--- title: Run time --- ### `svelte` The `svelte` package exposes [lifecycle functions](tutorial/onmount) and the [context API](tutorial/context-api). #### `onMount` ```js onMount(callback: () => void) ``` ```js onMount(callback: () => () => void) ``` --- The `onMount` function schedules a callback to run as soon as the component has been mounted to the DOM. It must be called during the component's initialisation (but doesn't need to live *inside* the component; it can be called from an external module). `onMount` does not run inside a [server-side component](docs#server-side-component-api). ```html ``` --- If a function is returned from `onMount`, it will be called when the component is unmounted. ```html ``` #### `beforeUpdate` ```js beforeUpdate(callback: () => void) ``` --- Schedules a callback to run immediately before the component is updated after any state change. > The first time the callback runs will be before the initial `onMount` ```html ``` #### `afterUpdate` ```js afterUpdate(callback: () => void) ``` --- Schedules a callback to run immediately after the component has been updated. ```html ``` #### `onDestroy` ```js onDestroy(callback: () => void) ``` --- Schedules a callback to run once the component is unmounted. Out of `onMount`, `beforeUpdate`, `afterUpdate` and `onDestroy`, this is the only one that runs inside a server-side component. ```html ``` #### `tick` ```js promise: Promise = tick() ``` --- Returns a promise that resolves once any pending state changes have been applied, or in the next microtask if there are none. ```html ``` #### `setContext` ```js setContext(key: any, context: any) ``` --- Associates an arbitrary `context` object with the current component and the specified `key`. The context is then available to children of the component (including slotted content) with `getContext`. Like lifecycle functions, this must be called during component initialisation. ```html ``` #### `getContext` ```js context: any = getContext(key: any) ``` --- Retrieves the context that belongs to the closest parent component with the specified `key`. Must be called during component initialisation. ```html ``` #### `createEventDispatcher` TODO ### `svelte/store` The `svelte/store` module exports functions for creating [stores](tutorial/writable-stores). --- To be considered a store, an object must have a `subscribe` method that returns an `unsubscribe` function. ```js const unsubscribe = store.subscribe(value => { console.log(value); }); // logs `value` // later... unsubscribe(); ``` --- Stores have special significance inside Svelte components. Their values can be read by prefixing the store's name with the `$` character, which causes Svelte to set up subscriptions and unsubscriptions automatically during the component's lifecycle. ```html ``` #### `writable` ```js store = writable(value: any) ``` ```js store = writable(value: any, (set: (value: any) => void) => () => void) ``` --- Creates a store with additional `set` and `update` methods. ```js import { writable } from 'svelte/store'; const count = writable(0); count.subscribe(value => { console.log(value); }); // logs '0' count.set(1); // logs '1' count.update(n => n + 1); // logs '2' ``` --- If a function is passed as the second argument, it will be called when the number of subscribers goes from zero to one (but not from one to two, etc). That function will be passed a `set` function which changes the value of the store. It must return a `stop` function that is called when the subscriber count goes from one to zero. ```js import { writable } from 'svelte/store'; const count = writable(0, () => { console.log('got a subscriber'); return () => console.log('no more subscribers'); }); count.set(1); // does nothing const unsubscribe = count.subscribe(value => { console.log(value); }); // logs 'got a subscriber', then '1' unsubscribe(); // logs 'no more subscribers' ``` #### `readable` ```js store = readable(value: any, (set: (value: any) => void) => () => void) ``` --- Creates a store whose value cannot be set from 'outside', the first argument is the store's initial value. The second argument to `readable` is the same as the second argument to `writable`, except that it is required with `readable` (since otherwise there would be no way to update the store value). ```js import { readable } from 'svelte/store'; const time = readable(new Date(), set => { const interval = setInterval(() => { set(new Date()); }, 1000); return () => clearInterval(interval); }); ``` #### `derived` ```js store = derived(a, callback: (a: any) => any) ``` ```js store = derived(a, callback: (a: any, set: (value: any) => void) => void, initial_value: any) ``` ```js store = derived([a, ...b], callback: ([a: any, ...b: any[]]) => any) ``` ```js store = derived([a, ...b], callback: ([a: any, ...b: any[]], set: (value: any) => void) => void, initial_value: any) ``` --- Derives a store from one or more other stores. Whenever those dependencies change, the callback runs. In the simplest version, `derived` takes a single store, and the callback returns a derived value. ```js import { derived } from 'svelte/store'; const doubled = derived(a, $a => $a * 2); ``` --- The callback can set a value asynchronously by accepting a second argument, `set`, and calling it when appropriate. In this case, you can also pass a third argument to `derived` — the initial value of the derived store before `set` is first called. ```js import { derived } from 'svelte/store'; const delayed = derived(a, ($a, set) => { setTimeout(() => set($a), 1000); }, 'one moment...'); ``` --- In both cases, an array of arguments can be passed as the first argument instead of a single store. ```js import { derived } from 'svelte/store'; const summed = derived([a, b], ([$a, $b]) => $a + $b); const delayed = derived([a, b], ([$a, $b], set) => { setTimeout(() => set($a + $b), 1000); }); ``` #### `get` ```js value: any = get(store) ``` --- Generally, you should read the value of a store by subscribing to it and using the value as it changes over time. Occasionally, you may need to retrieve the value of a store to which you're not subscribed. `get` allows you to do so. > This works by creating a subscription, reading the value, then unsubscribing. It's therefore not recommended in hot code paths. ```js import { get } from 'svelte/store'; const value = get(store); ``` ### `svelte/motion` The `svelte/motion` module exports two functions, `tweened` and `spring`, for creating writable stores whose values change over time after `set` and `update`, rather than immediately. #### `tweened` ```js store = tweened(value: any, options) ``` Tweened stores update their values over a fixed duration. The following options are available: * `delay` (`number`, default 0) — milliseconds before starting * `duration` (`number`, default 400) — milliseconds the tween lasts * `easing` (`function`, default `t => t`) — an [easing function](docs#svelte-easing) * `interpolator` (`function`) — see below `store.set` and `store.update` can accept a second `options` argument that will override the options passed in upon instantiation. Both functions return a Promise that resolves when the tween completes. If the tween is interrupted, the promise will never resolve. --- Out of the box, Svelte will interpolate between two numbers, two arrays or two objects (as long as the arrays and objects are the same 'shape', and their 'leaf' properties are also numbers). ```html ``` --- The `interpolator` option allows you to tween between *any* arbitrary values. It must be an `(a, b) => t => value` function, where `a` is the starting value, `b` is the target value, `t` is a number between 0 and 1, and `value` is the result. For example, we can use the [d3-interpolate](https://github.com/d3/d3-interpolate) package to smoothly interpolate between two colours. ```html {#each colors as c} {/each}