Subsystem

了解 subsystem的构建过程非常重要，比如很多Subsystem不需要手动创建和调用，只需直接继承它，运行时就会被调用。Subsystem本身是一套生命周期绑定机制，就像Android的Activity有非常多的生命周期，如果功能都写在activitiy里面，就会变得非常臃肿，难以迁移和优化, 于是Android推出很多 lifecycle 关联机制，这个subsystem是异曲同工的，不过我感觉subsystem的设计理念更加优雅。 这里简单记录

更多参考:

• https://www.cnblogs.com/shiroe/p/14819721.html

• https://www.cnblogs.com/yejianying/p/15029111.html

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Subsystem 初始化

• 获取派生类的工具方法

  /**
  * Returns an array of classes that were derived from the specified class.
  *
  * @param	ClassToLookFor				The parent class of the classes to return.
  * @param	Results						An output list of child classes of the specified parent class.
  * @param	bRecursive					If true, the results will include children of the children classes, recursively. Otherwise, only direct decedents will be included.
  */
  COREUOBJECT_API void GetDerivedClasses(const UClass* ClassToLookFor, TArray<UClass *>& Results, bool bRecursive = true);

• 递归获取派生类，加入初始化列表

  TArray<UClass*> SubsystemClasses;
  GetDerivedClasses(BaseType, SubsystemClasses, true);
  
  for (UClass* SubsystemClass : SubsystemClasses)
  {
      AddAndInitializeSubsystem(SubsystemClass);
  }

• 开始初始化，调用该类CDO 的ShouldCreateSubSystem 来判断是不是要实例化。

  const USubsystem* CDO = SubsystemClass->GetDefaultObject<USubsystem>();
  if (CDO->ShouldCreateSubsystem(Outer))
  {
      USubsystem* Subsystem = NewObject<USubsystem>(Outer, SubsystemClass);
      SubsystemMap.Add(SubsystemClass,Subsystem);
      Subsystem->InternalOwningSubsystem = this;
      Subsystem->Initialize(*this);

• 如果只想最外层的的派生类被实例化，可以参考：

  bool UCommonUIActionRouterBase::ShouldCreateSubsystem(UObject* Outer) const
  {
      TArray<UClass*> ChildClasses;
      GetDerivedClasses(GetClass(), ChildClasses, false);
  
      UE_LOG(LogUIActionRouter, Display, TEXT("Found %i derived classes when attemping to create action router (%s)"), ChildClasses.Num(), *GetClass()->GetName());
  
      // Only create an instance if there is no override implementation defined elsewhere
      return ChildClasses.Num() == 0;
  }