JSX

React 是怎么解析JSX语法的？
要提出两个方法

react.createElement

这里有三个入参：type, config, children
type: dom的类型，如div
config: dom的属性，如style={{color: #fff}}

    // 给props赋值config
    for (propName in config) {
      if (hasOwnProperty.call(config, propName) && !RESERVED_PROPS.hasOwnProperty(propName)) {
        props[propName] = config[propName];
      }
    }

children:当前dom的子元素，可能有多个，也可能没有。
源码中用var childrenLength = arguments.length - 2;来计算子元素的个数

  var childrenLength = arguments.length - 2; // 计算子元素的个数

  if (childrenLength === 1) {
    props.children = children;
  } else if (childrenLength > 1) {
    var childArray = Array(childrenLength);

    for (var i = 0; i < childrenLength; i++) {
      childArray[i] = arguments[i + 2];
    }

    {
      if (Object.freeze) {
        Object.freeze(childArray); // 冻结子元素
      }
    }

    props.children = childArray; // 将children赋值给props
  }

打印出一个virtual dom

image.png

react.createElement源码

function createElement(type, config, children) {
  var propName; // Reserved names are extracted

  var props = {};
  var key = null;
  var ref = null;
  var self = null;
  var source = null;

  if (config != null) {
    if (hasValidRef(config)) {
      ref = config.ref;

      {
        warnIfStringRefCannotBeAutoConverted(config);
      }
    }

    if (hasValidKey(config)) {
      key = '' + config.key;
    }

    self = config.__self === undefined ? null : config.__self;
    source = config.__source === undefined ? null : config.__source; // Remaining properties are added to a new props object

    for (propName in config) {
      if (hasOwnProperty.call(config, propName) && !RESERVED_PROPS.hasOwnProperty(propName)) {
        props[propName] = config[propName];
      }
    }
  } // Children can be more than one argument, and those are transferred onto
  // the newly allocated props object.


  var childrenLength = arguments.length - 2;

  if (childrenLength === 1) {
    props.children = children;
  } else if (childrenLength > 1) {
    var childArray = Array(childrenLength);

    for (var i = 0; i < childrenLength; i++) {
      childArray[i] = arguments[i + 2];
    }

    {
      if (Object.freeze) {
        Object.freeze(childArray);
      }
    }

    props.children = childArray;
  } // Resolve default props


  if (type && type.defaultProps) {
    var defaultProps = type.defaultProps;

    for (propName in defaultProps) {
      if (props[propName] === undefined) {
        props[propName] = defaultProps[propName];
      }
    }
  }

  {
    if (key || ref) {
      var displayName = typeof type === 'function' ? type.displayName || type.name || 'Unknown' : type;

      if (key) {
        defineKeyPropWarningGetter(props, displayName);
      }

      if (ref) {
        defineRefPropWarningGetter(props, displayName);
      }
    }
  }

  return ReactElement(type, key, ref, self, source, ReactCurrentOwner.current, props);
}

reactDom.render()

将JSX 翻译为 html。

ReactDOM.render(
  <div>
    <input type="text" style={inputStyle} value={value}/> 
    <button>{buttonName}</button>
  </div>,
  document.getElementById("container")
)

diff算法

tree

逐层比较，不会移动，遇到不同只会删除和创建

component

组件差异性比较，不会移动，遇到不同只会删除和创建

element (key)

可以根据key移动/添加/删除，只会对老的节点做向后移的操作
diff算法

为什么不建议用index做key

key的作用，如果key相同，则只会对比改变属性，反之，key不同，会卸载并重新加载。

<ul>
  <li key = "0">松子</li>
  <li key = "1">开心果</li>
  <li key = "2">核桃</li>
</ul>

如果上述列表顺序发生变化，但若用index做key，实际index不会发生变化：

<ul>
  <li key = "0">开心果</li>
  <li key = "1">核桃</li>
  <li key = "2">松子</li>
</ul>

如遇到非受控组件，则如果向数组头部添加元素，会导致value回显错误。

<ul>
  {demo.map(((it, index)=>{
      return <input key={index}></input>
  }))}
</ul>
<Button onClick={()=>setDemo(["girl"].concat(demo))}>交换</Button>

reactkey.gif