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Performance Management

Included are several utilities for you to manage the performance of your React app. The strategy is simple: avoid work where possible.

These utilities help you avoid work in two ways.

  1. By making components' shouldComponentUpdate fns both easy to create, and accurate (safe). If it compiles, the logic in shouldComponentUpdate will be what you expect.
  2. By allowing you to cache your own arbitrary data, and build on it in a way such that derivative data is also cached efficiently.

Reusability/Reusable is part of core (as of v1.4.0) where as the other utilities are part of the extra module.

libraryDependencies += "com.github.japgolly.scalajs-react" %%% "extra" % "1.7.7"

Contents

Reusability

Reusability is a typeclass that tests whether one instance can be used in place of another. It's used to compare properties and state of a component to avoid unnecessary updates.

An implicit instance of Reusability[A] means that reusability can be calculated for the entire type of A, all of its values.

Imagine a class with 8 fields - typical equality like == would compare all 8 fields (and if you're not using scalaz.Equal you've no way of knowing whether all those 8 fields have correct equals methods defined). When deciding whether a component needs updating, full equality comparison can be overkill (and slow) - in many cases it is sufficient to check only the ID field, the update-date, or the revision number. Reusability is designed for you to do just that.

Usage

When building your component, pass in Reusability.shouldComponentUpdate to .configure on the component builder.

It will not compile until it knows how to compare the reusability of your props and state. Out-of-the-box, it knows how to compare Scala primitives, Strings, Option, Either, Scala tuples, js.UndefOr, Scala and JS Dates, UUIDs, Sets, Lists, Vectors, and Scalaz classes \/ and \&/. For all other types, you'll need to teach it how. Use one of the following methods:

  • Reusability((A, B) => Boolean) to hand-write custom logic.
  • Reusability.by_== uses universal equality (ie. a1 == a2).
  • Reusability.byRef uses reference equality (ie. a1 eq a2).
  • Reusability.byRefOr_== uses reference equality and if different, tries universal equality.
  • Reusability.derive for ADTs of your own.
  • Reusability.deriveDebug as above, but with info useful for debugging (see below)
  • Reusability.caseClassExcept for case classes of your own where you want to exclude some fields.
  • Reusability.caseClassExceptDebug as above, but with info useful for debugging (see below)
  • Reusability.by(A => B) to use a subset (B) of the subject data (A).
  • Reusability.byIterator uses an Iterable's iterator to check each element in order.
  • Reusability.indexedSeq uses .length and .apply(index) to check each element in order.
  • Reusability.{double,float} exist and require a tolerance to be specified.
  • Reusability.{always,never,const(bool)} for a hard-coded reusability decision.

If you're using the Scalaz module, you also gain:

  • Reusability.byEqual uses a Scalaz Equal typeclass.
  • Reusability.byRefOrEqual uses reference equality and if different, tries using a Scalaz Equal typeclass.

If you're using the cats module you also have similar methods, but with the suffix byEq and using the equivalent Eq typeclass.

Example

The following component will only re-render when one of the following change:

  • props.name
  • props.age
  • props.pic.id
case class Picture(id: Long, url: String, title: String)
case class Props(name: String, age: Option[Int], pic: Picture)

implicit val picReuse   = Reusability.by((_: Picture).id)  // ← only check id
implicit val propsReuse = Reusability.derive[Props]        // ← check all fields

val component = ScalaComponent.builder[Props]("Demo")
  .render_P(p =>
    <.div(
      <.p("Name: ", p.name),
      <.p("Age: ", p.age.fold("Unknown")(_.toString)),
      <.img(^.src := p.pic.url, ^.title := p.pic.title))
  )
  .configure(Reusability.shouldComponentUpdate)                 // ← hook into lifecycle
  .build

Alternatively, picReuse could be written using caseClassExcept as follows.

// Not natural in this case but demonstrates how to use caseClassExcept
implicit val picReuse = Reusability.caseClassExcept[Picture]('url, 'title)

You can peek into reusability calculation by wrapping it with a logger:

implicit val loggedPicReuse =
    Reusability.caseClassExcept[Picture]('url, 'title).logNonReusable

Deriving instances with debug info

There are debug versions of the derivation macros:

  • Reusability.deriveDebug
  • Reusability.caseClassExceptDebug

Both methods accept the following parameters:

  • logNonReuse: Boolean - logs to the console details about when/why reusability fails (example below)
  • logCode: Boolean - prints the generated code to the screen during compilation so you can see exactly what's generated. If you're a macros=tinfoil-time guy/gal you could run this once and copy/paste the generated implementation into your codebase.

