diff --git a/packages/pigeon/CHANGELOG.md b/packages/pigeon/CHANGELOG.md
index afecc10659..2c96dc7816 100644
--- a/packages/pigeon/CHANGELOG.md
+++ b/packages/pigeon/CHANGELOG.md
@@ -1,3 +1,8 @@
+## 0.1.20
+
+* Implemented `@async` HostApi's for iOS.
+* Fixed async FlutterApi methods with void return.
+
 ## 0.1.19
 
 * Fixed a bug introduced in 0.1.17 where methods without arguments were
diff --git a/packages/pigeon/lib/ast.dart b/packages/pigeon/lib/ast.dart
index e3c3a96925..747a7ed56d 100644
--- a/packages/pigeon/lib/ast.dart
+++ b/packages/pigeon/lib/ast.dart
@@ -17,7 +17,8 @@ class Node {}
 /// Represents a method on an [Api].
 class Method extends Node {
   /// Parametric constructor for [Method].
-  Method({this.name, this.returnType, this.argType, this.isAsynchronous});
+  Method(
+      {this.name, this.returnType, this.argType, this.isAsynchronous = false});
 
   /// The name of the method.
   String name;
diff --git a/packages/pigeon/lib/generator_tools.dart b/packages/pigeon/lib/generator_tools.dart
index 0e948a2eaa..996e794a08 100644
--- a/packages/pigeon/lib/generator_tools.dart
+++ b/packages/pigeon/lib/generator_tools.dart
@@ -8,7 +8,7 @@ import 'dart:mirrors';
 import 'ast.dart';
 
 /// The current version of pigeon. This must match the version in pubspec.yaml.
-const String pigeonVersion = '0.1.19';
+const String pigeonVersion = '0.1.20';
 
 /// Read all the content from [stdin] to a String.
 String readStdin() {
@@ -72,9 +72,7 @@ class Indent {
     if (begin != null) {
       _sink.write(begin + newline);
     }
-    inc();
-    func();
-    dec();
+    nest(1, func);
     if (end != null) {
       _sink.write(str() + end);
       if (addTrailingNewline) {
@@ -83,6 +81,17 @@ class Indent {
     }
   }
 
+  /// Like `scoped` but writes the current indentation level.
+  void writeScoped(
+    String begin,
+    String end,
+    Function func, {
+    bool addTrailingNewline = true,
+  }) {
+    scoped(str() + begin ?? '', end, func,
+        addTrailingNewline: addTrailingNewline);
+  }
+
   /// Scoped increase of the ident level.  For the execution of [func] the
   /// indentation will be incremented by the given amount.
   void nest(int count, Function func) {
diff --git a/packages/pigeon/lib/objc_generator.dart b/packages/pigeon/lib/objc_generator.dart
index e264ef35e2..a0ce4ba9c2 100644
--- a/packages/pigeon/lib/objc_generator.dart
+++ b/packages/pigeon/lib/objc_generator.dart
@@ -71,6 +71,91 @@ String _propertyTypeForDartType(String type) {
   }
 }
 
+void _writeClassDeclarations(
+    Indent indent, List<Class> classes, String prefix) {
+  for (Class klass in classes) {
+    indent.writeln('@interface ${_className(prefix, klass.name)} : NSObject');
+    for (Field field in klass.fields) {
+      final HostDatatype hostDatatype = getHostDatatype(
+          field, classes, _objcTypeForDartType,
+          customResolver: (String x) => '${_className(prefix, x)} *');
+      final String propertyType = hostDatatype.isBuiltin
+          ? _propertyTypeForDartType(field.dataType)
+          : 'strong';
+      final String nullability =
+          hostDatatype.datatype.contains('*') ? ', nullable' : '';
+      indent.writeln(
+          '@property(nonatomic, $propertyType$nullability) ${hostDatatype.datatype} ${field.name};');
+    }
+    indent.writeln('@end');
+    indent.writeln('');
+  }
+}
+
+void _writeHostApiDeclaration(Indent indent, Api api, ObjcOptions options) {
+  final String apiName = _className(options.prefix, api.name);
+  indent.writeln('@protocol $apiName');
+  for (Method func in api.methods) {
+    final String returnTypeName = _className(options.prefix, func.returnType);
+    if (func.isAsynchronous) {
+      if (func.returnType == 'void') {
+        if (func.argType == 'void') {
+          indent.writeln(
+              '-(void)${func.name}:(void(^)(FlutterError *_Nullable))completion;');
+        } else {
+          final String argType = _className(options.prefix, func.argType);
+          indent.writeln(
+              '-(void)${func.name}:(nullable $argType *)input completion:(void(^)(FlutterError *_Nullable))completion;');
+        }
+      } else {
+        if (func.argType == 'void') {
+          indent.writeln(
+              '-(void)${func.name}:(void(^)($returnTypeName *_Nullable, FlutterError *_Nullable))completion;');
+        } else {
+          final String argType = _className(options.prefix, func.argType);
+          indent.writeln(
+              '-(void)${func.name}:(nullable $argType *)input completion:(void(^)($returnTypeName *_Nullable, FlutterError *_Nullable))completion;');
+        }
+      }
+    } else {
+      final String returnType =
+          func.returnType == 'void' ? 'void' : 'nullable $returnTypeName *';
+      if (func.argType == 'void') {
+        indent.writeln(
+            '-($returnType)${func.name}:(FlutterError *_Nullable *_Nonnull)error;');
+      } else {
+        final String argType = _className(options.prefix, func.argType);
+        indent.writeln(
+            '-($returnType)${func.name}:($argType*)input error:(FlutterError *_Nullable *_Nonnull)error;');
+      }
+    }
+  }
+  indent.writeln('@end');
+  indent.writeln('');
+  indent.writeln(
+      'extern void ${apiName}Setup(id<FlutterBinaryMessenger> binaryMessenger, id<$apiName> _Nullable api);');
+  indent.writeln('');
+}
+
+void _writeFlutterApiDeclaration(Indent indent, Api api, ObjcOptions options) {
+  final String apiName = _className(options.prefix, api.name);
+  indent.writeln('@interface $apiName : NSObject');
+  indent.writeln(
+      '- (instancetype)initWithBinaryMessenger:(id<FlutterBinaryMessenger>)binaryMessenger;');
+  for (Method func in api.methods) {
+    final String returnType = _className(options.prefix, func.returnType);
+    final String callbackType = _callbackForType(func.returnType, returnType);
+    if (func.argType == 'void') {
+      indent.writeln('- (void)${func.name}:($callbackType)completion;');
+    } else {
+      final String argType = _className(options.prefix, func.argType);
+      indent.writeln(
+          '- (void)${func.name}:($argType*)input completion:($callbackType)completion;');
+    }
+  }
+  indent.writeln('@end');
+}
+
 /// Generates the ".h" file for the AST represented by [root] to [sink] with the
 /// provided [options].
