Delphi SysUtils.Supports意外返回true

Question

我正在根据Spring4d的文档示例制作一个eventPublisher

不同之处在于订阅者必须明确订阅事件。

我想根据它们是否实现IEventHandler<TEventType>接口来触发它们的Handle过程。

发布传入事件时，我使用事件的类名和Spring4d的IEventHandler<TEventType>找到TType.FindType('IEventHandler<TEvent1>')类型引用

然后我遍历我的订阅者（实现IEventHandler接口的对象）并检查它是否支持IEventHandler类型。

问题是即使订户没有实现接口，Supports方法也会返回true。

此外，我尝试列出说TMyEventHandler2类型的接口。它包含IEventHandler<TEvent2> ??

我相信这是由于IEventHandler<TEvent2>和IEventHandler<TEvent1>共享相同GUID的限制

这有解决方法吗？

使用这些类和接口：

TEvent1 = class(TObject)
end;

TEvent2 = class(TObject)
end;

IEventHandler = interface(IInvokable)
[guid]
procedure Handle(aEvent : TObject);
end;

IEventHandler<T : class> = interface(IEventHandler)
[guid]
procedure Handle(aEvent : T);
end;

TMyEventHandler1 = class(TObject, IEventHandler, IEventHandler<TEvent1>)
public 
procedure Handle(AEvent : TObject); overload;
procedure Handle(AEvent : TEvent1); overload;
end;

TMyEventHandler2 = class(TObject, IEventHandler, IEventHandler<TEvent2>)
public 
procedure Handle(AEvent : TObject); overload;
procedure Handle(AEvent : TEvent2); overload;
end;

TEventPublisher = class(TObject)
public
  fSubscribers : IList<TValue>;
  procedure Subscribe(aSubscriber : TValue);  // Simply adds the subscriber to the list of subscribers
  procedure Publish(aEvent : TObject); // Publishes an event to the subscribers
end;

procedure TEventPublisher.Publish(const event: TObject; ownsObject: Boolean = True);
const
  IEventSubscriberName = 'IEventSubscriber<*>';
var
  consumerTypeName: string;
  consumerType    : TRttiType;
  intfType        : TRttiInterfaceType;
  subscriber      : TValue;
  subscribed      : IInterface;
  lEventSubscriber: IEventSubscriber;
  lIntfs          : IReadOnlyList<TRttiInterfaceType>;
begin

  consumerTypeName := StringReplace(IEventSubscriberName, '*', GetQualifiedClassName(event), []);
  consumerType     := TType.FindType(consumerTypeName);
  intfType         := consumerType as TRttiInterfaceType;

  for subscriber in fSubscribers do
  begin

    lIntfs := TType.GetType(subscriber.AsObject.ClassInfo).GetInterfaces();

    // lIntfs for TMyEventHandler2 containts IEventHandler<TEvent1> ???

    if Supports(subscriber.AsObject, intfType.GUID, subscribed) then
      if Supports(subscriber.AsObject, IEventSubscriber, lEventSubscriber) then
      begin
        intfType.GetMethod('Handle').Invoke(TValue.From(@subscribed, intfType.Handle), [event])
      end;
  end;

  if ownsObject then
    event.Free;
end;


lEventPublisher := TEventPublisher.Create;
lEventPublisher.Subscribe(TMyEventHandler1.Create);
lEventPublisher.Subscribe(TMyEventHandler2.Create);
lEventPublisher.Publish(TEvent1.Create); // Will both trigger TMyEventHandler1.Handle and TMyEventHandler2.Handle. Why ??

