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Copy file name to clipboardExpand all lines: src/guide/reactivity.md
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title: Reactivity in Depth
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title: リアクティブの探求
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type: guide
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order: 15
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---
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We've covered most of the basics - now it's time to take a deep dive! One of Vue's most distinct features is the unobtrusive reactivity system. Models are just plain JavaScript objects. When you modify them, the view updates. It makes state management very simple and intuitive, but it's also important to understand how it works to avoid some common gotchas. In this section, we are going to dig into some of the lower-level details of Vue's reactivity system.
When you pass a plain JavaScript object to a Vue instance as its `data`option, Vue will walk through all of its properties and convert them to getter/setters using [Object.defineProperty](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperty). This is an ES5-only and un-shimmable feature, which is why Vue doesn't support IE8 and below.
The getter/setters are invisible to the user, but under the hood they enable Vue to perform dependency-tracking and change-notification when properties are accessed or modified. One caveat is that browser consoles format getter/setters differently when converted data objects are logged, so you may want to install [vue-devtools](https://github.com/vuejs/vue-devtools)for a more inspection-friendly interface.
Every component instance has a corresponding **watcher**instance, which records any properties "touched" during the component's render as dependencies. Later on when a dependency's setter is triggered, it notifies the watcher, which in turn causes the component to re-render.
Due to the limitations of modern JavaScript (and the abandonment of `Object.observe`), Vue**cannot detect property addition or deletion**. Since Vue performs the getter/setter conversion process during instance initialization, a property must be present in the `data`object in order for Vue to convert it and make it reactive. For example:
Vue does not allow dynamically adding new root-level reactive properties to an already created instance. However, it's possible to add reactive properties to a nested object using the `Vue.set(object, key, value)`method:
Sometimes you may want to assign a number of properties to an existing object, for example using `Object.assign()`or`_.extend()`. However, new properties added to the object will not trigger changes. In such cases, create a fresh object with properties from both the original object and the mixin object:
Since Vue doesn't allow dynamically adding root-level reactive properties, this means you have to initialize you instances by declaring all root-level reactive data properties upfront, even just with an empty value:
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Vue は動的に新しいルートレベルのリアクティブなプロパティを追加することはできませので、前もってインスタンス全てのルートレベルのリアクティブな data を宣言して初期化する必要があります、空の値でもかまいません:
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```js
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var vm =newVue({
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data: {
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//declare message with an empty value
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//空の値として message を宣言する
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message:''
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},
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template:'<div>{{ message }}</div>'
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})
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//set `message` later
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//後で、`message` を追加する
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vm.message='Hello!'
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```
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If you don't declare `message`in the data option, Vue will warn you that the render function is trying to access a property that doesn't exist.
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data オプションで `message`を宣言していないと、Vue は render ファンクションが存在しないプロパティにアクセスしようとしていることを警告します。
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There are technical reasons behind this restriction - it eliminates a class of edge cases in the dependency tracking system, and also makes Vue instances play nicer with type checking systems. But there is also an important consideration in terms of code maintainability: the `data`object is like the schema for your component's state. Declaring all reactive properties upfront makes the component code easier to understand when revisited later or read by another developer.
In case you haven't noticed yet, Vue performs DOM updates **asynchronously**. Whenever a data change is observed, it will open a queue and buffer all the data changes that happen in the same event loop. If the same watcher is triggered multiple times, it will be pushed into the queue only once. This buffered de-duplication is important in avoiding unnecessary calculations and DOM manipulations. Then, in the next event loop "tick", Vue flushes the queue and performs the actual (already de-duped) work. Internally Vue uses`MutationObserver`if available for the asynchronous queuing and falls back to `setTimeout(fn, 0)`.
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もしあなたがまだ気づいていない場合、Vue は **非同期** に DOM 更新を実行します。データ変更が監視されている限り、Vue はキューをオープンし、同じイベントループで起こる全てのデータ変更をバッファリングします。同じウオッチャが複数回トリガされる場合、一度だけキューに押し込まれます。この重複除外バッファリングは不要な計算や DOM 操作を回避する上で重要です。そして、次のイベントループの "tick" で、Vue はキューをフラッシュし、実際の(すでに重複が除外された)作業を実行します。内部的には、Vue はもし非同期キューイング向けに`MutationObserver`が利用可能ならそれを使い、`setTimeout(fn, 0)` にフォールバックします。
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For example, when you set `vm.someData = 'new value'`, the component will not re-render immediately. It will update in the next "tick", when the queue is flushed. Most of the time we don't need to care about this, but it can be tricky when you want to do something that depends on the post-update DOM state. Although Vue.js generally encourages developers to think in a "data-driven" fashion and avoid touching the DOM directly, sometimes it might be necessary to get your hands dirty. In order to wait until Vue.js has finished updating the DOM after a data change, you can use `Vue.nextTick(callback)`immediately after the data is changed. The callback will be called after the DOM has been updated. For example:
There is also the `vm.$nextTick()`instance method, which is especially handy inside components, because it doesn't need global `Vue`and its callback's `this`context will be automatically bound to the current Vue instance:
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