factories.txt

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 íåêîòîðîì ñìûñëå â ïðîäîëæåíèå òåìû ïîäíÿòîé òóò -&nbsp;<a href="http://programmingmindstream.blogspot.ru/2014/08/blog-post_85.html">Êîðîòêî. Î âîçáóæäåíèè èñêëþ÷åíèé</a><br />
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Îò÷àñòè â ïðîäîëæåíèå âîò ýòîé òåìû -&nbsp;<a href="http://18delphi.blogspot.ru/2013/04/blog-post_7483.html">Ôàáðè÷íûé ìåòîä</a><br />
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Êàê ìîæíî ñîçäàâàòü îáúåêò?<br />
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Íó êîíå÷íî æå òàê:<br />
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<pre class="brush:delphi">type
 TmyObject = class
  public
   constructor Create(aSomeData: TSomeData);
 end;//TmyObject

...
var
 myObject : TmyObject;
...
 myObject := TmyObject.Create(aSomeData);
</pre>
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À ìîæíî òàê:
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<pre class="brush:delphi">type
 TmyObject = class
  protected
   constructor Create(aSomeData: TSomeData);
  public
   class function Make(aSomeData: TSomeData): TmyObject;
 end;//TmyObject

...
class function TmyObject.Make(aSomeData: TSomeData): TmyObject;
begin
 if IsValidData(aSomeData) then
  Result := Self.Create(aSomeData)
 else
  Result := nil;
end;
...
var
 myObject : TmyObject;
...
 myObject := TmyObject.Make(theConcreteData);
</pre>
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-- â ÷¸ì ðàçíèöà?&nbsp;
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À â òîì, ÷òî â "ôàáðè÷íîì ìåòîäå" ìîæíî âñòàâèòü íåêîòîðóþ áèçíåñ-ëîãèêó.<br />
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 íàøåì ñëó÷àå ýòî - IsValidData(aSomeData).<br />
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Íî ìîæíî ïîéòè äàëüøå:

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<pre class="brush:delphi">interface
...
type
 TmyObject = class
  protected
   constructor Create(aSomeData: TSomeData);
   procedure SomeMethodToOverride; virtual;
  public
   class function Make(aSomeData: TSomeData): TmyObject;
 end;//TmyObject
...
implementation
...
 TmySpecialObject = class(TmyObject)
  protected
   procedure SomeMethodToOverride; override;
 end;//TmySpecialObject

...
class function TmyObject.Make(aSomeData: TSomeData): TmyObject;
begin
 if IsMySpecialData(aSomeData) then
  Result := TmySpecialObject.Create(aSomeData) 
 else
 if IsValidData(aSomeData) then
  Result := Self.Create(aSomeData)
 else
  Result := nil;
end;
...
var
 myObject : TmyObject;
...
 myObject := TmyObject.Make(theConcreteData);
</pre>
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À ìîæíî ïîéòè åù¸ äàëüøå:
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<br />
<pre class="brush:delphi">interface
...
type
 TmyObject = class
  protected
   constructor Create(aSomeData: TSomeData);
   procedure SomeMethodToOverride; virtual;
  public
   class function Make(aSomeData: TSomeData): TmyObject;
 end;//TmyObject
...
implementation
...
 TmyNULLObject = class(TmyObject)
  protected
   procedure SomeMethodToOverride; override;
 end;//TmyNULLObject

 TmySpecialObject = class(TmyObject)
  protected
   procedure SomeMethodToOverride; override;
 end;//TmySpecialObject

...
class function TmyObject.Make(aSomeData: TSomeData): TmyObject;
begin
 if IsMySpecialData(aSomeData) then
  Result := TmySpecialObject.Create(aSomeData) 
 else
 if IsValidData(aSomeData) then
  Result := Self.Create(aSomeData)
 else
  Result := TmyNULLObject.Create(aSomeData);
end;
...
var
 myObject : TmyObject;
...
 myObject := TmyObject.Make(theConcreteData);
</pre>
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Íó è ìîæíî ïîéòè è åù¸ äàëüøå:

