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

Debug: Flutter Hot Restart

我們都知道在開發環境下，Flutter的 hot restart 對 UI快速成型是非常有幫助的，要讓現有的 App支援 Flutter並且開啟 Hot restart也不難。

Flutter(engine) 基礎庫

首先，在你的專案裡面拖入 Flutter.framework，這個庫是 Flutter Engine，承載了 Dart執行時和繪圖引擎。Flutter.framework和命令列工具版本是一一對應的，如果你不知道從哪裡找這個檔案，可以直接在 Flutter原始碼專案裡面進行一次 flutter run，然後你就能在 /<project>/ios/Flutter/目錄下面找到了，直接拖進專案即可。

Flutter ViewController

接下來需要把 Flutter的基礎程式碼引入現有工程，有了基礎的 Flutter ViewController才可以顯示 Flutter檢視。這一步很簡單，只需要在你現有的 ViewController中 Push過去就可以了：

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

但是還需要注意需要把 AppDelegate裡面的生命週期事件傳遞給 Flutter

Release Preview 1後文件中提到有 FlutterAppLifeCycleProvider這個協議，但是 0.5.1還沒有，所以這裡先野路子來了

1. 直接讓現有的 AppDelegate繼承 FlutterAppDelegate即可，但這帶來的負面影響是 root ViewController被設定為 Flutter ViewController

2. 自行改造 AppDelegate。很多中大型 App喜歡在開發中將業務模組化，幸好 Flutter APPDelegate並不是很難改造過來，也能支援模組 Delegate。首先讓你的 Delegate遵循 FlutterPluginRegistry協議

// AppDelegate 或者模組的 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 宣告和實現

@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實現

@interface DemoFlutterBaseAppDelegate ()

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

@end

@implementation DemoFlutterBaseAppDelegate

/*
 * ... 這裡寫轉發各種宣告週期事件給 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例項
- (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

這樣 Flutter的執行環境其實就準備好了，無論是 Hot Restart還是 AOT都可以支援。接下來我們實現 Debug Hot Restart
首先在你的 Flutter程式碼目錄下執行一遍 Flutter build bundle，這可以幫助我們打包出一個 Flutter Asset，然後把這個 flutter_assets 目錄拖入專案。

對你的專案進行一次 build，確保能夠得到一個 .app 檔案。然後新建一個資料夾叫做 Payload，把 .app檔案放入 Payload資料夾，然後壓縮成 zip檔案。這個檔案便可以被 Flutter命令列工具使用了。

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

然後效果如圖：

其實這裡並沒有實現 Hot Reload，主要是目前 flutter工具鏈支援度還不好，不過 Hot Restart也夠用了。

Release: Flutter AOT

Release模式和 Debug下不一樣，我們需要做幾件事情：

1. 把 Flutter.framework替換成 flutter/bin/cache/artifacts/engine/ios-release/Flutter.framework，因為上一步我們用的庫其實是 JIT Runtime
2. 在 Flutter程式碼專案下面執行 flutter build aot --release --target-platform ios --ios-arch armv7,arm64 然後我們可以在 build目錄下拿到一個打包好的 App.framework，不過別忘記在裡面放一個 Info.plist。並且把這個庫拖到工程裡面
3. 刪除工程裡面的 flutter_assets資料夾下的 isolate_snapshot_data、kernel_blob.bin、platform.dill、vm_snapshot_data這幾個檔案
4. 編譯打包給真機執行，效果如下：