07 July 2015

When building the first prototypes of what became the Delphi form designer I needed to solve a very simple problem, a palette of controls. When one of the palette items was selected and the form design was clicked, I needed to create control on the form in the location clicked. The problem was, how do you create such a list?

The solution we choose was to allow a reference to a class be treated as a first class value; a class reference. If you have a class called A then the identifier A is considered a constant reference to the class of type class of A.

Class references follow inheritance substitution rules. For example, if B derives A then a reference to class B can be passed when a class of A reference is expected. This allowed us to create TComponent, the base class of all Delphi components, and the corresponding class reference type TComponentClass.

With a class reference you can call any class method or constructor declared on that class. If the constructor or method is virtual, the most derived constructor or method is called. Given the A and B classes above, the statements:

var
  SomeClass: class of A;
  SomeA: A;
begin
  SomeClass := B;
  SomeA := SomeClass.Create();
end;

create an instance of the B class which derives from A. This allowed us to use an array of class references as the palette data structure. Something like,

type
  TPaletteItems = array of TComponentClass;

var
  PaletteItems: TPaletteItems = [TButton, TEdit, TCheckBox, TLabel];

Selecting an item in the palette was simply selecting an index into that array which selected the cooresponding class reference of which to construct an instance.

The early prototypes and early pre-release versions of Delphi I just hard-coded the array of classes as I was the only one writting them at the time. Eventually we allowed the array to be modified by calling a Register procedure.

The class reference was also useful for form deserialization as it allowed us to construct the correct class based on a registered class name but the genisis of the class reference was the palette of the form designer.



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