Review comments: Changes to cache PerResourcePatchGenerator instance

engine/src/main/java/com/google/android/fhir/sync/upload/patch/PatchGenerator.kt

@@ -45,7 +45,7 @@ internal object PatchGeneratorFactory {
  ): PatchGenerator =
  when (mode) {
  is PatchGeneratorMode.PerChange -> PerChangePatchGenerator
- is PatchGeneratorMode.PerResource -> PerResourcePatchGenerator(database)
+ is PatchGeneratorMode.PerResource -> PerResourcePatchGenerator.with(database)
  }
 }
 

engine/src/main/java/com/google/android/fhir/sync/upload/patch/PerResourcePatchGenerator.kt

@@ -32,7 +32,8 @@ import com.google.android.fhir.sync.upload.patch.PatchOrdering.orderByReferences
  * maintain an audit trail, but instead, multiple changes made to the same FHIR resource on the
  * client can be recorded as a single change on the server.
  */
-internal class PerResourcePatchGenerator(val database: Database) : PatchGenerator {
+internal class PerResourcePatchGenerator private constructor(val database: Database) :
+ PatchGenerator {
 
  override suspend fun generate(localChanges: List<LocalChange>): List<PatchMapping> {
  return generateSquashedChangesMapping(localChanges).orderByReferences(database)
@@ -144,4 +145,20 @@ internal class PerResourcePatchGenerator(val database: Database) : PatchGenerato
  mergedOperations.values.flatten().forEach(mergedNode::add)
  return objectMapper.writeValueAsString(mergedNode)
  }
+
+ companion object {
+
+ private lateinit var _instance: PerResourcePatchGenerator
+
+ @Synchronized
+ fun with(database: Database): PerResourcePatchGenerator {
+ if (!::_instance.isInitialized) {
+ _instance = PerResourcePatchGenerator(database)
+ } else if (_instance.database != database) {
+ _instance = PerResourcePatchGenerator(database)
+ }
+
+ return _instance
+ }
+ }
 }

engine/src/test/java/com/google/android/fhir/sync/upload/patch/PerResourcePatchGeneratorTest.kt

@@ -61,7 +61,7 @@ class PerResourcePatchGeneratorTest {
  fun setUp() {
  MockitoAnnotations.openMocks(this)
  runTest { whenever(database.getLocalChangeResourceReferences(any())).thenReturn(emptyList()) }
- patchGenerator = PerResourcePatchGenerator(database)
+ patchGenerator = PerResourcePatchGenerator.with(database)
  }
 
  @Test