useRef 鉤子解釋

The useRef hook in React is a powerful feature that allows you to create a mutable reference to a DOM element or any other value that persists for the entire lifecycle of a component. Here’s a detailed explanation of how it works, along with its use cases:

What is useRef?

• Persistent Storage: useRef provides a way to hold a mutable reference that does not trigger a re-render when updated. This is different from state, where updating a state variable will cause the component to re-render.

• Returns a Mutable Object: When you call useRef(initialValue), it returns a mutable object with a current property that you can modify. The initial value you pass to useRef is set to current, but you can change current at any time.

Basic Syntax

const myRef = useRef(initialValue);

Example of useRef

Here’s a simple example where useRef is used to access a DOM element:

import React, { useRef } from 'react';

function FocusInput() {
  const inputRef = useRef(null);

  const focusInput = () => {
    if (inputRef.current) {
      inputRef.current.focus();
    }
  };

  return (
    <div>
      <input ref={inputRef} type="text" />
      <button onClick={focusInput}>Focus Input</button>
    </div>
  );
}

export default FocusInput;

Explanation of the Example

1. Creating a Ref: const inputRef = useRef(null); creates a reference to hold a reference to the input element.

2. Assigning the Ref: The ref attribute of the input element is assigned to inputRef. This allows React to attach the input DOM element to the current property of inputRef.

3. Accessing the Ref: When the button is clicked, the focusInput function accesses the input element through inputRef.current and calls focus() on it.

Use Cases

1. Accessing DOM Elements: As shown in the example, useRef is commonly used to access and interact with DOM elements directly.

2. Storing Mutable Values: You can use useRef to store any mutable value that doesn’t require re-rendering when changed, such as a timer ID or a previous value.
   

   const timerRef = useRef();

   const startTimer = () => {
     timerRef.current = setTimeout(() => {
       // some action
     }, 1000);
   };

   const stopTimer = () => {
     clearTimeout(timerRef.current);
   };

1. Persisting Values Across Renders: Unlike state, a value held in useRef does not reset on re-renders. This can be useful for keeping track of values that are used in callbacks or effects.

2. Integrating with Third-party Libraries: When using third-party libraries that manipulate the DOM directly, useRef can provide a way to keep a reference to those DOM nodes.

Comparison with useState

• Re-renders: Updating a state variable with useState will trigger a re-render of the component, while updating a useRef will not.

• Storage: Use useRef for values that do not affect the rendering of the component, whereas useState should be used for values that do.

Key Points to Remember

• useRef can hold any value, not just DOM elements.
• The current property can be updated freely without causing a re-render.
• Ideal for accessing DOM nodes or storing mutable values that don’t need to trigger a render.

By understanding these concepts, you can effectively utilize the useRef hook in your React applications! If you have any specific use cases or questions about useRef, feel free to ask!

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