Title: Handle lots of classes at runtime
Question: Classes which can't support TPersistent or TPersistentClass had to implement another technique to manage the creation of obejcts at runtime
Answer:
If a class inherits from TPersistent we can use RegisterClass in several units so we can use the function GetClass to convert a class name to a class instance:
TFinanceClass = Class of TFinance;
var refClass: TFinanceClass;
refClass:= TFinanceClass(getClass(values['RegClass']));
dynObj:= refClass.Create;
dynObj.getCharge(9.2);
RegisterClass(TFinance);
If you can't inherit from TPersistent we can use a "Name=Value" pair of objects in a stringlist and loaded when a module is created or by user events. So you can determine which object should be created.
function initObjinList;
var //refClass: TFinanceClass;
dynObj: TFinance;
myClsname: string[240];
begin
with TStringList.Create do begin
clear;
values['RegClass']:= 'TRegularCharge2';
values['PrefClass']:= 'TPreferredCharge2';
values['TrialClass']:= 'TTrialCharge2';
myClsname:= 'TrialClass';
if indexofname(myClsname) -1 then begin
{refClass:= TFinanceClass(getClass(values['RegClass']));
dynObj:= refClass.Create;
dynObj.getCharge(9.2); }
case IndexOfname(myClsname) of
0: dynObj:= TRegularCharge2.create;
1: dynObj:= TPreferredCharge2.create;
2: dynObj:= TTrialCharge2.create;
end;
dynObj.getcharge(9.1);
dynObj.Free;
result:= true;
end;
Free;
end;
end;
The value is the class type and the name simplifies the maintainability of the classes.
syntax: mystringlist.values[name]:= value //write
myname:= mystringlist.values[name] //read
I made only one function to show the simplicity but registering classes in a StringList and choosen from a case of structure should be separated in differnt functions.
All the classes inherit from TFinance and in comparison to RegisterClass we don't use a ClassReference.
Descendants override the public or protected getCharge() methods to perform their actions.
TFinance = class
public
function getCharge(const Balance: double): double; virtual;
abstract;
end;
TRegularCharge2 = class(TFinance)
public
function getCharge(const Balance: double): double; override;
end;
TPreferredCharge2 = class(TFinance)
public
function getCharge(const Balance: double): double; override;
end;
TTrialCharge2 = class(TFinance)
public
function getCharge(const Balance: double): double; override;
end;