To better understand the Reactivity APIs, it is recommended to read the following chapters in the guide:
Takes an inner value and returns a reactive and mutable ref object, which has a single property .value that points to the inner value.
Type
function ref<T>(value: T): Ref<UnwrapRef<T>>
interface Ref<T> {
value: T
}
Details
The ref object is mutable - i.e. you can assign new values to .value. It is also reactive - i.e. any read operations to .value are tracked, and write operations will trigger associated effects.
If an object is assigned as a ref's value, the object is made deeply reactive with reactive(). This also means if the object contains nested refs, they will be deeply unwrapped.
To avoid the deep conversion, use shallowRef() instead.
Example
const count = ref(0) console.log(count.value) // 0 count.value = 1 console.log(count.value) // 1
See also
Takes a getter function and returns a readonly reactive ref object for the returned value from the getter. It can also take an object with get and set functions to create a writable ref object.
Type
// read-only
function computed<T>(
getter: (oldValue: T | undefined) => T,
// see "Computed Debugging" link below
debuggerOptions?: DebuggerOptions
): Readonly<Ref<Readonly<T>>>
// writable
function computed<T>(
options: {
get: (oldValue: T | undefined) => T
set: (value: T) => void
},
debuggerOptions?: DebuggerOptions
): Ref<T>
Example
Creating a readonly computed ref:
const count = ref(1) const plusOne = computed(() => count.value + 1) console.log(plusOne.value) // 2 plusOne.value++ // error
Creating a writable computed ref:
const count = ref(1)
const plusOne = computed({
get: () => count.value + 1,
set: (val) => {
count.value = val - 1
}
})
plusOne.value = 1
console.log(count.value) // 0
Debugging:
const plusOne = computed(() => count.value + 1, {
onTrack(e) {
debugger
},
onTrigger(e) {
debugger
}
})
See also
Returns a reactive proxy of the object.
Type
function reactive<T extends object>(target: T): UnwrapNestedRefs<T>
Details
The reactive conversion is "deep": it affects all nested properties. A reactive object also deeply unwraps any properties that are refs while maintaining reactivity.
It should also be noted that there is no ref unwrapping performed when the ref is accessed as an element of a reactive array or a native collection type like Map.
To avoid the deep conversion and only retain reactivity at the root level, use shallowReactive() instead.
The returned object and its nested objects are wrapped with ES Proxy and not equal to the original objects. It is recommended to work exclusively with the reactive proxy and avoid relying on the original object.
Example
Creating a reactive object:
const obj = reactive({ count: 0 })
obj.count++
Ref unwrapping:
const count = ref(1)
const obj = reactive({ count })
// ref will be unwrapped
console.log(obj.count === count.value) // true
// it will update `obj.count`
count.value++
console.log(count.value) // 2
console.log(obj.count) // 2
// it will also update `count` ref
obj.count++
console.log(obj.count) // 3
console.log(count.value) // 3
Note that refs are not unwrapped when accessed as array or collection elements:
const books = reactive([ref('Vue 3 Guide')])
// need .value here
console.log(books[0].value)
const map = reactive(new Map([['count', ref(0)]]))
// need .value here
console.log(map.get('count').value)
When assigning a ref to a reactive property, that ref will also be automatically unwrapped:
const count = ref(1)
const obj = reactive({})
obj.count = count
console.log(obj.count) // 1
console.log(obj.count === count.value) // true
See also
Takes an object (reactive or plain) or a ref and returns a readonly proxy to the original.
Type
function readonly<T extends object>( target: T ): DeepReadonly<UnwrapNestedRefs<T>>
Details
A readonly proxy is deep: any nested property accessed will be readonly as well. It also has the same ref-unwrapping behavior as reactive(), except the unwrapped values will also be made readonly.
To avoid the deep conversion, use shallowReadonly() instead.
Example
const original = reactive({ count: 0 })
const copy = readonly(original)
watchEffect(() => {
// works for reactivity tracking
console.log(copy.count)
})
// mutating original will trigger watchers relying on the copy
original.count++
// mutating the copy will fail and result in a warning
copy.count++ // warning!
Runs a function immediately while reactively tracking its dependencies and re-runs it whenever the dependencies are changed.
Type
function watchEffect(
effect: (onCleanup: OnCleanup) => void,
options?: WatchEffectOptions
): StopHandle
type OnCleanup = (cleanupFn: () => void) => void
interface WatchEffectOptions {
flush?: 'pre' | 'post' | 'sync' // default: 'pre'
onTrack?: (event: DebuggerEvent) => void
onTrigger?: (event: DebuggerEvent) => void
}
type StopHandle = () => void
Details
The first argument is the effect function to be run. The effect function receives a function that can be used to register a cleanup callback. The cleanup callback will be called right before the next time the effect is re-run, and can be used to clean up invalidated side effects, e.g. a pending async request (see example below).
The second argument is an optional options object that can be used to adjust the effect's flush timing or to debug the effect's dependencies.