 void generateObjcHeader(ObjcOptions options, Root root, StringSink sink) {
@@ -92,65 +177,13 @@ void generateObjcHeader(ObjcOptions options, Root root, StringSink sink) {
 
   indent.writeln('');
 
-  for (Class klass in root.classes) {
-    indent.writeln(
-        '@interface ${_className(options.prefix, klass.name)} : NSObject');
-    for (Field field in klass.fields) {
-      final HostDatatype hostDatatype = getHostDatatype(
-          field, root.classes, _objcTypeForDartType,
-          customResolver: (String x) => '${_className(options.prefix, x)} *');
-      final String propertyType = hostDatatype.isBuiltin
-          ? _propertyTypeForDartType(field.dataType)
-          : 'strong';
-      final String nullability =
-          hostDatatype.datatype.contains('*') ? ', nullable' : '';
-      indent.writeln(
-          '@property(nonatomic, $propertyType$nullability) ${hostDatatype.datatype} ${field.name};');
-    }
-    indent.writeln('@end');
-    indent.writeln('');
-  }
+  _writeClassDeclarations(indent, root.classes, options.prefix);
 
   for (Api api in root.apis) {
-    final String apiName = _className(options.prefix, api.name);
     if (api.location == ApiLocation.host) {
-      indent.writeln('@protocol $apiName');
-      for (Method func in api.methods) {
-        final String returnTypeName =
-            _className(options.prefix, func.returnType);
-        final String returnType =
-            func.returnType == 'void' ? 'void' : 'nullable $returnTypeName *';
-        if (func.argType == 'void') {
-          indent.writeln(
-              '-($returnType)${func.name}:(FlutterError *_Nullable *_Nonnull)error;');
-        } else {
-          final String argType = _className(options.prefix, func.argType);
-          indent.writeln(
-              '-($returnType)${func.name}:($argType*)input error:(FlutterError *_Nullable *_Nonnull)error;');
-        }
-      }
-      indent.writeln('@end');
-      indent.writeln('');
-      indent.writeln(
-          'extern void ${apiName}Setup(id<FlutterBinaryMessenger> binaryMessenger, id<$apiName> _Nullable api);');
-      indent.writeln('');
+      _writeHostApiDeclaration(indent, api, options);
     } else if (api.location == ApiLocation.flutter) {
-      indent.writeln('@interface $apiName : NSObject');
-      indent.writeln(
-          '- (instancetype)initWithBinaryMessenger:(id<FlutterBinaryMessenger>)binaryMessenger;');
-      for (Method func in api.methods) {
-        final String returnType = _className(options.prefix, func.returnType);
-        final String callbackType =
-            _callbackForType(func.returnType, returnType);
-        if (func.argType == 'void') {
-          indent.writeln('- (void)${func.name}:($callbackType)completion;');
-        } else {
-          final String argType = _className(options.prefix, func.argType);
-          indent.writeln(
-              '- (void)${func.name}:($argType*)input completion:($callbackType)completion;');
-        }
-      }
-      indent.writeln('@end');
+      _writeFlutterApiDeclaration(indent, api, options);
     }
   }
 
@@ -204,21 +237,56 @@ void _writeHostApiSource(Indent indent, ObjcOptions options, Api api) {
           indent.scoped('{', '}];', () {
             final String returnType =
                 _className(options.prefix, func.returnType);
-            indent.writeln('FlutterError *error;');
-            String call;
+            String syncCall;
             if (func.argType == 'void') {
-              call = '[api ${func.name}:&error]';
+              syncCall = '[api ${func.name}:&error]';
             } else {
               final String argType = _className(options.prefix, func.argType);
               indent.writeln('$argType *input = [$argType fromMap:message];');
-              call = '[api ${func.name}:input error:&error]';
+              syncCall = '[api ${func.name}:input error:&error]';
             }
-            if (func.returnType == 'void') {
-              indent.writeln('$call;');
-              indent.writeln('callback(wrapResult(nil, error));');
+            if (func.isAsynchronous) {
+              if (func.returnType == 'void') {
+                const String callback = 'callback(error));';
+                if (func.argType == 'void') {
+                  indent.writeScoped(
+                      '[api ${func.name}:^(FlutterError *_Nullable error) {',
+                      '}];', () {
+                    indent.writeln(callback);
+                  });
+                } else {
+                  indent.writeScoped(
