为现有 iOS项目集成 Flutter

Flutter：0.5.1
Xcode 9.4.1
Flutter工程：flutter/examples/hello_world

Debug: Flutter Hot Restart

我们都知道在开发环境下，Flutter的 hot restart 对 UI快速成型是非常有帮助的，要让现有的 App支持 Flutter并且开启 Hot restart也不难。

Flutter(engine) 基础库

首先，在你的项目里面拖入 Flutter.framework，这个库是 Flutter Engine，承载了 Dart运行时和绘图引擎。Flutter.framework和命令行工具版本是一一对应的，如果你不知道从哪里找这个文件，可以直接在 Flutter源码项目里面进行一次 flutter run，然后你就能在 /<project>/ios/Flutter/目录下面找到了，直接拖进项目即可。

Flutter ViewController

接下来需要把 Flutter的基础代码引入现有工程，有了基础的 Flutter ViewController才可以显示 Flutter视图。这一步很简单，只需要在你现有的 ViewController中 Push过去就可以了：

- (void)jumpToFlutter {
    FlutterViewController *viewController = [FlutterViewController new];
    [self.navigationController pushViewController:viewController animated:YES];
}

但是还需要注意需要把 AppDelegate里面的生命周期事件传递给 Flutter

Release Preview 1后文档中提到有 FlutterAppLifeCycleProvider这个协议，但是 0.5.1还没有，所以这里先野路子来了

1. 直接让现有的 AppDelegate继承 FlutterAppDelegate即可，但这带来的负面影响是 root ViewController被设置为 Flutter ViewController

2. 自行改造 AppDelegate。很多中大型 App喜欢在开发中将业务模块化，幸好 Flutter APPDelegate并不是很难改造过来，也能支持模块 Delegate。首先让你的 Delegate遵循 FlutterPluginRegistry协议

// AppDelegate 或者模块的 Delegate

@interface DemoFlutterBaseAppDelegate : NSObject <ModularApplicationDelegate, FlutterPluginRegistry>

/**
 FlutterBinaryMessenger
 this determines which view controller is the flutter view controller
 nomally, flutter view controller provides the binary messages
 @return root Flutter ViewController
 */
- (NSObject<FlutterBinaryMessenger> *)binaryMessenger;

/**
 FlutterTextureRegistry
 this determines which view controller is the flutter view controller
 nomally, flutter view controller provides the custom textures
 @return root Flutter ViewController
 */
- (NSObject<FlutterTextureRegistry> *)textures;

@end

// Registrar 声明和实现

@interface DemoFlutterAppDelegateRegistrar : NSObject<FlutterPluginRegistrar>

@property(nonatomic, copy) NSString *pluginKey;
@property(nonatomic, strong) DemoFlutterBaseAppDelegate *appDelegate;

- (instancetype)initWithPlugin:(NSString*)pluginKey appDelegate:(DemoFlutterBaseAppDelegate*)delegate;

@end

@implementation DemoFlutterAppDelegateRegistrar
- (instancetype)initWithPlugin:(NSString*)pluginKey appDelegate:(DemoFlutterBaseAppDelegate*)appDelegate {
    self = [super init];
    NSAssert(self, @"Super init cannot be nil");
    _pluginKey = [pluginKey copy];
    _appDelegate = appDelegate;
    return self;
}


- (NSObject<FlutterBinaryMessenger>*)messenger {
    return [_appDelegate binaryMessenger];
}

- (NSObject<FlutterTextureRegistry>*)textures {
    return [_appDelegate textures];
}

- (void)publish:(NSObject*)value {
    _appDelegate.pluginPublications[_pluginKey] = value;
}

- (void)addMethodCallDelegate:(NSObject<FlutterPlugin>*)delegate
                      channel:(FlutterMethodChannel*)channel {
    [channel setMethodCallHandler:^(FlutterMethodCall* call, FlutterResult result) {
        [delegate handleMethodCall:call result:result];
    }];
}

