Move handling of missing values from Executor to Field Resolvers

Sources/Apollo/ApolloStore.swift

@@ -184,14 +184,17 @@ public class ApolloStore {
     fileprivate let cache: NormalizedCache
 
     fileprivate lazy var loader: DataLoader<CacheKey, Record> = DataLoader(self.cache.loadRecords)
-    fileprivate lazy var executor = GraphQLExecutor { object, info in
-        return object[info.cacheKeyForField]
-      } resolveReference: { [weak self] reference in
-        guard let self = self else {
-          return .immediate(.failure(ApolloStore.Error.notWithinReadTransaction))
-        }
-        return self.loadObject(forKey: reference.key)
+    fileprivate lazy var readExecutor = GraphQLExecutor { object, info in
+      guard let value = object[info.cacheKeyForField] else {
+        throw JSONDecodingError.missingValue
+      }
+      return value
+    } resolveReference: { [weak self] reference in
+      guard let self = self else {
+        return .immediate(.failure(ApolloStore.Error.notWithinReadTransaction))
       }
+      return self.loadObject(forKey: reference.key)
+    }
 
     fileprivate init(store: ApolloStore) {
       self.cache = store.cache
@@ -226,7 +229,7 @@ public class ApolloStore {
     ) throws -> Accumulator.FinalResult {
       let object = try loadObject(forKey: key).get()
 
-      return try executor.execute(
+      return try readExecutor.execute(
         selectionSet: type,
         on: object,
         withRootCacheReference: CacheReference(key),
@@ -293,7 +296,13 @@ public class ApolloStore {
     ) throws {
       let normalizer = GraphQLResultNormalizer()
       let executor = GraphQLExecutor { object, info in
-        return object[info.responseKeyForField]
+        guard let value = object[info.responseKeyForField] else {
+          guard info.field.type.isNullable else {
+            throw JSONDecodingError.missingValue
+          }
+          return NSNull()
+        }
+        return value
       }
 
       let records = try executor.execute(

Sources/Apollo/GraphQLExecutor.swift

@@ -3,8 +3,13 @@ import Foundation
 import ApolloAPI
 #endif
 
-/// A field resolver is responsible for resolving a value for a field.
-typealias GraphQLFieldResolver = (_ object: JSONObject, _ info: FieldExecutionInfo) -> JSONValue?
+/// A field resolver is responsible for resolving a value for a field from a JSON object.
+///
+/// Because the field resolver should have context on the source of JSON object, it is responsible
+/// for knowing how to handle missing fields. It must either return a valid value or throw an error.
+/// If the object has no value for the field, the resolver can return `NSNull` or throw a
+/// `JSONDecodingError`.
+typealias GraphQLFieldResolver = (_ object: JSONObject, _ info: FieldExecutionInfo) throws -> JSONValue
 
 /// A reference resolver is responsible for resolving an object based on its key. These references are
 /// used in normalized records, and data for these objects has to be loaded from the cache for execution to continue.
@@ -308,10 +313,7 @@ final class GraphQLExecutor {
     }
 
     return PossiblyDeferred {
-      guard let value = fieldResolver(object, fieldInfo) else {
-        throw JSONDecodingError.missingValue
-      }
-      return value
+      try fieldResolver(object, fieldInfo)
     }.flatMap {
       return self.complete(fields: fieldInfo,
                            withValue: $0,

Sources/Apollo/GraphQLResponse.swift

@@ -41,7 +41,10 @@ public final class GraphQLResponse<Data: RootSelectionSet> {
     }
 
     let executor = GraphQLExecutor { object, info in
-      return object[info.responseKeyForField]
+      guard let value = object[info.responseKeyForField] else {
+        throw JSONDecodingError.missingValue
+      }
+      return value
     }
 
     executor.shouldComputeCachePath = computeCachePaths

Tests/ApolloTests/GraphQLExecutor_ResultNormalizer_FromResponse_Tests.swift

@@ -9,7 +9,10 @@ class GraphQLExecutor_ResultNormalizer_FromResponse_Tests: XCTestCase {
 
   private static let executor: GraphQLExecutor = {
     let executor = GraphQLExecutor { object, info in
-      return object[info.responseKeyForField]
+      guard let value = object[info.responseKeyForField] else {
+        throw JSONDecodingError.missingValue
+      }
+      return value
     }
 
     executor.shouldComputeCachePath = true

Tests/ApolloTests/GraphQLExecutor_SelectionSetMapper_FromResponse_Tests.swift

@@ -11,7 +11,10 @@ class GraphQLExecutor_SelectionSetMapper_FromResponse_Tests: XCTestCase {
 
   private static let executor: GraphQLExecutor = {
     let executor = GraphQLExecutor { object, info in
-      return object[info.responseKeyForField]
+      guard let value = object[info.responseKeyForField] else {
+        throw JSONDecodingError.missingValue
+      }
+      return value
     }
     executor.shouldComputeCachePath = false
     return executor