+                      '[api ${func.name}:input completion:^(FlutterError *_Nullable error) {',
+                      '}];', () {
+                    indent.writeln(callback);
+                  });
+                }
+              } else {
+                const String callback =
+                    'callback(wrapResult([output toMap], error));';
+                if (func.argType == 'void') {
+                  indent.writeScoped(
+                      '[api ${func.name}:^($returnType *_Nullable output, FlutterError *_Nullable error) {',
+                      '}];', () {
+                    indent.writeln(callback);
+                  });
+                } else {
+                  indent.writeScoped(
+                      '[api ${func.name}:input completion:^($returnType *_Nullable output, FlutterError *_Nullable error) {',
+                      '}];', () {
+                    indent.writeln(callback);
+                  });
+                }
+              }
             } else {
-              indent.writeln('$returnType *output = $call;');
-              indent.writeln('callback(wrapResult([output toMap], error));');
+              indent.writeln('FlutterError *error;');
+              if (func.returnType == 'void') {
+                indent.writeln('$syncCall;');
+                indent.writeln('callback(wrapResult(nil, error));');
+              } else {
+                indent.writeln('$returnType *output = $syncCall;');
+                indent.writeln('callback(wrapResult([output toMap], error));');
+              }
             }
           });
         });
@@ -246,7 +314,7 @@ void _writeFlutterApiSource(Indent indent, ObjcOptions options, Api api) {
     indent.writeln('self = [super init];');
     indent.write('if (self) ');
     indent.scoped('{', '}', () {
-      indent.writeln('self.binaryMessenger = binaryMessenger;');
+      indent.writeln('_binaryMessenger = binaryMessenger;');
     });
     indent.writeln('return self;');
   });
@@ -312,19 +380,19 @@ void generateObjcSource(ObjcOptions options, Root root, StringSink sink) {
   indent.addln('');
 
   indent.format(
-      '''static NSDictionary* wrapResult(NSDictionary *result, FlutterError *error) {
+      '''static NSDictionary<NSString*, id>* wrapResult(NSDictionary *result, FlutterError *error) {
 \tNSDictionary *errorDict = (NSDictionary *)[NSNull null];
 \tif (error) {
-\t\terrorDict = [NSDictionary dictionaryWithObjectsAndKeys:
-\t\t\t\t(error.code ? error.code : [NSNull null]), @"${Keys.errorCode}",
-\t\t\t\t(error.message ? error.message : [NSNull null]), @"${Keys.errorMessage}",
-\t\t\t\t(error.details ? error.details : [NSNull null]), @"${Keys.errorDetails}",
-\t\t\t\tnil];
+\t\terrorDict = @{
+\t\t\t\t@"${Keys.errorCode}": (error.code ? error.code : [NSNull null]),
+\t\t\t\t@"${Keys.errorMessage}": (error.message ? error.message : [NSNull null]),
+\t\t\t\t@"${Keys.errorDetails}": (error.details ? error.details : [NSNull null]),
+\t\t\t\t};
 \t}
-\treturn [NSDictionary dictionaryWithObjectsAndKeys:
-\t\t\t(result ? result : [NSNull null]), @"${Keys.result}",
-\t\t\terrorDict, @"${Keys.error}",
-\t\t\tnil];
+\treturn @{
+\t\t\t@"${Keys.result}": (result ? result : [NSNull null]),
+\t\t\t@"${Keys.error}": errorDict,
+\t\t\t};
 }''');
   indent.addln('');
 
diff --git a/packages/pigeon/platform_tests/ios_unit_tests/ios/Runner.xcodeproj/project.pbxproj b/packages/pigeon/platform_tests/ios_unit_tests/ios/Runner.xcodeproj/project.pbxproj
index 20ab37a48e..cca797a82d 100644
--- a/packages/pigeon/platform_tests/ios_unit_tests/ios/Runner.xcodeproj/project.pbxproj
+++ b/packages/pigeon/platform_tests/ios_unit_tests/ios/Runner.xcodeproj/project.pbxproj
@@ -9,6 +9,8 @@
 /* Begin PBXBuildFile section */
 		0D50126D23FF759100CD5B95 /* messages.m in Sources */ = {isa = PBXBuildFile; fileRef = 0D50126B23FF759100CD5B95 /* messages.m */; };
 		0D50127523FF75B100CD5B95 /* RunnerTests.m in Sources */ = {isa = PBXBuildFile; fileRef = 0D50127423FF75B100CD5B95 /* RunnerTests.m */; };
+		0D8C35E825D45A3000B76435 /* async_handlers.m in Sources */ = {isa = PBXBuildFile; fileRef = 0D8C35E725D45A3000B76435 /* async_handlers.m */; };
+		0D8C35EB25D45A7900B76435 /* AsyncHandlersTest.m in Sources */ = {isa = PBXBuildFile; fileRef = 0D8C35EA25D45A7900B76435 /* AsyncHandlersTest.m */; };
 		1498D2341E8E89220040F4C2 /* GeneratedPluginRegistrant.m in Sources */ = {isa = PBXBuildFile; fileRef = 1498D2331E8E89220040F4C2 /* GeneratedPluginRegistrant.m */; };
