React源码解析之React.children.map()

摘要：一例子看到一个有趣的现象，就是多层嵌套的数组经过后，平铺成了，接下来以该例解析二作用源码进行基本的判断和初始化后，调用该方法就是重命名了，即解析注意，该数组在里面滚了一圈后，会结果三作用的包裹器源码第一次第二次如果字符串中有连续多个的话

一、例子

function ChildrenDemo(props) {
  console.log(props.children, "children30");
  console.log(React.Children.map(props.children, item => [item, [item, [item]]]), "children31");
  // console.log(React.Children.map(props.children,item=>item),"children31")
  return props.children;
}

export default ()=>(
  
    1
    2
  
)

props.children ：

React.Children.map(props.children, item => [item, [item, [item]]] ：

看到一个有趣的现象，就是多层嵌套的数组[item, [item, [item]]]经过map()后，平铺成[item,item,item]了，接下来以该例解析React.Child.map()

二、React.Children.map()
作用：
https://zh-hans.reactjs.org/docs/react-api.html#reactchildren

源码：

// React.Children.map(props.children,item=>[item,[item,] ])
function mapChildren(children, func, context) {
  if (children == null) {
    return children;
  }
  const result = [];
  //进行基本的判断和初始化后，调用该方法
  //props.children,[],null,(item)=>{return [item,[item,] ]},undefined
  mapIntoWithKeyPrefixInternal(children, result, null, func, context);
  return result;
}

export {
  //as就是重命名了，map即mapChildren
  forEachChildren as forEach,
  mapChildren as map,
  countChildren as count,
  onlyChild as only,
  toArray,
};

解析：
注意result，该数组在里面滚了一圈后，会return结果

三、mapIntoWithKeyPrefixInternal()
作用：
getPooledTraverseContext()/traverseAllChildren()/releaseTraverseContext()的包裹器

源码：

//第一次：props.children , [] , null , (item)=>{return [item,[item,] ]} , undefined
//第二次：[item,[item,] ] , [] , .0 , c => c , undefined
function mapIntoWithKeyPrefixInternal(children, array, prefix, func, context) {
  let escapedPrefix = "";
  //如果字符串中有连续多个 / 的话，在匹配的字串后再加 /
  if (prefix != null) {
    escapedPrefix = escapeUserProvidedKey(prefix) + "/";
  }
  //从pool中找一个对象
  //[],"",(item)=>{return [item,[item,] ]},undefined

  //traverseContext=
  // {
  //  result:[],
  //  keyPrefix:"",
  //  func:(item)=>{return [item,[item,] ]},
  //  context:undefined,
  //  count:0,
  // }
  const traverseContext = getPooledTraverseContext(
    array,
    escapedPrefix,
    func,
    context,
  );
  //将嵌套的数组展平
  traverseAllChildren(children, mapSingleChildIntoContext, traverseContext);
  releaseTraverseContext(traverseContext);
}

解析：

① escapeUserProvidedKey()
这个函数一般是第二层递归时，会用到

作用：
在/后再加一个/

源码：

const userProvidedKeyEscapeRegex = //+/g;
function escapeUserProvidedKey(text) {
  //如果字符串中有连续多个 / 的话，在匹配的字串后再加 /
  return ("" + text).replace(userProvidedKeyEscapeRegex, "$&/");
}

解析：
react对key定义的一个规则：
如果字符串中有连续多个/的话，在匹配的字串后再加/

例：

let a="aa/a/"
console.log(a.replace(//+/g, "$&/")); //  aa//a//

② getPooledTraverseContext()

作用：
创建一个对象池，复用Object，从而减少很多对象创建带来的内存占用和gc（垃圾回收）的损耗

源码：

//对象池的最大容量为10
const POOL_SIZE = 10;
//对象池
const traverseContextPool = [];
//[],"",(item)=>{return [item,[item,] ]},undefined
function getPooledTraverseContext(
  mapResult,
  keyPrefix,
  mapFunction,
  mapContext,
) {
  //如果对象池内存在对象，则出队一个对象，
  //并将arguments的值赋给对象属性
  //最后返回该对象
  if (traverseContextPool.length) {
    const traverseContext = traverseContextPool.pop();
    traverseContext.result = mapResult;
    traverseContext.keyPrefix = keyPrefix;
    traverseContext.func = mapFunction;
    traverseContext.context = mapContext;
    traverseContext.count = 0;
    return traverseContext;
  }
  //如果不存在，则返回一个新对象
  else {
    //{
    // result:[],
    // keyPrefix:"",
    // func:(item)=>{return [item,[item,] ]},
    // context:undefined,
    // count:0,
    // }
    return {
      result: mapResult,
      keyPrefix: keyPrefix,
      func: mapFunction,
      context: mapContext,
      count: 0,
    };
  }
}

