Aide-mémoire Vue de The Dark Side | Réactivité des pièces

Bonjour à la communauté DEV.to !

Cet article inclura plusieurs aspects de Vue 3 qui sont principalement utilisés ou qui sont plutôt du côté obscur et auxquels on ne prête pas attention comme ils sont censés le faire.

Comme je vais décrire Vue 3, je vais utiliser l'API de composition et non l'API d'options à l'ancienne. Le concept des deux méthodes est le même, vous pouvez donc vous adapter assez rapidement à l'API de composition. Je ne suis pas en mesure de dicter quoi que ce soit et chaque programmeur est libre de choisir la façon dont il souhaite écrire son programme mais à titre personnel, je préfère l'API de composition pour sa syntaxe concise et sa meilleure gestion du code. Donc, si vous avez toujours peur de passer à l'API de composition, je vous suggère d'essayer car cela en vaudra la peine.

C'est ce que vous ressentirez après avoir lu cet article. J'espère :).

Table des matières

• Réactivité
  • Réf
  • Réactif
  • Réf peu profonde
  • Réactif peu profond
  • Réactif peu profond
  • Lecture seule
  • Lecture superficielle seule
• Calculé
  • Inscriptible calculé
• Regarder
  • Montre
    • Immédiat
    • Une fois
    • Regarder la fonction Getter
    • Regarder plusieurs valeurs
  • Effet de montre

Réactivité

React n'est pas le seul qui devrait venir à l'esprit lorsque l'on parle de réactivité. La réactivité fait référence à la capacité d'une entité (à partir d'une page Web) à réagir en fonction des modifications des données. Vous connaissez peut-être ce concept sous le nom de MVVM. MVVM est la forme abrégée de Model-View-View-Model. Comme son nom l'indique, lorsque les données changent, la vue change et vice-versa.

Pour utiliser la puissance de réactivité de Vue, nous allons aborder certaines options.

Réf

Vous pouvez considérer une référence comme un type spécial de variable que vous pouvez utiliser dans votre application Vue. Cette description n'est vraie que lorsque vous commencez à travailler avec Vue pour la première fois, car cela devient un peu plus complexe par la suite.

Définir une référence est aussi simple que ceci :

const message = ref("Hello World")

Utilisez-le dans votre modèle en utilisant la syntaxe d'interpolation :

<span>{{ message }}</span>

Si vous vous demandez pourquoi je l'ai appelé une variable mais déclaré un message à l'aide d'un mot-clé const, vous en avez parfaitement le droit.

Comme vous le savez, vous ne pouvez pas modifier la valeur de constant, car c'est le but du mot-clé const. Mais il y a une petite chose subtile que vous devriez savoir. Bien que le mot-clé const ne permet pas de modifier les données d'une variable, il ne se soucie pas des données imbriquées ! C'est également le cas de la réf. Pour mieux comprendre cette situation, essayez le code ci-dessous :

const o = {
  a: 1,
  b: 2,
  c: 3
}
console.log(o.a) // 1
o.a = 4
console.log(o.a) // 4

Comme vous pouvez le voir, je peux changer la valeur de o.a puisque les objets ne sont que des références et non une valeur entière en eux-mêmes. Ainsi, lorsque je modifie la valeur de a à l'intérieur de l'objet, la limitation const n'est pas appliquée. Bien sûr, si vous souhaitez attribuer une valeur à o lui-même, cela générera une erreur et ne vous permettra pas de le faire. Par exemple, le code ci-dessous est erroné :

const o = {
  a: 1,
  b: 2,
  c: 3
}
o = "hello"

C'est le même cas lors de l'utilisation de ref (et d'autres éléments que vous verrez plus tard ici). Lorsque vous invoquez une fonction ref, elle transforme tout ce qu'elle a reçu en objet. C’est ce qu’on appelle l’emballage. Essayez ce code :

const message = ref("Hello World")
console.log(message)

Vous devriez voir quelque chose comme l'image ci-dessous :

