Array – Create an instance from a class array

My class definition is:

TAnimal = class(TInterfacedObject)
public
 constructor Create; overload; 
 constructor Create(param: string); overload;
end;

IAnimal = interface
 procedure DoSomething;
end;

TDog = class(TAnimal, IAnimal)
public
 procedure DoSomething;
end;

TCat = class(TAnimal, IAnimal)
public< br /> procedure DoSomething;
end;

Sample code:

procedure TForm1.DogButtonPressed(Sender: TObject);
 var
 myDog: TDog;
 I: Integer;
begin
 myDog := TDog.Create('123');
 I := Length(myQueue); 
 SetLength(myQueue, I+1);
 myQueue[I] := TDog; //Probably not the way to do it...?? 
end;

procedure TForm1.CatButtonPressed(Sender: TObject);
var
 myCat: TCat;
 I: Integer;
begin 
 myCat := TCat.Create('123');
 I := Length(myQueue);
 SetLength(myQueue, I+1);
 myQueue[I]: = TCat; //Probably not the way to do it...??
end;

procedure TForm1.OnProcessQueueButtonPressed(Sender: TObject);
var
 MyInterface: IAnimal; //Interface variable
 I: Integer;
begin
 for I := Low(myQueue) to High(myQueue) do
 begin
 MyInterface: = myQueue[I].Create('123'); //Create instance of relevant class
 MyInterface.DoSomething;
 end;
end;

So, suppose You have a form with three buttons. The “dog” button, the “cat” button and the “processing queue” button. When you press the “dog” button or the “cat” button, the related classes will be added to the array to Act as a queue. Then press the “Process Queue” button, the program gradually executes the array, creates objects of related classes, and then calls the interface methods implemented in the class. Remember my sample code, how to achieve it?

The obvious way is to add the class name as a string to the string array, and then use the if statement in the OnProcessQueueButtonPressed process, for example:

procedure TForm1.OnProcessQueueButtonPressed(Sender: TObject);
var
 MyInterface: IAnimal; //Interface variable
 I: Integer;
begin
 for I := Low(myQueue ) to High(myQueue) do
 begin
 if myQueue[I] ='TDog' then
 MyInterface := TDog.Create('123');
 if myQueue[I ] ='TCat' then
 MyInterface := TCat.Create('123'); 
 MyInterface.DoSomething;
 end;
end;

I Try to avoid this, because every time I add a new class, I have to remember to add an if block for the new class.

You can use class reference performs this operation. Define the class reference type as follows:

type
 TAnimalClass = class of TAnimal;

and Arrange TAnimal to support the interface:

type
 IAnimal = interface
 procedure DoSomething;
 end;

 TAnimal = class(TInterfacedObject, IAnimal)
 public
 constructor Create; overload;
 constructor Create(param: string); overload;
 procedure DoSomething; virtual; abstract;
 end;

 TDog = class(TAnimal)
 public
 procedure DoSomething; override;
 end;

 TCat = class(TAnimal)
 public
 procedure DoSomething; override;
 end;

Using an array will lead to code confusion. It is better to use a list object.

var
 myQueue: TList;

Now You can write the following code:

procedure TForm1.DogButtonPressed(Sender: TObject);
begin
 myQueue.Add(TDog);
 end;

procedure TForm1.CatButtonPressed(Sender: TObject);
begin
 myQueue.Add(TCat);
end;

 procedure TForm1.OnProcessQueueButtonPressed(Sender: TObject);
var
 AnimalClass: TAnimalClass;
 Animal: IAnimal;
begin
 for AnimalClass in myQueue do
 begin 
 Animal := AnimalClass.Create('123'); 
 Animal.DoSomething;
 end;
 myQueue.Clear;
end;

You need to create and destroy an instance of myQueue in the appropriate place. I fake Assuming you already know how to do this.

When using class references, a better nuance is that you usually provide virtual constructors in the base class. This is because when you use class references to create When instantiating, you will call the constructor declared in the base class. If the constructor is not virtual, the derived class constructor code will not be executed.

Of course, using class references in this way will Make the interface meaningless.