- (void)addApplicationDelegate:(NSObject<FlutterPlugin>*)delegate {
    [_appDelegate.pluginDelegates addObject:delegate];
}

- (NSString*)lookupKeyForAsset:(NSString*)asset {
    return [FlutterDartProject lookupKeyForAsset:asset];
}

- (NSString*)lookupKeyForAsset:(NSString*)asset fromPackage:(NSString*)package {
    return [FlutterDartProject lookupKeyForAsset:asset fromPackage:package];
}

@end


// AppDelegate实现

@interface DemoFlutterBaseAppDelegate ()

@property(nonatomic, strong) DemoFlutterBaseViewController *rootController;
@property(readonly, nonatomic) NSMutableArray* pluginDelegates;
@property(readonly, nonatomic) NSMutableDictionary* pluginPublications;

@end

@implementation DemoFlutterBaseAppDelegate

/*
 * ... 这里写转发各种声明周期事件给 plugin
 */
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    for (id<FlutterPlugin> plugin in _pluginDelegates) {
        if ([plugin respondsToSelector:_cmd]) {
            [plugin application:application didFinishLaunchingWithOptions:launchOptions];
        }
    }
    return YES;
}

#pragma mark - getters for flutter

// 返回 FlutterViewController实例
- (FlutterViewController *)rootController {
    // ...
}

- (NSObject<FlutterBinaryMessenger> *)binaryMessenger
{
    if ([self.rootController conformsToProtocol:@protocol(FlutterBinaryMessenger)]) {
        return (NSObject<FlutterBinaryMessenger> *)self.rootController;
    }
    return nil;
}

- (NSObject<FlutterTextureRegistry> *)textures
{
    if ([self.rootController conformsToProtocol:@protocol(FlutterTextureRegistry)]) {
        return (NSObject<FlutterTextureRegistry> *)self.rootController;
    }
    return nil;
}

- (NSObject<FlutterPluginRegistrar>*)registrarForPlugin:(NSString*)pluginKey {
    NSAssert(self.pluginPublications[pluginKey] == nil, @"Duplicate plugin key: %@", pluginKey);
    self.pluginPublications[pluginKey] = [NSNull null];
    return [[DemoFlutterAppDelegateRegistrar alloc] initWithPlugin:pluginKey appDelegate:self];
}

- (BOOL)hasPlugin:(NSString*)pluginKey {
    return _pluginPublications[pluginKey] != nil;
}

- (NSObject*)valuePublishedByPlugin:(NSString*)pluginKey {
    return _pluginPublications[pluginKey];
}

@end

这样 Flutter的运行环境其实就准备好了，无论是 Hot Restart还是 AOT都可以支持。接下来我们实现 Debug Hot Restart
首先在你的 Flutter代码目录下执行一遍 Flutter build bundle，这可以帮助我们打包出一个 Flutter Asset，然后把这个 flutter_assets 目录拖入项目。

对你的项目进行一次 build，确保能够得到一个 .app 文件。然后新建一个文件夹叫做 Payload，把 .app文件放入 Payload文件夹，然后压缩成 zip文件。这个文件便可以被 Flutter命令行工具使用了。

flutter run --use-application-binary /path/to/Payload.zip

然后效果如图：

其实这里并没有实现 Hot Reload，主要是目前 flutter工具链支持度还不好，不过 Hot Restart也够用了。

Release: Flutter AOT

Release模式和 Debug下不一样，我们需要做几件事情：

1. 把 Flutter.framework替换成 flutter/bin/cache/artifacts/engine/ios-release/Flutter.framework，因为上一步我们用的库其实是 JIT Runtime
2. 在 Flutter代码项目下面执行 flutter build aot --release --target-platform ios --ios-arch armv7,arm64 然后我们可以在 build目录下拿到一个打包好的 App.framework，不过别忘记在里面放一个 Info.plist。并且把这个库拖到工程里面
3. 删除工程里面的 flutter_assets文件夹下的 isolate_snapshot_data、kernel_blob.bin、platform.dill、vm_snapshot_data这几个文件
4. 编译打包给真机运行，效果如下：