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ElTree组件的内部原理

ElTree组件的内部原理

作者: 爱写Bug的程序猿 | 来源:发表于2021-02-06 21:12 被阅读0次

    ElTree组件的内部原理

    最近一直在做Element3的Tree组件开发,这里就记录一下Tree组件的思想和内部实现原理,也对想要学习源码的童鞋的一个帮助吧。

    设计思想

    在设计Tree组件的时候是采用两颗树进行互相映射的方案进行设计的,一颗树是用户自定义节点构成的树RawNode,另一颗是内部进行渲染的树TreeNode。当RawNode某个节点的值变更后Mapper就会得到通知,然后通过通知的内容对TreeNode进行更改。

    核心思想

    整个组件的难点就是在Mapper这里,他需要完成:

    • 节点转换与映射
    • 变更监听
    • 响应更改

    而这里需要注意的两点是:

    • RawNode变更后Mapper要对TreeNode进行修改,但是在修改TreeNode后不能在通知变更去修改RawNode
    • 在监听节点的时候,需要对存储子节点的数组进行监听(children)

    这里就引出了一个重要的问题如何监听一个节点的变化?

    这里就要说一下ES6的一个新的类Proxy,顾名思义就是代理,他可以对一个对象进行拦截如:

    这里Proxy还有个特性就是,在修改原始对象的时候不会触发拦截,通过这个特性就很好解决了注意点1

    Proxy还有一个特性就是,每次在修改前一定会触发get操作,因为他是先获取,在修改

    测试先行

    TDD 测试先行,我们先想一下需要什么方法,然后先假设有在测试里用一下,然后在去写这个方法

    其实很多时候我们内心就是这样的,每次在开发类或者函数之前肯定要先想接口,然后在实现,而TDD只是将内心的活动带到了现实里,这样做有几点好处

    1、之后可以自动测试

    2、理清楚了接口

    3、在你对代码大动刀戈的时候他可以起到一定的指引作用,这一点在开发Tree的时候是感受颇深

    • 首先需要一个TreeNode类作为树的节点
    • 一个可以监听对象的的工具类 Watcher
    • 一个可以事件通知的工具类 Event
    • 一个需要对象映射Mapper类

    TreeNode Spec

    我们需要传入id、label、children来实现节点的创建,这也是TreeNode必须要传入的

    describe('TreeNode.js', ()=>{
        it('init a node', ()=>{ // 初始化一个节点
            const root = new TreeNode(1, 'Node1', [
                new TreeNode(2, 'Node2', [])
            ])
            
            expect(root.id).toBe(1)
            expect(root.label).toBe('Node1')
            expect(root.children[0].id).toBe(2)
            expect(root.children[0].label).toBe('Node')
        })
    })
    

    Watcher Spec

    这里我们要将对象的操作进行拦截,这样才能知道这个对象的变化

    • 监听节点内哪个属性修改了什么值
    • 监听存放孩子节点的数组
      • 节点的增加
      • 节点的修改(这里需要注意一下,修改就是指针变化)
      • 节点的删除
    describe('Watcher.js', ()=>{
        it('listern a node prop change', ()=>{
            const root = {
                label: 'Node1',
            }
            const watcher = new Watcher(root)
            const _root = watcher.proxy // 拿到代理后的对象
            
            const changeHandler = jest.fn()
            watcher.bindHandler('change', changeHandler)
            const addHandler = jest.fn()
            watcher.bindHandler('add', addHandler)
            
            _root.label = "Test"
            expect(changeHandler).toHaveBeenCalledTimes(1)
            
            _root.disabled = true
            expect(addHandler).toHaveBeenCalledTimes(1)
        })
        
        it('listen a node children node pointer and length change', ()=>{
            const root = {
                label: 'Node1',
                children:[
                    {
                        label: 'Node2'
                    }
                ]
            }
            const watcher = new Watcher(root)
            const _root = watcher.proxy // 拿到代理后的对象
            
            const childrenChangeHandler = jest.fn()
            watcher.bindHandler('array/change', childrenChangeHandler)
            const addHandler = jest.fn()
            watcher.bindHandler('array/', addHandler)
            
            _root.label = 'Test'
            expect(childrenChangeHandler).toHaveBeenCalledTimes(1)
            expect(childrenChangeHandler).toHaveBeenNthCalledWith(1, {
              target: root,
              key: 'label',
              value: 'Test',
              currentNode: root
            })
            
