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PODelphi Dependency Injection: Framework vs Delegating Constructor
 

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  1. PODelphi Dependency Injection: Framework vs Delegating Constructor
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    <p>Why would you use a Dependency Injection Framework when you can simple use the following pattern?</p> <pre><code>unit uSomeServiceIntf; interface type ISomeService = interface procedure SomeMethod; end; var CreateSomeService: function: ISomeService; implementation end. </code></pre> <hr> <pre><code>unit uSomeServiceImpl; interface type TSomeService = class(TInterfacedObject, ISomeService) procedure DoSomething; end; function CreateSomeService: ISomeService; implementation function CreateSomeService: ISomeService; begin Result := TSomeService.Create; end; procedure TSomeService.DoSomeThing; begin ... end; end. </code></pre> <hr> <pre><code>unit uInitializeSystem; interface procedure Initialze; implementation uses uSomeServiceIntf, uSomeServiceImpl; procedure Initialze; begin uSomeServiceIntf.CreateSomeService := uSomeServiceImpl.CreateSomeService; end; end. </code></pre> <p>I am trying to grasp the benefits of using a framework instead of doing this but so far I only see the benefits of this simple approach:</p> <p>1) Parameterized constructors are easier to implement. E.g.: var CreateSomeOtherService: function(aValue: string);</p> <p>2) Faster (no lookups necessary in a container)</p> <p>3) Simplier</p> <p>This is how I would use it:</p> <pre><code>unit uBusiness; interface [...] implementation uses uSomeServiceIntf; [...] procedure TMyBusinessClass.DoSomething; var someService: ISomeService; begin someService := CreateSomeService; someService.SomeMethod; end; end. </code></pre> <p>What would be your reasoning to use a DI framework instead of this approach?</p> <p>How this would look like using a DI framework?</p> <p>As far as I know if you would use a DI framework than you would register the concrete class against the interface and then consumers of the system would ask an implementation for the given framework. So there would be a registering call:</p> <pre><code>DIFramework.Register(ISomeInterface, TSomeInterface) </code></pre> <p>and when you need an ISomeInterface implementation you can ask the DI framework for it:</p> <pre><code>var someInterface: ISomeInterface; begin someInteface := DIFrameWork.Get(ISomeInterface) as ISomeInterface; </code></pre> <p>Now obviously if you do need to pass parameters to create an ISomeInterface the whole thing gets more complicated with the DIFramework (but simple with the approach described above).</p>
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    1. USmenjaraz
    1. USRM.
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      1. USJørn E. Angeltveit
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    1. COMaybe you could add a source code example using a DI container to make the differences in declaration and usage clearer?
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      1. USmjn
    2. CONot sure what you mean by "convenience"?. Influence?
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      1. USWarren P
    3. CO@Warren, maybe the OP wants to know the reasons to choose either the pattern exeplified or the DI container...
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      1. USFabricio Araujo
 

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