Comme vous pouvez le voir lors de la journalisation de la variable de message, vous ne recevez pas directement Hello World, mais elle se trouve à l'intérieur d'un objet et vous pouvez accéder à votre valeur réelle en utilisant la clé de valeur de l'objet susmentionné. Cela permet à Vue de surveiller les changements et de faire le truc MVVM ! :)

Lorsque vous accédez à votre référence dans un modèle Vue, il n'est pas nécessaire d'y accéder comme message.value. Vue est suffisamment intelligent pour restituer la valeur réelle à l'intérieur du modèle au lieu de l'objet. Mais si vous souhaitez accéder ou modifier la valeur d'une référence dans votre script, vous devez le faire en utilisant .value :

message.value = "Adnan!"
console.log(message.value) // Adnan!

Réactif

Comme vous l'avez vu lors de l'utilisation d'une référence, Vue enveloppe vos données dans un objet et vous permet d'y accéder via .value. C'est généralement le cas le plus utilisé. Vous pouvez envelopper presque tout en utilisant une référence et la rendre réactive.

Si vous vous demandez comment Vue surveille les changements de valeur et restitue la vue encore et encore, vous devriez consulter les proxys JavaScript.

Si votre valeur est un objet lui-même, vous pouvez utiliser réactif au lieu de ref. La fonction réactive n'enveloppera pas votre valeur et rendra l'objet lui-même réactif et observable.

const o = reactive({count: 0})

Si vous essayez d'imprimer la constante o vous verrez qu'il s'agit bien de votre objet sans aucun changement majeur :

Now you may manipulate the key count as you would normally do in JavaScript and Vue will render the changes as soon as possible.

Here is an example:

const increase = () => {
    o.count++
}

If you had ref instead of reactive it would have looked like this:

const o = ref({count: 0})

const increase = () => {
    o.value.count++
}

If you are still unsure which one to use, keep in mind that ref is a safe option to use.

Shallow Ref

Give that you have a ref like below:

const state = ref({
  names: {
    adnan: 'babakan',
    arian: 'amini',
    ata: 'parvin',
    mahdi: 'salehi'
  }
})

And printed my last name in your template as below:

<span>{{ state.names.adnan }}</span>

If you every changed my last name like this:

state.value.names.adnan = 'masruri'

Your template will be updated to show masruri instead of babakan. This is due to the fact that ref makes a deeply watched object and the changes to the view (template) are triggered for nested data as well.

There is an option to prevent such behaviour if that's what you want. To do so you may use shallowRef. A shallowRef acts exactly like ref does, with an exception of not watching for deep changes.

const state = shallowRef({
  names: {
    adnan: 'babakan',
    arian: 'amini',
    ata: 'parvin',
    mahdi: 'salehi'
  }
})

onMounted(() => {
  state.value.names.adnan = 'masruri'
})

The code above will result in your template showing babakan as it is not watched. But changing the .value entirely will trigger changes. So the code below will result in your template getting updated as well:

const state = shallowRef({
  names: {
    adnan: 'babakan',
    arian: 'amini',
    ata: 'parvin',
    mahdi: 'salehi'
  }
})

onMounted(() => {
  state.value = {
    names: {
      adnan: 'masruri',
      arian: 'amini',
      ata: 'parvin',
      mahdi: 'salehi'
    }
  }
})

This is a great option for performance-related concerns.

Shallow Reactive

So far we've known that ref wraps the data and watches it deeply and shallowRef wraps the data and watches it shallowly. Now tell me this, if reactive makes an object reactive, what does shallowReactive do?

const state = shallowReactive({
  names: {
    adnan: 'babakan',
    arian: 'amini',
    ata: 'parvin',
    mahdi: 'salehi',
  },
})

onMounted(() => {
  state.names.adnan = 'masruri'
})

As you might have guessed the template won't be updated.

Trigger Ref

Given that you are using a shallowRef and changed a value and now want your template to be updated according to the new data as well, you may use the triggerRef function:

const state = shallowRef({
  names: {
    adnan: 'babakan',
    arian: 'amini',
    ata: 'parvin',
    mahdi: 'salehi'
  }
})

onMounted(() => {
  state.value.names.adnan = 'masruri'
  triggerRef(state)
})

Now the template will also show masruri. This is more like changing from an automatic gear to a manual gear if you will.

