有一个例子:
type TDelegate = procedure of object; I1 = interface ['{31D4A1C7-668B-4969-B043-0EC93B673569}'] procedure P1; end; TC1 = class(TInterfacedobject,I1) procedure P1; end; ... var obj: TC1; int: I1; d: TDelegate; begin obj := TC1.Create; ... int := obj; // "int" may contains TSomeAnotherObjectWhichImplementsI1 d := obj.P1; // <- that's fine d := int.P1; // <- compiler error end;
那我怎么做最后一次操作呢?
我不知道哪个类型的对象将出现在“int”变量中,所以我不能使用类型转换.但我知道它会出现什么(因为如果你实现了一个接口,你必须实现它的所有方法).那么为什么我不能只获得指向这种方法的指针呢?也许有另一种方式?
谢谢.
解决方法
Maybe there is another way?
最好的方法是更改期望TDelegate也接受i1的代码.如果您编写代码,那么更改是微不足道的,而且它基本上是您可以做的最好的.如果您无法更改期望TDelegate的代码,并且您绝对需要从接口调用该过程,则可能需要创建一个适配器对象,如下所示:
TDelegateAdapter = class private Fi1: i1; public constructor Create(Ani1: i1); procedure P; end; constructor TDelegateAdapter.Create(Ani1: i1); begin Fi1 := Ani1; end; procedure TDelegateAdapter.P; begin Fi1.P1; end;
然后在需要分配TDelegate的代码中,执行以下操作:
var Adapter: TDelegateAdapter; Intf: i1; // assumed assigned ObjectExpectingDelegate: TXObject; // assumed assigned begin Adapter := TDelegateAdapter.Create(Intf); try ObjectExpectingDelegate.OnSomething := Adapter.P; try ObjectExpectingDelegate.PerformWork; finally ObjectExpectingDelegate.OnSomething := nil; end; finally Adapter.Free; end; end;
编辑
如果您使用的是支持匿名方法的Delphi版本,则可以使用此类匿名方法实现Delegate适配器,每个过程签名只需要一个“适配器”. Delphi使用Interfaces在幕后实现匿名方法,因此运行时性能很好,无需担心.
下面的代码是匿名委托适配器的演示控制台实现.直接看看最后的开始 – 结束块,看看魔术.
program Project29; {$APPTYPE CONSOLE} uses SysUtils; type // This is the type of the anonymous method I want to use TnoparamsProc = reference to procedure; // This implements the "delegate" adapter using an anonymous method TAnonymousDelegateAdapter = class private noparamsProc: TnoparamsProc; public constructor Create(anoparamsProc: TnoparamsProc); procedure AdaptedDelegate; end; { TAnonymousDelegateAdapter } procedure TAnonymousDelegateAdapter.AdaptedDelegate; begin noparamsProc; end; constructor TAnonymousDelegateAdapter.Create(anoparamsProc: TnoparamsProc); begin noparamsProc := anoparamsProc; end; // --------- test code follows ---------- type // Interface defining a single method. ISomething = interface procedure Test; end; // Implementation of the interface above TSomethingImp = class(TInterfacedobject,ISomething) public procedure Test; end; // DeFinition of delegate TnoparamsDelegate = procedure of object; { TSomethingImp } procedure TSomethingImp.Test; begin WriteLn('Test'); end; // ---- Test program to see it all in action. --- var intf: ISomething; Dlg: TnoparamsDelegate; begin intf := TSomethingImp.Create; // Here I'll create the anonymous delegate adapter,notice the "begin - end" // in the constructor call; That's the anonymous method. Runtime performance // of anonymous methods is very good,so you can use this with no warries. // My anonymous method uses the "intf" variable and calls the method "Test" // on it. Because of that the "intf" variable is "captured",so it doesn't run // out of scope as long as the anonymous method itself doesn't run out of scope. // In other words,you don't risk having your interface freed because it's reference // count reaches zero. If you want to use an other interface,replace the code // in the begin-end. with TAnonymousDelegateAdapter.Create(procedure begin intf.Test; end) do try Dlg := AdaptedDelegate; Dlg; finally Free; end; Readln; end.
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