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History is littered with hundreds of conflicts over the future of a community, group, location or business that were "resolved" when one of the parties stepped ahead and destroyed what was there. With the original point of contention destroyed, the debates would fall to the wayside. Archive Team believes that by duplicated condemned data, the conversation and debate can continue, as well as the richness and insight gained by keeping the materials. Our projects have ranged in size from a single volunteer downloading the data to a small-but-critical site, to over 100 volunteers stepping forward to acquire terabytes of user-created data to save for future generations.
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Our collection has grown to the point of having sub-collections for the type of data we acquire. If you are seeking to browse the contents of these collections, the Wayback Machine is the best first stop. Otherwise, you are free to dig into the stacks to see what you may find.
The Archive Team Panic Downloads are full pulldowns of currently extant websites, meant to serve as emergency backups for needed sites that are in danger of closing, or which will be missed dearly if suddenly lost due to hard drive crashes or server failures.
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React
v17.0.2 LanguagesGitHub
render() function as creating a tree of React elements. On the next state or props update, that render() function will return a different tree of React elements. React then needs to figure out how to efficiently update the UI to match the most recent tree.
There are some generic solutions to this algorithmic problem of generating the minimum number of operations to transform one tree into another. However, the state of the art algorithms have a complexity in the order of O(n3) where n is the number of elements in the tree.
If we used this in React, displaying 1000 elements would require in the order of one billion comparisons. This is far too expensive. Instead, React implements a heuristic O(n) algorithm based on two assumptions:
(一)Two elements of different types will produce different trees.
(二)The developer can hint at which child elements may be stable across different renders with a key prop.
In practice, these assumptions are valid for almost all practical use cases.
<a>to<img>, or from <Article>to<Comment>, or from <Button>to<div> - any of those will lead to a full rebuild.
When tearing down a tree, old DOM nodes are destroyed. Component instances receive componentWillUnmount(). When building up a new tree, new DOM nodes are inserted into the DOM. Component instances receive UNSAFE_componentWillMount() and then componentDidMount(). Any state associated with the old tree is lost.
Any components below the root will also get unmounted and have their state destroyed. For example, when diffing:
<div>
<Counter />
</div>
<span>
<Counter />
</span>Counter and remount a new one.
Note:
These methods are considered legacy and you should avoid them in new code:
●UNSAFE_componentWillMount()
<div className="before" title="stuff" />
<div className="after" title="stuff" />className on the underlying DOM node.
When updating style, React also knows to update only the properties that changed. For example:
<div style={{color: 'red', fontWeight: 'bold'}} />
<div style={{color: 'green', fontWeight: 'bold'}} />color style, not the fontWeight.
After handling the DOM node, React then recurses on the children.
UNSAFE_componentWillReceiveProps(), UNSAFE_componentWillUpdate() and componentDidUpdate() on the underlying instance.
Next, the render() method is called and the diff algorithm recurses on the previous result and the new result.
Note:
These methods are considered legacy and you should avoid them in new code:
●UNSAFE_componentWillUpdate()
●UNSAFE_componentWillReceiveProps()
<ul>
<li>first</li>
<li>second</li>
</ul>
<ul>
<li>first</li>
<li>second</li>
<li>third</li>
</ul><li>first</li> trees, match the two <li>second</li> trees, and then insert the <li>third</li> tree.
If you implement it naively, inserting an element at the beginning has worse performance. For example, converting between these two trees works poorly:
<ul>
<li>Duke</li>
<li>Villanova</li>
</ul>
<ul>
<li>Connecticut</li>
<li>Duke</li>
<li>Villanova</li>
</ul><li>Duke</li> and <li>Villanova</li> subtrees intact. This inefficiency can be a problem.
key attribute. When children have keys, React uses the key to match children in the original tree with children in the subsequent tree. For example, adding a key to our inefficient example above can make the tree conversion efficient:
<ul>
<li key="2015">Duke</li>
<li key="2016">Villanova</li>
</ul>
<ul>
<li key="2014">Connecticut</li>
<li key="2015">Duke</li>
<li key="2016">Villanova</li>
</ul>'2014' is the new one, and the elements with the keys '2015' and '2016' have just moved.
In practice, finding a key is usually not hard. The element you are going to display may already have a unique ID, so the key can just come from your data:
<li key={item.id}>{item.name}</li>render for all components, it doesn’t mean React will unmount and remount them. It will only apply the differences following the rules stated in the previous sections.
We are regularly refining the heuristics in order to make common use cases faster. In the current implementation, you can express the fact that a subtree has been moved amongst its siblings, but you cannot tell that it has moved somewhere else. The algorithm will rerender that full subtree.
Because React relies on heuristics, if the assumptions behind them are not met, performance will suffer.
(一)The algorithm will not try to match subtrees of different component types. If you see yourself alternating between two component types with very similar output, you may want to make it the same type. In practice, we haven’t found this to be an issue.
(二)Keys should be stable, predictable, and unique. Unstable keys (like those produced by Math.random()) will cause many component instances and DOM nodes to be unnecessarily recreated, which can cause performance degradation and lost state in child components.
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Installation
●Getting Started
●Add React to a Website
●Create a New React App
●CDN Links
●Release Channels
Main Concepts
●1. Hello World
●2. Introducing JSX
●3. Rendering Elements
●4. Components and Props
●5. State and Lifecycle
●6. Handling Events
●7. Conditional Rendering
●8. Lists and Keys
●9. Forms
●10. Lifting State Up
●11. Composition vs Inheritance
●12. Thinking In React
Advanced Guides
●Accessibility
●Code-Splitting
●Context
●Error Boundaries
●Forwarding Refs
●Fragments
●Higher-Order Components
●Integrating with Other Libraries
●JSX In Depth
●Optimizing Performance
●Portals
●Profiler
●React Without ES6
●React Without JSX
●Reconciliation
●Refs and the DOM
●Render Props
●Static Type Checking
●Strict Mode
●Typechecking With PropTypes
●Uncontrolled Components
●Web Components
API Reference
●React
●React.Component
●ReactDOM
●ReactDOMServer
●DOM Elements
●SyntheticEvent
●Test Utilities
●Test Renderer
●JS Environment Requirements
●Glossary
Hooks
●1. Introducing Hooks
●2. Hooks at a Glance
●3. Using the State Hook
●4. Using the Effect Hook
●5. Rules of Hooks
●6. Building Your Own Hooks
●7. Hooks API Reference
●8. Hooks FAQ
Testing
●Testing Overview
●Testing Recipes
●Testing Environments
Contributing
●How to Contribute
●Codebase Overview
●Implementation Notes
●Design Principles
FAQ
●AJAX and APIs
●Babel, JSX, and Build Steps
●Passing Functions to Components
●Component State
●Styling and CSS
●File Structure
●Versioning Policy
●Virtual DOM and Internals
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