MobX-utils

Utility functions and common patterns for MobX

This package provides utility functions and common MobX patterns build on top of MobX. It is encouraged to take a peek under the hood and read the sources of these utilities. Feel free to open a PR with your own utilities. For large new features, please open an issue first.

Installation

NPM: npm install mobx-utils --save

CDN: https://unpkg.com/mobx-utils/mobx-utils.umd.js

API

fromPromise

lib/from-promise.js:103-107

fromPromise takes a Promise and returns an object with 3 observable properties that track the status of the promise. The returned object has the following observable properties:

• value: either the initial value, the value the Promise resolved to, or the value the Promise was rejected with. use .state if you need to be able to tell the difference
• state: one of "pending", "fulfilled" or "rejected"
• promise: (not observable) the original promise object and the following method:
• case({fulfilled, rejected, pending}): maps over the result using the provided handlers, or returns undefined if a handler isn't available for the current promise state.

Parameters

• promise IThenable<T> The promise which will be observed
• initialValue [T] Optional predefined initial value (optional, default undefined)
• modifier [any] MobX modifier, e.g. asFlat, to be applied to the resolved value (optional, default IDENTITY)

Examples

const fetchResult = fromPromise(fetch("http://someurl"))

// combine with when..
when(
  () => fetchResult.state !== "pending"
  () => {
    console.log("Got ", fetchResult.value)
  }
)

// or a mobx-react component..
const myComponent = observer(({ fetchResult }) => {
  switch(fetchResult.state) {
     case "pending": return <div>Loading...</div>
     case "rejected": return <div>Ooops... {fetchResult.value}</div>
     case "fulfilled": return <div>Gotcha: {fetchResult.value}</div>
  }
})

// or using the case method instead of switch:

const myComponent = observer(({ fetchResult }) =>
  fetchResult.case({
    pending:   () => <div>Loading...</div>
    rejected:  error => <div>Ooops.. {error}</div>
    fulfilled: value => <div>Gotcha: {value}</div>
  }))

Note that the status strings are available as constants:
`mobxUtils.PENDING`, `mobxUtils.REJECTED`, `mobxUtil.FULFILLED`

Returns IPromiseBasedObservable<T>

lazyObservable

lib/lazy-observable.js:37-58

lazyObservable creates an observable around a fetch method that will not be invoked util the observable is needed the first time. The fetch method receives a sink callback which can be used to replace the current value of the lazyObservable. It is allowed to call sink multiple times to keep the lazyObservable up to date with some external resource.

Note that it is the current() call itself which is being tracked by MobX, so make sure that you don't dereference to early.

Parameters

• fetch
• initialValue [T] optional initialValue that will be returned from current as long as the sink has not been called at least once (optional, default undefined)
• modifier [any] optional mobx modifier that determines the the comparison and recursion strategy of the observable, for example asFlat or asStructure (optional, default IDENTITY)

Examples

const userProfile = lazyObservable(
  sink => fetch("/myprofile").then(profile => sink(profile))
)

// use the userProfile in a React component:
const Profile = observer(({ userProfile }) =>
  userProfile.current() === undefined
  ? <div>Loading user profile...</div>
  : <div>{userProfile.current().displayName}</div>
)

// triggers refresh the userProfile
userProfile.refresh()

fromResource

lib/from-resource.js:67-101

fromResource creates an observable which current state can be inspected using .current(), and which can be kept in sync with some external datasource that can be subscribed to.

The created observable will only subscribe to the datasource if it is in use somewhere, (un)subscribing when needed. To enable fromResource to do that two callbacks need to be provided, one to subscribe, and one to unsubscribe. The subscribe callback itself will receive a sink callback, which can be used to update the current state of the observable, allowing observes to react.

Whatever is passed to sink will be returned by current(). The values passed to the sink will not be converted to observables automatically, but feel free to do so. It is the current() call itself which is being tracked, so make sure that you don't dereference to early.

For inspiration, an example integration with the apollo-client on github

The following example code creates an observable that connects to a dbUserRecord, which comes from an imaginary database and notifies when it has changed.

Parameters

• subscriber
• unsubscriber [IDisposer] (optional, default NOOP)
• initialValue [T] the data that will be returned by get() until the sink has emitted its first data (optional, default undefined)

Examples

function createObservableUser(dbUserRecord) {
  let currentSubscription;
  return fromResource(
    (sink) => {
      // sink the current state
      sink(dbUserRecord.fields)
      // subscribe to the record, invoke the sink callback whenever new data arrives
      currentSubscription = dbUserRecord.onUpdated(() => {
        sink(dbUserRecord.fields)
      })
    },
    () => {
      // the user observable is not in use at the moment, unsubscribe (for now)
      dbUserRecord.unsubscribe(currentSubscription)
    }
  )
}

// usage:
const myUserObservable = createObservableUser(myDatabaseConnector.query("name = 'Michel'"))

// use the observable in autorun
autorun(() => {
  // printed everytime the database updates its records
  console.log(myUserObservable.current().displayName)
})

// ... or a component
const userComponent = observer(({ user }) =>
  <div>{user.current().displayName}</div>
)

createViewModel

lib/create-view-model.js:127-129

createViewModel takes an object with observable properties (model) and wraps a viewmodel around it. The viewmodel proxies all enumerable property of the original model with the following behavior:

• as long as no new value has been assigned to the viewmodel property, the original property will be returned.
• any future change in the model will be visible in the viewmodel as well unless the viewmodel property was dirty at the time of the attempted change.
• once a new value has been assigned to a property of the viewmodel, that value will be returned during a read of that property in the future. However, the original model remain untouched until submit() is called.

