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How are Vue3 computed properties implemented? Let’s talk about implementation principles

青灯夜游
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2022-04-13 21:02:48 3885browse

How are Vue3 computed properties implemented? The following article will give you a brief analysis of the implementation principles of calculated properties in Vue3.0. I hope it will be helpful to you!

How are Vue3 computed properties implemented? Let’s talk about implementation principles

Computed properties are a very practical API inVue.jsdevelopment. It allows users to define a calculation method and then respond based on some dependencies. The data calculates the new value and returns it. When dependencies change, calculated properties can be automatically recalculated to obtain new values, so they are very convenient to use.

In Vue.js 2.x, I believe you are already familiar with the application of computed properties. We can define the computed property in the component object. With Vue.js 3.0, although the usage of Vue.js 2.x can also be used in components, we can also use the calculated property API alone.

Computed properties are essentially calculations of dependencies, so why don’t we use functions directly? How is the API for calculating properties implemented in Vue.js 3.0? This article will analyze the implementation principle of calculated properties. (Learning video sharing:vuejs tutorial)

Computed property API: computed

Vue.js 3.0 provides a The computed function serves as a computed attribute API. Let's first take a look at how it is used.

Let’s take a simple example:

const count = ref(1) const plusOne = computed(() => count.value + 1) console.log(plusOne.value) // 2 plusOne.value++ // error count.value++ console.log(plusOne.value) // 3

As you can see from the code, we first used the ref API to create a responsive object count, and then used the computed API to create another responsive object. Object plusOne, its value is count.value 1. When we modify count.value, plusOne.value will automatically change.

Note that if we directly modify plusOne.value here, an error will be reported. This is because if what we pass to computed is a function, then this is a getter function, and we can only get its value, not Modify it directly.

In the getter function, we will recalculate the new value based on the responsive object, which is why it is called a calculated property, and this responsive object is a calculated property dependence.

Of course, sometimes we also want to be able to directly modify the return value of computed, then we can pass in an object to computed:

const count = ref(1) const plusOne = computed({ get: () => count.value + 1, set: val => { count.value = val - 1 } }) plusOne.value = 1 console.log(count.value) // 0

In this example, combined with the above code, you can see that, We passed in an object with a getter function and a setter function to the computed function. The getter function returns count.value 1 as before; as for the setter function, please note that when we modify the value of plusOne.value here, the setter function will be triggered. , in fact, the setter function will actually modify the dependency value count.value of the calculated attribute according to the passed in parameters, because once the dependent value is modified, we will re-execute the getter when we get the calculated attribute, so the value obtained in this way There has been a change.

Okay, we now know the two ways to use the computed API, let’s see how it is implemented:

function computed(getterOrOptions) { // getter 函数 let getter // setter 函数 let setter // 标准化参数 if (isFunction(getterOrOptions)) { // 表面传入的是 getter 函数,不能修改计算属性的值 getter = getterOrOptions setter = (process.env.NODE_ENV !== 'production') ? () => { console.warn('Write operation failed: computed value is readonly') } : NOOP } else { getter = getterOrOptions.get setter = getterOrOptions.set } // 数据是否脏的 let dirty = true // 计算结果 let value let computed // 创建副作用函数 const runner = effect(getter, { // 延时执行 lazy: true, // 标记这是一个 computed effect 用于在 trigger 阶段的优先级排序 computed: true, // 调度执行的实现 scheduler: () => { if (!dirty) { dirty = true // 派发通知,通知运行访问该计算属性的 activeEffect trigger(computed, "set" /* SET */, 'value') } } }) // 创建 computed 对象 computed = { __v_isRef: true, // 暴露 effect 对象以便计算属性可以停止计算 effect: runner, get value() { // 计算属性的 getter if (dirty) { // 只有数据为脏的时候才会重新计算 value = runner() dirty = false } // 依赖收集,收集运行访问该计算属性的 activeEffect track(computed, "get" /* GET */, 'value') return value }, set value(newValue) { // 计算属性的 setter setter(newValue) } } return computed }

As you can see from the code, the computed function The process mainly does three things:standardize parameters,create side-effect functionsandcreate computed objects.Let’s analyze these steps in detail.

The first isstandardized parameters. The computed function accepts two types of parameters, one is a getter function, and the other is an object with getter and setter functions. By judging the type of the parameters, we initialize the getter and setter functions defined inside the function.

The next step is to create theside effect function runner. Computed internally creates a side effect function through effect, which is a layer of encapsulation of the getter function. In addition, we should pay attention to the second parameter here, which is the configuration object of the effect function. Among them, lazy is true, which means that the runner returned by the effect function will not be executed immediately; computed is true, which means that this is a computed effect, which is used for the priority sorting of the trigger stage, which we will analyze later; scheduler means that its scheduling is run. method, we will also analyze it later.

