Surrogate binary messenger (flutter/engine#9347)

Created FlutterSurrogateBinaryMessenger to make sure that channels are
holding onto engines and not viewcontrollers.  This doesn't change the
public API but makes clients do what we want them to be doing, using
Engine for FlutterBinaryMessenger.

engine/src/flutter/ci/licenses_golden/licenses_flutter

@@ -664,6 +664,7 @@ FILE: ../../../flutter/shell/platform/darwin/common/framework/Source/FlutterChan
 FILE: ../../../flutter/shell/platform/darwin/common/framework/Source/FlutterCodecs.mm
 FILE: ../../../flutter/shell/platform/darwin/common/framework/Source/FlutterStandardCodec.mm
 FILE: ../../../flutter/shell/platform/darwin/common/framework/Source/FlutterStandardCodec_Internal.h
+FILE: ../../../flutter/shell/platform/darwin/common/framework/Source/flutter_channels_unittest.mm
 FILE: ../../../flutter/shell/platform/darwin/common/framework/Source/flutter_codecs_unittest.mm
 FILE: ../../../flutter/shell/platform/darwin/common/framework/Source/flutter_standard_codec_unittest.mm
 FILE: ../../../flutter/shell/platform/darwin/ios/framework/Flutter.podspec
@@ -699,6 +700,7 @@ FILE: ../../../flutter/shell/platform/darwin/ios/framework/Source/FlutterPlatfor
 FILE: ../../../flutter/shell/platform/darwin/ios/framework/Source/FlutterPlatformViews_Internal.mm
 FILE: ../../../flutter/shell/platform/darwin/ios/framework/Source/FlutterPluginAppLifeCycleDelegate.mm
 FILE: ../../../flutter/shell/platform/darwin/ios/framework/Source/FlutterPluginAppLifeCycleDelegate_internal.h
+FILE: ../../../flutter/shell/platform/darwin/ios/framework/Source/FlutterSurrogateBinaryMessenger.h
 FILE: ../../../flutter/shell/platform/darwin/ios/framework/Source/FlutterTextInputDelegate.h
 FILE: ../../../flutter/shell/platform/darwin/ios/framework/Source/FlutterTextInputPlugin.h
 FILE: ../../../flutter/shell/platform/darwin/ios/framework/Source/FlutterTextInputPlugin.mm

engine/src/flutter/shell/platform/darwin/BUILD.gn

@@ -53,6 +53,7 @@ executable("flutter_channels_unittests") {
   testonly = true
 
   sources = [
+    "common/framework/Source/flutter_channels_unittest.mm",
     "common/framework/Source/flutter_codecs_unittest.mm",
     "common/framework/Source/flutter_standard_codec_unittest.mm",
   ]

engine/src/flutter/shell/platform/darwin/common/framework/Source/FlutterChannels.mm

@@ -3,9 +3,19 @@
 // found in the LICENSE file.
 
 #include "flutter/shell/platform/darwin/common/framework/Headers/FlutterChannels.h"
+#import "flutter/shell/platform/darwin/ios/framework/Source/FlutterSurrogateBinaryMessenger.h"
 
 #pragma mark - Basic message channel
 
+static NSObject<FlutterBinaryMessenger>* getSurrogate(NSObject<FlutterBinaryMessenger>* messenger) {
+  if ([messenger conformsToProtocol:@protocol(FlutterSurrogateBinaryMessenger)]) {
+    NSObject<FlutterSurrogateBinaryMessenger>* surrogate =
+        (NSObject<FlutterSurrogateBinaryMessenger>*)messenger;
+    return [surrogate surrogateBinaryMessenger];
+  }
+  return messenger;
+}
+
 @implementation FlutterBasicMessageChannel {
   NSObject<FlutterBinaryMessenger>* _messenger;
   NSString* _name;
@@ -32,7 +42,7 @@
   self = [super init];
   NSAssert(self, @"Super init cannot be nil");
   _name = [name retain];
-  _messenger = [messenger retain];
+  _messenger = [getSurrogate(messenger) retain];
   _codec = [codec retain];
   return self;
 }
@@ -172,7 +182,7 @@ NSObject const* FlutterMethodNotImplemented = [NSObject new];
   self = [super init];
   NSAssert(self, @"Super init cannot be nil");
   _name = [name retain];
-  _messenger = [messenger retain];
+  _messenger = [getSurrogate(messenger) retain];
   _codec = [codec retain];
   return self;
 }
@@ -251,7 +261,7 @@ NSObject const* FlutterEndOfEventStream = [NSObject new];
   self = [super init];
   NSAssert(self, @"Super init cannot be nil");
   _name = [name retain];
-  _messenger = [messenger retain];
+  _messenger = [getSurrogate(messenger) retain];
   _codec = [codec retain];
   return self;
 }

