WeakRef对象允许您保留对另一个对象的弱引用，而不会阻止被弱引用对象被GC回收
WeakRef对象包含对对象的弱引用，这个弱引用被称为该WeakRef对象的target或者是referent。对对象的弱引用是指当该对象应该被GC回收时不会阻止GC的回收行为。而与此相反的，一个普通的引用（默认是强引用）会将与之对应的对象保存在内存中。只有当该对象没有任何的强引用时，JavaScript引擎GC才会销毁该对象并且回收该对象所占的内存空间。如果上述情况发生了，那么你就无法通过任何的弱引用来获取该对象。
备注： 在使用前请阅读Avoid where possible，对于WeakRef对象的使用要慎重考虑，能不使用就尽量不要使用
undefined正确使用WeakRef对象需要仔细的考虑，最好尽量避免使用。避免依赖于规范没有保证的任何特定行为也是十分重要的。何时、如何以及是否发生垃圾回收取决于任何给定JavaScript引擎的实现。GC在一个JavaScript引擎中的行为有可能在另一个JavaScript引擎中的行为大相径庭，或者甚至在同一类引擎，不同版本中GC的行为都有可能有较大的差距。GC目前还是JavaScript引擎实现者不断改进和改进解决方案的一个难题。
以下是WeakRef提案的作者在其解释文件（explainer document）中提出的一些具体观点
Garbage collectors are complicated. If an application or library depends on GC cleaning up a WeakRef or calling a finalizer [cleanup callback] in a timely, predictable manner, it's likely to be disappointed: the cleanup may happen much later than expected, or not at all. Sources of variability include:
- One object might be garbage-collected much sooner than another object, even if they become unreachable at the same time, e.g., due to generational collection.
- Garbage collection work can be split up over time using incremental and concurrent techniques.
- Various runtime heuristics can be used to balance memory usage, responsiveness.
- The JavaScript engine may hold references to things which look like they are unreachable (e.g., in closures, or inline caches).
- Different JavaScript engines may do these things differently, or the same engine may change its algorithms across versions.
- Complex factors may lead to objects being held alive for unexpected amounts of time, such as use with certain APIs.
关于WeakRefs的一些说明
WeakRefs 有相同的目标，那么他们的target对象是一样的。对其中一个调用deref的结果将与对另一个调用deref的结果匹配（在同一个作业中），您不会从其中一个获取目标对象，而是从另一个获取未定义的对象。undefined这个例子演示了在一个DOM元素中启动一个计数器，当这个元素不存在时停止:
class Counter {
constructor(element) {
// Remember a weak reference to the DOM element
this.ref = new WeakRef(element);
this.start();
}
start() {
if (this.timer) {
return;
}
this.count = 0;
const tick = () => {
// Get the element from the weak reference, if it still exists
const element = this.ref.deref();
if (element) {
element.textContent = ++this.count;
} else {
// The element doesn't exist anymore
console.log("The element is gone.");
this.stop();
this.ref = null;
}
};
tick();
this.timer = setInterval(tick, 1000);
}
stop() {
if (this.timer) {
clearInterval(this.timer);
this.timer = 0;
}
}
}
const counter = new Counter(document.getElementById("counter"));
counter.start();
setTimeout(() => {
document.getElementById("counter").remove();
}, 5000);
| 规范 |
|---|
| WeakRefs proposal |
{{Compat("javascript.builtins.WeakRef")}}