with a view model implementation, make it subscribe (via your favorite subscription pattern) to changes in the state of the view model. Whenever the view model properties update, queue a state update on the React component, picking each property you are interested in from your view model. Something like so:
Basically, we can explicitly use the component private state to queue a re-render in a natural manner, without forcing one. Note that, in this implementation, the whole view model instance that is going to be used throughout the lifetime of the component is passed down as a prop by the parent, not its individual properties or the next immutable instance of it.
The view model is a bundle of state to the view, so if the result of rendering the component is dependent on a stateful prop (the view model) then the render function cannot be pure. The approach proposed here where
state is leveraged as a way to bind a view to a view model is, however, compatible with React semantics. Does this approach stretch the intended use of
state, however, by wiring it up in such a manner to the rest of the application?
- The React docs, https://reactjs.org/docs/hello-world.html
- L. Bain. ReactJS: Props vs. State. 2017.
Note: not to be confused with the idea that a React component is called “pure” if it has no
state, whenever the
rendermethod of the class, or the function that represents the component is called, its output is meant to be entirely determined by the props and state that have been passed in. In this sense React components are pure functions on props and state. ↩