【Vue】vue中computed源码详解

默认computed也是一个watcher，具备缓存，只有当依赖的属性发生变化才会更新视图。

原理图：

流程：computed watcher在defineReactive的get中订阅属性的变化（4），在defineReactive的set时触发notify（4），notify调用每个订阅了改属性变化的watcher的update（3），update中将computed watcher 的dirty赋值为true（2），notify中其中一个watcher为渲染watcher，此时执行渲染，渲染时因为模板中使用了computed watcher对应的计算属性，故而触发计算属性的get方法createComputedGetter，在createComputedGetter中调用computed watcher 的evaluate方法（1），evaluate方法调用computed watcher的this.get（5），进而调用用户在计算属性上定义的方法进行计算。

源码：

1、定义computed 的getter方法（在dirty为true时，使用缓存数据）（src\core\instance\state.js）：

const computedWatcherOptions = { lazy: true }
function initComputed (vm: Component, computed: Object) {
    ...
    if (!isSSR) {
      // create internal watcher for the computed property.
      watchers[key] = new Watcher(
        vm,
        getter || noop,//将用户定义传到watcher中
        noop,
        computedWatcherOptions//lazy:true懒watcher
      )
    }

    // component-defined computed properties are already defined on the
    // component prototype. We only need to define computed properties defined
    // at instantiation here.
    if (!(key in vm)) {
      defineComputed(vm, key, userDef)
    } else if (process.env.NODE_ENV !== 'production') {
      if (key in vm.$data) {
        warn(`The computed property "${key}" is already defined in data.`, vm)
      } else if (vm.$options.props && key in vm.$options.props) {
        warn(`The computed property "${key}" is already defined as a prop.`, vm)
      }
    }
  }
}

export function defineComputed (
  target: any,
  key: string,
  userDef: Object | Function
) {
  const shouldCache = !isServerRendering()
  if (typeof userDef === 'function') {
    sharedPropertyDefinition.get = shouldCache
      ? createComputedGetter(key)//创建计算属性的getter，不是用用户传的
      : createGetterInvoker(userDef)
    sharedPropertyDefinition.set = noop
  } else {
    sharedPropertyDefinition.get = userDef.get
      ? shouldCache && userDef.cache !== false
        ? createComputedGetter(key)
        : createGetterInvoker(userDef.get)
      : noop
    sharedPropertyDefinition.set = userDef.set || noop
  }
  if (process.env.NODE_ENV !== 'production' &&
      sharedPropertyDefinition.set === noop) {
    sharedPropertyDefinition.set = function () {
      warn(
        `Computed property "${key}" was assigned to but it has no setter.`,
        this
      )
    }
  }
  Object.defineProperty(target, key, sharedPropertyDefinition)
}

function createComputedGetter (key) {
  return function computedGetter () {//用户取值的时候会调用此方法
    const watcher = this._computedWatchers && this._computedWatchers[key]
    if (watcher) {
      if (watcher.dirty) {//dirty为true会去进行求值，这儿的dirty起到了缓存的作用
        watcher.evaluate()
      }
      if (Dep.target) {
        watcher.depend()
      }
      return watcher.value
    }
  }
}

2、computed的getter方法中dirty的赋值时机（src\core\observer\watcher.js）：

update () {
    /* istanbul ignore else */
    if (this.lazy) {
      this.dirty = true
    } else if (this.sync) {
      this.run()
    } else {
      queueWatcher(this)
    }
  }

3、update的调用时机（调用当前属性的订阅中每一个watcher的update）（src\core\observer\dep.js）：

notify () {
    ...
    for (let i = 0, l = subs.length; i < l; i++) {
      subs[i].update()
    }
  }

4、notify的调用时机(src\core\observer\index.js中 defineReactive的set)以及watcher在defineReactive的get订阅属性的变化（其中dep.target的赋值是在new Watcher时的this.get）：

export function defineReactive (
  obj: Object,
  key: string,
  val: any,
  customSetter?: ?Function,
  shallow?: boolean
) {
  ...
  Object.defineProperty(obj, key, {
    enumerable: true,
    configurable: true,
    get: function reactiveGetter () {
      const value = getter ? getter.call(obj) : val
      if (Dep.target) {
        dep.depend()
        if (childOb) {
          childOb.dep.depend()
          if (Array.isArray(value)) {
            dependArray(value)
          }
        }
      }
      return value
    },
    set: function reactiveSetter (newVal) {
      ...
      dep.notify()
    }
  })
}

dep.target的赋值是在new Watcher时调用this.get（src\core\observer\watcher.js）：

constructor (
    vm: Component,
    expOrFn: string | Function,
    cb: Function,
    options?: ?Object,
    isRenderWatcher?: boolean
  ) {
    ...
    this.value = this.lazy
      ? undefined
      : this.get()
  }

get () {
    pushTarget(this)
    let value
    const vm = this.vm
    try {
      value = this.getter.call(vm, vm)
    } catch (e) {
      if (this.user) {
        handleError(e, vm, `getter for watcher "${this.expression}"`)
      } else {
        throw e
      }
    } finally {
      // "touch" every property so they are all tracked as
      // dependencies for deep watching
      if (this.deep) {
        traverse(value)
      }
      popTarget()
      this.cleanupDeps()
    }
    return value
  }

5、使用用户定义的计算属性的getter方法计算值（src\core\observer\watcher.js）：

get () {
    pushTarget(this)
    let value
    const vm = this.vm
    try {
      value = this.getter.call(vm, vm)
    } catch (e) {
      if (this.user) {
        handleError(e, vm, `getter for watcher "${this.expression}"`)
      } else {
        throw e
      }
    } finally {
      // "touch" every property so they are all tracked as
      // dependencies for deep watching
      if (this.deep) {
        traverse(value)
      }
      popTarget()
      this.cleanupDeps()
    }
    return value
  }