            _root.disabled = true
            expect(addHandler).toHaveBeenCalledTimes(1)
            expect(childrenChangeHandler).toHaveBeenNthCalledWith(1, {
              target: root,
              key: 'disabled',
              value: true,
              currentNode: root
            })
        })
    })
    

    Event Spec

    这里要实现简单的事件的监听和发送,你也可以理解为订阅与推送

    describe('Event.js', ()=>{
        it('listen a event', ()=>{
            const event = new Event()
            const cb = jest.fn()
            event.on('ev1', cb)
            event.emit('ev1', 1, 2, 3)
    
            expect(cb).toHaveBeenCalledTimes(1)
            expect(cb).toHaveBeenCalledWith(1, 2, 3)
        })
    })
    

    Mapper Spec

    这里就是最终将上面的功能都集合到一起

    • 节点转换与映射
    • 变更监听
    • 响应更改
    describe('Mapper.js', () => {
      it('mapper a tree', () => {
        const rawNode = {
          text: 'Node1',
          childs: [
            {
              text: 'Node11',
              childs: [
                {
                  text: 'Node111',
                  childs: []
                }
              ]
            }
          ]
        }
    
        const mapper = new TreeMapper(rawNode, {
          label: 'text',
          children: 'childs'
        })
    
        const rawNodeProxy = mapper.rawNodeProxy
        const treeNodeProxy = mapper.treeNodeProxy
        
        rawNodeProxy.text = "Test"
    
        expect(rawNodeProxy.text).toEqual(treeNodeProxy.label)
        expect(rawNodeProxy.childs[0].text).toEqual(treeNodeProxy.children[0].label)
        expect(rawNodeProxy.childs[0].childs[0].text).toEqual(
          treeNodeProxy.children[0].children[0].label
        )
      })
      ...
    })
    

    实现原理

    TreeNode

    这个利用ES6的Class简单的实现了一下TreeNode

    class TreeNode {
      constructor(id, label, children) {
        this.id = id
        this.label = label;
        this.children = children ?? [];
      }
    }
    

    Watcher

    这里主要是通过Proxy进行代理拦截,然后通过Event推送出去

    class Watcher {
      constructor(target) {
        this.event = new Event(); // 用于变更通知
        this.toProxy = new WeakMap(); // WeakMap 有个特别好的特性,可以自动移除未引用的对象
        this.toRaw = new WeakMap();
        this.proxy = this.reactive(target, target);
      }
    
      reactive(target, lastTarget) { // 嵌套响应式
        if (!isObject(target) || this.toRaw.has(target)) { // 如果当前是代理,或者不是对象则返回
          return target;
        }
    
        if (this.toProxy.has(target)) { // 如果当前对象以及代理则返回代理
          return this.toProxy.get(target);
        }
    
        const currentNode = isArray(lastTarget) ? target : lastTarget; // 获取当前的节点
    
        const handler = {
          get: this.createGetter(),
          set: this.createSetter(currentNode),
          deleteProperty: this.createDeleteProperty(currentNode),
        };
    
        const observer = new Proxy(target, handler);
        this.toProxy.set(target, observer); // 建立原始对象和代理
        this.toRaw.set(observer, target);   // 对象的映射关系
        return observer;
      }
    
      bindHandler(type, callback) { // 绑定一个通知
        this.event.on(type, callback);
      }
    
      createGetter() {
        return (target, key) => {
          const res = Reflect.get(target, key);
          return isObject(res) ? this.reactive(res, target) : res;
                 // 如果是对象,则继续嵌套代理,如果不是对象则返回这个值
        };
      }
    
      createSetter(currentNode) {
        return (target, key, value) => {
          if (this.toRaw.has(value)) { // 如果写入的是已经被代理的对象,则先转换为普通对象
            value = this.toRaw.get(value);
          }
          if (isArray(target)) { 
            if (key === "length") {
              this.event.emit("array/changeLength", {
                target,
                key,
                value,
                currentNode,
              });
            } else {
              if (Reflect.has(target, key)) {
                // 修改
                this.event.emit("array/change", {
                  target,
                  key,
                  value,
                  currentNode,
                });
              } else {
                // 新增
                this.event.emit("array/append", {
                  target,
                  key,
                  value,
                  currentNode,
                });
              }
            }
          } else {
            if (Reflect.has(target, key)) {
              // 修改
              this.event.emit("change", { target, key, value, currentNode });
            } else {
              // 新增
              this.event.emit("add", { target, key, value, currentNode });
            }
          }
          return Reflect.set(target, key, value);
        };
      }
    
      createDeleteProperty(currentNode) {
        return (target, key) => {
          if (isArray(target)) {
            this.event.emit("array/delete", { target, key, currentNode });
          } else {
            this.event.emit("delete", { target, key, currentNode });
          }
          return Reflect.deleteProperty(target, key);
        };
      }
    }
    

