android - FMX系统消息传递以发送指针(？)

Question

我正在使用C++ Builder 10.2。
在Android中，我想将各种消息(包括主线程)发送到主GUI线程。在Windows中，我可以发布一条消息，并将LPARAM或WPARAM分配给结构或类的某些实例的地址。
我正在尝试使用System.Messaging.TMessageManager做同样的事情，类似于此处的示例:System.Messaging (C++)。但是我只能发送“简单”类型，例如UnicodeString或int。我什至没有想出如何发送一个指针，假设它甚至有可能。
我想发送一个这样的结构/类实例:

class TSendResult
{
public:
    String Message;
    unsigned int Value;
    int Errno;

    __fastcall TSendResult(void);
    __fastcall ~TSendResult();
};

如果可以做到，该怎么写？我设法得到一个版本进行编译，但是出现了链接器错误:

error: undefined reference to 'vtable for System::Messaging::TMessage__1<TSendResult>'

表单构造函数:

__fastcall TForm1::TForm1(TComponent* Owner)
    : TForm(Owner)
{
    TMessageManager* MessageManager = TMessageManager::DefaultManager;
    TMetaClass* MessageClass = __classid(TMessage__1<TSendResult>);
    TMessageListenerMethod ShowReceivedMessagePointer = &(this->MMReceiveAndCallBack);
    MessageManager->SubscribeToMessage(MessageClass, ShowReceivedMessagePointer);
}

按钮点击处理程序:

void __fastcall TForm1::SpeedButton1Click(TObject *Sender)
{
    ...

    TSendResult *SPtr = new TSendResult();
    SPtr->Message = "All good";
    SPtr->Value = 10;
    SPtr->Errno = 0;
    TMessageManager* MessageManager = TMessageManager::DefaultManager;
    TMessage__1<TSendResult>* Message = new TMessage__1<TSendResult>(*SPtr); // <-- this doesn't look right...
    MessageManager->SendMessage(Sender, Message, false);
}

捕获消息的功能:

void __fastcall TForm1::MMReceiveAndCallBack(System::TObject* const Sender,
        System::Messaging::TMessageBase* const M)
{
    TMessage__1<TSendResult>* Message = dynamic_cast<TMessage__1<TSendResult>*>(M);
    if (Message) {
        ShowMessage(Message->Value.Message);
    }
}

Answer 1

TMessage__1<T>是Delphi通用 TMessage<T> 类的C++类实现。不幸的是，在C++中使用Delphi泛型类时有一个已记录的限制，这就是为什么出现链接器错误的原因:
How to Handle Delphi Generics in C++

Delphi generics are exposed to C++ as templates. However, it is important to realize that the instantiations occur on the Delphi side, not in C++. Therefore, you can only use these template for types that were explicitly instantiated in Delphi code.

...

If C++ code attempts to use a Delphi generic for types that were not instantiated in Delphi, you'll get errors at link time.

这就是TMessage__1<UnicodeString>有效而TMessage__1<TSendResult>不起作用的原因，因为Delphi RTL中存在TMessage<UnicodeString>的实例化。您正在看的谁写了C++ example，可能都不知道此限制，而只是按原样翻译了Delphi example。
话虽如此，您有两种选择:

向您的C++项目中添加一个Delphi .pas单元，将TSendResult实现为Delphi record，并为其定义TMessage<TSendResult>的实例化。然后，您可以在C++代码中使用该单元(编译.hpp文件时，C++ Builder会为您生成一个C++ .pas文件)，例如:

unit MyMessageTypes;

interface

uses
  System.Messaging;

type
  TSendResult = record
    Message: String;
    Value: UInt32;
    Errno: Integer;
  end;

  TSendResultMsg = TMessage<TSendResult>;

implementation

initialization
  TSendResultMsg.Create.Free;
finalization

end.

#include "MyMessageTypes.hpp"

__fastcall TForm1::TForm1(TComponent* Owner)
    : TForm(Owner)
{
    TMessageManager::DefaultManager->SubscribeToMessage(__classid(TSendResultMsg), &MMReceiveAndCallBack);
}

void __fastcall TForm1::SpeedButton1Click(TObject *Sender)
{
    ...

    TSendResult Res;
    Res.Message = _D("All good");
    Res.Value = 10;
    Res.Errno = 0;

    TSendResultMsg *Message = new TSendResultMsg(Res);
    TMessageManager::DefaultManager->SendMessage(this, Message, true);
}

void __fastcall TForm1::MMReceiveAndCallBack(System::TObject* const Sender,
    System::Messaging::TMessageBase* const M)
{
    const TSendResultMsg* Message = static_cast<const TSendResultMsg*>(M);
    ShowMessage(Message->Value.Message);
}

而不是根本不使用TMessage__1，而是可以直接从 TSendResult 派生TMessageBase，例如:

class TSendResultMsg : public TMessageBase
{
public:
    String Message;
    unsigned int Value;
    int Errno;
};

__fastcall TForm1::TForm1(TComponent* Owner)
    : TForm(Owner)
{
    TMessageManager::DefaultManager->SubscribeToMessage(__classid(TSendResultMsg), &MMReceiveAndCallBack);
}

void __fastcall TForm1::SpeedButton1Click(TObject *Sender)
{
    ...

    TSendResultMsg *Message = new TSendResultMsg;
    Message->Message = _D("All good");
    Message->Value = 10;
    Message->Errno = 0;
    TMessageManager::DefaultManager->SendMessage(this, Message, true);
}

void __fastcall TForm1::MMReceiveAndCallBack(System::TObject* const Sender,
    System::Messaging::TMessageBase* const M)
{
    const TSendResultMsg* Message = static_cast<const TSendResultMsg*>(M);
    ShowMessage(Message->Message);
}