Unfortunately Scala doesn't accept named or default arguments so you can't call them like this:

// Scala doesn't allow this unfortunately
implicit val reusability: Reusability[Person] =
  Reusability.deriveDebug[Person](logCode = true)

We just have to have an annoying little syntax degradation:

// Ok: no args means logNonReuse & logCode are both true
implicit val reusability: Reusability[Person] =
  Reusability.deriveDebug[Person]

// Ok: here we specify logNonReuse=false & logCode=true
implicit val reusability: Reusability[Person] =
  Reusability.deriveDebug[Person](false, true)

// Even without arguments, Scala will reject this :(
implicit val reusability: Reusability[Person] =
  Reusability.deriveDebug[Person](logNonReuse = false, logCode = true)

logNonReuse is quite useful so here's an example. If you used it to derive an instance for case class Person(id: Int, name: String, age: Int) and passed in Person(1, "Bob", 80) and Person(3, "Bob", 60) you would see this in your console:

Instance of japgolly.scalajs.react.example.Person not-reusable for the following reasons:
  .id values not reusable
    A: 1
    B: 3
  .age values not reusable
    A: 80
    B: 60

Monitoring

There exist two mixins, out-of-the-box, to help you monitor reusability. Use them instead of shouldComponentUpdate.

  1. ReusabilityOverlay.install - Adds an overlay beside each mounted instance of the component, showing how many updates were prevented and how many were rendered. You can hover over it for some detail, and click it to print more to the JS console. Live demo.
  2. shouldComponentUpdateAndLog - Logs each callback evaluation to the console.

Usage:

// No monitoring
.configure(Reusability.shouldComponentUpdate)

// Display stats on screen, clickable for detail
.configure(ReusabilityOverlay.install)

// Log to console
.configure(Reusability.shouldComponentUpdateAndLog("MyComponent"))

You can also call .logNonReusable on any reusability instance to get a new reusability which emits a warning about non-reusability to aid quick debugging.

Global Monitoring

If you're in dev-mode (i.e. you're using fastOptJS and not fullOptJS) you can also install some kind of monitoring globally. The ScalaJsReactConfig.DevOnly.overrideReusability settings allows you to override the behaviour of Reusability.shouldComponentUpdate. Because it only affects fastOptJS, it's removed at compile-time from fullOptJS.

The most common use case is that during development you can call ReusabilityOverlay.overrideGloballyInDev() at the start of your JS main method which will provide a little GUI around all components with Reusability.shouldComponentUpdate that shows you went Reusability prevented updates, when it re-rendered, and why (either by hovering over the overlay, or clicking it to get a full log in the console). You can also call ScalaJsReactConfig.DevOnly.overrideReusability yourself and provide your own implementation if desirable.

Reusable

Note: if the following doesn't make much sense to you, try watching this talk: https://www.youtube.com/watch?v=9RvWDDZK3D8 which builds up to Reusable and its purpose

There are a cases when you cannot universally define reusability for all inhabitants of a type, but you can for values that you produce. Enter: Reusable[A]

Reusable[A] is a single value of A with reusability. It promises that whoever provides the value will also specify the value's reusability. Where as Reusability[A] is for ∀a∈A, and Reusable[A] is for ∃a∈A.

Usage

To create a Reusable value, use one of the following methods:

  • Reusable(A)((A, A) => Boolean) to hand-write custom logic.
  • Reusable.implicitly when there is an implicit Reusability instance available.
  • Reusable.explicitly when to explicitly provide a Reusability instance.
  • Reusable.by_== uses universal equality (ie. a1 == a2).
  • Reusable.byRef uses reference equality (ie. a1 eq a2).
  • Reusable.byRefOr_== uses reference equality and if different, tries universal equality.
  • Reusable.callback{,Option}ByRef uses reference equality over Callback and CallbackOption
  • Reusable.byRefIso - Compare by reference through an isomorphism
  • Reusable.{always,never,const(bool)} for a hard-coded reusability decision.

Mapping without affecting reusability

A common use case is to have a non-reusable value deterministically derived from a reusable seed value.

Reusable instances have a .map function that lazily maps the value without affecting its reusability.

Example below.