 		3B3967161E833CAA004F5970 /* AppFrameworkInfo.plist in Resources */ = {isa = PBXBuildFile; fileRef = 3B3967151E833CAA004F5970 /* AppFrameworkInfo.plist */; };
 		978B8F6F1D3862AE00F588F7 /* AppDelegate.m in Sources */ = {isa = PBXBuildFile; fileRef = 7AFFD8EE1D35381100E5BB4D /* AppDelegate.m */; };
@@ -47,6 +49,9 @@
 		0D50127223FF75B100CD5B95 /* RunnerTests.xctest */ = {isa = PBXFileReference; explicitFileType = wrapper.cfbundle; includeInIndex = 0; path = RunnerTests.xctest; sourceTree = BUILT_PRODUCTS_DIR; };
 		0D50127423FF75B100CD5B95 /* RunnerTests.m */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.objc; path = RunnerTests.m; sourceTree = "<group>"; };
 		0D50127623FF75B100CD5B95 /* Info.plist */ = {isa = PBXFileReference; lastKnownFileType = text.plist.xml; path = Info.plist; sourceTree = "<group>"; };
+		0D8C35E625D45A3000B76435 /* async_handlers.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = async_handlers.h; sourceTree = "<group>"; };
+		0D8C35E725D45A3000B76435 /* async_handlers.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = async_handlers.m; sourceTree = "<group>"; };
+		0D8C35EA25D45A7900B76435 /* AsyncHandlersTest.m */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.objc; path = AsyncHandlersTest.m; sourceTree = "<group>"; };
 		1498D2321E8E86230040F4C2 /* GeneratedPluginRegistrant.h */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.h; path = GeneratedPluginRegistrant.h; sourceTree = "<group>"; };
 		1498D2331E8E89220040F4C2 /* GeneratedPluginRegistrant.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = GeneratedPluginRegistrant.m; sourceTree = "<group>"; };
 		3B3967151E833CAA004F5970 /* AppFrameworkInfo.plist */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = text.plist.xml; name = AppFrameworkInfo.plist; path = Flutter/AppFrameworkInfo.plist; sourceTree = "<group>"; };
@@ -86,6 +91,7 @@
 			children = (
 				0D50127423FF75B100CD5B95 /* RunnerTests.m */,
 				0D50127623FF75B100CD5B95 /* Info.plist */,
+				0D8C35EA25D45A7900B76435 /* AsyncHandlersTest.m */,
 			);
 			path = RunnerTests;
 			sourceTree = "<group>";
@@ -123,6 +129,8 @@
 		97C146F01CF9000F007C117D /* Runner */ = {
 			isa = PBXGroup;
 			children = (
+				0D8C35E625D45A3000B76435 /* async_handlers.h */,
+				0D8C35E725D45A3000B76435 /* async_handlers.m */,
 				0D50126C23FF759100CD5B95 /* messages.h */,
 				0D50126B23FF759100CD5B95 /* messages.m */,
 				7AFFD8ED1D35381100E5BB4D /* AppDelegate.h */,
@@ -283,6 +291,7 @@
 			buildActionMask = 2147483647;
 			files = (
 				0D50127523FF75B100CD5B95 /* RunnerTests.m in Sources */,
+				0D8C35EB25D45A7900B76435 /* AsyncHandlersTest.m in Sources */,
 			);
 			runOnlyForDeploymentPostprocessing = 0;
 		};
@@ -294,6 +303,7 @@
 				0D50126D23FF759100CD5B95 /* messages.m in Sources */,
 				97C146F31CF9000F007C117D /* main.m in Sources */,
 				1498D2341E8E89220040F4C2 /* GeneratedPluginRegistrant.m in Sources */,
+				0D8C35E825D45A3000B76435 /* async_handlers.m in Sources */,
 			);
 			runOnlyForDeploymentPostprocessing = 0;
 		};
diff --git a/packages/pigeon/platform_tests/ios_unit_tests/ios/Runner/async_handlers.h b/packages/pigeon/platform_tests/ios_unit_tests/ios/Runner/async_handlers.h
new file mode 100644
index 0000000000..a69bdb6a97
--- /dev/null
+++ b/packages/pigeon/platform_tests/ios_unit_tests/ios/Runner/async_handlers.h
@@ -0,0 +1,27 @@
+// Autogenerated from Pigeon (v0.1.20), do not edit directly.
+// See also: https://pub.dev/packages/pigeon
+#import <Foundation/Foundation.h>
+@protocol FlutterBinaryMessenger;
+@class FlutterError;
+@class FlutterStandardTypedData;
+
+NS_ASSUME_NONNULL_BEGIN
+
+@class Value;
+
+@interface Value : NSObject
+@property(nonatomic, strong, nullable) NSNumber *number;
+@end
+
+@interface Api2Flutter : NSObject
+- (instancetype)initWithBinaryMessenger:(id<FlutterBinaryMessenger>)binaryMessenger;
+- (void)calculate:(Value *)input completion:(void (^)(Value *, NSError *_Nullable))completion;
+@end
+@protocol Api2Host
+- (void)calculate:(nullable Value *)input
+       completion:(void (^)(Value *_Nullable, FlutterError *_Nullable))completion;
+@end
+
+extern void Api2HostSetup(id<FlutterBinaryMessenger> binaryMessenger, id<Api2Host> _Nullable api);
+
+NS_ASSUME_NONNULL_END
diff --git a/packages/pigeon/platform_tests/ios_unit_tests/ios/Runner/async_handlers.m b/packages/pigeon/platform_tests/ios_unit_tests/ios/Runner/async_handlers.m
new file mode 100644
index 0000000000..73f062e979
--- /dev/null
+++ b/packages/pigeon/platform_tests/ios_unit_tests/ios/Runner/async_handlers.m
@@ -0,0 +1,88 @@
+// Autogenerated from Pigeon (v0.1.20), do not edit directly.