engine/src/flutter/shell/platform/darwin/common/framework/Source/flutter_channels_unittest.mm

@@ -0,0 +1,107 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "flutter/shell/platform/darwin/common/framework/Headers/FlutterChannels.h"
+#import "flutter/shell/platform/darwin/ios/framework/Source/FlutterSurrogateBinaryMessenger.h"
+#include "gtest/gtest.h"
+
+@interface MockBinaryMessenger : NSObject <FlutterBinaryMessenger>
+@property(nonatomic, assign) int sendOnChannel_message_count;
+@end
+
+@implementation MockBinaryMessenger
+- (void)sendOnChannel:(NSString*)channel message:(NSData* _Nullable)message {
+  _sendOnChannel_message_count++;
+}
+
+- (void)sendOnChannel:(NSString*)channel
+              message:(NSData* _Nullable)message
+          binaryReply:(FlutterBinaryReply _Nullable)callback {
+}
+
+- (void)setMessageHandlerOnChannel:(NSString*)channel
+              binaryMessageHandler:(FlutterBinaryMessageHandler _Nullable)handler {
+}
+@end
+
+@interface MockSurrogateBinaryMessenger
+    : NSObject <FlutterBinaryMessenger, FlutterSurrogateBinaryMessenger>
+@property(nonatomic, strong) MockBinaryMessenger* surrogate;
+@property(nonatomic, assign) int sendOnChannel_message_count;
+@end
+
+@implementation MockSurrogateBinaryMessenger
+- (instancetype)init {
+  self = [super init];
+  if (self) {
+    _surrogate = [[[MockBinaryMessenger alloc] init] retain];
+  }
+  return self;
+}
+
+- (void)dealloc {
+  [_surrogate release];
+  [super dealloc];
+}
+
+- (void)sendOnChannel:(NSString*)channel message:(NSData* _Nullable)message {
+  _sendOnChannel_message_count++;
+}
+
+- (void)sendOnChannel:(NSString*)channel
+              message:(NSData* _Nullable)message
+          binaryReply:(FlutterBinaryReply _Nullable)callback {
+}
+
+- (void)setMessageHandlerOnChannel:(NSString*)channel
+              binaryMessageHandler:(FlutterBinaryMessageHandler _Nullable)handler {
+}
+
+- (NSObject<FlutterBinaryMessenger>*)surrogateBinaryMessenger {
+  return _surrogate;
+}
+
+@end
+
+TEST(FlutterChannels, BasicMessageChannelUsesSurrogate) {
+  MockSurrogateBinaryMessenger* binaryMessenger =
+      [[[MockSurrogateBinaryMessenger alloc] init] autorelease];
+  NSObject<FlutterMessageCodec>* codec = [FlutterStandardMessageCodec sharedInstance];
+  FlutterBasicMessageChannel* channel =
+      [[[FlutterBasicMessageChannel alloc] initWithName:@"channel-name"
+                                        binaryMessenger:binaryMessenger
+                                                  codec:codec] autorelease];
+  ASSERT_TRUE(channel != nil);
+  [channel sendMessage:nil];
+  ASSERT_EQ(1, binaryMessenger.surrogate.sendOnChannel_message_count);
+  ASSERT_EQ(0, binaryMessenger.sendOnChannel_message_count);
+}
+
+TEST(FlutterChannels, MethodChannelUsesSurrogate) {
+  MockSurrogateBinaryMessenger* binaryMessenger =
+      [[[MockSurrogateBinaryMessenger alloc] init] autorelease];
+  NSObject<FlutterMethodCodec>* codec = [FlutterStandardMethodCodec sharedInstance];
+  FlutterMethodChannel* channel = [[[FlutterMethodChannel alloc] initWithName:@"channel-name"
+                                                              binaryMessenger:binaryMessenger
+                                                                        codec:codec] autorelease];
+  ASSERT_TRUE(channel != nil);
+  [channel invokeMethod:@"foo" arguments:nil];
+  ASSERT_EQ(1, binaryMessenger.surrogate.sendOnChannel_message_count);
+  ASSERT_EQ(0, binaryMessenger.sendOnChannel_message_count);
+}
+
+TEST(FlutterChannels, EventChannelRetainsSurrogate) {
+  MockSurrogateBinaryMessenger* binaryMessenger =
+      [[[MockSurrogateBinaryMessenger alloc] init] autorelease];
+  NSObject<FlutterMethodCodec>* codec = [FlutterStandardMethodCodec sharedInstance];
+  NSUInteger binaryMessengerRetainCount = [binaryMessenger retainCount];
+  NSUInteger surrogateRetainCount = [binaryMessenger.surrogate retainCount];
+
+  FlutterEventChannel* channel = [[[FlutterEventChannel alloc] initWithName:@"channel-name"
+                                                            binaryMessenger:binaryMessenger
+                                                                      codec:codec] autorelease];
+  ASSERT_TRUE(channel != nil);
+  ASSERT_EQ(binaryMessengerRetainCount, binaryMessenger.retainCount);
+  ASSERT_EQ(surrogateRetainCount + 1, binaryMessenger.surrogate.retainCount);
+}

engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterSurrogateBinaryMessenger.h

@@ -0,0 +1,9 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+@protocol FlutterBinaryMessenger;
+
+@protocol FlutterSurrogateBinaryMessenger
+- (NSObject<FlutterBinaryMessenger>*)surrogateBinaryMessenger;
+@end

engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterViewController.mm

@@ -992,20 +992,20 @@ constexpr CGFloat kStandardStatusBarHeight = 20.0;
 #pragma mark - FlutterBinaryMessenger
 
 - (void)sendOnChannel:(NSString*)channel message:(NSData*)message {
-  [_engine.get() sendOnChannel:channel message:message];
+  [self.surrogateBinaryMessenger sendOnChannel:channel message:message];
 }
 
 - (void)sendOnChannel:(NSString*)channel
               message:(NSData*)message
           binaryReply:(FlutterBinaryReply)callback {
   NSAssert(channel, @"The channel must not be null");
-  [_engine.get() sendOnChannel:channel message:message binaryReply:callback];
+  [self.surrogateBinaryMessenger sendOnChannel:channel message:message binaryReply:callback];
 }
 
 - (void)setMessageHandlerOnChannel:(NSString*)channel
               binaryMessageHandler:(FlutterBinaryMessageHandler)handler {
   NSAssert(channel, @"The channel must not be null");
-  [_engine.get() setMessageHandlerOnChannel:channel binaryMessageHandler:handler];
+  [self.surrogateBinaryMessenger setMessageHandlerOnChannel:channel binaryMessageHandler:handler];
 }
 
 #pragma mark - FlutterTextureRegistry
@@ -1048,4 +1048,9 @@ constexpr CGFloat kStandardStatusBarHeight = 20.0;
   return [_engine.get() valuePublishedByPlugin:pluginKey];
 }
 
+#pragma mark - FlutterSurrogateBinaryMessenger
+- (NSObject<FlutterBinaryMessenger>*)surrogateBinaryMessenger {
+  return _engine.get();
+}
+
 @end

engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterViewController_Internal.h

@@ -11,8 +11,9 @@
 #include "flutter/shell/common/shell.h"
 #include "flutter/shell/platform/darwin/ios/framework/Headers/FlutterViewController.h"
 #include "flutter/shell/platform/darwin/ios/framework/Source/FlutterPlatformViews_Internal.h"
+#import "flutter/shell/platform/darwin/ios/framework/Source/FlutterSurrogateBinaryMessenger.h"
 
-@interface FlutterViewController ()
+@interface FlutterViewController () <FlutterSurrogateBinaryMessenger>
 
 - (fml::WeakPtr<FlutterViewController>)getWeakPtr;
 - (flutter::FlutterPlatformViewsController*)platformViewsController;