    如果看过Vue3的Reactivity这个库,你会发现我这里的Watcher与Reactivity有些类似,其实Watcher的内部就是借鉴了一些Vue3Reactive的实现。

    Event

    简单的实现一个事件通知

    class Event {
      constructor() {
        this.events = new Map();
      }
    
      on(name, callback) {
        if (!this.events.has(name)) {
          this.events.set(name, new Set([callback]));
          return;
        }
        this.events.get(name).add(callback);
      }
    
      emit(name, ...args) {
        if (this.events.has(name)) {
          this.events.get(name).forEach((cb) => cb(...args));
        }
      }
    }
    

    Mapper

    这个组件的实现大概是这几步

    1、转换RawNode -> TreeNode

    2、对RawNode进行Watcher监听

    3、对TreeNode进行Watcher监听

    4、当发生变更通知后,对原数据进行修改

    
    class Mapper {
      constructor(rawNode, keyMap) {
        this.toTreeNode = new WeakMap();
        this.toRawNode = new WeakMap();
        this.toRawNodeKey = keyMap;
        this.toTreeNodeKey = reversalNodeKeyMap(keyMap); // 反向 NodeKey
        // 初始化
    
        this.rawNode = rawNode;
        this.treeNode = this.convertToTreeNode(rawNode);
        // 生成TreeNode
    
        this.rawNodeWatcher = new Watcher(this.rawNode);
        this.treeNodeWatcher = new Watcher(this.treeNode);
        this.withRawNodeHandler();
        this.withTreeNodeHandler();
        // 对 rawNode 与 treeNode 分别进行响应式处理
      }
    
      convertToTreeNode(rawNode) {
        const treeNode = new TreeNode(
          rawNode[this.toRawNodeKey.id],
          rawNode[this.toRawNodeKey.label],
          this.convertToTreeNodes(rawNode[this.toRawNodeKey.children]),
          { isChecked: rawNode[this.toRawNodeKey.isChecked] }
        );
        this.toTreeNode.set(rawNode, treeNode);
        this.toRawNode.set(treeNode, rawNode);
        return treeNode;
      }
    
      convertToRawNode(treeNode) {
        const rawNode = {
          [this.toRawNodeKey.id]: treeNode.id,
          [this.toRawNodeKey.label]: treeNode.label,
          [this.toRawNodeKey.children]: this.convertToRawNodes(treeNode.children),
        };
        this.toTreeNode.set(rawNode, treeNode);
        this.toRawNode.set(treeNode, rawNode);
        return rawNode;
      }
    
      convertToTreeNodes(rawNodes) {
        return rawNodes?.map((node) => this.convertToTreeNode(node));
      }
    
      convertToRawNodes(treeNodes) {
        return treeNodes?.map((node) => this.convertToRawNode(node));
      }
    
      withRawNodeHandler() {
        this.rawNodeWatcher.bindHandler(
          "array/append",
          ({ currentNode, value }) => {
            const currentTreeNode = this.toTreeNode.get(currentNode);
            this.forTreeNodeAppendChild(
              currentTreeNode,
              this.convertToTreeNode(value)
            );
          }
        );
    
        this.rawNodeWatcher.bindHandler("array/delete", ({ currentNode, key }) => {
          const currentTreeNode = this.toTreeNode.get(currentNode);
          this.forTreeNodeRemoveChild(currentTreeNode, key);
        });
    
        this.rawNodeWatcher.bindHandler(
          "array/change",
          ({ currentNode, key, value }) => {
            const currentTreeNode = this.toTreeNode.get(currentNode);
            this.forTreeNodeUpdateChild(
              currentTreeNode,
              key,
              this.toTreeNode.get(value) ?? this.convertToTreeNode(value)
            );
          }
        );
    