+// See also: https://pub.dev/packages/pigeon
+#import "async_handlers.h"
+#import <Flutter/Flutter.h>
+
+#if !__has_feature(objc_arc)
+#error File requires ARC to be enabled.
+#endif
+
+static NSDictionary<NSString *, id> *wrapResult(NSDictionary *result, FlutterError *error) {
+  NSDictionary *errorDict = (NSDictionary *)[NSNull null];
+  if (error) {
+    errorDict = @{
+      @"code" : (error.code ? error.code : [NSNull null]),
+      @"message" : (error.message ? error.message : [NSNull null]),
+      @"details" : (error.details ? error.details : [NSNull null]),
+    };
+  }
+  return @{
+    @"result" : (result ? result : [NSNull null]),
+    @"error" : errorDict,
+  };
+}
+
+@interface Value ()
++ (Value *)fromMap:(NSDictionary *)dict;
+- (NSDictionary *)toMap;
+@end
+
+@implementation Value
++ (Value *)fromMap:(NSDictionary *)dict {
+  Value *result = [[Value alloc] init];
+  result.number = dict[@"number"];
+  if ((NSNull *)result.number == [NSNull null]) {
+    result.number = nil;
+  }
+  return result;
+}
+- (NSDictionary *)toMap {
+  return [NSDictionary
+      dictionaryWithObjectsAndKeys:(self.number ? self.number : [NSNull null]), @"number", nil];
+}
+@end
+
+@interface Api2Flutter ()
+@property(nonatomic, strong) NSObject<FlutterBinaryMessenger> *binaryMessenger;
+@end
+
+@implementation Api2Flutter
+- (instancetype)initWithBinaryMessenger:(NSObject<FlutterBinaryMessenger> *)binaryMessenger {
+  self = [super init];
+  if (self) {
+    _binaryMessenger = binaryMessenger;
+  }
+  return self;
+}
+
+- (void)calculate:(Value *)input completion:(void (^)(Value *, NSError *_Nullable))completion {
+  FlutterBasicMessageChannel *channel =
+      [FlutterBasicMessageChannel messageChannelWithName:@"dev.flutter.pigeon.Api2Flutter.calculate"
+                                         binaryMessenger:self.binaryMessenger];
+  NSDictionary *inputMap = [input toMap];
+  [channel sendMessage:inputMap
+                 reply:^(id reply) {
+                   NSDictionary *outputMap = reply;
+                   Value *output = [Value fromMap:outputMap];
+                   completion(output, nil);
+                 }];
+}
+@end
+void Api2HostSetup(id<FlutterBinaryMessenger> binaryMessenger, id<Api2Host> api) {
+  {
+    FlutterBasicMessageChannel *channel =
+        [FlutterBasicMessageChannel messageChannelWithName:@"dev.flutter.pigeon.Api2Host.calculate"
+                                           binaryMessenger:binaryMessenger];
+    if (api) {
+      [channel setMessageHandler:^(id _Nullable message, FlutterReply callback) {
+        Value *input = [Value fromMap:message];
+        [api calculate:input
+            completion:^(Value *_Nullable output, FlutterError *_Nullable error) {
+              callback(wrapResult([output toMap], error));
+            }];
+      }];
+    } else {
+      [channel setMessageHandler:nil];
+    }
+  }
+}
diff --git a/packages/pigeon/platform_tests/ios_unit_tests/ios/RunnerTests/AsyncHandlersTest.m b/packages/pigeon/platform_tests/ios_unit_tests/ios/RunnerTests/AsyncHandlersTest.m
new file mode 100644
index 0000000000..f32e04697d
--- /dev/null
+++ b/packages/pigeon/platform_tests/ios_unit_tests/ios/RunnerTests/AsyncHandlersTest.m
@@ -0,0 +1,139 @@
+#import <Flutter/Flutter.h>
+#import <XCTest/XCTest.h>
+#import "async_handlers.h"
+
+///////////////////////////////////////////////////////////////////////////////////////////
+@interface Value ()
++ (Value*)fromMap:(NSDictionary*)dict;
+- (NSDictionary*)toMap;
+@end
+
+///////////////////////////////////////////////////////////////////////////////////////////
+@interface MockBinaryMessenger : NSObject<FlutterBinaryMessenger>
+@property(nonatomic, copy) NSNumber* result;
+@property(nonatomic, retain) FlutterStandardMessageCodec* codec;
+@property(nonatomic, retain) NSMutableDictionary<NSString*, FlutterBinaryMessageHandler>* handlers;
+@end
+
+///////////////////////////////////////////////////////////////////////////////////////////
+@implementation MockBinaryMessenger
+
+- (instancetype)init {
+  self = [super init];
+  if (self) {
+    _codec = [FlutterStandardMessageCodec sharedInstance];
+    _handlers = [[NSMutableDictionary alloc] init];
+  }
+  return self;
+}
+
+- (void)cleanupConnection:(FlutterBinaryMessengerConnection)connection {
+}
+
+- (void)sendOnChannel:(nonnull NSString*)channel message:(NSData* _Nullable)message {
+}
+
+- (void)sendOnChannel:(nonnull NSString*)channel
+              message:(NSData* _Nullable)message
