Why is it possible to improve performance by setting fixed and unique keys for elements at the same level in virtual DOM

horse23333 2020-11-07 21:04:17
possible improve performance setting fixed


One 、 written words

key Give Way React know , Currently newly generated React The elements of the elements , Whether it can be generated before React Find the corresponding in the element . If there is , Then just take it and use it . Suppose you insert an entry at the head of the list , By comparison ,React Know that the items have not changed except for the head .React Yes DOM During operation , Just insert a node in the head . The rest of the nodes don't move them .

If developer No settings key, that React Will set it up key Is the index value of the item in the list . Suppose the head inserts an item , Two React A comparison of trees ,React Look at your first element and the previous one key equally , It's just props Changed the ( Here we assume that the contents of each item in the list are different ). That only needs to be done for the first DOM Node update . Compare the second element ,React Look, you're the same as the second element before , It's just props Changed the , That only needs to be done for the second one DOM Node update . Compare the third element ,React Look, you're the same as the third element before , It's just props Changed the , That only needs to be done to the third one DOM Node update ... Finally, there is an extra element left ,React Take a look at your key I haven't seen it before , Create a new one DOM Nodes are inserted into the above DOM The end of the nodes .

In conclusion : If the original list had n term . Developer settings key The overhead of value is to create a DOM Node overhead .

Developers don't set key The overhead of value is to create a DOM Node overhead and update n Cost of nodes .

Two 、 Code

For a better understanding of key The role of , You can click the link to see the code and page effect .

 https://codesandbox.io/s/nervous-bash-3ico9?file=/src/App.js

 

Reference material :

[1] Understand the virtual DOM And key The function of attributes .https://time.geekbang.org/course/detail/100009301-9441

[2] Coordinate .https://zh-hans.reactjs.org/docs/reconciliation.html

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