Examples

// Explicit reusability
val i: Reusable[Int] =
  Reusable(readSensor)((a, b) => Math.abs(a - b) < 100)

// Variable that's reusable each time it's used without an update
var statusVdom: ReusableVal[VdomElement] =
  Reusable.byRef(<.h1("Initialising..."))
// Example update:
def enterReadyState: Unit =
  statusVdom = Reusable.byRef(<.h1("Ready."))

// VDOM reusable based on seed Int
def renderCalculation(n: Int): Reusable[VdomElement] =
  Reusable.implicitly(n).map { n =>
    val result = expensiveCalculation(n)
    <.div("n=$n, result=$result")
  }

Reusable functions

In React, components typically pass callbacks to their children via component properties. Due to the ease of function creation in Scala it is often the case that functions are created inline and thus provide no means of determining whether a component can safely skip its update.

The solution here is to use Reusable[A => B] (shorthand: A ~=> B). It is a function that has been created in such a way that allows it to be both reused, and curried in a way that allows reuse.

Usage

  1. Just wrap Reusable.fn around your function.
  2. Store the Reusable.fn as a val somewhere outside of your render function, usually in the body of your backend class.
  3. Replace the callback (say A => B) in components' props, to take a A ~=> B.
  4. Treat the A ~=> B as you would a normal function.

For three or more arguments the result of Reusable.fn(…) is curried (or Schönfinkel'ed!), and each curried argument must have Reusability. Eg. Reusable.fn((A, B) => C) returns a A ~=> (B ~=> C).

Example

In this example personEditor will only rerender if props.name changes, or the curried PersonId in its props.update function changes (which it won't - observable from the code).

type State = Map[PersonId, PersonData]
type PersonId = Long
type PersonData = String

val topComponent = ScalaComponent.builder[State]("Demo")
  .initialStateFromProps(identity)
  .renderBackend[Backend]
  .build

class Backend(bs: BackendScope[_, State]) {

  val updateUser = Reusable.fn((id: PersonId, data: PersonData) =>         // ← Create a 2-arg fn
    bs.modState(_.updated(id, data)))

  def render(state: State) =
    <.div(
      state.toVdomArray { case (id, name) =>
        personEditor.withKey(id.toString)(PersonEditorProps(name, updateUser(id))) // ← Apply 1 arg
      }
    )
}

case class PersonEditorProps(name: String, update: String ~=> Callback)   // ← Notice the ~=>

implicit val propsReuse = Reusability.derive[PersonEditorProps]

val personEditor = ScalaComponent.builder[PersonEditorProps]("PersonEditor")
  .render_P(p =>
    <.input.text(
      ^.onChange ==> ((e: ReactEventFromInput) => p.update(e.target.value)),
      ^.value := p.name))                                                // ← Use as normal
  .configure(Reusability.shouldComponentUpdate)                          // ← shouldComponentUpdate like magic
  .build

WARNING!

DO NOT feed the Reusable.fn(...) constructor a function directly derived from a component's props or state. Access to props/state on the right-hand side of the function args is ok but if the function itself is a result of the props/state, the function will forever be based on data that can go stale.

Example:

@Lenses case class Person(name: String, age: Int)
case class Props(person: StateSnapshot[Person], other: Other)

// THIS IS BAD!!
Reusable.fn($.props.runNow().person setStateL Props.name)

// It is equivalent to:
val g: String => Callback  = $.props.runNow().person setStateL Person.name // ← $.props is evaluated once here
val f: String ~=> Callback = Reusable.fn(g)                                // ← …and never again.

Alternatives:

Ensure the scope is only used on the right-hand side of the function:

Reusable.fn(str => $.props.flatMap(_.person.setStateL(Person.name)(str)))

Tricks

To cater for some common use cases, there are few convenience methods that are useful to know. For these examples imagine $ to be your component's scope instance, eg. BackendScope[P, S], StateAccessPure[S] or similar.

  1. Reusable.fn.state($).{set,mod}.

    You'll find that if you try Reusable.fn($.xxxState) Scala will fail to infer the correct types. Use Reusable.fn.state($).xxx instead to get the types that you expect.

    Example: instead of Reusable.fn($.setState) use Reusable.fn.state($).set and you will correctly get a S ~=> Callback.