+          binaryReply:(FlutterBinaryReply _Nullable)callback {
+  if (self.result) {
+    Value* output = [[Value alloc] init];
+    output.number = self.result;
+    NSDictionary* outputDictionary = [output toMap];
+    callback([_codec encode:outputDictionary]);
+  }
+}
+
+- (FlutterBinaryMessengerConnection)setMessageHandlerOnChannel:(nonnull NSString*)channel
+                                          binaryMessageHandler:
+                                              (FlutterBinaryMessageHandler _Nullable)handler {
+  _handlers[channel] = [handler copy];
+  return _handlers.count;
+}
+
+@end
+
+///////////////////////////////////////////////////////////////////////////////////////////
+@interface MockApi2Host : NSObject<Api2Host>
+@property(nonatomic, copy) NSNumber* output;
+@end
+
+///////////////////////////////////////////////////////////////////////////////////////////
+@implementation MockApi2Host
+
+- (void)calculate:(Value* _Nullable)input
+       completion:(nonnull void (^)(Value* _Nullable, FlutterError* _Nullable))completion {
+  if (self.output) {
+    Value* output = [[Value alloc] init];
+    output.number = self.output;
+    completion(output, nil);
+  } else {
+    completion(nil, [FlutterError errorWithCode:@"hey" message:@"ho" details:nil]);
+  }
+}
+
+@end
+
+///////////////////////////////////////////////////////////////////////////////////////////
+@interface AsyncHandlersTest : XCTestCase
+@end
+
+///////////////////////////////////////////////////////////////////////////////////////////
+@implementation AsyncHandlersTest
+
+- (void)testAsyncHost2Flutter {
+  MockBinaryMessenger* binaryMessenger = [[MockBinaryMessenger alloc] init];
+  binaryMessenger.result = @(2);
+  Api2Flutter* api2Flutter = [[Api2Flutter alloc] initWithBinaryMessenger:binaryMessenger];
+  Value* input = [[Value alloc] init];
+  input.number = @(1);
+  XCTestExpectation* expectation = [self expectationWithDescription:@"calculate callback"];
+  [api2Flutter calculate:input
+              completion:^(Value* _Nonnull output, NSError* _Nullable error) {
+                XCTAssertEqual(output.number.intValue, 2);
+                [expectation fulfill];
+              }];
+  [self waitForExpectationsWithTimeout:1.0 handler:nil];
+}
+
+- (void)testAsyncFlutter2Host {
+  MockBinaryMessenger* binaryMessenger = [[MockBinaryMessenger alloc] init];
+  MockApi2Host* mockApi2Host = [[MockApi2Host alloc] init];
+  mockApi2Host.output = @(2);
+  Api2HostSetup(binaryMessenger, mockApi2Host);
+  NSString* channelName = @"dev.flutter.pigeon.Api2Host.calculate";
+  XCTAssertNotNil(binaryMessenger.handlers[channelName]);
+
+  Value* input = [[Value alloc] init];
+  input.number = @(1);
+  NSData* inputEncoded = [binaryMessenger.codec encode:[input toMap]];
+  XCTestExpectation* expectation = [self expectationWithDescription:@"calculate callback"];
+  binaryMessenger.handlers[channelName](inputEncoded, ^(NSData* data) {
+    NSDictionary* outputMap = [binaryMessenger.codec decode:data];
+    Value* output = [Value fromMap:outputMap[@"result"]];
+    XCTAssertEqual(output.number.intValue, 2);
+    [expectation fulfill];
+  });
+  [self waitForExpectationsWithTimeout:1.0 handler:nil];
+}
+
+- (void)testAsyncFlutter2HostError {
+  MockBinaryMessenger* binaryMessenger = [[MockBinaryMessenger alloc] init];
+  MockApi2Host* mockApi2Host = [[MockApi2Host alloc] init];
+  Api2HostSetup(binaryMessenger, mockApi2Host);
+  NSString* channelName = @"dev.flutter.pigeon.Api2Host.calculate";
+  XCTAssertNotNil(binaryMessenger.handlers[channelName]);
+
+  Value* input = [[Value alloc] init];
+  input.number = @(1);
+  NSData* inputEncoded = [binaryMessenger.codec encode:[input toMap]];
+  XCTestExpectation* expectation = [self expectationWithDescription:@"calculate callback"];
+  binaryMessenger.handlers[channelName](inputEncoded, ^(NSData* data) {
+    NSDictionary* outputMap = [binaryMessenger.codec decode:data];
+    XCTAssertNotNil(outputMap[@"error"]);
+    [expectation fulfill];
+  });
+  [self waitForExpectationsWithTimeout:1.0 handler:nil];
+}
+
+@end
diff --git a/packages/pigeon/platform_tests/ios_unit_tests/ios/RunnerTests/RunnerTests.m b/packages/pigeon/platform_tests/ios_unit_tests/ios/RunnerTests/RunnerTests.m
index 926b7ebc92..f077fd464a 100644
--- a/packages/pigeon/platform_tests/ios_unit_tests/ios/RunnerTests/RunnerTests.m
+++ b/packages/pigeon/platform_tests/ios_unit_tests/ios/RunnerTests/RunnerTests.m
@@ -1,11 +1,3 @@
-//
-//  RunnerTests.m
-//  RunnerTests
-//
-//  Created by Aaron Clarke on 2/20/20.