        this.rawNodeWatcher.bindHandler("change", ({ currentNode, key, value }) => {
          const currentTreeNode = this.toTreeNode.get(currentNode);
          this.forTreeNodeUpdateValue(
            currentTreeNode,
            this.toTreeNodeKey[key],
            value
          );
        });
    
        this.rawNodeWatcher.bindHandler("add", ({ currentNode, key, value }) => {
          const currentTreeNode = this.toTreeNode.get(currentNode);
          this.forTreeNodeUpdateValue(
            currentTreeNode,
            this.toTreeNodeKey[key],
            value
          );
        });
      }
    
      withTreeNodeHandler() {
        this.treeNodeWatcher.bindHandler(
          "array/append",
          ({ currentNode, value }) => {
            const currentRawNode = this.toRawNode.get(currentNode);
            this.forRawNodeAppendChild(
              currentRawNode,
              this.convertToRawNode(value)
            );
          }
        );
    
        this.treeNodeWatcher.bindHandler("array/delete", ({ currentNode, key }) => {
          const currentRawNode = this.toRawNode.get(currentNode);
          this.forRawNodeRemoveChild(currentRawNode, key);
        });
    
        this.treeNodeWatcher.bindHandler(
          "array/change",
          ({ currentNode, key, value }) => {
            const currentRawNode = this.toRawNode.get(currentNode);
            this.forRawNodeUpdateChild(currentRawNode, key, value);
          }
        );
    
        this.treeNodeWatcher.bindHandler(
          "change",
          ({ currentNode, key, value }) => {
            const currentRawNode = this.toRawNode.get(currentNode);
            this.forRawNodeUpdateValue(
              currentRawNode,
              this.toRawNodeKey[key],
              value
            );
          }
        );
    
        this.treeNodeWatcher.bindHandler("add", ({ currentNode, key, value }) => {
          const currentRawNode = this.toRawNode.get(currentNode);
          this.forRawNodeUpdateValue(currentRawNode, this.toRawNodeKey[key], value);
        });
      }
    
      forTreeNodeAppendChild(currentTreeNode, newTreeNode) {
        currentTreeNode.children.push(newTreeNode);
      }
    
      forTreeNodeUpdateValue(currentTreeNode, key, value) {
        if (key === "children") {
          currentTreeNode[key] = this.convertToTreeNodes(value);
        } else {
          currentTreeNode[key] = value;
        }
      }
    
      forTreeNodeRemoveChild(currentTreeNode, index) {
        // TODO: 这里还得对toRawNode 与 toTreeNode 进行处理,不然会内存泄漏
        currentTreeNode.children.splice(index, 1);
      }
    
      forTreeNodeUpdateChild(currentTreeNode, index, childNode) {
        currentTreeNode.children[index] = childNode;
      }
    
      forRawNodeAppendChild(currentRawNode, newRawNode) {
        currentRawNode[this.toRawNodeKey.children].push(newRawNode);
      }
    
      forRawNodeUpdateValue(currentRawNode, key, value) {
        if (key === this.toRawNodeKey["children"]) {
          currentRawNode[key] = this.convertToRawNodes(value);
        } else if (Reflect.has(currentRawNode, key)) {
          currentRawNode[key] = value;
        }
      }
    
      forRawNodeRemoveChild(currentRawNode, index) {
        currentRawNode[this.toRawNodeKey.children].splice(index, 1);
      }
    
      forRawNodeUpdateChild(currentRawNode, index, childNode) {
        currentRawNode[this.toRawNodeKey.children][index] = childNode;
      }
    }
    
    

    结束

    在开发Tree的时候其实,还要比上面写的要复杂,这里是简化了一些操作,实际上在真正的Tree里他还需要将TreeNode进行Vue的reactive响应化,但是在开发的过程中,Vue的响应化和Watcher的响应化有一些冲突,所以会有一些很坑人的事情发生,不过最后也用了一些方法解决了,这里你可以去看Tree的源码学习。这里我说了这么多其实想说的是,当你遇到大坑的时候,将这个大坑解决掉,可以加深对这个坑所对应的问题理解,这就像是摸着坑过河吧。

    最后,也是非常希望更多人可以加入我们花果山团队,来和我们一起做开源吧

    https://github.com/hug-sun/element3

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