Weak Cache

Weak cache is a Map implementation that uses WeakReferences for holding
values and Finalizer to manage it's storage.

You can use this to cache data for a small amount of time until next garbage
collection cycle.

Note: Values cannot be numbers, strings, booleans, records, null,
dart:ffi pointers, dart:ffi structs, or dart:ffi unions.

Features

• Uses WeakReference for storing values.
• Uses Finalizer to remove objects from internal storage upon their deletion.
• Allows you iterate over keys/values.

  While iterating, all stored values are temporarily made into strong
  references, to prevent concurrent edit of storage, while iterating over it.

• Optimized containsValue via internal managed Expando.
• Implements full Map<K, V> interface.
• WeakCache itself can be safely garbage collected and doesn't hold unto any
  stored data.

Usage

Create cache, add values, and they'll be removed once there no more strong
references to them.

// ID - Object cache
final cache = WeakCache<int, Object>();

Object? obj = Object();
cache[0] = obj;
// ...
obj = null;
// ...
// After garbage collection cache[0] will be removed.
cache[0] == null;

See example for detailed test case.