《GOF设计模式》—桥接（BRIDGE）—Delphi源码示例：创建正确的Implementor对象（缺省的实现）

示例：创建正确的Implementor对象（缺省的实现）
说明：
创建正确Implementor对象的另外一种方法是首先选择一个缺省的实现，然后根据需要改变这个实现。
例如，如果一个collection的大小超出了一定的阈值时，它将会切换它的实现，使之更适用于表目较多的collection。

代码：

unit uBridge3;

interface

uses
Dialogs;

type
TTable = class;

{抽象类}
TCollection = class
private
FImp: TTable;
FCount: integer;
procedure SetCount(const Value: integer);
public
constructor Create;
destructor Destroy; override;
//---
procedure Operation; virtual;
property Count: integer read FCount write SetCount;
end;
TCollectionA = class(TCollection)
public
procedure Operation; override;
end;

{实现类}
TTable = class
procedure OperationImp; virtual; abstract;
end;
THash = class(TTable)
procedure OperationImp; override;
end;
TLink = class(TTable)
procedure OperationImp; override;
end;

implementation

constructor TCollection.Create;
begin
FImp := TLink.Create;
end;

destructor TCollection.Destroy;
begin
FImp.Free;
//---
inherited;
end;

procedure TCollection.Operation;
begin
FImp.OperationImp;
end;

procedure TCollection.SetCount(const Value: integer);
begin
if FCount <> Value then
begin
FCount := Value;
//---
FImp.Free;
if FCount < 100 then
FImp := TLink.Create
else
FImp := THash.Create;
end;
end;

procedure THash.OperationImp;
begin
ShowMessage('Hash');
end;

procedure TLink.OperationImp;
begin
ShowMessage('Link');
end;

procedure TCollectionA.Operation;
begin
inherited;
end;

end.

procedure TForm1.Button1Click(Sender: TObject);
var
ACollection: TCollection;
begin
ACollection := TCollectionA.Create;
try
ACollection.Operation;
//---
ACollection.Count := 200;
ACollection.Operation;
finally
ACollection.Free;
end;
end;