前言

首先要知道vue2 是2013年 基于 ES5开发出来的，我们常说的重渲染就是重新运行render函数

vue2 的响应式原理是利⽤ES5 的⼀个 API ，Object.defineProperty()对数据进⾏劫持结合发布订阅模式的⽅式来实现的。

// euv.js
/**
 * 观察某个对象的所有属性
 * @param {Object} obj
 */
function observe(obj) {
  for (const key in obj) {
    let internalValue = obj[key];
    let funcs = [];
    Object.defineProperty(obj, key, {
      get: function () {
        //  依赖收集，记录：是哪个函数在用我
        if (window.__func && !funcs.includes(window.__func)) {
          funcs.push(window.__func);
        }
        return internalValue;
      },
      set: function (val) {
        internalValue = val;
        // 派发更新，运行：执行用我的函数
        for (var i = 0; i < funcs.length; i++) {
          funcs[i]();
        }
      },
    });
  }
}

function autorun(fn) {
  window.__func = fn;
  fn();
  window.__func = null;
}

// index.js
var user = {
  name: '有骨气',
  birth: '1998-4-7',
};

observe(user); // 观察

// 显示姓氏
function showFirstName() {
  document.querySelector('#firstName').textContent = '姓：' + user.name[0];
}

// 显示名字
function showLastName() {
  document.querySelector('#lastName').textContent = '名：' + user.name.slice(1);
}

// 显示年龄
function showAge() {
  var birthday = new Date(user.birth);
  var today = new Date();
  today.setHours(0), today.setMinutes(0), today.setMilliseconds(0);
  thisYearBirthday = new Date(
    today.getFullYear(),
    birthday.getMonth(),
    birthday.getDate()
  );
  var age = today.getFullYear() - birthday.getFullYear();
  if (today.getTime() < thisYearBirthday.getTime()) {
    age--;
  }
  document.querySelector('#age').textContent = '年龄：' + age;
}
autorun(showFirstName);
autorun(showLastName);
autorun(showAge);

var obj = {
    a: 1,
    b: 2,
    c: {
        d: 3,
        e: 4
    },
    f: [
        {
            a: 1,
            b: 2
        },
        3, 4, 5, 6
    ]
}

// 利用Vue提供的静态方法 .observable, 将一个普通对象转化为响应式对象
var reactiveObj = Vue.observable(obj)
console.log(reactiveObj)

<template>
    <div class="demo-wrapper">
        <p>obj.a -> {{ obj.a }}， obj.b -> {{ obj.b }}</p>

        <!-- 非响应式式数据操作 -->
        <!-- 
            <button @click="obj.b = 2">add obj.b</button>
            <button @click="delete obj.a">delete obj.a</button>
        -->
        <!-- 响应式数据操作 -->
        <button @click="$set(obj, 'b', 2)">add obj.b</button>
        &nbsp;
        <button @click="$delete(obj, 'a')">delete obj.a</button>
    </div>
</template>

<script>
export default {
    data() {
        return {
            obj: {
                a: 1,
            },
        };
    },
};
</script>

<script>
export default {
    data() {
        return {
            arr: [1, 2, 3, 4],
        };
    },
    created() {
        window.lesson4 = this;
    },
    mounted() {
        this.arr[0] = 8; // 不是响应式的
        this.arr.length = 2; //不是响应式的
    }
};
</script>

<script>
export default {
    data() {
        return {
            arr: [1, 2, 3, 4],
        };
    },
    created() {
        window.lesson4 = this;
    },
    mounted() {
				// 操作一：通过索引修改数组项
        this.$set(arr, 0, 8); // 响应式的
        // 或
        Vue.set(arr, 0, 8);  // 响应式的
        // 或
        this.arr.splice(0, 1, 8) //响应式的
      
        // 操作二：修改数组长度为2
        this.arr.splice(2) // 响应式的
    }
};
</script>

<script>
  export default{
    data() {
      return {
        arr: [1,2,3,4]
      }
    },
    mounted() {
      console.log(this.arr._proto_ === Array.prototype)  // => false
      console.log(this.arr._proto_._proto_ === Array.prototype)  //=> true
    }
  }
</script>

<script>
export default {
    data() {
        return {
            a: 1,
            b: 2,
            c: 3,
            d: 4,
        };
    },
    methods: {
        changeAllData() {
            this.$nextTick(function () {
                var pre = document.querySelector("pre");
                console.log(pre.textContent);
            });

            this.a = this.b = this.c = this.d = 10;

            this.$nextTick(function () {
                var pre = document.querySelector("pre");
                console.log(pre.textContent);
            });
        },
    },
    render(h) {
        console.log('render function')
        return h('div', [
            h('pre', `${this.a}, ${this.b}, ${this.c}, ${this.d}`),
            h('button', {
                on: {
                    click: () => {
                        this.changeAllData()
                    }
                }
            }, 'change all data')
        ])
    }
};
</script>

<template>
  <span>{{a}}</span>
</template>
<script>
	export default {
    data() {
      return {
        a: 'hello'
      }
    },
    mounted() {
      this.a = 'world'
      console.log(this.$el.textContent)  // -> 'hello'
      this.$nextTick(function() {
        console.log(this.$el.textContent) // -> 'world'
      })
    }
  }
</script>

<script>
	export default {
    data() {
      return {
        a: 'hello'
      }
    },
    // $nextTick 结合 async/await语法使用
    async mounted() {
      this.a = 'world'
      console.log(this.$el.textContent)  // -> 'hello'
      await this.$nextTick()
      console.log(this.$el.textContent) // -> 'world'
    }
  }
</script>