VUE计算属性原理

Posted 2020-12-25 liuyinlei

代码：

var obj = {};
var Dep = null;
function defineReactive(obj,key,val){
    var deps = [];
    //obj.a读写
    Object.defineProperty(obj,key,{
        //观察者模式
        get:function(){
            if(Dep){
                //console.log("Dep is:",Dep);
                deps.push(Dep);
            }
            console.log("valx is:",val);
            return val;
        },
        set: function(newVal){
            val = newVal;
            deps.forEach(func => func());
        }
    });
}
//obj.b只读
function defineComputed(obj,key,func){
    func = func.bind(obj);
    var value;
    Dep = function(){
        value = func();
    };
    value = func(); //执行了 this.a-->get-->打印val = 0;
    Dep = null;
    // console.log("Dep isx:",Dep);
    // 大量闭包
    Object.defineProperty(obj,key,{
        get:function(){
            return value
        }
    });
}
defineReactive(obj,‘a‘,0);
defineComputed(obj,‘b‘,function(){
    var a = this.a;
    return a+1;
});
console.log(obj.b);
// console.log("obj.a=",obj.a);
console.log("obj.a=",obj.a);
obj.a +=1;  //obj.a = obj.a + 1;
console.log(obj.b);
obj.a +=1;
console.log(obj.b);
obj.a +=1;
console.log(obj.b);

 很绕,难点是下面代码会执行两次a的get函数。第二次在set函数中的执行value = func(); func函数有this.a中执行了get。并+1赋值给闭包变量value。set函数结束之后。obj.a + 1;而

obj.a +=1;  //obj.a = obj.a + 1;