一. flutter与iOS的消息传递

Flutter和原生交互主要通过channel进行通信，主要包括三类（以flutter开头的类均是原生端类）

下面以MethodChannel 和FlutterMethodChannel为例进行说明
1. flutter端

 //1.初始化channel
 final platform = const  MethodChannel('samples.flutter.io/test');

  //2.设置回调方法
  void initState() {
    super.initState();
    platform.setMethodCallHandler(invokeFlutterMethod);
  }

  Future< Null > invokeFlutterMethod(MethodCall call) async{
    setState(() {
      content = call.arguments['text'];
    });
  }

  //3.在合适的地方（按钮点击事件），调用iOS方法
  Future _invokeIOSMethod() async {
    try {
      await platform.invokeMethod('invokeIOSMethod',{'text':'flutter传过来的参数'});
    } on PlatformException catch (e) {    }
  }

2.iOS端

    //1.初始化FlutterViewController页面（官方推荐方式）
    FlutterEngine *flutterEngine =
    ((AppDelegate *)UIApplication.sharedApplication.delegate).flutterEngine;
    FlutterViewController *flutterVC =
    [[FlutterViewController alloc] initWithEngine:flutterEngine nibName:nil bundle:nil];
    flutterVC.modalPresentationStyle = UIModalPresentationFullScreen;
    [self presentViewController:flutterVC animated:YES completion:nil];

    //2.初始化channel
    FlutterMethodChannel* channel = [FlutterMethodChannel
                                     methodChannelWithName:@"samples.flutter.io/test"
                                     binaryMessenger:flutterVC.binaryMessenger];

    //3.调用Flutter方法
    [channel invokeMethod:@"invokeFlutterMethod" arguments:@{@"text":@"ios传给flutter的参数"}];

    //4.设置回调（等待Flutter调用）
    [channel setMethodCallHandler:^(FlutterMethodCall* call, FlutterResult result) {
        if ([call.method isEqualToString:@"invokeIOSMethod"]) {
            [flutterVC dismissViewControllerAnimated:YES completion:nil];
        }
    }];

二.在iOS端切换flutter页面

❌闪退？返回控制器不销毁？内存泄漏？
✅解决方案：
官方人员（Dan Field）给出了答案
通过验证官方的例子，现总结如下：
1. 使用setInitialRoute 设置flutter页面
自己的理解：重新初始化FlutterViewController内部会重新创建FlutterEngine，导致flutter的代码重新加载main函数，然后通过window.defaultRouteName获取新的路由值（这里路由值为full），根据路由值加载对应页面

//FullScreenViewController继承自FlutterViewController
  FullScreenViewController *flutterViewController =
      [[FullScreenViewController alloc] init];
  [flutterViewController setInitialRoute:@"full"];
//在交互的过程中，官方代码动画过度都设置的NO，说是有问题，猜测是过渡不自然，
 [self.navigationController
      pushViewController:flutterViewController
                animated:NO];

2.使用全局的FlutterEngine设置flutter页面
实现过程：使用一个全局的channel来管理页面切换，iOS通过发送channel回调来改变flutter页面，如：[[self reloadMessageChannel] sendMessage:@"full"];，具体看官方代码
自己的理解：下面第一行代码这种设置路由纯属多余，虽然路由值设置成功了（这里只能说是路由值，具体的路由页面可不一定切换成功了），因为用的是全局FlutterEngine获取的FlutterViewController，此时是不加载flutter的main函数的。这种情况要想切换页面必须通过channel进行回调flutter端切换路由页面的方法。所以我觉得下面的第一行代码纯属多余，即使注释掉也不影响。暂且把它当作通过FlutterEngine设置路由值的一种方法吧。

  [[self engine].navigationChannel invokeMethod:@"setInitialRoute"
                                      arguments:@"full"];
  [[self reloadMessageChannel] sendMessage:@"full"];

  FullScreenViewController *flutterViewController =
      [[FullScreenViewController alloc] initWithEngine:[self engine]
                                               nibName:nil
                                                bundle:nil];
  [self.navigationController
      pushViewController:flutterViewController
                animated:NO]; // Animating this is problematic.

⚠️直接使用 FlutterViewController，在连续返回然后进入的操作下可能会闪退，建议在 FlutterViewController消失的情况下执行以下代码

-(void)viewWillDisappear:(BOOL)animated {
  [super viewWillDisappear:animated];
  if (self.isMovingFromParentViewController) {
    // Note that if we were doing things that might cause the VC
    // to disappear (like using the image_picker plugin)
    // we shouldn't do this.  But in this case we know we're
    // just going back to the navigation controller.
    // If we needed Flutter to tell us when we could actually go away,
    // we'd need to communicate over a method channel with it.
    [self.engine setViewController:nil];
  }
}

• 附加知识点

1. 常量构造函数的作用？
   理解：const修饰的构造函数，要求成员变量必须都是final的，并且在构造函数中要初始化
   在类内部修饰const如果修饰成员变量，必须和static配合使用

class Hello {
  const Hello(this.name,this.age);
  
  final String name;
  final int age;
}