Add a feature flag for the event cache.

libraries/featureflag/api/src/main/kotlin/io/element/android/libraries/featureflag/api/FeatureFlags.kt

@@ -161,4 +161,11 @@ enum class FeatureFlags(
         defaultValue = { buildMeta -> buildMeta.buildType != BuildType.RELEASE },
         isFinished = false,
     ),
+    EventCache(
+        key = "feature.event_cache",
+        title = "Use SDK Event cache",
+        description = "Warning: you must kill and restart the app for the change to take effect.",
+        defaultValue = { false },
+        isFinished = false,
+    ),
 }

libraries/matrix/impl/src/main/kotlin/io/element/android/libraries/matrix/impl/RustMatrixClientFactory.kt

@@ -109,9 +109,7 @@ class RustMatrixClientFactory @Inject constructor(
             .addRootCertificates(userCertificatesProvider.provides())
             .autoEnableBackups(true)
             .autoEnableCrossSigning(true)
-            // TODO Add a feature flag to enable persistent storage
-            // See https://github.com/matrix-org/matrix-rust-sdk/pull/4396
-            .useEventCachePersistentStorage(false)
+            .useEventCachePersistentStorage(featureFlagService.isFeatureEnabled(FeatureFlags.EventCache))
             .roomKeyRecipientStrategy(
                 strategy = if (featureFlagService.isFeatureEnabled(FeatureFlags.OnlySignedDeviceIsolationMode)) {
                     CollectStrategy.IdentityBasedStrategy