This is usable for both shallowRef and shallowReactive.

Read Only

The readonly function receives a ref or a reactive as an argument and returns an exact copy that is only possible to be read from. This is used when you want to make sure your data is safe and is not possible to change when watching for it.

Example:

<template>
  <div>
    {{ infoReadOnly }}
  </div>
</template>

<script setup>
const info = ref({
  first: 'Adnan',
  last: 'Babakan'
})

const infoReadOnly = readonly(info)

onMounted(() => {
  info.value.first = 'Arian'
})
</script>

Though I've changed the value of info, since infoReadOnly is actually a live copy of info my changes are reflected in infoReadOnly as well. Yet you may not change the values using infoReadOnly directly:

const info = ref({
  first: 'Adnan',
  last: 'Babakan'
})

const infoReadOnly = readonly(info)

onMounted(() => {
  infoReadOnly.value.first = 'Arian'
})

This won't change the data and will warn you in the console as below:

Shallow Read Only

If we have ref and shallowRef, reactive and shallowReactive, why not have a shallowReadonly?

A shallowReadonly only makes the root level elements readonly whilst you can change the nested data:

const stateShallowReadonly = shallowReadonly({
  name: 'Adnan',
  friends: [
    { name: 'Arian' },
    { name: 'Ata' },
    { name: 'Mahdi' }
  ]
})

onMounted(() => {
  stateShallowReadonly.name = 'Brad'
})

The code above will warn you and won't change the value of name since it is a direct property.

But you can freely change anything inside friends since it is nested:

const stateShallowReadonly = shallowReadonly({
  name: 'Adnan',
  friends: [
    { name: 'Arian' },
    { name: 'Ata' },
    { name: 'Mahdi' }
  ]
})

onMounted(() => {
  stateShallowReadonly.friends[0].name = 'Brad'
})

Computed

Man, I love computed in Vue! You can imagine it as a glass in which you can mix your potions and still have your potions standing there intact!

A computed is like a ref or reactive that can be accessed and watched but not changed. Then what's the difference between a computed and a readonly you might ask. A computed can be a mixture of stuff. For example:

const state = ref({
  first: 'Adnan',
  last: 'Babakan'
})

const fullName = computed(() => state.value.first + ' ' + state.value.last)

Now you have a fullName which you may access its value inside a template with {{ fullName }} or inside your script using fullName.value.

The value of fullName will always depend on the state.value.first and state.value.last and will change if those guys change.

A computed receives a function that returns a value and can depend on multiple reactive data.

Writable Computed

Though a computed is mostly used to read a combination of data, the possibility to make a computed writable is also there.

Instead of passing a function to computed you may pass an object including two properties called get and set that both are functions.

For instance:

const state = ref({
  first: 'Adnan',
  last: 'Babakan'
})

const fullName = computed({
  get: () => state.value.first + ' ' + state.value.last,
  set: (value) => {
    const [first, last] = value.split(' ')
    state.value.first = first
    state.value.last = last
  }
})

Now if you try to write the value of fullName like below:

fullName.value = 'Ata Parvin'

It will split your string into 2 parts using a space and assign the first part to state.value.first and the second to state.value.last.

This is not a good way to determine which part of a name is a first name and which is a last name, but for the sake of demonstration, this is the only thing that came to my mind. :D

Watching

Watching is something that you will probably need a lot. Watching is referred to the act in which you want to run something in case a reactive data changes. In different systems there are various naming for this act, sometimes they are called hooks as well. But in Vue, we will call them watches.

Watch

The first thing you will encounter. A watch function receives two arguments. The reactive data to watch and the function to be invoked when the data changes respectively.

Here is a simple watch:

const count = ref(0)

const increase = () => {
  count.value++
}

watch(count, () => {
  console.log('Count changed to ' + count.value)
})

Now whenever the value of count is changed, you will see the log Count changed to ((count)) in your console.

