Tutorials on Beforeupdate

In Svelte, a component's beforeUpdate lifecycle method is called before the component is updated as a result of a state change. When the component receives new prop values or has its local state modified, the component's beforeUpdate lifecycle method is called before any updates to the DOM are made. Once the beforeUpdate lifecycle method finishes executing, the DOM will be updated with these data changes, which will allow the subsequent call to afterUpdate to have access to a completely synced DOM. Being able to schedule a callback to run at this phase of a component's lifecycle ensures that the value of certain DOM properties, such as a container element's scrollTop , can be cached prior to being updated. Then, these cached values can be used to revert those properties back to what they were originally. Here's a template of how the beforeUpdate lifecycle method is used inside of a component: Note : beforeUpdate is called before the component's initial onMount . Below, I'm going to show you: Suppose you are formatting text within an editable element, such as an <input /> or <textarea /> via a set of controls in a floating tooltip. When you create a selection by dragging your cursor over a subset of the text and click on any one of the tooltip options to transform the text, the selected text should be modified with the selection and focus on the editable element still intact. By default, clicking anywhere outside of an editable element will cause it to lose its focus and current text selection. This does not happen when clicking on any of the buttons in the tooltip because of additional code that preserves the selection and focus. Visit this simple Svelte REPL demo to see how both the focus and text selection can be immediately restored using the beforeUpdate lifecycle method: Demo - beforeUpdate Lifecycle Method - Restore Input Cursor Position In this demo, there are four buttons, each of which transform the input field's text selection differently: When any one of these buttons is clicked while there is a text selection in the input field, the transformText function is called. This function transforms the text selection and sets value to the newly transformed text. Because value is updated with a new value, the beforeUpdate lifecycle method will be called, which checks if input exists (since beforeUpdate is called before the component is mounted) and caches the start and end positions of the text selection. After the DOM is updated and the new value is rendered within the input field, the afterUpdate lifecycle method will be called, which will recreate the text selection based on the cached positions and place the focus back on the input field. If you have built React applications, then you will have probably noticed how beforeUpdate can be used similarly to getDerivedStateFromProps / shouldComponentUpdate / getSnapshotBeforeUpdate . Try rewriting some of the those components that use getDerivedStateFromProps / shouldComponentUpdate / getSnapshotBeforeUpdate as Svelte components that use beforeUpdate . Because beforeUpdate is often used in tandem with afterUpdate , check out the afterUpdate blog post for more examples using beforeUpdate . If you want to learn more about Svelte, then check out Fullstack Svelte :

Modern applications involve many DOM manipulations. Components (and elements) are constantly being removed, added, updated, etc. to deliver incredible user experiences and/or to optimize performance. Sometimes, you may need to execute code when a component is added (such as automatically focusing an input field when a form is loaded like the G-Mail login page) or when a component is removed (such as removing all event listeners associated with that element to prevent memory leaks). Frameworks/libraries such as React.js and Svelte provide component lifecycle methods that allow developers to predictably execute code at these very moments/situations. Having such access into each stage a component undergoes provides more control over how and when the component should be rendered. Commonly, functions can be scheduled to be invoked when these events occur in the lifecycle of a component instance: Svelte offers four lifecycle methods: This is the order of phases for a cycle involving beforeUpdate and afterUpdate : Additionally, only one lifecycle method is ran during server-side rendering: Coincidentally, the lifecycle methods of Svelte and React.js are quite similar: Notice that all of these methods happen as soon as the component's data is updated (either new props are received or state has changed) and before any rendering occurs. As previously mentioned, beforeUpdate is also called after the props or state has been modified and before the DOM is updated. Essentially, beforeUpdate can be customized to perform the same duties as any of these three lifecycle methods. To explore these similarities in-depth and understand why lifecycle methods are important in the development of components, let's walkthrough two versions of a simple application (one version using React.js and another using Svelte) and observe these lifecycle methods. Open both demos side-by-side, and open the developer console for both. React Demo Svelte Demo The application is a simple form that asks the user if they have any allergies. If they do, then an input field asking the user to mention their allergies appears beneath the radio button group. The lifecycle methods are located in the <FormInput /> component, and each method will log a message to the console when it is executed. Note : In the React demo, you will notice two different versions of the <FormInput /> component: src/FormInput_Class.js and src/FormInput_Hooks.js . In this post, we will be using the src/FormInput_Class.js version of the component since the useEffect hook only encompasses the lifecycle methods componentDidMount , componentDidUpdate and componentWillUnmount . Feel free to use this version of the component. Answer "Yes" to the question by clicking the "Yes" option in the radio button group. The input field should appear beneath the radio button group, like so: Check the developer consoles. When a component is mounted in Svelte, it will call beforeUpdate before the initial onMount . Then, once the component has rendered, onMount is called, followed by afterUpdate . The additional beforeUpdate call is caused by the readonly this binding ( bind:this ) that references the input field ( inputElement ). When a component is mounted in React, it will call getDerivedStateFromProps before the initial mount (before the render method is called). Then, once the component has rendered, componentDidMount (the React equivalent of Svelte's onMount ) is called. Type the letter "a" into the input field. Check the developer consoles. When a component's state is changed in Svelte, it will call beforeUpdate , followed by afterUpdate . Notice that the "X" button that appears when the input contains text is not yet rendered into the DOM when beforeUpdate is called. Once beforeUpdate finishes, the DOM is updated with the new state, and then afterUpdate is executed and has access to this newly rendered "X" button. When a component's state is changed in React, it will call getDerivedStateFromProps (always called before the render method), followed by shouldComponentUpdate and getSnapshotBeforeUpdate . Notice that the "X" button is not yet rendered into the DOM when any of these methods are called (like beforeUpdate ). Once these methods finish, the render method is called, the DOM is updated with the new state, and then componentDidUpdate (the React equivalent of Svelte's afterUpdate ) is executed and has access to this newly rendered "X" button. Inside of the input field, clear it by pressing the "X" button on the right. Check the developer consoles. Basically, the same methods are called in the exact same order. The only difference here is that the "X" button has not yet been removed from the DOM when beforeUpdate (also getDerivedStateFromProps , shouldComponentUpdate and getSnapshotBeforeUpdate ) are called. Once the component re-renders and the "X" button is removed, afterUpdate (also componentDidUpdate ) recognize that this button is removed. Answer "No" to the question by clicking the "No" option in the radio button group. Check the developer consoles. When a component is unmounted ("destroyed") in Svelte, it will call onDestroy , followed by the unmount method returned from onMount . Since both methods are called just before the component is unmounted, the component (and its constituent elements) are still in the DOM when these methods are called. Once these methods finish, the component will no longer exist in the DOM. Essentially, both of these methods perform the same function, but onDestroy can run inside a server-side component unlike the unmount method returned from onMount . When a component is unmounted in React, it will call componentWillUnmount . This method, like onDestroy , is called just before the component is unmounted, so the component (and its constituent elements) are still in the DOM when it is called. Once this method finishes, the component will no longer exist in the DOM. Experiment! Apply these lifecycle methods in your Svelte applications! Build custom Svelte lifecycle methods using beforeUpdate , afterUpdate , onMount and onDestroy .