解析：
在每次map()的过程中，每次递归都会用到traverseContext 对象，
创建traverseContextPool对象池的目的，就是**复用里面的对象，
以减少内存消耗**，并且在map()结束时，
将复用的对象初始化，并push进对象池中（releaseTraverseContext），以供下次map()时使用

③ mapSingleChildIntoContext()
mapSingleChildIntoContext是traverseAllChildren(children, mapSingleChildIntoContext, traverseContext)的第二个参数，为避免讲traverseAllChildren要调头看这个 API，就先分析下

作用：
递归仍是数组的child；
将单个ReactElement的child加入result中

源码：

//bookKeeping:traverseContext=
// {
//  result:[],
//  keyPrefix:"",
//  func:(item)=>{return [item,[item,] ]},
//  context:undefined,
//  count:0,
// }

//child:1

//childKey:.0
function mapSingleChildIntoContext(bookKeeping, child, childKey) {
  //解构赋值
  const {result, keyPrefix, func, context} = bookKeeping;
  //func:(item)=>{return [item,[item,] ]},
  //item即1
  //第二个参数bookKeeping.count++很有意思，压根儿没用到，但仍起到计数的作用
  let mappedChild = func.call(context, child, bookKeeping.count++);
  //如果根据React.Children.map()第二个参数callback，执行仍是一个数组的话，
  //递归调用mapIntoWithKeyPrefixInternal，继续之前的步骤，
  //直到是单个ReactElement
  if (Array.isArray(mappedChild)) {
    //mappedChild:[item,[item,] ]
    //result:[]
    //childKey:.0
    //func:c => c
    mapIntoWithKeyPrefixInternal(mappedChild, result, childKey, c => c);
  }
  //当mappedChild是单个ReactElement并且不为null的时候
  else if (mappedChild != null) {
    if (isValidElement(mappedChild)) {
      //赋给新对象除key外同样的属性，替换key属性
      mappedChild = cloneAndReplaceKey(
        mappedChild,
        // Keep both the (mapped) and old keys if they differ, just as
        // traverseAllChildren used to do for objects as children
        //如果新老keys是不一样的话，两者都保留，像traverseAllChildren对待objects做的那样
        keyPrefix +
          (mappedChild.key && (!child || child.key !== mappedChild.key)
            ? escapeUserProvidedKey(mappedChild.key) + "/"
            : "") +
          childKey,
      );
    }
    //result即map时，return的result
    result.push(mappedChild);
  }
}

解析：
（1）让child调用func 方法，所得的结果如果是数组的话继续递归；如果是单个ReactElement的话，将其放入result数组中

（2）cloneAndReplaceKey()字如其名，就是赋给新对象除key外同样的属性，替换key属性

简单看下源码：

export function cloneAndReplaceKey(oldElement, newKey) {
  const newElement = ReactElement(
    oldElement.type,
    newKey,
    oldElement.ref,
    oldElement._self,
    oldElement._source,
    oldElement._owner,
    oldElement.props,
  );

  return newElement;
}

（3）isValidElement() 判断是否为ReactElement
简单看下源码：

export function isValidElement(object) {
  return (
    typeof object === "object" &&
    object !== null &&
    object.$$typeof === REACT_ELEMENT_TYPE
  );
}

④ traverseAllChildren()

作用：
traverseAllChildrenImpl的触发器

源码：

// children, mapSingleChildIntoContext, traverseContext
function traverseAllChildren(children, callback, traverseContext) {
  if (children == null) {
    return 0;
  }

  return traverseAllChildrenImpl(children, "", callback, traverseContext);
}

⑤ traverseAllChildrenImpl()