By default, watchers will run just prior to component rendering. Setting flush: 'post' will defer the watcher until after component rendering. See Callback Flush Timing for more information. In rare cases, it might be necessary to trigger a watcher immediately when a reactive dependency changes, e.g. to invalidate a cache. This can be achieved using flush: 'sync'. However, this setting should be used with caution, as it can lead to problems with performance and data consistency if multiple properties are being updated at the same time.
The return value is a handle function that can be called to stop the effect from running again.
Example
const count = ref(0) watchEffect(() => console.log(count.value)) // -> logs 0 count.value++ // -> logs 1
Side effect cleanup:
watchEffect(async (onCleanup) => {
const { response, cancel } = doAsyncWork(id.value)
// `cancel` will be called if `id` changes
// so that previous pending request will be cancelled
// if not yet completed
onCleanup(cancel)
data.value = await response
})
Stopping the watcher:
const stop = watchEffect(() => {})
// when the watcher is no longer needed:
stop()
Options:
watchEffect(() => {}, {
flush: 'post',
onTrack(e) {
debugger
},
onTrigger(e) {
debugger
}
})
See also
Alias of watchEffect() with flush: 'post' option.
Alias of watchEffect() with flush: 'sync' option.
Watches one or more reactive data sources and invokes a callback function when the sources change.
Type
// watching single source
function watch<T>(
source: WatchSource<T>,
callback: WatchCallback<T>,
options?: WatchOptions
): StopHandle
// watching multiple sources
function watch<T>(
sources: WatchSource<T>[],
callback: WatchCallback<T[]>,
options?: WatchOptions
): StopHandle
type WatchCallback<T> = (
value: T,
oldValue: T,
onCleanup: (cleanupFn: () => void) => void
) => void
type WatchSource<T> =
| Ref<T> // ref
| (() => T) // getter
| T extends object
? T
: never // reactive object
interface WatchOptions extends WatchEffectOptions {
immediate?: boolean // default: false
deep?: boolean // default: false
flush?: 'pre' | 'post' | 'sync' // default: 'pre'
onTrack?: (event: DebuggerEvent) => void
onTrigger?: (event: DebuggerEvent) => void
once?: boolean // default: false (3.4+)
}
Types are simplified for readability.
Details
watch() is lazy by default - i.e. the callback is only called when the watched source has changed.
The first argument is the watcher's source. The source can be one of the following:
The second argument is the callback that will be called when the source changes. The callback receives three arguments: the new value, the old value, and a function for registering a side effect cleanup callback. The cleanup callback will be called right before the next time the effect is re-run, and can be used to clean up invalidated side effects, e.g. a pending async request.
When watching multiple sources, the callback receives two arrays containing new / old values corresponding to the source array.
The third optional argument is an options object that supports the following options:
immediate: trigger the callback immediately on watcher creation. Old value will be undefined on the first call.deep: force deep traversal of the source if it is an object, so that the callback fires on deep mutations. See Deep Watchers.flush: adjust the callback's flush timing. See Callback Flush Timing and watchEffect().onTrack / onTrigger: debug the watcher's dependencies. See Watcher Debugging.once: run the callback only once. The watcher is automatically stopped after the first callback run.
Compared to watchEffect(), watch() allows us to:
Example
Watching a getter:
const state = reactive({ count: 0 })
watch(
() => state.count,
(count, prevCount) => {
/* ... */
}
)
Watching a ref:
const count = ref(0)
watch(count, (count, prevCount) => {
/* ... */
})
When watching multiple sources, the callback receives arrays containing new / old values corresponding to the source array:
watch([fooRef, barRef], ([foo, bar], [prevFoo, prevBar]) => {
/* ... */
})
When using a getter source, the watcher only fires if the getter's return value has changed. If you want the callback to fire even on deep mutations, you need to explicitly force the watcher into deep mode with { deep: true }. Note in deep mode, the new value and the old will be the same object if the callback was triggered by a deep mutation:
const state = reactive({ count: 0 })
watch(
() => state,
(newValue, oldValue) => {
// newValue === oldValue
},
{ deep: true }
)
When directly watching a reactive object, the watcher is automatically in deep mode:
const state = reactive({ count: 0 })
watch(state, () => {
/* triggers on deep mutation to state */
})
watch() shares the same flush timing and debugging options with watchEffect():
watch(source, callback, {
flush: 'post',
onTrack(e) {
debugger
},
onTrigger(e) {
debugger
}
})
Stopping the watcher:
const stop = watch(source, callback) // when the watcher is no longer needed: stop()
Side effect cleanup:
watch(id, async (newId, oldId, onCleanup) => {
const { response, cancel } = doAsyncWork(newId)
// `cancel` will be called if `id` changes, cancelling
// the previous request if it hasn't completed yet
onCleanup(cancel)
data.value = await response
})
See also
© 2013–present Yuxi Evan You
Licensed under the MIT License.
https://vuejs.org/api/reactivity-core