The callback function also receives two arguments which are passed to it when the watch is triggered. The first argument holds the new value and the second one holds the old value. Here is an example:

const count = ref(0)

const increase = () => {
  count.value++
}

watch(count, (newValue, oldValue) => {
  console.log('Counter changed from ' + oldValue + ' to ' + newValue)
})

Note: Be careful when using the newValue and oldValue with objects as objects are passed by reference.

To be more accurate, a watch function receives a third argument as well. This third argument is an object that holds some options which can change the behaviour of the watching action.

Immediate

An immediate watch function is triggered at the instance it's created as well as when a change happens. You can think of it as the difference between a while loop and a do...while loop if you know what I mean. In other words, even if there is never a change, your callback function will run at least once:

watch(count, () => {
  console.log('Count changed to ' + count.value)
}, {
  immediate: true,
})

The value for immediate can be true or false. And the default value is false.

Once

If you want your watcher to run only once, you may define the once option and set its value to true. The default value is false.

watch(count, () => {
  console.log('Count changed to ' + count.value)
}, {
  once: true,
})

This will only trigger once when the value of count changes.

Advanced Watchers

Previously we've mentioned that watchers accept a reactive data as the first argument. While this is true, this is not the whole case.

A watch function can receive a getter function or an array of reactive objects and getter functions. This is used for when we need to watch for multiple data changes, and/or when we need to watch the result of two or more things when affecting each other. Let's have some examples.

Watching Getter Function

Take the code below as an example:

<template>
  <div>
    <div>
      <div>Timer one: {{ timerOne }}</div>
      <div>Timer two: {{ timerTwo }}</div>
    </div>
    <button @click="timerOne++">Accelerate timer one</button>
    <button @click="timerTwo++">Accelerate timer two</button>
  </div>
</template>

<script setup>
const timerOne = ref(0)
const timerTwo = ref(0)

onMounted(() => {
  setInterval(() => {
    timerOne.value++
    timerTwo.value++
  }, 1000)
})

watch(() => timerOne.value - timerTwo.value, () => {
  console.log('There was a change in the gap between timerOne and timerTwo. Gap is ' + (Math.abs(timerOne.value - timerTwo.value)) + ' seconds.')
})
</script>

It's a simple code that makes 2 refs holding a number and increasing both of them 1 by 1 each second. Logically the difference of these two refs are always equal to zero unless one gets changes out of its turn. As both increase the difference stays 0 so the watched won't get triggered as it only watches for the changes to the result of timerOne.value - timerTwo.value.

Yet there are two buttons that each adds 1 to timerOne and timerTwo respectively. When you click on any of those buttons the difference will be more or less than 0 thus the watch being triggered and logging the gap between these two timers.

Watching Multiple Values

Here is an example of an array of reactive data being passed to the first argument of the watch function:

<template>
  <div>
    <div>
      <div>Counter one: {{ counterOne }}</div>
      <div>Counter two: {{ counterTwo }}</div>
    </div>
    <button @click="counterOne++">Increase counter one</button>
    <button @click="counterTwo++">Increase counter two</button>
  </div>
</template>

<script setup>
const counterOne = ref(0)
const counterTwo = ref(0)

watch([
  counterOne,
  counterTwo,
], () => {
  console.log('One of the counters changes')
})
</script>

No matter which ref changes, the watcher will be triggered.

Watch Effect

A watchEffect function acts almost exactly like a watch function with a main difference. In a watchEffect you don't need to define what to watch and any reactive data that is used inside the callback you provide your watchEffect is watched automatically. For example:

const count = ref(0)

watchEffect(() => {
  console.log(count.value)
})

In case our count is changed the watchEffect will trigger its callback function since count.value is used inside it. This is good if you have complex logic to watch for.

---

Hope this was useful and you've enjoyed it. In case you spot any mistakes or feel like there should be an improvement, kindly let me know.

---

BTW! Check out my free Node.js Essentials E-book here:

NodeJS Essentials | Free E-Book

Adnan Babakan (he/him) ・ Sep 11 '20

#node #javascript #books

Feel free to contact me if you have any questions or suggestions.

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