-//  Copyright © 2020 The Chromium Authors. All rights reserved.
-//
-
 #import <XCTest/XCTest.h>
 #import "messages.h"
 
diff --git a/packages/pigeon/pubspec.yaml b/packages/pigeon/pubspec.yaml
index 1fd88de9f9..408779eaad 100644
--- a/packages/pigeon/pubspec.yaml
+++ b/packages/pigeon/pubspec.yaml
@@ -1,5 +1,5 @@
 name: pigeon
-version: 0.1.19 # This must match the version in lib/generator_tools.dart
+version: 0.1.20 # This must match the version in lib/generator_tools.dart
 description: Code generator tool to make communication between Flutter and the host platform type-safe and easier.
 homepage: https://github.com/flutter/packages/tree/master/packages/pigeon
 dependencies:
diff --git a/packages/pigeon/run_tests.sh b/packages/pigeon/run_tests.sh
index f45d79ccb5..c6b5b20c52 100755
--- a/packages/pigeon/run_tests.sh
+++ b/packages/pigeon/run_tests.sh
@@ -134,6 +134,7 @@ pushd $PWD
 cd e2e_tests/test_objc/
 flutter pub get
 popd
+test_pigeon_ios ./pigeons/async_handlers.dart
 test_null_safe_dart ./pigeons/message.dart
 test_pigeon_android ./pigeons/voidflutter.dart
 test_pigeon_android ./pigeons/voidhost.dart
@@ -152,8 +153,6 @@ test_pigeon_ios ./pigeons/void_arg_host.dart
 test_pigeon_ios ./pigeons/void_arg_flutter.dart
 test_pigeon_ios ./pigeons/list.dart
 test_pigeon_ios ./pigeons/all_datatypes.dart
-# Not implemented yet.
-# test_pigeon_ios ./pigeons/async_handlers.dart
 
 ###############################################################################
 # iOS unit tests on generated code.
@@ -163,8 +162,15 @@ pub run pigeon \
   --dart_out /dev/null \
   --objc_header_out platform_tests/ios_unit_tests/ios/Runner/messages.h \
   --objc_source_out platform_tests/ios_unit_tests/ios/Runner/messages.m
+pub run pigeon \
+  --input pigeons/async_handlers.dart \
+  --dart_out /dev/null \
+  --objc_header_out platform_tests/ios_unit_tests/ios/Runner/async_handlers.h \
+  --objc_source_out platform_tests/ios_unit_tests/ios/Runner/async_handlers.m
 clang-format -i platform_tests/ios_unit_tests/ios/Runner/messages.h
 clang-format -i platform_tests/ios_unit_tests/ios/Runner/messages.m
+clang-format -i platform_tests/ios_unit_tests/ios/Runner/async_handlers.h
+clang-format -i platform_tests/ios_unit_tests/ios/Runner/async_handlers.m
 pushd $PWD
 cd platform_tests/ios_unit_tests
 flutter build ios --simulator
diff --git a/packages/pigeon/test/objc_generator_test.dart b/packages/pigeon/test/objc_generator_test.dart
index 1acbcc5680..041d811354 100644
--- a/packages/pigeon/test/objc_generator_test.dart
+++ b/packages/pigeon/test/objc_generator_test.dart
@@ -431,4 +431,190 @@ void main() {
     expect(code, contains('@interface Foobar'));
     expect(code, matches('@property.*NSDictionary.*field1'));
   });
+
+  test('async void(input) HostApi header', () {
+    final Root root = Root(apis: <Api>[
+      Api(name: 'Api', location: ApiLocation.host, methods: <Method>[
+        Method(
+            name: 'doSomething',
+            argType: 'Input',
+            returnType: 'void',
+            isAsynchronous: true)
+      ])
+    ], classes: <Class>[
+      Class(
+          name: 'Input',
+          fields: <Field>[Field(name: 'input', dataType: 'String')]),
+      Class(
+          name: 'Output',
+          fields: <Field>[Field(name: 'output', dataType: 'String')]),
+    ]);
+    final StringBuffer sink = StringBuffer();
+    generateObjcHeader(ObjcOptions(header: 'foo.h', prefix: 'ABC'), root, sink);
+    final String code = sink.toString();
+    expect(
+        code,
+        contains(
+            '(void)doSomething:(nullable ABCInput *)input completion:(void(^)(FlutterError *_Nullable))completion'));
+  });
+
+  test('async output(input) HostApi header', () {
+    final Root root = Root(apis: <Api>[
+      Api(name: 'Api', location: ApiLocation.host, methods: <Method>[
+        Method(
+            name: 'doSomething',
+            argType: 'Input',
+            returnType: 'Output',
+            isAsynchronous: true)
+      ])
+    ], classes: <Class>[
+      Class(
+          name: 'Input',
+          fields: <Field>[Field(name: 'input', dataType: 'String')]),
+      Class(
+          name: 'Output',
+          fields: <Field>[Field(name: 'output', dataType: 'String')]),
+    ]);
+    final StringBuffer sink = StringBuffer();
+    generateObjcHeader(ObjcOptions(header: 'foo.h', prefix: 'ABC'), root, sink);
+    final String code = sink.toString();
+    expect(
+        code,
+        contains(
+            '(void)doSomething:(nullable ABCInput *)input completion:(void(^)(ABCOutput *_Nullable, FlutterError *_Nullable))completion'));
+  });
+
+  test('async output(void) HostApi header', () {