Finally, the computed object is created and returned. This object also has getter and setter functions. When the computed object is accessed, the getter will be triggered, and then it will be judged whether it is dirty. If so, the runner will be executed, and then dependency collection will be done. When we set the computed object directly, the setter will be triggered, that is, the setter function defined inside the computed function will be executed.

The operating mechanism of computed properties

The logic of the computed function will be a little convoluted, but it doesn’t matter, we can combine it with a method that applies computed properties Examples to understand the operating mechanism of the entire computed property. Before analysis, we need to remember two important variables inside computed. The first dirty indicates whether the value of a calculated attribute is "dirty" and is used to determine whether it needs to be recalculated. The second value indicates each calculation of the calculated attribute. the final result.

Now, let’s look at this example:

 

可以看到,在这个例子中我们利用 computed API 创建了计算属性对象 plusOne,它传入的是一个 getter 函数,为了和后面计算属性对象的 getter 函数区分,我们把它称作 computed getter。另外,组件模板中引用了 plusOne 变量和 plus 函数。

组件渲染阶段会访问 plusOne,也就触发了 plusOne 对象的 getter 函数:

get value() { // 计算属性的 getter if (dirty) { // 只有数据为脏的时候才会重新计算 value = runner() dirty = false } // 依赖收集,收集运行访问该计算属性的 activeEffect track(computed, "get" /* GET */, 'value') return value }

由于默认 dirty 是 true,所以这个时候会执行 runner 函数,并进一步执行 computed getter,也就是 count.value + 1,因为访问了 count 的值,并且由于 count 也是一个响应式对象,所以就会触发 count 对象的依赖收集过程。

请注意,由于是在 runner 执行的时候访问 count,所以这个时候的 activeEffect 是 runner 函数。runner 函数执行完毕,会把 dirty 设置为 false,并进一步执行 track(computed,"get",'value') 函数做依赖收集,这个时候 runner 已经执行完了,所以 activeEffect 是组件副作用渲染函数。

所以你要特别注意这是两个依赖收集过程:对于 plusOne 来说,它收集的依赖是组件副作用渲染函数;对于 count 来说,它收集的依赖是 plusOne 内部的 runner 函数。

然后当我们点击按钮的时候,会执行 plus 函数,函数内部通过 count.value++ 修改 count 的值,并派发通知。请注意,这里不是直接调用 runner 函数,而是把 runner 作为参数去执行 scheduler 函数。我们来回顾一下 trigger 函数内部对于 effect 函数的执行方式:

const run = (effect) => { // 调度执行 if (effect.options.scheduler) { effect.options.scheduler(effect) } else { // 直接运行 effect() } }

computed API 内部创建副作用函数时,已经配置了 scheduler 函数,如下:

scheduler: () => { if (!dirty) { dirty = true // 派发通知,通知运行访问该计算属性的 activeEffect trigger(computed, "set" /* SET */, 'value') } }

它并没有对计算属性求新值,而仅仅是把 dirty 设置为 true,再执行 trigger(computed, "set" , 'value'),去通知执行 plusOne 依赖的组件渲染副作用函数,即触发组件的重新渲染。

在组件重新渲染的时候,会再次访问 plusOne,我们发现这个时候 dirty 为 true,然后会再次执行 computed getter,此时才会执行 count.value + 1 求得新值。这就是虽然组件没有直接访问 count,但是当我们修改 count 的值的时候,组件仍然会重新渲染的原因。

通过下图可以直观的展现上述过程:

How are Vue3 computed properties implemented? Let’s talk about implementation principles

通过以上分析,我们可以看出 computed 计算属性有两个特点:

  • 延时计算,只有当我们访问计算属性的时候,它才会真正运行 computed getter 函数计算;

  • 缓存,它的内部会缓存上次的计算结果 value,而且只有 dirty 为 true 时才会重新计算。如果访问计算属性时 dirty 为 false,那么直接返回这个 value。

现在,我们就可以回答开头提的问题了。和单纯使用普通函数相比,计算属性的优势是:只要依赖不变化,就可以使用缓存的 value 而不用每次在渲染组件的时候都执行函数去计算,这是典型的空间换时间的优化思想。

嵌套计算属性

计算属性也支持嵌套,我们可以针对上述例子做个小修改,即不在渲染函数中访问 plusOne,而在另一个计算属性中访问:

const count = ref(0) const plusOne = computed(() => { return count.value + 1 }) const plusTwo = computed(() => { return plusOne.value + 1 }) console.log(plusTwo.value)

从代码中可以看到,当我们访问 plusTwo 的时候,过程和前面都差不多,同样也是两个依赖收集的过程。对于 plusOne 来说,它收集的依赖是 plusTwo 内部的 runner 函数;对于 count 来说,它收集的依赖是 plusOne 内部的 runner 函数。

