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81 lines
1.9 KiB
81 lines
1.9 KiB
---
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title: svelte/action
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---
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Actions are functions that are called when an element is created. They can return an object with a `destroy` method that is called after the element is unmounted:
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```svelte
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<script>
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/** @type {import('svelte/action').Action} */
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function foo(node) {
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// the node has been mounted in the DOM
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return {
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destroy() {
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// the node has been removed from the DOM
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}
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};
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}
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</script>
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<div use:foo />
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```
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An action can have a parameter. If the returned value has an `update` method, it will be called immediately after Svelte has applied updates to the markup whenever that parameter changes.
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> Don't worry that we're redeclaring the `foo` function for every component instance — Svelte will hoist any functions that don't depend on local state out of the component definition.
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```svelte
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<script>
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/** @type {string} */
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export let bar;
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/** @type {import('svelte/action').Action<HTMLElement, string>} */
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function foo(node, bar) {
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// the node has been mounted in the DOM
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return {
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update(bar) {
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// the value of `bar` has changed
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},
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destroy() {
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// the node has been removed from the DOM
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}
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};
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}
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</script>
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<div use:foo={bar} />
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```
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## Attributes
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Sometimes actions emit custom events and apply custom attributes to the element they are applied to. To support this, actions typed with `Action` or `ActionReturn` type can have a last parameter, `Attributes`:
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```svelte
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<script>
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/**
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* @type {import('svelte/action').Action<HTMLDivElement, { prop: any }, { 'on:emit': (e: CustomEvent<string>) => void }>}
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*/
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function foo(node, { prop }) {
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// the node has been mounted in the DOM
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//...LOGIC
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node.dispatchEvent(new CustomEvent('emit', { detail: 'hello' }));
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return {
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destroy() {
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// the node has been removed from the DOM
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}
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};
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}
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</script>
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<div use:foo={{ prop: 'someValue' }} on:emit={handleEmit} />
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```
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## Types
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> TYPES: svelte/action
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