作用：
核心递归函数，目的是展平嵌套数组

源码：

// children, "", mapSingleChildIntoContext, traverseContext
function traverseAllChildrenImpl(
  children,
  nameSoFar,
  callback,
  //traverseContext=
  // {
  //  result:[],
  //  keyPrefix:"",
  //  func:(item)=>{return [item,[item,] ]},
  //  context:undefined,
  //  count:0,
  // }
  traverseContext,
) {
  const type = typeof children;

  if (type === "undefined" || type === "boolean") {
    //以上所有的被认为是null
    // All of the above are perceived as null.
    children = null;
  }
  //调用func的flag
  let invokeCallback = false;

  if (children === null) {
    invokeCallback = true;
  } else {
    switch (type) {
      case "string":
      case "number":
        invokeCallback = true;
        break;
      case "object":
        //如果props.children是单个ReactElement/PortalElement的话
        //递归traverseAllChildrenImpl时，1和2作为child
        //必会触发invokeCallback=true
        switch (children.$$typeof) {
          case REACT_ELEMENT_TYPE:
          case REACT_PORTAL_TYPE:
            invokeCallback = true;
        }
    }
  }

  if (invokeCallback) {
    callback(
      traverseContext,
      children,
      //如果只有一个子节点，也将它放在数组中来处理
      // If it"s the only child, treat the name as if it was wrapped in an array
      // so that it"s consistent if the number of children grows.
      //.$=0

      //1 key=".0"
      nameSoFar === "" ? SEPARATOR + getComponentKey(children, 0) : nameSoFar,
    );
    return 1;
  }

  let child;
  let nextName;
  //有多少个子节点
  let subtreeCount = 0; // Count of children found in the current subtree.
  const nextNamePrefix =
    //.
    nameSoFar === "" ? SEPARATOR : nameSoFar + SUBSEPARATOR;

  if (Array.isArray(children)) {
    for (let i = 0; i < children.length; i++) {
      //1
      child = children[i];
      //不手动设置key的话第一层第一个是.0，第二个是.1
      nextName = nextNamePrefix + getComponentKey(child, i);

      subtreeCount += traverseAllChildrenImpl(
        child,
        nextName,
        callback,
        traverseContext,
      );
    }
  } else {
    const iteratorFn = getIteratorFn(children);
    if (typeof iteratorFn === "function") {
      if (__DEV__) {
        // Warn about using Maps as children
        if (iteratorFn === children.entries) {
          warning(
            didWarnAboutMaps,
            "Using Maps as children is unsupported and will likely yield " +
              "unexpected results. Convert it to a sequence/iterable of keyed " +
              "ReactElements instead.",
          );
          didWarnAboutMaps = true;
        }
      }

      const iterator = iteratorFn.call(children);
      let step;
      let ii = 0;
      while (!(step = iterator.next()).done) {
        child = step.value;
        nextName = nextNamePrefix + getComponentKey(child, ii++);
        subtreeCount += traverseAllChildrenImpl(
          child,
          nextName,
          callback,
          traverseContext,
        );
      }
    }
    //如果是一个纯对象的话，throw error
    else if (type === "object") {
      let addendum = "";
      if (__DEV__) {
        addendum =
          " If you meant to render a collection of children, use an array " +
          "instead." +
          ReactDebugCurrentFrame.getStackAddendum();
      }
      const childrenString = "" + children;
      invariant(
        false,
        "Objects are not valid as a React child (found: %s).%s",
        childrenString === "[object Object]"
          ? "object with keys {" + Object.keys(children).join(", ") + "}"
          : childrenString,
        addendum,
      );
    }
  }

  return subtreeCount;
}

解析：
分为两部分：
（1）children是Object，并且$$typeof是REACT_ELEMENT_TYPE/REACT_PORTAL_TYPE

调用callback 即mapSingleChildIntoContext ，复制除key外的属性，替换key属性，将其放入到result中

（2）children是Array
循环children，再用traverseAllChildrenImpl 执行child

三、流程图

四、根据React.Children.map()的算法出一道面试题

数组扁平化处理：
实现一个flatten方法，使得输入一个数组，该数组里面的元素也可以是数组，该方法会输出一个扁平化的数组

// Example
let givenArr = [[1, 2, 2], [3, 4, 5, 5], [6, 7, 8, 9, [11, 12, [12, 13, [14]]]], 10];
let outputArr = [1,2,2,3,4,5,5,6,7,8,9,11,12,12,13,14,10]
// 实现flatten方法使得flatten(givenArr)——>outputArr

解法一：根据上面的流程图使用递归

function flatten(arr){
    var res = [];
    for(var i=0;i
解法二：ES6
function flatten(array) {
      //只要数组中的元素有一个嵌套数组，就合并
      while(array.some(item=>Array.isArray(item)))
        array=[].concat(...array)

      console.log(array) //[1,2,2,3,4,5,5,6,7,8,9,11,12,12,13,14,10]
      return array
    }


（完）