接着当我们修改 count 的值时,它会派发通知,先运行 plusOne 内部的 scheduler 函数,把 plusOne 内部的 dirty 变为 true,然后执行 trigger 函数再次派发通知,接着运行 plusTwo 内部的 scheduler 函数,把 plusTwo 内部的 dirty 设置为 true。

然后当我们再次访问 plusTwo 的值时,发现 dirty 为 true,就会执行 plusTwo 的 computed getter 函数去执行 plusOne.value + 1,进而执行 plusOne 的 computed gette 即 count.value + 1 + 1,求得最终新值 2。

得益于 computed 这种巧妙的设计,无论嵌套多少层计算属性都可以正常工作。

计算属性的执行顺序

我们曾提到计算属性内部创建副作用函数的时候会配置 computed 为 true,标识这是一个 computed effect,用于在 trigger 阶段的优先级排序。我们来回顾一下 trigger 函数执行 effects 的过程:

const add = (effectsToAdd) => { if (effectsToAdd) { effectsToAdd.forEach(effect => { if (effect !== activeEffect || !shouldTrack) { if (effect.options.computed) { computedRunners.add(effect) } else { effects.add(effect) } } }) } } const run = (effect) => { if (effect.options.scheduler) { effect.options.scheduler(effect) } else { effect() } } computedRunners.forEach(run) effects.forEach(run)

在添加待运行的 effects 的时候,我们会判断每一个 effect 是不是一个 computed effect,如果是的话会添加到 computedRunners 中,在后面运行的时候会优先执行 computedRunners,然后再执行普通的 effects。

那么为什么要这么设计呢?其实是考虑到了一些特殊场景,我们通过一个示例来说明:

import { ref, computed } from 'vue' import { effect } from '@vue/reactivity' const count = ref(0) const plusOne = computed(() => { return count.value + 1 }) effect(() => { console.log(plusOne.value + count.value) }) function plus() { count.value++ } plus()

这个示例运行后的结果输出:

1 3 3

在执行 effect 函数时运行 console.log(plusOne.value + count.value),所以第一次输出 1,此时 count.value 是 0,plusOne.value 是 1。

后面连续输出两次 3 是因为, plusOne 和 count 的依赖都是这个 effect 函数,所以当我们执行 plus 函数修改 count 的值时,会触发并执行这个 effect 函数,因为 plusOne 的 runner 也是 count 的依赖,count 值修改也会执行 plusOne 的 runner,也就会再次执行 plusOne 的依赖即 effect 函数,因此会输出两次。

那么为什么两次都输出 3 呢?这就跟先执行 computed runner 有关。首先,由于 plusOne 的 runner 和 effect 都是 count 的依赖,当我们修改 count 值的时候, plusOne 的 runner 和 effect 都会执行,那么此时执行顺序就很重要了。

这里先执行 plusOne 的 runner,把 plusOne 的 dirty 设置为 true,然后通知它的依赖 effect 执行 plusOne.value + count.value。这个时候,由于 dirty 为 true,就会再次执行 plusOne 的 getter 计算新值,拿到了新值 2, 再加上 1 就得到 3。执行完 plusOne 的 runner 以及依赖更新之后,再去执行 count 的普通effect 依赖,从而去执行 plusOne.value + count.value,这个时候 plusOne dirty 为 false, 直接返回上次的计算结果 2,然后再加 1 就又得到 3。

如果我们把 computed runner 和 effect 的执行顺序换一下会怎样呢?我来告诉你,会输出如下结果:

1 2 3

第一次输出 1 很好理解,因为流程是一样的。第二次为什么会输出 2 呢?我们来分析一下,当我们执行 plus 函数修改 count 的值时,会触发 plusOne 的 runner 和 effect 的执行,这一次我们先让 effect 执行 plusOne.value + count.value,那么就会访问 plusOne.value,但由于 plusOne 的 runner 还没执行,所以此时 dirty 为 false,得到的值还是上一次的计算结果 1,然后再加 1 得到 2。

接着再执行 plusOne 的 runner,把 plusOne 的 dirty 设置为 true,然后通知它的依赖 effect 执行 plusOne.value + count.value,这个时候由于 dirty 为 true,就会再次执行 plusOne 的 getter 计算新值,拿到了 2,然后再加上 1 就得到 3。

知道原因后,我们再回过头看例子。因为 effect 函数依赖了 plusOne 和 count,所以 plusOne 先计算会更合理,这就是为什么我们需要让 computed runner 的执行优先于普通的 effect 函数。

总结

本文分析了计算属性的工作机制、计算属性嵌套代码的执行顺序,以及计算属性的两个特点——延时计算和缓存。

本文转载自:https://juejin.cn/post/7085524315063451684

作者:风度前端

(学习视频分享:web前端开发

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