This time I will bring you a detailed explanation of the use cases of computed in Vue.js. What are the precautions for using computed in Vue.js? Here are the actual cases, let’s take a look.
JS properties:
JavaScriptOne property is Object<a href="http://www.php.cn/wiki/60.html" target="_blank">.defineProperty </a>, it can do many things, but I only focus on one of these methods in this article:
var person = {}; Object.defineProperty (person, 'age', { get: function () { console.log ("Getting the age"); return 25; } }); console.log ("The age is ", person.age); // Prints: // // Getting the age // The age is 25
A basically responsive Vue.js
Vue.js internally builds an object that can convert ordinary objects into values that can be observed (responsive properties). The following shows you a simplified version of how to add response attributes:function defineReactive (obj, key, val) { Object.defineProperty (obj, key, { get: function () { return val; }, set: function (newValue) { val = newValue; } }) }; // 创建一个对象 var person = {}; // 添加可响应的属性"age"和"country" defineReactive (person, 'age', 25); defineReactive (person, 'country', 'Brazil'); // 现在你可以随意使用person.age了 if (person.age < 18) { return 'minor'; } else { return 'adult'; } // 设置person.country的值 person.country = 'Russia';
Declaring a computed property
Let’s create a function defineComputed that defines a computed property. This function is the same as when you usually use computed.defineComputed ( person, // 计算属性就声明在这个对象上 'status', // 计算属性的名称 function () { // 实际返回计算属性值的函数 console.log ("status getter called") if (person.age < 18) { return 'minor'; } else { return 'adult'; } }, function (newValue) { // 当计算属性值更新时调用的函数 console.log ("status has changed to", newValue) } }); // 我们可以像使用一般的属性一样使用计算属性 console.log ("The person's status is: ", person.status);
function defineComputed (obj, key, computeFunc, updateCallback) { Object.defineProperty (obj, key, { get: function () { // 执行计算函数并且返回值 return computeFunc (); }, set: function () { // 什么也不做,不需要设定计算属性的值 } }) }
// 我希望最终函数执行后是这个效果:每当person.age更新值的时候,person.status也同步更新 person.age = 17; // console: status 的值为 minor person.age = 22; // console: status 的值为 adult
Add a dependency
Let’s add A global variable Dep:var Dep = { target: null };
function defineComputed (obj, key, computeFunc, updateCallback) { var onDependencyUpdated = function () { // TODO } Object.defineProperty (obj, key, { get: function () { // 将onDependencyUpdated 这个函数传给Dep.target Dep.target = onDependencyUpdated; var value = computeFunc (); Dep.target = null; }, set: function () { // 什么也不做,不需要设定计算属性的值 } }) }
function defineReactive (obj, key, val) { // 所有的计算属性都依赖这个数组 var deps = []; Object.defineProperty (obj, key, { get: function () { // 检查是否调用了计算属性,如果调用了,Department.target将等于一个onDependencyUpdated函数 if (Dep.target) { // 把onDependencyUpdated函数push到deos中 deps.push (target); } return val; }, set: function (newValue) { val = newValue; // 通知所有的计算属性,告诉它们有个响应属性更新了 deps.forEach ((changeFunction) => { changeFunction (); }); } }) };
var onDependencyUpdated = function () { // 再次计算 计算属性的值 var value = computeFunc (); updateCallback (value); }
person.age = 22; defineComputed ( person, 'status', function () { if (person.age > 18) { return 'adult'; } }, function (newValue) { console.log ("status has changed to", newValue) } }); console.log ("Status is ", person.status);
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