+    final Root root = Root(apis: <Api>[
+      Api(name: 'Api', location: ApiLocation.host, methods: <Method>[
+        Method(
+            name: 'doSomething',
+            argType: 'void',
+            returnType: 'Output',
+            isAsynchronous: true)
+      ])
+    ], classes: <Class>[
+      Class(
+          name: 'Output',
+          fields: <Field>[Field(name: 'output', dataType: 'String')]),
+    ]);
+    final StringBuffer sink = StringBuffer();
+    generateObjcHeader(ObjcOptions(header: 'foo.h', prefix: 'ABC'), root, sink);
+    final String code = sink.toString();
+    expect(
+        code,
+        contains(
+            '(void)doSomething:(void(^)(ABCOutput *_Nullable, FlutterError *_Nullable))completion'));
+  });
+
+  test('async void(void) HostApi header', () {
+    final Root root = Root(apis: <Api>[
+      Api(name: 'Api', location: ApiLocation.host, methods: <Method>[
+        Method(
+            name: 'doSomething',
+            argType: 'void',
+            returnType: 'void',
+            isAsynchronous: true)
+      ])
+    ], classes: <Class>[]);
+    final StringBuffer sink = StringBuffer();
+    generateObjcHeader(ObjcOptions(header: 'foo.h', prefix: 'ABC'), root, sink);
+    final String code = sink.toString();
+    expect(
+        code,
+        contains(
+            '(void)doSomething:(void(^)(FlutterError *_Nullable))completion'));
+  });
+
+  test('async output(input) HostApi source', () {
+    final Root root = Root(apis: <Api>[
+      Api(name: 'Api', location: ApiLocation.host, methods: <Method>[
+        Method(
+            name: 'doSomething',
+            argType: 'Input',
+            returnType: 'Output',
+            isAsynchronous: true)
+      ])
+    ], classes: <Class>[
+      Class(
+          name: 'Input',
+          fields: <Field>[Field(name: 'input', dataType: 'String')]),
+      Class(
+          name: 'Output',
+          fields: <Field>[Field(name: 'output', dataType: 'String')]),
+    ]);
+    final StringBuffer sink = StringBuffer();
+    generateObjcSource(ObjcOptions(header: 'foo.h', prefix: 'ABC'), root, sink);
+    final String code = sink.toString();
+    expect(
+        code,
+        contains(
+            '[api doSomething:input completion:^(ABCOutput *_Nullable output, FlutterError *_Nullable error) {'));
+  });
+
+  test('async void(input) HostApi source', () {
+    final Root root = Root(apis: <Api>[
+      Api(name: 'Api', location: ApiLocation.host, methods: <Method>[
+        Method(
+            name: 'doSomething',
+            argType: 'Input',
+            returnType: 'void',
+            isAsynchronous: true)
+      ])
+    ], classes: <Class>[
+      Class(
+          name: 'Input',
+          fields: <Field>[Field(name: 'input', dataType: 'String')]),
+      Class(
+          name: 'Output',
+          fields: <Field>[Field(name: 'output', dataType: 'String')]),
+    ]);
+    final StringBuffer sink = StringBuffer();
+    generateObjcSource(ObjcOptions(header: 'foo.h', prefix: 'ABC'), root, sink);
+    final String code = sink.toString();
+    expect(
+        code,
+        contains(
+            '[api doSomething:input completion:^(FlutterError *_Nullable error) {'));
+  });
+
+  test('async void(void) HostApi source', () {
+    final Root root = Root(apis: <Api>[
+      Api(name: 'Api', location: ApiLocation.host, methods: <Method>[
+        Method(
+            name: 'doSomething',
+            argType: 'void',
+            returnType: 'void',
+            isAsynchronous: true)
+      ])
+    ], classes: <Class>[]);
+    final StringBuffer sink = StringBuffer();
+    generateObjcSource(ObjcOptions(header: 'foo.h', prefix: 'ABC'), root, sink);
+    final String code = sink.toString();
+    expect(
+        code, contains('[api doSomething:^(FlutterError *_Nullable error) {'));
+  });
+
+  test('async output(void) HostApi source', () {
+    final Root root = Root(apis: <Api>[
+      Api(name: 'Api', location: ApiLocation.host, methods: <Method>[
+        Method(
+            name: 'doSomething',
+            argType: 'void',
+            returnType: 'Output',
+            isAsynchronous: true)
+      ])
+    ], classes: <Class>[
+      Class(
+          name: 'Output',
+          fields: <Field>[Field(name: 'output', dataType: 'String')]),
+    ]);
+    final StringBuffer sink = StringBuffer();
+    generateObjcSource(ObjcOptions(header: 'foo.h', prefix: 'ABC'), root, sink);
+    final String code = sink.toString();
+    expect(
+        code,
+        contains(
+            '[api doSomething:^(ABCOutput *_Nullable output, FlutterError *_Nullable error) {'));
+  });
 }