【Bug】Memory Leak cause by toVue3ComponentInstance

[Dennis]

[Cause]
i found the logic in toVue3ComponentInstance function
it use a WeakMap to cache to the map of Vue2 instance to fake-Vue3 instance
But the fake-Vue3 instance has a property named proxy that references to the Vue2 instance.
This proxy forms a strong reference, which prevent Vue2 instance to be GC,
And If the Vue2 instance is not GC, then the weakmap mapping will not be automatically deleted,
then the fake-vue3 instance will keep by the WeakMap obj, also the strong reference to vue2 instance will be keep too

This is the result of the Memlab memory leak test

[Affect]
All instances created after Vue-composition-API registration

[Solution]
there are to way to fix it;

1. replace all strong reference of vue2 instance with WeakRef

function toVue3ComponentInstance(vm) {
  if (instanceMapCache.has(vm)) {
    console.info('get vm from cache');
    return instanceMapCache.get(vm);
  }
// use WeakRef to reference vue2 instance
  let weakRef = new WeakRef(vm);
  const vmProxy = {
    get proxy() {
        return weakRef.deref() || {};
    },
    set proxy(val) {
        const vm = weakRef.deref() || {};
        const newVal = { ...vm, ...val };
        weakRef = new WeakRef(newVal);
    }
  }.proxy;
  var instance = {
    proxy: vmProxy,
    update: vmProxy.$forceUpdate,
    type: vmProxy.$options,
    uid: vmProxy._uid,
    // $emit is defined on prototype and it expected to be bound
    emit: vmProxy.$emit.bind(vmProxy),
    parent: null,
    root: null, // to be immediately set
  };
  bindCurrentScopeToVM(instance);
  // map vm.$props =
  var instanceProps = [
    'data',
    'props',
    'attrs',
    'refs',
    'vnode',
    'slots',
  ];
  instanceProps.forEach(function(prop) {
    proxy(instance, prop, {
      get: function() {
        return vmProxy['$'.concat(prop)];
      },
    });
  });
  proxy(instance, 'isMounted', {
    get: function() {
      // @ts-expect-error private api
      return vmProxy._isMounted;
    },
  });
  proxy(instance, 'isUnmounted', {
    get: function() {
      // @ts-expect-error private api
      return vmProxy._isDestroyed;
    },
  });
  proxy(instance, 'isDeactivated', {
    get: function() {
      // @ts-expect-error private api
      return vmProxy._inactive;
    },
  });
  proxy(instance, 'emitted', {
    get: function() {
      // @ts-expect-error private api
      return vmProxy._events;
    },
  });
  instanceMapCache.set(vm, instance);
  vm.$on('hook:destroyed', function() { console.info('composition api cache has vm', instanceMapCache.has(vm))  });
  if (vmProxy.$parent) {
    instance.parent = toVue3ComponentInstance(vm.$parent);
  }
  if (vmProxy.$root) {
    instance.root = toVue3ComponentInstance(vm.$root);
  }
  return instance;
}

2. delete the map manually

function toVue3ComponentInstance(vm) {
...
  instanceMapCache.set(vm, instance);
  // add this line
  vm.$on('hook:destroyed', function() { instanceMapCache.delete(vm) });
...
}

The first method is not very elegant, so I recommend using the second method

[Other]
Since this library has stopped updating now,
I just fix it in my own project.

hope this can help u if u meet the problem too.