FlutterEngine使用不当导致App崩溃

 笔者公司的项目最近开始使用了Flutter，由于老项目维护的时间比较长了。我们采用了原生嵌入一部分Flutter页面的混合开发方式。项目开发完成测试阶段我们遇到了一个问题。就是原生页面跳转到FlutterViewController(flutter页面的根控制器，作为flutter的宿主页面)承载的页面时候。在返回原生页面，重复几次会导致APP的崩溃。下图是官网给的示例

// 1.创建引擎
// 2.引擎启动
// 3.引擎注册
// 4.创建FlutterViewController
self.flutterEngine = [[FlutterEngine alloc] initWithName:@"my flutter engine"];
  // Runs the default Dart entrypoint with a default Flutter route.
  [self.flutterEngine run];
  // Used to connect plugins (only if you have plugins with iOS platform code).
  [GeneratedPluginRegistrant registerWithRegistry:self.flutterEngine];
FlutterViewController *flutterViewController =
        [[FlutterViewController alloc] initWithEngine:flutterEngine nibName:nil bundle:nil];

笔者项目也是按照这个文档步骤来的，后来经过反复的查找，定位到为题所在,在初始化引擎的方法列表里，allowHeadlessExecution控制gluttervc的生命周期。设置为NO表示，当FlutterViewController销毁后，就停掉flutterEngine。

/**
 * Initialize this FlutterEngine with a `FlutterDartProject`.
 *
 * If the FlutterDartProject is not specified, the FlutterEngine will attempt to locate
 * the project in a default location (the flutter_assets folder in the iOS application
 * bundle).
 *
 * A newly initialized engine will not run the `FlutterDartProject` until either
 * `-runWithEntrypoint:` or `-runWithEntrypoint:libraryURI:` is called.
 *
 * @param labelPrefix The label prefix used to identify threads for this instance. Should
 *   be unique across FlutterEngine instances, and is used in instrumentation to label
 *   the threads used by this FlutterEngine.
 * @param project The `FlutterDartProject` to run.
 * @param allowHeadlessExecution Whether or not to allow this instance to continue
 *   running after passing a nil `FlutterViewController` to `-setViewController:`.
 */
- (instancetype)initWithName:(NSString*)labelPrefix
                     project:(nullable FlutterDartProject*)project
      allowHeadlessExecution:(BOOL)allowHeadlessExecution;