vue计算函数新旧变化
# 前文只是自己的一次无聊想法
很基础的内容
大佬不要嘲笑
# computed变化
vue对computed进行了响应优化
这里拿了2.5.6和2.5.17做对比
2.5.17提示的是RangeError: Maximum call stack size exceeded
具体改变在于哪呢？
在源码的Watcher.prototype.update位置
看注释和代码来说，大概意思是早期是判断deps是否为0，如果为0则再下轮访问刷新，否则立即刷新
但是新版无论是否进行notify，都会下轮访问才进行刷新
也就是说内部修改变量已经无法触发刷新了
新版则是收集dep之后，一旦网页访问compute函数，则同时对所有函数的user watch收集的dep也进行一次依赖响应
来让依赖也拿到渲染watch
一旦对其变量进行修改
则变量会对user watch做一个dirty处理
然后触发渲染watch刷新
渲染watch获取computed属性，触发重新计算值
而旧版的逻辑则是
一旦变量更新通知user watch,获取值之后，再由user watch通知渲染watch进行更新
```javascript
Watcher.prototype.update = function update () {
var this$1 = this;
/* istanbul ignore else */
if (this.computed) {
// A computed property watcher has two modes: lazy and activated.
// It initializes as lazy by default, and only becomes activated when
// it is depended on by at least one subscriber, which is typically
// another computed property or a component's render function.
if (this.dep.subs.length === 0) {
// In lazy mode, we don't want to perform computations until necessary,
// so we simply mark the watcher as dirty. The actual computation is
// performed just-in-time in this.evaluate() when the computed property
// is accessed.
this.dirty = true;
} else {
// In activated mode, we want to proactively perform the computation
// but only notify our subscribers when the value has indeed changed.
this.getAndInvoke(function () {
this$1.dep.notify();
});
}
} else if (this.sync) {
this.run();
} else {
queueWatcher(this);
}
};
```
:):lol:loveliness:
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