Answer 1

发生这种情况是因为如果你在一个通用接口上放置一个guid，那么该接口的每个特化都将具有相同的guid，而不管它的泛型类型参数。

我通常通过提供一种方法来解决这个问题，在接口中提供有关它的信息（比如Spring.Collections.IEnumerable有一个ElementType属性来获取IEnumerable<T>的实际类型）。

所以实现看起来像这样：

program GenericEventPublisher;

{$APPTYPE CONSOLE}

uses
  Spring,
  Spring.Collections,
  System.SysUtils;

type
  IEventHandler = interface
    ['{2E4BD8F4-4EB8-4B33-84F4-B70F42EF9208}']
    procedure Handle(const event: TObject);
  end;

  IEventHandler<T: class> = interface
    ['{82B7521E-D719-4051-BE2C-2EC449A92B22}']
    procedure Handle(const event: T);
    function GetHandledClass: TClass;
  end;

  IEventPublisher = interface
    ['{2A460EF0-AE27-480F-ACEA-1B897F2DE056}']
    procedure Subscribe(const subscriber: IEventHandler);
    procedure Publish(const event: TObject; ownsObject: Boolean = True);
  end;

  TEventHandlerBase<T: class> = class(TInterfacedObject, IEventHandler, IEventHandler<T>)
  private
    function GetHandledClass: TClass;
    procedure Handle(const event: TObject); overload;
  public
    procedure Handle(const event: T); overload; virtual; abstract;
  end;

  TEvent1 = class
  end;

  TEvent2 = class
  end;

  TMyEventHandler1 = class(TEventHandlerBase<TEvent1>)
  public
    procedure Handle(const event: TEvent1); override;
  end;

  TMyEventHandler2 = class(TEventHandlerBase<TEvent2>)
  public
    procedure Handle(const event: TEvent2); override;
  end;

  TEventPublisher = class(TInterfacedObject, IEventPublisher)
  private
    fSubscribers: IList<IEventHandler>;
  public
    constructor Create;
    procedure Subscribe(const subscriber: IEventHandler);
    procedure Publish(const event: TObject; ownsObject: Boolean = True);
  end;

{ TEventPublisher }

constructor TEventPublisher.Create;
begin
  fSubscribers := TCollections.CreateList<IEventHandler>;
end;

procedure TEventPublisher.Publish(const event: TObject; ownsObject: Boolean);
var
  subscriber: IEventHandler;
  eventSubscriber: IEventHandler<TObject>;
begin
  for subscriber in fSubscribers do
    if Supports(subscriber, IEventHandler<TObject>, eventSubscriber)
      and (eventSubscriber.GetHandledClass = event.ClassType) then
        eventSubscriber.Handle(event);

  if ownsObject then
    event.Free;
end;

procedure TEventPublisher.Subscribe(const subscriber: IEventHandler);
begin
  fSubscribers.Add(subscriber)
end;

{ TEventHandlerBase<T> }

function TEventHandlerBase<T>.GetHandledClass: TClass;
begin
  Result := T;
end;

procedure TEventHandlerBase<T>.Handle(const event: TObject);
begin
  Assert(event is T);
  Handle(T(event));
end;

{ TMyEventHandler1 }

procedure TMyEventHandler1.Handle(const event: TEvent1);
begin
  Writeln(event.ClassName, ' handled by ', ClassName);
end;

{ TMyEventHandler2 }

procedure TMyEventHandler2.Handle(const event: TEvent2);
begin
  Writeln(event.ClassName, ' handled by ', ClassName);
end;

var
  eventPublisher: IEventPublisher;
begin
  eventPublisher := TEventPublisher.Create;
  eventPublisher.Subscribe(TMyEventHandler1.Create);
  eventPublisher.Subscribe(TMyEventHandler2.Create);
  eventPublisher.Publish(TEvent1.Create);
  eventPublisher.Publish(TEvent2.Create);
end.

由于接口上存在类约束，因此无论T的类型如何，都可以确保接口是二进制兼容的（因为它们只能是对象）。对通用事件处理程序使用基类型也减少了要写入的额外代码。它只是将非泛型Handle方法重定向到必须在具体实现中实现的泛型方法。

此外，由于基类实现了两个接口，我们不需要将处理程序存储在TValue列表中，但可以使用非泛型接口类型并轻松访问它们而无需RTTI。

现在Publish方法使用一个小技巧调用Support与IEventHandler<TObject> - 因为eventSubscriber是那种类型我们可以将event参数传递给它的Handle方法恰好是正确的 - 这是因为我之前解释的二进制兼容性因为我们只是处理不同的类作为T的类型 - 如果我们没有那个类约束，故事会完全不同。