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<pre class="brush:delphi">interface
...
type
 ImyInterface = interface
  procedure SomeMethodToOverride;
 end;//ImyInterface

 TmyObject = class(TIntefacedObject, ImyInterface)
  protected
   constructor Create(aSomeData: TSomeData);
   procedure SomeMethodToOverride; virtual;
  public
   class function Make(aSomeData: TSomeData): ImyInterface;
 end;//TmyObject
...
implementation
...
 TmyNULLObject = class(TIntefacedObject, ImyInterface)
  protected
   procedure SomeMethodToOverride;
   // - Òóò ïîíÿòíîå äåëî override óæå íå íóæåí
   constructor Create(aSomeData: TSomeData);
  public
   class function Make(aSomeData: TSomeData): ImyInterface;
   // - à òóò âîîáùå ãîâîðÿ ìîæíî "çàáàáàõàòü ñèíãëåòîí"
 end;//TmyNULLObject

 TmySpecialObject = class(TmyObject)
  protected
   procedure SomeMethodToOverride; override;
 end;//TmySpecialObject

...
class function TmyObject.Make(aSomeData: TSomeData): ImyInterface;
begin
 if IsMySpecialData(aSomeData) then
  Result := TmySpecialObject.Create(aSomeData) 
 else
 if IsValidData(aSomeData) then
  Result := Self.Create(aSomeData)
 else
  Result := TmyNULLObject.Make(aSomeData);
end;
...
var
 myObject : ImyInterface;
...
 myObject := TmyObject.Make(theConcreteData);
</pre>
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Íó âîò ñîáñòâåííî è âñ¸.<br />
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Íàäåþñü, ÷òî ýòî êîìó-íèáóäü ïîíðàâèòñÿ.
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Îïÿòü æå îãîâîðþñü, ÷òî ýòî âñåãî ëèøü "ìàêåò".<br />
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Äà!<br />
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È åù¸ îäíà êðàòêàÿ ðåìàðêà.<br />
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Êàê ñäåëàòü òàê, ÷òîáû "íå ïðîøëè ìèìî ôàáðèêè"?<br />
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Ò.å. ÷òîáû íå âûçâàëè "ïàðàçèòíûé óìîë÷àòåëüíûé êîíñòðóêòîð". Êîòîðûé îò TObject.<br />
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À âîò òàê:<br />
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<pre class="brush:delphi">type
 TmyObject = class
  protected
   constructor Create(aSomeData: TSomeData); overload;
  public
   class function Make(aSomeData: TSomeData): TmyObject;
   constructor Create; overload;
 end;//TmyObject
...
constructor TmyObject.Create;
begin
 Assert(false, 'Íàäî âûçûâàòü ôàáðè÷íûé ìåòîä, à íå óíàñëåäîâàííûé êîíñòðóêòîð');
end;
</pre>
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À ìîæíî ñäåëàòü åù¸ "âåñåëåå", âîò òàê:
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<pre class="brush:delphi">type
 TmyObject = class
  protected
   constructor InternalCreate(aSomeData: TSomeData);
  public
   class function Make(aSomeData: TSomeData): TmyObject;
   procedure Create;
 end;//TmyObject
...
procedure TmyObject.Create;
begin
 Assert(false, 'Íàäî âûçûâàòü ôàáðè÷íûé ìåòîä, à íå óíàñëåäîâàííûé êîíñòðóêòîð');
end;
</pre>
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-- òîãäà îøèáî÷íûé êîä âîîáùå êîìïèëèðîâàòüñÿ íå áóäåò.<br />
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À ìîæíî íàâåðíîå âîîáùå òàê:

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<pre class="brush:delphi">type
 TmyObject = class
  protected
   constructor InternalCreate(aSomeData: TSomeData);
  public
   class function Create(aSomeData: TSomeData): TmyObject;
 end;//TmyObject
</pre>
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