  2. Reusable.fn.state($ zoomStateL lens)

Lenses provide an abstraction over read-and-write field access. Using Monocle, you can annotate your case classes with @Lenses to gain automatic lenses. $ zoomStateL lens will then narrow the scope of its state to the field targeted by the given lens. This can then be used with Reusable.fn.state as follows:

@Lenses
case class Person(name: String, age: Int)

class Backend($: BackendScope[_, Person]) {

  val nameSetter: String ~=> Callback =
    Reusable.fn.state($ zoomStateL Person.name).set

StateSnapshot with Reusability

StateSnapshot is supports reusability. Begin with StateSnapshot.withReuse.

  • StateSnapshot.withReuse(s)(reusable setStateFn) - Provide a current value, and update function manually.
  • StateSnapshot.withReuse.prepare(setStateFn) - Provides a reusable S => StateSnapshot[S] with a stable update function. This should be stored in a component backend, as a val, and reused with different values of S.
  • StateSnapshot.withReuse.prepareVia($) - As above but gets the update function from $ which is usually a BackendScope.
  • StateSnapshot.withReuse.zoom(…) - Zooms into a subset of the total state. For example, you could create a StateSnapshot[Age] from Person.
    • StateSnapshot.withReuse.zoom(…).prepare(setStateFn)
    • StateSnapshot.withReuse.zoom(…).prepareVia($)

Example

@Lenses case class State(name: String, desc: String)

class Backend(bs: BackendScope[State, State]) {
  val ssName = StateSnapshot.withReuse.zoomL(State.name).prepareVia(bs)
  val ssDesc = StateSnapshot.withReuse.zoomL(State.desc).prepareVia(bs)

  def render(s: State) =
    <.div(
      stringEditor(ssName(s)),
      stringEditor(ssDesc(s)))
}

val topComponent = ScalaComponent.builder[State]("Demo")
  .initialState_P(identity)
  .renderBackend[Backend]
  .build

lazy val stringEditor = ScalaComponent.builder[StateSnapshot[String]]("StringEditor")
  .render_P(p =>
    <.input.text(
      ^.value := p.value,
      ^.onChange ==> ((e: ReactEventFromInput) => p.setState(e.target.value))))
  .configure(Reusability.shouldComponentUpdate)
  .build

Px

Px is a mechanism for caching data with dependencies. It's basically a performance-focused, lightweight implementation of pull-based FRP; pull-based meaning that in the chain A→B→C, an update to A doesn't affect C until the value of C is requested. Values are only compared when they are set or modified. When data is retrieved, only the revision number (an integer) is compared to determine if an update is required.

NOTE: Px does not have Reusability. Details below.

What does Px mean? I don't know, I just needed a name and I liked the way @lihaoyi's Rx type name looked in code. You can consider this "Performance eXtension". If this were Java it'd be named AutoRefreshOnRequestDependentCachedVariable.

Initial instances

DSL

Px comes in two flavours: those with reusable values, and those without. If its values are reusable then when its underlying value A changes, it will compare the new A value to the previous A (using Reusability[A]) and discard the change if it can. If its values are reusable, all changes to the underlying value (including duplicates) are accepted.

Create a non-derivative Px using one of these:

  1. Manual Update - A variable in the traditional sense.

Doesn't change until you explicitly call .set().

val num = Px(123).withReuse.manualUpdate
num.set(666)
  1. Manual Refresh - The value of a zero-param function. The value will not update until you explicitly call refresh().
case class State(name: String, age: Int)

class ComponentBackend($: BackendScope[User, State]) {

  val user     = Px.props($).withReuse.manualRefresh
  val stateAge = Px.state($).map(_.age).withReuse.manualRefresh

  def render: VdomElement = {
    // Every render cycle, refresh Pxs. Unnecessary changes will be discarded.
    // This is a shortcut for:
    //   user.refresh(); stateAge.refresh()
    Px.refresh(user, stateAge)

    <.div(
      "Age is ", stateAge.value(),
      UserInfoComponent(user),
      SomeOtherComponent(user, stateAge))
  }
}
  1. Auto Refresh - The value of a zero-param function. The function will be called every time the value is requested, and the value updated if necessary.
// Suppose this is updated by some process that periodically pings the server
object InternalGlobalState {
  var usersOnline = 0
}

class ComponentBackend($: BackendScope[Props, _]) {
  val usersOnline = Px(InternalGlobalState.usersOnline).withReuse.autoRefresh

  // Only updated when the InternalGlobalState changes
  val coolGraphOfUsersOnline: Px[VdomElement] =
    for (u <- usersOnline) yield
      <.div(
        <.h3("Users online: ", u),
        coolgraph(u))

  def render(p: Props): VdomElement =
    <.div(
      "Hello ", p.username,
      coolGraphOfUsersOnline.value())
}
  1. Constants - Px.constByValue(A) & Px.constByNeed(=> A) create constant values.

