Скачать или смотреть Understanding React: Why Changing ClassNames on Click Does Not Re-Render as Expected

Discover why changing an element's className in React can lead to unexpected re-rendering issues, and learn how to handle it correctly.
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Understanding React: Why Changing ClassNames on Click Does Not Re-Render as Expected

When developing with React, one may encounter several puzzling behaviors related to component rendering and state management. A common situation arises when developers try to change an element's className on click but notice unexpected re-rendering. In this guide, we will delve into a specific example involving a grid of elements where toggling selection through className manipulation leads to confusion about state updates. Let's explore the underlying problem and, more importantly, how to effectively solve it.

The Problem: Class Name Changes and State Management

In a React application with a grid composed of div elements, each div can be toggled for selection based on user interaction. Here’s a summarized overview of the issue:

The user can click a grid tile to change its background color by adjusting its className (from grid-tile to grid-tileb).

The application needs to maintain a selection state (selectedTiles) in the component's state.

When the grid size changes (for instance, when the user selects a smaller grid), the previously selected tiles should reflect this change; ideally, they should lose their selection.

However, users noticed that even when selectedTiles was set to an empty array after resizing the grid, the visual appearance of the selected tiles persisted unexpectedly.

This behavior led to confusion as the expected visual feedback from the state changes did not align with the actual state of selectedTiles.

The Solution: Proper State Management and Render Logic

Upon analyzing the code, it becomes apparent that the main culprit is the direct manipulation of the className. Let’s walk through a revamped approach to effectively handle this scenario.

1. Move Away from Direct DOM Manipulation

In React, it is essential to manage all changes through state to ensure that the component re-renders appropriately. The first step to resolving the issue is removing direct alterations to className via the DOM. Here’s how to do that:

Original Code Snippet

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Instead, we will let React handle the rendering of class names based on state.

2. Properly Update State

We will restructure the function handling tile selection to use setState appropriately. Instead of manipulating the className directly, update the selected items through the state:

Updated selectTile Function

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3. Class Name Rendering Logic

The grid rendering logic needs to rely on the updated state, ensuring that class names are assigned based on whether a tile is selected or not:

Example Check

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4. Handling Grid Resizing

When resizing occurs, we must reset the selection state accordingly. Ensure that if the grid is resized smaller than the previous dimensions, previously selected tiles are properly handled:

Clearing Selection Example

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Conclusion

With these adjustments, your React component will reliably manage class changes and re-renders based on internal state without unexpected behavior. The core takeaway is to maintain the integrity of state management in React—let React handle what it does best: rendering changes based on state rather than direct DOM manipulation.

By understanding the nuances of state and re-rendering in React, you can build more predictable and user-friendly interfaces. Happy coding!