The viewmodel exposes the following additional methods, besides all the enumerable properties of the model:

• submit(): copies all the values of the viewmodel to the model and resets the state
• reset(): resets the state of the viewmodel, abandoning all local modifications
• resetProperty(propName): resets the specified property of the viewmodel
• isDirty: observable property indicating if the viewModel contains any modifications
• isPropertyDirty(propName): returns true if the specified property is dirty
• model: The original model object for which this viewModel was created

You may use observable arrays, maps and objects with createViewModel but keep in mind to assign fresh instances of those to the viewmodel's properties, otherwise you would end up modifying the properties of the original model. Note that if you read a non-dirty property, viewmodel only proxies the read to the model. You therefore need to assign a fresh instance not only the first time you make the assignment but also after calling reset() or submit().

Parameters

• model T

Examples

class Todo {
  \@observable title = "Test"
}

const model = new Todo()
const viewModel = createViewModel(model);

autorun(() => console.log(viewModel.model.title, ",", viewModel.title))
// prints "Test, Test"
model.title = "Get coffee"
// prints "Get coffee, Get coffee", viewModel just proxies to model
viewModel.title = "Get tea"
// prints "Get coffee, Get tea", viewModel's title is now dirty, and the local value will be printed
viewModel.submit()
// prints "Get tea, Get tea", changes submitted from the viewModel to the model, viewModel is proxying again
viewModel.title = "Get cookie"
// prints "Get tea, Get cookie" // viewModel has diverged again
viewModel.reset()
// prints "Get tea, Get tea", changes of the viewModel have been abandoned

whenWithTimeout

lib/guarded-when.js:32-51

Like normal when, except that this when will automatically dispose if the condition isn't met within a certain amount of time.

Parameters

• expr
• action
• timeout [number] maximum amount when spends waiting before giving up (optional, default 10000)
• onTimeout [any] the ontimeout handler will be called if the condition wasn't met within the given time (optional, default ())

Examples

test("expect store to load", t => {
  const store = {
    items: [],
    loaded: false
  }
  fetchDataForStore((data) => {
    store.items = data;
    store.loaded = true;
  })
  whenWithTimeout(
    () => store.loaded
    () => t.end()
    2000,
    () => t.fail("store didn't load with 2 secs")
  )
})

Returns IDisposer disposer function that can be used to cancel the when prematurely. Neither action or onTimeout will be fired if disposed

keepAlive

lib/keep-alive.js:31-36

MobX normally suspends any computed value that is not in use by any reaction, and lazily re-evaluates the expression if needed outside a reaction while not in use. keepAlive marks a computed value as always in use, meaning that it will always fresh, but never disposed automatically.

Parameters

• target Object an object that has a computed property, created by @computed or extendObservable
• property string the name of the property to keep alive
• _1
• _2

Examples

const obj = observable({
  number: 3,
  doubler: function() { return this.number * 2 }
})
const stop = keepAlive(obj, "doubler")

Returns IDisposer stops this keep alive so that the computed value goes back to normal behavior

keepAlive

lib/keep-alive.js:31-36

Parameters

• computedValue IComputedValue<any> created using the computed function
• _1
• _2

Examples

const number = observable(3)
const doubler = computed(() => number.get() * 2)
const stop = keepAlive(doubler)
// doubler will now stay in sync reactively even when there are no further observers
stop()
// normal behavior, doubler results will be recomputed if not observed but needed, but lazily

Returns IDisposer stops this keep alive so that the computed value goes back to normal behavior

queueProcessor

lib/queue-processor.js:22-40

queueProcessor takes an observable array, observes it and calls processor once for each item added to the observable array, optionally deboucing the action

Parameters

• observableArray Array<T> observable array instance to track
• processor
• debounce [number] optional debounce time in ms. With debounce 0 the processor will run synchronously (optional, default 0)

Examples

const pendingNotifications = observable([])
const stop = queueProcessor(pendingNotifications, msg => {
  // show Desktop notification
  new Notification(msg);
})

// usage:
pendingNotifications.push("test!")

Returns IDisposer stops the processor

chunkProcessor

lib/chunk-processor.js:27-52

chunkProcessor takes an observable array, observes it and calls processor once for a chunk of items added to the observable array, optionally deboucing the action. The maximum chunk size can be limited by number. This allows both, splitting larger into smaller chunks or (when debounced) combining smaller chunks and/or single items into reasonable chunks of work.

Parameters

• observableArray Array<T> observable array instance to track
• processor
• debounce [number] optional debounce time in ms. With debounce 0 the processor will run synchronously (optional, default 0)
• maxChunkSize [number] optionally do not call on full array but smaller chunks. With 0 it will process the full array. (optional, default 0)

Examples

const trackedActions = observable([])
const stop = chunkProcessor(trackedActions, chunkOfMax10Items => {
  sendTrackedActionsToServer(chunkOfMax10Items);
}, 100, 10)

// usage:
trackedActions.push("scrolled")
trackedActions.push("hoveredButton")
// when both pushes happen within 100ms, there will be only one call to server

Returns IDisposer stops the processor