feat: Trim MMKV on memoryWarning - GC safe via WeakRef!

package/src/MMKV.ts

@@ -1,9 +1,9 @@
-import { AppState } from 'react-native';
 import { createMMKV } from './createMMKV';
 import { createMockMMKV } from './createMMKV.mock';
 import { isTest } from './PlatformChecker';
 import type { Configuration } from './NativeMmkv';
 import type { Listener, MMKVInterface, NativeMMKV } from './Types';
+import { addMemoryWarningListener } from './MemoryWarningListener';
 
 const onValueChangedListeners = new Map<string, ((key: string) => void)[]>();
 
@@ -26,10 +26,7 @@ export class MMKV implements MMKVInterface {
       : createMMKV(configuration);
     this.functionCache = {};
 
-    AppState.addEventListener('memoryWarning', () => {
-      // when we have a memory warning, delete unused keys by trimming MMKV
-      this.trim();
-    });
+    addMemoryWarningListener(this);
   }
 
   private get onValueChangedListeners() {

package/src/MemoryWarningListener.ts

@@ -0,0 +1,29 @@
+import { AppState } from 'react-native';
+import type { NativeEventSubscription } from 'react-native';
+import { MMKVInterface } from './Types';
+
+export function addMemoryWarningListener(mmkv: MMKVInterface): void {
+  if (global.WeakRef != null && global.FinalizationRegistry != null) {
+    // 1. Weakify MMKV so we can safely use it inside the memoryWarning event listener
+    const weakMmkv = new WeakRef(mmkv);
+    const listener = AppState.addEventListener('memoryWarning', () => {
+      // 0. Everytime we receive a memoryWarning, we try to trim the MMKV instance (if it is still valid)
+      weakMmkv.deref()?.trim();
+    });
+    // 2. Add a listener to when the MMKV instance is deleted
+    const finalization = new FinalizationRegistry(
+      (l: NativeEventSubscription) => {
+        // 3. When MMKV is deleted, this listener will be called with the memoryWarning listener.
+        l.remove();
+      }
+    );
+    // 2.1. Bind the listener to the actual MMKV instance.
+    finalization.register(mmkv, listener);
+  } else {
+    // WeakRef/FinalizationRegistry is not implemented in this engine.
+    // Just add the listener, even if it retains MMKV strong forever.
+    AppState.addEventListener('memoryWarning', () => {
+      mmkv.trim();
+    });
+  }
+}