<div id="app">
  <p>{{fullName}}:{{fullName}}-{{formBY}}</p>
</div>

const vm = new Vue({
    el: "#app",
    data() {
        return {
            firstName: "Shiina",
            lastName: "Mashiro",
            formBY: "flytree-cnblogs",
            arr: [1, 2, 3, ["a"]],
        };
    },
    computed: {
        fullName() {
            return this.firstName + this.lastName;
        }
    },
    watch: {
        firstName(newValue, oldValue) {
            console.log(newValue, oldValue)
        }
    }
});

setTimeout(() => {
    vm.firstName = 'flytree'
}, 1000);

// 路径 /scr/core/observer/index.js
export function defineReactive() {
    const dep = new Dep()

    const property = Object.getOwnPropertyDescriptor(obj, key)
    if (property && property.configurable === false) {
        return
    }

    // cater for pre-defined getter/setters
    const getter = property && property.get
    const setter = property && property.set
    if ((!getter || setter) && arguments.length === 2) {
        val = obj[key]
    }

    let childOb = !shallow && observe(val)
    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) {
            const value = getter ? getter.call(obj) : val
            /* eslint-disable no-self-compare */
            if (newVal === value || (newVal !== newVal && value !== value)) {
                return
            }
            /* eslint-enable no-self-compare */
            if (process.env.NODE_ENV !== 'production' && customSetter) {
                customSetter()
            }
            // #7981: for accessor properties without setter
            if (getter && !setter) return
            if (setter) {
                setter.call(obj, newVal)
            } else {
                val = newVal
            }
            childOb = !shallow && observe(newVal)
            dep.notify()
        }
    })
}

export default class Dep {
  constructor () {
    this.id = uid++
    this.subs = []
  }

  addSub (sub) {
    this.subs.push(sub)
  }

  removeSub (sub) {
    remove(this.subs, sub)
  }

  depend () {
    if (Dep.target) {
      Dep.target.addDep(this)
    }
  }

  notify () {
    // stabilize the subscriber list first
    const subs = this.subs.slice()
    if (process.env.NODE_ENV !== 'production' && !config.async) {
      // subs aren't sorted in scheduler if not running async
      // we need to sort them now to make sure they fire in correct
      // order
      subs.sort((a, b) => a.id - b.id)
    }
    for (let i = 0, l = subs.length; i < l; i++) {
      subs[i].update()
    }
  }
}

export default class Watcher {
    constructor(vm, expOrFn, cb, options, isRenderWatcher) {
        this.vm = vm
        if (isRenderWatcher) {
            vm._watcher = this
        }
        vm._watchers.push(this)
        // options
        if (options) {
            this.deep = !!options.deep
            this.user = !!options.user
            this.lazy = !!options.lazy
            this.sync = !!options.sync
            this.before = options.before
        } else {
            this.deep = this.user = this.lazy = this.sync = false
        }
        this.cb = cb
        this.id = ++uid // uid for batching
        this.active = true
        this.dirty = this.lazy // for lazy watchers
        this.deps = []
        this.newDeps = []
        this.depIds = new Set()
        this.newDepIds = new Set()
        this.expression = process.env.NODE_ENV !== 'production'
            ? expOrFn.toString()
            : ''
        // parse expression for getter
        if (typeof expOrFn === 'function') {
            // 渲染watcher时就gettr就传入了 _update(_render())
            this.getter = expOrFn
        } else {
            this.getter = parsePath(expOrFn)
            if (!this.getter) {
                this.getter = noop
                process.env.NODE_ENV !== 'production' && warn(
                    `Failed watching path: "${expOrFn}" ` +
                    'Watcher only accepts simple dot-delimited paths. ' +
                    'For full control, use a function instead.',
                    vm
                )
            }
        }
        // 在计算属性创建watcher的时候lazy为true
        this.value = this.lazy
            ? undefined
            : this.get()
    }

    /**
     * Evaluate the getter, and re-collect dependencies.
     */
    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
    }

    /**
     * Add a dependency to this directive.
     */
    addDep(dep) {
        const id = dep.id
        if (!this.newDepIds.has(id)) {
            this.newDepIds.add(id)
            this.newDeps.push(dep)
            if (!this.depIds.has(id)) {
                dep.addSub(this)
            }
        }
    }

    /**
     * Clean up for dependency collection.
     */
    cleanupDeps() {
        let i = this.deps.length
        while (i--) {
            const dep = this.deps[i]
            if (!this.newDepIds.has(dep.id)) {
                dep.removeSub(this)
            }
        }
        let tmp = this.depIds
        this.depIds = this.newDepIds
        this.newDepIds = tmp
        this.newDepIds.clear()
        tmp = this.deps
        this.deps = this.newDeps
        this.newDeps = tmp
        this.newDeps.length = 0
    }

    /**
     * Subscriber interface.
     * Will be called when a dependency changes.
     */
    update() {
        /* istanbul ignore else */
        if (this.lazy) {
            this.dirty = true
        } else if (this.sync) {
            this.run()
        } else {
            queueWatcher(this)
        }
    }

    /**
     * Scheduler job interface.
     * Will be called by the scheduler.
     */
    run() {
        if (this.active) {
            const value = this.get()
            if (
                value !== this.value ||
                // Deep watchers and watchers on Object/Arrays should fire even
                // when the value is the same, because the value may
                // have mutated.
                isObject(value) ||
                this.deep
            ) {
                // set new value
                const oldValue = this.value
                this.value = value
                if (this.user) {
                    const info = `callback for watcher "${this.expression}"`
                    invokeWithErrorHandling(this.cb, this.vm, [value, oldValue], this.vm, info)
                } else {
                    this.cb.call(this.vm, value, oldValue)
                }
            }
        }
    }

    /**
     * Evaluate the value of the watcher.
     * This only gets called for lazy watchers.
     */
    evaluate() {
        this.value = this.get()
        this.dirty = false
    }

    /**
     * Depend on all deps collected by this watcher.
     */
    depend() {
        let i = this.deps.length
        while (i--) {
            this.deps[i].depend()
        }
    }

    /**
     * Remove self from all dependencies' subscriber list.
     */
    teardown() {
        if (this.active) {
            // remove self from vm's watcher list
            // this is a somewhat expensive operation so we skip it
            // if the vm is being destroyed.
            if (!this.vm._isBeingDestroyed) {
                remove(this.vm._watchers, this)
            }
            let i = this.deps.length
            while (i--) {
                this.deps[i].removeSub(this)
            }
            this.active = false
        }
    }
}

export function lifecycleMixin (Vue) {
  Vue.prototype._update = function (vnode, hydrating) {
    const vm = this
    const prevEl = vm.$el
    const prevVnode = vm._vnode
    const restoreActiveInstance = setActiveInstance(vm)
    vm._vnode = vnode
    // Vue.prototype.__patch__ is injected in entry points
    // based on the rendering backend used.
    if (!prevVnode) {
      // initial render
      vm.$el = vm.__patch__(vm.$el, vnode, hydrating, false /* removeOnly */)
    } else {
      // updates
      vm.$el = vm.__patch__(prevVnode, vnode)
    }
    restoreActiveInstance()
    // update __vue__ reference
    if (prevEl) {
      prevEl.__vue__ = null
    }
    if (vm.$el) {
      vm.$el.__vue__ = vm
    }
    // if parent is an HOC, update its $el as well
    if (vm.$vnode && vm.$parent && vm.$vnode === vm.$parent._vnode) {
      vm.$parent.$el = vm.$el
    }
    // updated hook is called by the scheduler to ensure that children are
    // updated in a parent's updated hook.
  }
}