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POBetter way to implement filtered enumerator on TList<TMyObject>
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  1. POBetter way to implement filtered enumerator on TList<TMyObject>
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    copied!<p>Using Delphi 2010, let's say I've got a class declared like this:</p> <pre><code>TMyList = TList&lt;TMyObject&gt; </code></pre> <p>For this list Delphi kindly provides us with an enumerator, so we can write this:</p> <pre><code>var L:TMyList; E:TMyObject; begin for E in L do ; end; </code></pre> <p>The trouble is, I'd like to write this:</p> <pre><code>var L:TMyList; E:TMyObject; begin for E in L.GetEnumerator('123') do ; end; </code></pre> <p>That is, I want the ability to provide multiple enumerators for the same list, using some criteria. Unfortunately the implementation of <code>for X in Z</code> requires the presence of a function <code>Z.GetEnumerator</code>, with no parameters, that returns the given enumerator! To circumvent this problem I'm defining an interface that implements the "GetEnumerator" function, then I implement a class that implements the interface and finally I write a function on TMyList that returns the interface! And I'm returning an interface because I don't want to be bothered with manually freeing the very simple class... Any way, this requires a LOT of typing. Here's how this would look like:</p> <pre><code>TMyList = class(TList&lt;TMyObject&gt;) protected // Simple enumerator; Gets access to the "root" list TSimpleEnumerator = class protected public constructor Create(aList:TList&lt;TMyObject&gt;; FilterValue:Integer); function MoveNext:Boolean; // This is where filtering happens property Current:TTipElement; end; // Interface that will create the TSimpleEnumerator. Want this // to be an interface so it will free itself. ISimpleEnumeratorFactory = interface function GetEnumerator:TSimpleEnumerator; end; // Class that implements the ISimpleEnumeratorFactory TSimpleEnumeratorFactory = class(TInterfacedObject, ISimpleEnumeratorFactory) function GetEnumerator:TSimpleEnumerator; end; public function FilteredEnum(X:Integer):ISimpleEnumeratorFactory; end; </code></pre> <p>Using this I can finally write:</p> <pre><code>var L:TMyList; E:TMyObject; begin for E in L.FilteredEnum(7) do ; end; </code></pre> <p>Do you know a better way of doing this? Maybe Delphi does support a way of calling GetEnumerator with a parameter directly?</p> <p><strong>Later Edit:</strong></p> <p>I decided to use Robert Love's idea of implementing the enumerator using anonymous methods and using gabr's "record" factory to save yet an other class. This allows me to create a brand new enumerator, complete with code, using just a few lines of code in a function, no new class declaration required.</p> <p>Here's how my generic enumerator is declared, in a library unit:</p> <pre><code>TEnumGenericMoveNext&lt;T&gt; = reference to function: Boolean; TEnumGenericCurrent&lt;T&gt; = reference to function: T; TEnumGenericAnonim&lt;T&gt; = class protected FEnumGenericMoveNext:TEnumGenericMoveNext&lt;T&gt;; FEnumGenericCurrent:TEnumGenericCurrent&lt;T&gt;; function GetCurrent:T; public constructor Create(EnumGenericMoveNext:TEnumGenericMoveNext&lt;T&gt;; EnumGenericCurrent:TEnumGenericCurrent&lt;T&gt;); function MoveNext:Boolean; property Current:T read GetCurrent; end; TGenericAnonEnumFactory&lt;T&gt; = record public FEnumGenericMoveNext:TEnumGenericMoveNext&lt;T&gt;; FEnumGenericCurrent:TEnumGenericCurrent&lt;T&gt;; constructor Create(EnumGenericMoveNext:TEnumGenericMoveNext&lt;T&gt;; EnumGenericCurrent:TEnumGenericCurrent&lt;T&gt;); function GetEnumerator:TEnumGenericAnonim&lt;T&gt;; end; </code></pre> <p>And here's a way to use it. On any class I can add a function like this (and I'm intentionally creating an enumerator that doesn't use a <code>List&lt;T&gt;</code> to show the power of this concept):</p> <pre><code>type Form1 = class(TForm) protected function Numbers(From, To:Integer):TGenericAnonEnumFactory&lt;Integer&gt;; end; // This is all that's needed to implement an enumerator! function Form1.Numbers(From, To:Integer):TGenericAnonEnumFactory&lt;Integer&gt;; var Current:Integer; begin Current := From - 1; Result := TGenericAnonEnumFactory&lt;Integer&gt;.Create( // This is the MoveNext implementation function :Boolean begin Inc(Current); Result := Current &lt;= To; end , // This is the GetCurrent implementation function :Integer begin Result := Current; end ); end; </code></pre> <p>And here's how I'd use this new enumerator:</p> <pre><code>procedure Form1.Button1Click(Sender: TObject); var N:Integer; begin for N in Numbers(3,10) do Memo1.Lines.Add(IntToStr(N)); end; </code></pre>
 

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