Delphi Property详解

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 Property
 Keyword    Defines controlled access to class fields    System unit
   ?.Property Name : Type read Getter|nodefault;
   ?.Property Name : Type write Setter;
   ?.Property Name : Type read Getter write Setter;
   ?.Property Name : Type Index Constant read Getter {default : Constant|nodefault;} {stored : Boolean};
   ?.Property Name : Type Index Constant write Setter {default : Constant|nodefault;} {stored : Boolean};
   ?.Property Name : Type Index Constant read Getter write Setter {default : Constant|nodefault;} {stored : Boolean};
   ?.Property Name[Index : IndexType] : BaseType read Getter {default;}
   ?.Property Name[Index : IndexType] : BaseType write Setter; {default;}
   ?.Property Name[Index : IndexType] : BaseType read Getter write Setter; {default;}
   .Property Name : Type read Getter implements Interfaces...;
   .Property Name; // Redeclared base class property
   .Property Name : Type; // Dispinterface only
   .Property Name : Type readonly; // Dispinterface only
   .Property Name : Type writeonly; // Dispinterface only

Description

The Property keyword defines a controlled access to class fields. 
 
It is an Object Oriented concept that internals of an object should be hidden from the outside. Whilst you can allow fields (data) in a class to be directly externally accessible (by placing in the public or published sections), this is unwise. Instead, Property can be used to define how the data is read and written. 
 
Versions 1, 2 and 3 These basic forms define read, write or read and write access to class fields. The data Type is returned from the field or method called Getter. The data is updated via the Setter field or method. 
 
Note that you must use a different name for the Name and for Getter and Setter. For example: Property Age read fAge write fAge; You would use field names when there is no vetting or retrieval processing required. When using a method to read or write, the read or written value can be a lot simpler than the stored value. The stored value can even be entirely different. 
 
Versions 4, 5 and 6 Using the Index keyword tells Delphi to pass the Constant value as the argument to the Getter and Setter methods. These must be functions that take this constant index value as an argument. 
 
For example: Property Item2 : string Index 2 read ItemGetter; where ItemGetter is defined as : Function ItemGetter(Index : Integer) : string; Default provides run time information for the property. NoDefault does not. Stored is beyond the scope of Delphi Basics. 
 
Versions 7, 8 and 9 This is a generalised version of versions 4,5 and 6. It requests the user to provide the index value for the Getter and Setter methods. 
 
Default allows the Getter and Setter method calls to be replaced as in the following example : myValue := MyClass.Getter(23); can be replaced by : myValue := MyCLass[23]; 
 
Version 10 Allows the implementation of an Interface method to be delegated to a property. Access to the property invokes the interface implementation. 
 
Version 11 By redclaring a parent class property, you can do so in a public or published clause, thereby raising the access rights of the property. 
 
Versions 12, 13 and 14 Are relevant for dispinterfaces, outside the scope of delphi basics.

Related commands

Index		Principally defines indexed class data properties
Private		Starts the section of private data and methods in a class
Protected		Starts a section of class private data accesible to sub-classes
Public		Starts an externally accessible section of a class
Published		Starts a published externally accessible section of a class

 

Example code : Illustrating basic and indexed properties

// Full Unit code.
// -----------------------------------------------------------
// You must store this code in a unit called Unit1 with a form
// called Form1 that has an OnCreate event called FormCreate.

unit Unit1;

interface

uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs;

type
  // Class with Indexed properties
TRectangle = class
private
fArea : LongInt;
fCoords : array[0..3] of Longint;
function GetCoord(Index: Integer): Longint;
procedure SetCoord(Index: Integer; Value: Longint);
public
    Property Area : Longint read fArea;
    Property Left : Longint Index 0 read GetCoord write SetCoord;
    Property Top : Longint Index 1 read GetCoord write SetCoord;

Property Right : Longint Index 2 read GetCoord write SetCoord;
    Property Bottom : Longint Index 3 read GetCoord write SetCoord;
    Property Coords[Index: Integer] : Longint read GetCoord write SetCoord;

constructor Create;
end;

// The form class itself
TForm1 = class(TForm)
procedure FormCreate(Sender: TObject);
end;

var
Form1: TForm1;

implementation

{$R *.dfm}

// TRectangle property 'Getter' routine
// TRectangle constructor
constructor TRectangle.Create;
begin
  // Give default rectangle coordinates
left := 0;
right := 100;
top := 0;
bottom := 100;
fArea := 100 * 100;
end;

function TRectangle.GetCoord(Index: Integer): Longint;
begin
  // Only allow valid index values
if (Index >= 0) and (Index <= 3)
then Result := fCoords[Index]
else Result := -1;
end;

// TRectangle property 'Setter' routine
procedure TRectangle.SetCoord(Index, Value: Integer);
begin
  // Only allow valid index values
if (Index >= 0) and (Index <= 3)
then
begin
    // Save the new value
fCoords[Index] := Value;

// And recreate the rectangle area
fArea := (right - left) * (bottom - top);
end;
end;

// Main line code
procedure TForm1.FormCreate(Sender: TObject);
var
myRect : TRectangle;
i : Integer;

begin
  // Create my little rectangle with default coordinates
myRect := TRectangle.Create;

// And set the corner coordinates
myRect.Left := 22; // Left using direct method
myRect.Top := 33;
myRect.SetCoord(2,44); // Right using indexed method
myRect.SetCoord(3,55);

// And ask for these values
for i:= 0 to 3 do
ShowMessageFmt('myRect coord %d = %d',[i,myRect.GetCoord(i)]);

// And show the rectangle area
ShowMessageFmt('myRect area = %d',[myRect.Area]);
end;

end.

Show full unit code

myRect coord 0 = 22
myRect coord 1 = 33
myRect coord 2 = 44
myRect coord 3 = 55
myRect area = 484