Mobx: Simple, scalable state management

seems like a very convoluted and inefficient way to have getter and setters without saying those words

The concept of tracking data is actually quite simple — when you define some object property as observable it is replaced with a getter & setter that invoke a callback when you get/set that property. Then you can bind a function that will be called when data is changed (via setter).

You can also wrap some code into special autorun function, that will track what observable data was accessed, and when that data changes, that code will be called again.

But wait, there's more =) It works with arrays, objects and other types, check out the docs!

can you elaborate more why this approach is convoluted?

"With mutable data structures, it is trivial to guarantee that there is only one version of a certain domain object in memory."

That statement refers to the fact that you loose (automatic) referential integrity when you start using Immutable data.

Take the following scenario: you have an app with Users and Tasks. Tasks can be assigned to users. When you express this using immutable data you have two problems. The first one is linking tasks to users. If you would link Task1 to Person1 and after that change the name of Person1, you would get a new person, Person1v2. However Task1 would still refer to the old, unmodified Person1, so then you have suddenly two versions of the same person in memory. A fresh and a stale one. In other words, you lost referential integrity.

Now the usual way to fix is, is to not use real references, but to normalize data and store only a key to the person. The effect of this is that you have to always lookup the person related to Task1 again from the state tree. If you have a reference (variable) to that person somewhere, make sure it is short-lived because you won't know if you are still referring to the correct one.

The second problem is that when you have correctly normalized your data and always do fresh lookups, you will still not know _when_ to make these lookups. If you have a TaskView that renders the name of the related Person, and the related person changes, the TaskView will not detect this automatically. The related person is not even a prop of the TaskView component so pushing a new state tree through your app won't repaint the TaskView if you are using PureRenderMixin (which is usually the purpose of using immutable state trees in the first place). For that issue there is a solution as well; you can introduce selectors (or lenses) that query the state tree to detect if the relevant person is changed. So now we already have three new concepts (no refs, normalization, lookups, selectors) to solve a problem that mutable data doesn't have in the first place.

By using mutable data and observing it using MobX these problems are solved (imho) more elegantly: references are never stale because if you would modify Person1, Task1 would still refer to the 'latest' person1, because it is still the same object. Secondly, because it is the same object, fine grained object observers can be established automatically for you, making sure the TaskView gets updated as soon as something relevant changes. This means that you have less concepts to learn and maintain (no copy-on-write, no need to assign a unique key to everything, no data normalization, no lookups, no selectors to make sure your views are always in sync with the state). That saves a significant amount of boring yet error prone work when your application's state model becomes more complex than the average Todo app.

So that is the long story behind that short comment. I hope it clarifies the statement!

There is no way for me to succinctly express how great I think this project is but let me try. IMHO this is the most sophisticated yet simple solution to state management for web apps out there. I have worked on several large Angular and React apps and worked with several flux frameworks. I have also written my own Flux like architecture seeking to improve and simplify state management by reducing boilerplate and increasing performance, so I have thought about all the issues involved a lot.

It is one thing to put together a set of observations like the Flux architecture that makes a lot of sense and simplifies app development. It is entirely another thing, as we have seen in the past couple years, to implement an elegantly simple and clean expression of that. It is deceivingly simple but in my experience even though it seems to have a bit of magic behind it (which I generally dislike) Mobservable now MobX is probably the simplest and ultimate progression of state management for React and other like apps. It is the reason I stopped development on my own state management system. Once I envisioned how to remove even more boilerplate even more odd conventions I came to realize Mobservable achieved it all, and the performance was just as good too. It really deserves the attention of any serious app developer and it's not limited to React either.