WeakRef

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

实例方法

WeakRef.prototype.deref()

返回当前实例的 WeakRef 对象所绑定的 target 对象，如果该 target 对象已被 GC 回收则返回undefined

正确使用 WeakRef 对象需要仔细的考虑，最好尽量避免使用。

• 如果您的代码刚刚为目标对象创建了 WeakRef，或者从 WeakRef 的 deref 方法获取了目标对象，在当前 JavaScript job (包括在脚本作业末尾运行的任何 promise reaction 作业) 结束之前，不会回收该目标对象
• 如果多个 WeakRefs 有相同的目标，那么他们的 target 对象是一样的。
• 如果一个对象是 WeakRef 的 target，又在 FinalizationRegistry,那么该 target 就会在调用与注册表关联的任何清理回调之前或者同时被清理。如果清理回调调用对象的 WeakRef 上的 deref，它将收到 undefined
• 你不能更改 WeakRef 的 target，它将始终是第一次指定的 target 或者在回收该 target 时会定义
• WeakRef 可能永远不会从 deref 返回 undefined，即使没有什么东西能很好地保存 target，因为 GC 可能永远不会决定回收对象。

用途

缓存数据

// 缓存
function makeWeakCached(f) {
  const cache = new Map();
  return key => {
    const ref = cache.get(key);
    if (ref) {
      const cached = ref.deref();
      if (cached !== undefined) return cached;
    }

    const fresh = f(key);
    cache.set(key, new WeakRef(fresh));
    return fresh;
  };
}

const getImageCached = makeWeakCached(getImage);

FinalizationRegistry

FinalizationRegistry (清理器注册表) 对象可以让你在对象被垃圾回收时请求一个回调。

清理回调不应被用于必要的程序逻辑

正确使用需要仔细考虑，如果可能的话，最好避免使用。

const registry = new FinalizationRegistry((heldValue) => {
  // ....
});

实例方法

• FinalizationRegistry.prototype.register(target, heldValue, unregisterToken)

  • target

    要注册的目标值

  • heldValue

    传递给清理回调函数的参数。不可以是 target

  • unregisterToken

    unregister 方法使用以注销目标值。必须是对象或未注册的 Symbol。如果未提供，则无法注销目标。

• FinalizationRegistry.prototype.unregister(unregisterToken)

  注销目标

  class Thingy {
    static #cleanup = (label) => {
      //               ^^^^^−−−−− held value
      console.error(
        `The "release" method was never called for the object with the label "${label}"`,
      );
    };
    #registry = new FinalizationRegistry(Thingy.#cleanup);
  
    /**
     * Constructs a `Thingy` instance.
     * Be sure to call `release` when you're done with it.
     *
     * @param label A label for the `Thingy`.
     */
    constructor(label) {
      //                            vvvvv−−−−− held value
      this.#registry.register(this, label, this);
      //          target −−−−−^^^^         ^^^^−−−−− unregister token
    }
  
    /**
     * Releases resources held by this `Thingy` instance.
     */
    release() {
      this.#registry.unregister(this);
      //                        ^^^^−−−−− unregister token
    }
  }
  class Thingy {
    static #cleanup = (file) => {
      //               ^^^^−−−−− held value
      console.error(
        `The "release" method was never called for the "Thingy" for the file "${file.name}"`,
      );
    };
    #registry = new FinalizationRegistry(Thingy.#cleanup);
    #file;
  
    /**
     * Constructs a `Thingy` instance for the given file.
     * Be sure to call `release` when you're done with it.
     *
     * @param filename The name of the file.
     */
    constructor(filename) {
      this.#file = File.open(filename);
      //                            vvvvv−−−−− held value
      this.#registry.register(this, label, this.#file);
      //          target −−−−−^^^^         ^^^^^^^^^^−−−−− unregister token
    }
  
    /**
     * Releases resources held by this `Thingy` instance.
     */
    release() {
      if (this.#file) {
        this.#registry.unregister(this.#file);
        //                        ^^^^^^^^^^−−−−− unregister token
        File.close(this.#file);
        this.#file = null;
      }
    }
  }
  

  class Thingy {
    static #cleanup = (file) => {
      //               ^^^^−−−−− held value
      console.error(
        `The "release" method was never called for the "Thingy" for the file "${file.name}"`,
      );
    };
    #registry = new FinalizationRegistry(Thingy.#cleanup);
    #file;
  
    /**
     * Constructs a `Thingy` instance for the given file.
     * Be sure to call `release` when you're done with it.
     *
     * @param filename The name of the file.
     */
    constructor(filename) {
      this.#file = File.open(filename);
      //                            vvvvv−−−−− held value
      this.#registry.register(this, label, this.#file);
      //          target −−−−−^^^^         ^^^^^^^^^^−−−−− unregister token
    }
  
    /**
     * Releases resources held by this `Thingy` instance.
     */
    release() {
      if (this.#file) {
        this.#registry.unregister(this.#file);
        //                        ^^^^^^^^^^−−−−− unregister token
        File.close(this.#file);
        this.#file = null;
      }
    }
  }
  

用途

缓存

function makeWeakCached(f) {
  const cache = new Map();
  const cleanup = new FinalizationRegistry(key => {
    const ref = cache.get(key);
    if (ref && !ref.deref()) cache.delete(key);
  });

  return key => {
    const ref = cache.get(key);
    if (ref) {
      const cached = ref.deref();
      if (cached !== undefined) return cached;
    }

    const fresh = f(key);
    cache.set(key, new WeakRef(fresh));
    cleanup.register(fresh, key);
    return fresh;
  };
}

const getImageCached = makeWeakCached(getImage);