Idea: introduce abstraction for computeds with arguments

See https://github.com/mobxjs/mobx/issues/1388#issuecomment-472748989 and https://github.com/mobxjs/mobx/issues/1388#issuecomment-472751685. This could potentially replace createTransformer as well.

Signature: memoizedComputed<T>(expression: T, invalidationStrategy?: "auto" | "never" | number = "auto"): T

strategies:

• "auto": remove entries from the memoization table as soon as they become unobserved
• "never": we don’t care, cache forever (that is, until the computeds and all things they depend on are GC-ed)
• number: remove entries after they have not been hit a the specified amount of time. Probably not needed in a first versoin.

Example:

class Todos {
  @observable todos = []
  
  private todosByUserCache = new Map()

  getAllTodosByUser = memoizedComputed((userId) => {
    if (this.todosByUserCache.has(userId))
      return this.todosByUserCache.get(userId).get()
      
    const computedFilter = computed(() => this.todos.filter(todo => todo.user === userId))
    this.todosByUserCache.set(userId, computedFilter)
    return todosByUserCache.get()
  })
}

const todos = new Todos()
const myTodos: Todo[] = todos.getAllTodosByUser("17")

[mweststrate]

Released as 5.4.0!

[dreamair]

For one-object-argument functions (a User object instead of just the userId in your example) a WeakMap could help automatically cleaning up entries with the argument-object being GCed. It would be like an “extension” to the User indirectly holding a computed member for all his todos without depending on them. I don’t see this working for multiple arguments or primitives though.