These Pxs do not have the ability to change.

Derivative instances

Derivative Pxs are created by:

  • calling .map
  • calling .flatMap
  • using in a for comprehension
  • using Px.applyn

Example:

val project     : Px[Project]      = Px.props($).withReuse.manualRefresh
val viewSettings: Px[ViewSettings] = Px.state($).map(_.viewSettings).withReuse.manualRefresh

// Using .map
val columns   : Px[Columns]    = viewSettings.map(_.columns)
val textSearch: Px[TextSearch] = project map TextSearch.apply

// Using Px.applyn
val widgets: Px[Widgets] = Px.apply2(project, textSearch)(Widgets.apply)

// For comprehension
val rows: Px[Rows] =
  for {
    vs <- viewSettings
    p  <- project
    ts <- textSearch
  } yield
    new Rows(vs, p, ts.index)

// column.value()     will only change when viewSettings.refresh() is called and its state changes.
// textSearch.value() will only change when project.refresh() is called and the project changes.
// widgets.value()    will only change when either project or textSearch changes.
// rows.value()       will only change when viewSettings, project or textSearch changes.

Px doesn't have Reusability

For Reusability to work it needs to compare two immutable values; Px is mutable.

If you have (a: Px[T], b: Px[T]) you might assume that if they are the same by reference equality (a eq b) and the revisions line up then they have Reusability. No. A Px is useless unless you call .value() and it's these values you would need to compare in shouldComponentUpdate. Comparing a.value() and b.value() will not work because .value() always returns the latest value; you would need to know which value in its history was seen by your component.

In short, do not use Px in a component's props or state. Instead of Px[A], just use the A.

// BAD!
case class Component2Props(count: Px[Int])
class Component1Backend {
  val px: Px[Int] = ...
  def render: VdomElement =
    Component2(Component2Props(px))
}

// Good
case class Component2Props(count: Int)
class Component1Backend {
  val px: Px[Int] = ...
  def render: VdomElement =
    Component2(Component2Props(px.value()))
}

There's also a convenience import in Px.AutoValue that avoids the need to call .value() on your Pxs, if you're into that kind of thing.

// Also good
import Px.AutoValue._

case class Component2Props(count: Int)
class Component1Backend {
  val px: Px[Int] = ...
  def render: VdomElement =
    Component2(Component2Props(px))  // .value() called implicitly
}

React.Profiler

React provides React.Profiler as a tool for you to (manually) measure selected parts of your application. From the official doc:

The Profiler measures how often a React application renders and what the "cost" of rendering is. Its purpose is to help identify parts of an application that are slow and may benefit from optimizations such as memoization.

Note: Profiling adds some additional overhead, so it is disabled in the production build. To opt into production profiling, React provides a special production build with profiling enabled. Read more about how to use this build at fb.me/react-profiling

Using React.Profiler in scalajs-react is mostly the same as using it in plain JS. For example, let's say you wanted to profile the following:

def render =
  App(
    Navigation(navProps),
    Main(mainProps)
  )

You could profile Navigation and Main like this:

private def onRender(data: React.Profiler.OnRenderData) = Callback {
  println(s"${data.id} took ${data.actualDuration.toMillis} ms")
}

def render =
  App(
    React.Profiler("Navigation", onRender)(
      Navigation(navProps)
    ),
    React.Profiler("Main", onRender)(
      Main(mainProps)
    )
  )

Experimental API

Some additional unstable React Profiler API has been exposed in the scalajs-react facade. Note that the methods below may be changed without warning:

  • React.Profiler.unstable_trace(name)(body) - Executes the given body in a "zone" that is detected by the Profiler and returned to you in the interactions value of the callback.

    Example:

    def onRed =
  React.Profiler.unstable_trace("Red") {
    doExpensiveThing()
  }

def onBlue =
  React.Profiler.unstable_trace("Blue") {
    doExpensiveThing()
  }

private def onRender(data: React.Profiler.OnRenderData) = Callback {
  if (data.interactions.exists(_.name == "Red"))
    println(s"Red path ${data.id} took ${data.actualDuration.toMillis} ms")
}