fix: eliminate latency of instantiation of client on iOS with explicit region (#508)

Author: wooj2

AWSClientRuntime/Sources/AWSClientConfiguration.swift

@@ -10,7 +10,7 @@ public protocol AWSRuntimeConfiguration {
     var region: String? { get set }
     var signingRegion: String? {get set}
     var endpointResolver: EndpointResolver {get set}
-    var regionResolver: RegionResolver {get set}
+    var regionResolver: RegionResolver? {get set}
     var frameworkMetadata: FrameworkMetadata? {get set}
 }
 

codegen/smithy-aws-swift-codegen/src/main/kotlin/software/amazon/smithy/aws/swift/codegen/AWSServiceConfig.kt

@@ -32,10 +32,19 @@ class AWSServiceConfig(writer: SwiftWriter, serviceName: String) : ServiceConfig
             writer.write("runtimeConfig: \$N", ClientRuntimeTypes.Core.SDKRuntimeConfiguration)
         }
         writer.indent()
-        writer.write("self.regionResolver = regionResolver ?? DefaultRegionResolver()")
-        writer.write("let defaultRegion = self.regionResolver.resolveRegion()")
-        writer.write("self.region = region ?? defaultRegion")
-        writer.write("self.signingRegion = signingRegion ?? defaultRegion")
+        writer.openBlock("if let region = region {", "} else {") {
+            writer.write("self.region = region")
+            writer.write("self.regionResolver = nil")
+            writer.write("self.signingRegion = signingRegion ?? region")
+        }
+        writer.indent()
+        writer.write("let resolvedRegionResolver = regionResolver ?? DefaultRegionResolver()")
+        writer.write("let region = resolvedRegionResolver.resolveRegion()")
+        writer.write("self.region = region")
+        writer.write("self.regionResolver = resolvedRegionResolver")
+        writer.write("self.signingRegion = signingRegion ?? region")
+        writer.dedent()
+        writer.write("}")
         writer.write("self.endpointResolver = endpointResolver ?? DefaultEndpointResolver()")
         writer.openBlock("if let credProvider = credentialsProvider {", "} else {") {
             writer.write("self.credentialsProvider = try \$N.fromCustom(credProvider)", AWSClientRuntimeTypes.Core.AWSCredentialsProvider)
@@ -90,7 +99,7 @@ class AWSServiceConfig(writer: SwiftWriter, serviceName: String) : ServiceConfig
                 "\$T",
                 "The region to send requests to. (Required)"
             ),
-            ConfigField(REGION_RESOLVER, AWSClientRuntimeTypes.Core.RegionResolver, documentation = "The region resolver uses an array of region providers to resolve the region."),
+            ConfigField(REGION_RESOLVER, AWSClientRuntimeTypes.Core.RegionResolver, "\$T", documentation = "The region resolver uses an array of region providers to resolve the region."),
             ConfigField(SIGNING_REGION_CONFIG_NAME, SwiftTypes.String, "\$T", "The region to sign requests in. (Required)")
         ).sortedBy { it.memberName }
     }

codegen/smithy-aws-swift-codegen/src/test/kotlin/software/amazon/smithy/aws/swift/codegen/RestJsonProtocolGeneratorTests.kt

@@ -121,7 +121,7 @@ class RestJsonProtocolGeneratorTests {
                     public var endpointResolver: AWSClientRuntime.EndpointResolver
                     public var frameworkMetadata: AWSClientRuntime.FrameworkMetadata?
                     public var region: Swift.String?
-                    public var regionResolver: AWSClientRuntime.RegionResolver
+                    public var regionResolver: AWSClientRuntime.RegionResolver?
                     public var signingRegion: Swift.String?
             
                     public init(
@@ -133,10 +133,17 @@ class RestJsonProtocolGeneratorTests {
                         signingRegion: Swift.String? = nil,
                         runtimeConfig: ClientRuntime.SDKRuntimeConfiguration
                     ) throws {
-                        self.regionResolver = regionResolver ?? DefaultRegionResolver()
-                        let defaultRegion = self.regionResolver.resolveRegion()
-                        self.region = region ?? defaultRegion
-                        self.signingRegion = signingRegion ?? defaultRegion
+                        if let region = region {
+                            self.region = region
+                            self.regionResolver = nil
+                            self.signingRegion = signingRegion ?? region
+                        } else {
+                            let resolvedRegionResolver = regionResolver ?? DefaultRegionResolver()
+                            let region = resolvedRegionResolver.resolveRegion()
+                            self.region = region
+                            self.regionResolver = resolvedRegionResolver
+                            self.signingRegion = signingRegion ?? region
+                        }
                         self.endpointResolver = endpointResolver ?? DefaultEndpointResolver()
                         if let credProvider = credentialsProvider {
                             self.credentialsProvider = try AWSClientRuntime.AWSCredentialsProvider.fromCustom(credProvider)