Building a Delphi Plugin System with RTTI and Custom Attributes

Problem: Adding features without recompiling the host

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In a Delphi desktop application you often want to let third‑party code add new commands or tools without rebuilding the main executable. A traditional approach uses interfaces and explicit registration, which forces the host to know every plugin type at compile time.

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Thesis: RTTI and custom attributes give a loosely coupled, discoverable plug‑in model

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By enabling Run‑Time Type Information (RTTI) and defining a custom attribute, the host can scan a package or DLL, find methods marked with the attribute, create instances and invoke them. The host never needs a compile‑time reference to the plugin.

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How the mechanism works

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1. Define the export attribute

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First declare an attribute class that can be applied to methods. The attribute carries no data; its presence is the signal.

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type\n  [AttributeUsage(TAttributeTargets.Method)]\n  TPluginExport = class(TCustomAttribute)\n  end;\n

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2. Mark the plugin entry point

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In each plugin unit, annotate the method that the host should call.

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type\n  TSamplePlugin = class\n  public\n    [TPluginExport]\n    procedure Execute;\n  end;\n\nprocedure TSamplePlugin.Execute;\nbegin\n  ShowMessage('Plugin executed');\nend;\n

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3. Host code that discovers and calls the attribute

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Using TRttiContext the host enumerates all types in the loaded module, checks each method for the attribute, creates an instance and invokes the method.

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var\n  ctx: TRttiContext;\n  rType: TRttiType;\n  rMethod: TRttiMethod;\n  attr: TCustomAttribute;\n  instance: TObject;\nbegin\n  ctx := TRttiContext.Create;\n  try\n    for rType in ctx.GetTypes do\n    begin\n      for rMethod in rType.GetMethods do\n      begin\n        for attr in rMethod.GetAttributes do\n        begin\n          if attr is TPluginExport then\n          begin\n            instance := rType.AsInstance.MetaclassType.Create;\n            try\n              rMethod.Invoke(instance, []);\n            finally\n              instance.Free;\n            end;\n          end;\n        end;\n      end;\n    end;\nfinally\n  ctx.Free;\n  end;\nend;\n

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Configuration and build steps

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1. Enable RTTI for the units that contain plugins. In the source file add {$RTTI EXPLICIT METHODS([]) PROPERTIES([]) FIELDS([])} or set Project → Options → Delphi Compiler → Linking → RTTI to "Explicit".
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2. Compile the plugin as a runtime package (.bpl) or DLL and place it where the host can locate it (e.g., the application folder or a known plug‑in directory).
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3. In the host, load the package with LoadPackage (or LoadLibrary for a DLL) before running the scan.
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Trade‑offs and limitations

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• Executable size. RTTI emits metadata for every marked type, increasing the binary size. For a typical VCL project the increase is a few hundred kilobytes; you can limit it by disabling RTTI for units that do not need it ({$RTTI EXCLUDE}).
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• Discovery reliability. If the attribute is misspelled, the method is not marked, or the plugin fails to load, the host will simply skip it without error. Adding a validation step that logs the number of discovered plugins helps catch missing attributes early.
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• Visibility. Only members that have RTTI (public, published, or those explicitly enabled) can be scanned. Marking a method in a strictly private section will not work unless you expand RTTI visibility.
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Practical verification

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1. Build the host and a sample plugin.
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2. Run the host and check the plugin’s ShowMessage appears when the scan code executes (e.g., from a button click).
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3. Compare the file size of the host executable with RTTI enabled versus disabled (Project → Options → Delphi Compiler → Linking → RTTI). The difference shows the metadata overhead.
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4. Optionally, add a logging line inside the discovery loop that outputs the class name of each discovered plugin to the console or a memo; an empty list indicates a configuration problem.
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Actionable closing

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Start small: create a single attribute, apply it to a test method, and implement the host scan as shown. Once the basic discovery works, expand the attribute to carry configuration data (e.g., a name or version) and refine the host lifecycle (caching discovered instances, handling unload). With RTTI enabled only where needed, you gain a flexible plug‑in system without sacrificing too much binary size or introducing tight compile‑time dependencies.