I'm trying to figure out the best way to use dependency injection for some legacy code which will take a long to refactor and has to be done gradually. Most of the old classes use a "Parent" property for determining various things and the parent property was often pass in via a constructor argument as follows:
constructor TParentObject.Create;
begin
FChildObject := TChildObject.Create(Self);
end;
constructor TChildObject.Create(AParent: TParentObject)
begin
FParent := AParent;
end;
This is fairly typical of our legacy code base. However when moving to interfaces and constructor injection, the Parent is not known by the Spring4D framework when creating the Child object. So it will just get a new parent but not the existing one. Of course I can create a property getter/setter but this would indicate an "optional" property for the class which is really a mandatory property. See the code below for more explanation:
unit uInterfaces;
interface
uses
Spring.Collections;
type
IChildObject = interface;
IParentObject = interface
['{8EA8F9A2-E627-4546-8008-0A77DA2B16F1}']
function GetSomethingRequiredByChild: string;
procedure SetSomethingRequiredByChild(const Value: string);
property SomethingRequiredByChild: string read GetSomethingRequiredByChild write SetSomethingRequiredByChild;
function GetChild: IChildObject;
property Child: IChildObject read GetChild;
end;
// This introduces a property getter/setter
// However it also implies that Parent can be NIL which it cannot
IChildObject = interface
['{ECCA09A6-4A52-4BE4-A72E-2801160A9086}']
function GetParent: IParentObject;
procedure SetParent(const Value: IParentObject);
property Parent: IParentObject read GetParent write SetParent;
end;
TParentObject = class(TInterfacedObject, IParentObject)
private
FChild: IChildObject;
FSomethingRequiredByChild: string;
function GetChild: IChildObject;
function GetSomethingRequiredByChild: string;
procedure SetSomethingRequiredByChild(const Value: string);
public
constructor Create;
end;
TChildObject = class(TInterfacedObject, IChildObject)
private
FParent: IParentObject;
function GetParent: IParentObject;
procedure SetParent(const Value: IParentObject);
public
// This requries a Parent object, but how does the Spring4D resolve the correct parent?
constructor Create(const AParent: IParentObject);
end;
implementation
uses
Spring.Services;
{ TParentObject }
constructor TParentObject.Create;
begin
// Here is the old way...
FChild := TChildObject.Create(Self); // Old way of doing it
// This is the Service Locator way...
FChild := ServiceLocator.GetService<IChildObject>;
// I would prefer that the Parent is assigned somehow by the Service Locator
// IS THIS POSSIBLE - or am I dreaming?
FChild.Parent := Self;
end;
function TParentObject.GetChild: IChildObject;
begin
Result := FChild;
end;
function TParentObject.GetSomethingRequiredByChild: string;
begin
Result := FSomethingRequiredByChild;
end;
procedure TParentObject.SetSomethingRequiredByChild(const Value: string);
begin
FSomethingRequiredByChild := Value;
end;
{ TChildObject }
constructor TChildObject.Create(const AParent: IParentObject);
begin
FParent := AParent;
end;
function TChildObject.GetParent: IParentObject;
begin
Result := FParent;
end;
procedure TChildObject.SetParent(const Value: IParentObject);
begin
FParent := Value;
end;
end.
Maybe there is some methodology which can be used that I'm not aware of to set the parent object using the DI framework?
I hope this question is clear what I'm trying to achieve. I'm happy to provide more description/code example where necessary.
