React diff算法面试考点超详细讲解

var a1 =new WzElement('div', { class: 'a1Class' }, ['a1'], "a1")；
var a2 =new WzElement('div', { class: 'a2Class' }, ['a2'], "a2")
let root = a1.render();//js模拟dom生成
步骤2）
let pathchs = diff(a1, a2); //获取当前的两个dom的差异
步骤3）
reloadDom(root, pathchs);//根据差异重新渲染

  _createDom( tag, props, children, key ){
      let dom = document.createElement( tag );
      for( let propKey in props ){
          dom.setAttribute( propKey, props[propKey] );
      }
      if( !key ){
          dom.setAttribute( "key", key );
      }
      children.forEach( item => {
          if( item instanceof WzElement ){//
              var root = this._createDom(
                  item.tag,
                  item.props,
                  item.children,
                  item.key
              )
              dom.appendChild( root ); 
          }else{
              var childNode = document.createTextNode( item );
              dom.appendChild( childNode ); 
          }
      });
      return dom;
  }

//判断当前对象
function dfs(oldElement, newElement, index, patches) {
    //如果新的对象为空，无需要对比
    //如果新的对象，key跟tag都不同，说明元素变了，直接替换。
    //如果新的对象，key跟tag都相同，则遍历子集，观察子集是否不同,观察元素属性是否不同
    var curPatches = [];
    if (!newElement) {
    } else if (oldElement.key != newElement.key || oldElement.tag != newElement.tag) {
        curPatches.push({
            type: stateType.Change,
            node: newElement
        });
    } else {
        var propsDiff = diffProps(oldElement.props, newElement.props);
        if (propsDiff.length > 0) {
            curPatches.push({
                type: stateType.DiffPropsList,
                node: newElement
            });
        }
        diffChildren(oldElement.children,  newElement.children,index, patches);//对比子集是否不同
    }
    if (curPatches.length > 0) {
        if (!patches[index]) {
            patches[index] = []
        }
        patches[index] = patches[index].concat(curPatches);
    }
    return patches;
}
//对比子集是否不同
function diffChildren(oldChild, newChild, index, patches) {
    let { changeList, resultList } = listDiff(oldChild, newChild, index, patches);
    if (changeList.length > 0) {
        if (!patches[index]) {
            patches[index] = []
        }
        patches[index] = patches[index].concat(changeList);
    }
    let last = null;
    oldChild && oldChild.forEach((item, i) => {
        let child = item && item.children;
        if (child) {
            if ( last && last.children != null) {//有子节点
                index = index + last.children.length + 1;
            } else {
                index += 1;
            }
            let keyIndex = resultList.indexOf( item.key ) ;
            let node = newChild[keyIndex]
            //只遍历新旧中都存在的节点，其他新增或者删除的没必要遍历
            if ( node ) {
                dfs(item, node, index, patches)
            }
        } else {
            index += 1;
        }
        last = item
    });
}

var num = 0;
//根据virtDom的结果渲染页面
function reloadDom( node, patchs ){
    var changes = patchs[ num ];
    let childNodes = node && node.childNodes;
    if (!childNodes) num += 1
    if( changes != null  ){
        changeDom( node, changes );
    }
    //保持更diff算法的num一直
    var last = null
    childNodes && childNodes.forEach(( item, i ) => {
        if( childNodes ){
            if ( last && last.children != null) {//有子节点
                num = num + last.children.length + 1;
            } else {
                num += 1;
            }
        }
        reloadDom( item, patchs );
        last = item;
    })
}
//执行每个dom的变化
function changeDom( node, changes ){
    changes && changes.forEach( change => {
        let {type} = change;
        switch( type ){
            case stateType.Change:
                node.parentNode && node.parentNode.replaceChild( change.node.create() , node );
                break;
            case stateType.Move:
                let fromNode = node.childNodes[change.from];
                let toNode =  node.childNodes[change.to];
                let formClone = fromNode.cloneNode(true);
                let toClone = toNode.cloneNode(true);
                node.replaceChild( fromNode, toClone ) ;
                node.replaceChild( toNode, formClone ) ;
                break;
            case stateType.Add:
                node.insertBefore( change.node.create(),  node.childNodes[ change.index ]  )
                break;
            case stateType.Del:
                node.childNodes[change.index ].remove();
                break;
            case stateType.DiffPropsList:
                let {props} = change.node;
                for( let key in props ){
                    if( key == stateType.DelProps ){
                        node.removeAttribute( );
                    } else {
                        node.setAttribute( key, props[key] );
                    }
                }
                break;
            default:
                break;
        }
    });
}