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168 lines
7.7 KiB
168 lines
7.7 KiB
---
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title: Lifecycle hooks
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---
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<!-- - onMount/onDestroy
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- mention that `$effect` might be better for your use case
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- beforeUpdate/afterUpdate with deprecation notice?
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- or skip this entirely and only have it in the reference docs? -->
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In Svelte 5, the component lifecycle consists of only two parts: Its creation and its destruction. Everything in-between — when certain state is updated — is not related to the component as a whole; only the parts that need to react to the state change are notified. This is because under the hood the smallest unit of change is actually not a component, it's the (render) effects that the component sets up upon component initialization. Consequently, there's no such thing as a "before update"/"after update" hook.
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## `onMount`
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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).
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`onMount` does not run inside a component that is rendered on the server.
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```svelte
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<script>
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import { onMount } from 'svelte';
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onMount(() => {
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console.log('the component has mounted');
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});
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</script>
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```
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If a function is returned from `onMount`, it will be called when the component is unmounted.
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```svelte
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<script>
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import { onMount } from 'svelte';
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onMount(() => {
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const interval = setInterval(() => {
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console.log('beep');
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}, 1000);
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return () => clearInterval(interval);
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});
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</script>
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```
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> [!NOTE] This behaviour will only work when the function passed to `onMount` _synchronously_ returns a value. `async` functions always return a `Promise`, and as such cannot _synchronously_ return a function.
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## `onDestroy`
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> EXPORT_SNIPPET: svelte#onDestroy
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Schedules a callback to run immediately before the component is unmounted.
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Out of `onMount`, `beforeUpdate`, `afterUpdate` and `onDestroy`, this is the only one that runs inside a server-side component.
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```svelte
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<script>
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import { onDestroy } from 'svelte';
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onDestroy(() => {
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console.log('the component is being destroyed');
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});
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</script>
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```
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## `tick`
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While there's no "after update" hook, you can use `tick` to ensure that the UI is updated before continuing. `tick` returns a promise that resolves once any pending state changes have been applied, or in the next microtask if there are none.
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```svelte
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<script>
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import { tick } from 'svelte';
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$effect.pre(() => {
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console.log('the component is about to update');
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tick().then(() => {
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console.log('the component just updated');
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});
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});
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</script>
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```
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## Deprecated: `beforeUpdate` / `afterUpdate`
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Svelte 4 contained hooks that ran before and after the component as a whole was updated. For backwards compatibility, these hooks were shimmed in Svelte 5 but not available inside components that use runes.
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```svelte
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<script>
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import { beforeUpdate, afterUpdate } from 'svelte';
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beforeUpdate(() => {
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console.log('the component is about to update');
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});
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afterUpdate(() => {
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console.log('the component just updated');
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});
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</script>
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```
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Instead of `beforeUpdate` use `$effect.pre` and instead of `afterUpdate` use `$effect` instead - these runes offer more granular control and only react to the changes you're actually interested in.
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### Chat window example
