Array Properties

Array properties are indexed properties. They can represent things like items in a list, child controls of a control, and pixels of a bitmap.

The declaration of an array property includes a parameter list that specifies the names and types of the indexes. For example, property Objects[Index: Integer]: TObject read GetObject write SetObject;

property Pixels[X, Y: Integer]: TColor read GetPixel write SetPixel; property Values[const Name: string]: string read GetValue write SetValue;

The format of an index parameter list is the same as that of a procedure's or function's parameter list, except that the parameter declarations are enclosed in brackets instead of parentheses. Unlike arrays, which can use only ordinal-type indexes, array properties allow indexes of any type.

For array properties, access specifiers must list methods rather than fields. The method in a read specifier must be a function that takes the number and type of parameters listed in the property's index parameter list, in the same order, and whose result type is identical to the property's type. The method in a write specifier must be a procedure that takes the number and type of parameters listed in the property's index parameter list, in the same order, plus an additional value or const parameter of the same type as the property.

For example, the access methods for the array properties above might be declared as function GetObject(Index: Integer): TObject; function GetPixel(X, Y: Integer): TColor; function GetValue(const Name: string): string; procedure SetObject(Index: Integer; Value: TObject); procedure SetPixel(X, Y: Integer; Value: TColor); procedure SetValue(const Name, Value: string);

An array property is accessed by indexing the property identifier. For example, the statements if Collection.0bjects[0] = nil then Exit; Canvas.Pixels[10, 20] := clRed; Params.Values['PATH'] := 'C:\BIN';

correspond to if Collection.GetObject(O) = nil then Exit; Canvas.SetPixel(10, 20, clRed); Params.SetValue('PATH', 'C:\BIN');

The definition of an array property can be followed by the default directive, in which case the array property becomes the default property of the class. For example, type

TStringArray = class public property Strings[Index: Integer]: string ...; default; end;

If a class has a default property, you can access that property with the abbreviation object[index], which is equivalent to[index]. For example, given the declaration above, StringArray.Strings[7] can be abbreviated to StringArray[7]. A class can have only one default property. Changing or hiding the default property in descendant classes may lead to unexpected behavior, since the compiler always binds to properties statically.

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Project Management Made Easy

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