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To implement a chat window that autoscrolls to the bottom when new messages appear (but only if you were _already_ scrolled to the bottom), we need to measure the DOM before we update it.
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In Svelte 4, we do this with `beforeUpdate`, but this is a flawed approach — it fires before _every_ update, whether it's relevant or not. In the example below, we need to introduce checks like `updatingMessages` to make sure we don't mess with the scroll position when someone toggles dark mode.
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With runes, we can use `$effect.pre`, which behaves the same as `$effect` but runs before the DOM is updated. As long as we explicitly reference `messages` inside the effect body, it will run whenever `messages` changes, but _not_ when `theme` changes.
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`beforeUpdate`, and its equally troublesome counterpart `afterUpdate`, are therefore deprecated in Svelte 5.
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- [Before](/playground/untitled#H4sIAAAAAAAAE31WXa_bNgz9K6yL1QmWOLlrC-w6H8MeBgwY9tY9NfdBtmlbiywZkpyPBfnvo2zLcZK28AWuRPGI5OGhkEuQc4EmiL9eAskqDOLg97oOZoE9125jDigs0t6oRqfOsjap5rXd7uTO8qpW2sIFEsyVxn_qjFmcAcstar-xPN3DFXKtKgi768IVgQku0ELj3Lgs_kZjWIEGNpAzYXDlHWyJFZI1zJjeh4O5uvl_DY8oUkVeVoFuJKYls-_CGYS25Aboj0EtWNqel0wWoBoLTGZgmdgDS9zW4Uz4NsrswPHoyutN4xInkylstnBxdmIhh8m7xzqmoNE2Wq46n1RJQzEbq4g-JQSl7e-HDx-GdaTy3KD9E3lRWvj5Zu9QX1QN20dj7zyHz8s-1S6lW7Cpz3RnXTcm04hIlfdFuO8p2mQ5-3a06cqjrn559bF_2NHOnRZ5I1PLlXQNyQT-hedMHeUEDyjtdMxsa4n2eIbNhlTwhyRthaOKOmYtniwF6pwt0wXa6MBEg0OibZec27gz_dk3UrZ6hB2LLYoiv521Yd8Gt-foTrfhiCDP0lC9VUUhcDLU49Xe_9943cNvEArHfAjxeBTovvXiNpFynfEDpIIZs9kFbg52QbeNHWZzebz32s7xHco3nJAJl1nshmhz8dYOQJDyZetnbb2gTWe-vEeWlrfpZMavr56ldb29eNt6UXvgwgFbp_WC0tl2RK25rGk6lYz3nUI2lzvBXGHhPZPGWmKUXFNBKqdaW259wl_aHbiqoVIZdpE60Nax6IOujT0LbFFxIVTCxCRR2XloUcYNvSbnGHKBp763jHoj59xiZWJI0Wm0P_m3MSS985xkasn-cFq20xTDy3J5KFcjgUTD69BHdcHIjz431z28IqlxGcPSfdFnrGDZn6gD6lyo45zyHAD-btczf-98nhQxHEvKfeUtOVkSejD3q-9X7JbzjGtsdUxlKdFU8qGsT78uaw848syWMXz85Waq2Gnem4mAn3prweq4q6Y3JEpnqMmnPoFRgmd3ySW0LLRqSKlwYHriCvJvUs2yjMaaoA-XzTXLeGMe45zmhv_XAno3Mj0xF7USuqNvnE9H343QHlq-eAgxpbTPNR9yzUkgLjwSR0NK4wKoxy-jDg-9vy8sUSToakzW-9fX13Em9Q8T6Z26uZhBN36XUYo5q7ggLXBZoub2Ofv7g6GCZfTxe034NCjiudXj7Omla0eTfo7QBPOcYxbE7qG-vl3_B1G-_i_JCAAA)
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- [After](/playground/untitled#H4sIAAAAAAAAE31WXa-jNhD9K7PsdknUQJLurtRLPqo-VKrU1327uQ8GBnBjbGSb5KZR_nvHgMlXtyIS9njO-MyZGZRzUHCBJkhez4FkNQZJ8HvTBLPAnhq3MQcUFmlvVKszZ1mbTPPGbndyZ3ndKG3hDJZne7hAoVUNYY8JV-RBPgIt2AprhA18MpZZnIQ50_twuvLHNRrDSjRXj9fwiCJTBLIKdCsxq5j9EM4gtBU3QD8GjWBZd14xWYJqLTCZg2ViDyx1W4cz4dv0hsiB49FRHkyfsCgws3GjcTKZwmYLZ2feWc9o1W8zJQ2Fb62i5JUQRNRHgs-fx3WsisKg_RN5WVn4-WrvUd9VA9tH4-AcwbfFQIpkLWByvWzqSe2sk3kyjUlOec_XPU-3TRaz_75tuvKoi19e3OvipSpamVmupJM2F_gXnnJ1lBM8oLQjHceys8R7PMFms4HwD2lRhzeEe-EsvluSrHe2TJdo4wMTLY48XKwPzm0KGm2r5ajFtRYU4TWOY7-ddWHfxhDP0QkQhnf5PWRnVVkKnIx8fZsOb5dR16nwG4TCCRdCMphWQ7z1_DoOcp3zA2SCGbPZBa5jd0G_TRxmc36Me-mG6A7l60XIlMs8ce2-OXtrDyBItdz6qVjPadObzx-RZdV1nJjx64tXad1sz962njceOHfAzmk9JzrbXqg1lw3NkZL7vgE257t-uMDcO6attSSokpmgFqVMO2U93e_dDlzOUKsc-3t6zNZp6K9cG3sS2KGSUqiUiUmq8tNYoJwbmvpTAoXA96GyjCojI26xNglk6DpwOPm7NdRYp4ia0JL94bTqRiGB5WJxqFY37RGPoz3c6i4jP3rcUA7wmhqNywQW7om_YQ2L4UQdUBdCHSPiOQJ8bFcxHzeK0jKBY0XcV95SkCWlD9t-9eOM3TLKucauiyktJdpaPqT19ddF4wFHntsqgS-_XE01e48GMwnw02AtWZP02QyGVOkcNfk072CU4PkduZSWpVYt9SkcmJ64hPwHpWF5ziVls3wIFmmW89Y83vMeGf5PBxjcyPSkXNy10J18t3x6-a6CDtBq6SGklNKeazFyLahB3PVIGo2UbhOgGi9vKjzW_j6xVFFD17difXx5ebll0vwvkcGpn4sZ9MN3vqFYsJoL6gUuK9TcPrO_PxgzWMRfflSEr2NHPJf6lj1957rRpH8CNMG84JgHidUtXt4u_wK21LXERAgAAA==)
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<!-- prettier-ignore -->
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```svelte
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<script>
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import { ---beforeUpdate, afterUpdate,--- tick } from 'svelte';
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---let updatingMessages = false;---
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let theme = +++$state('dark')+++;
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let messages = +++$state([])+++;
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let viewport;
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---beforeUpdate(() => {---
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+++$effect.pre(() => {+++
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---if (!updatingMessages) return;---
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+++messages;+++
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const autoscroll = viewport && viewport.offsetHeight + viewport.scrollTop > viewport.scrollHeight - 50;
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if (autoscroll) {
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tick().then(() => {
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viewport.scrollTo(0, viewport.scrollHeight);
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});
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}
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---updatingMessages = false;---
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});
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function handleKeydown(event) {
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if (event.key === 'Enter') {
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const text = event.target.value;
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if (!text) return;
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---updatingMessages = true;---
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messages = [...messages, text];
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event.target.value = '';
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}
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}
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function toggle() {
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toggleValue = !toggleValue;
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}
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</script>
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<div class:dark={theme === 'dark'}>
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<div bind:this={viewport}>
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{#each messages as message}
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<p>{message}</p>
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{/each}
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</div>
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<input +++onkeydown+++={handleKeydown} />
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<button +++onclick+++={toggle}> Toggle dark mode </button>
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</div>
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```
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