Dart Reflection

Reflection in Dart allows programs to examine, introspect, and modify their own structure and behavior at runtime. It's a powerful feature, but it comes with limitations and performance considerations.

Understanding Dart Reflection

Dart's reflection capabilities are provided through the dart:mirrors library. This library offers tools for introspecting types, invoking methods, and accessing object properties dynamically.

Key Concepts

• Mirrors: Objects that reflect other objects or types.
• Introspection: Examining object properties and methods at runtime.
• Invocation: Calling methods or accessing properties dynamically.

Using Reflection in Dart

To use reflection, you need to import the dart:mirrors library. Here's a basic example:

import 'dart:mirrors';

void main() {
  var obj = MyClass();
  var mirror = reflect(obj);
  
  // Invoke a method dynamically
  mirror.invoke(Symbol('myMethod'), []);
}

class MyClass {
  void myMethod() {
    print('Method called via reflection');
  }
}
    

Limitations and Considerations

While powerful, reflection in Dart has some important limitations:

• It's not supported in Flutter or when compiling to JavaScript.
• Using reflection can impact performance and increase code size.
• It bypasses static type checking, potentially introducing runtime errors.

Practical Applications

Despite its limitations, reflection can be useful in certain scenarios:

• Implementing plugin systems or dependency injection frameworks.
• Creating generic serialization/deserialization mechanisms.
• Building testing and mocking frameworks.

Reflection and Metadata

Dart's reflection capabilities work well with metadata annotations. You can use reflection to read and process metadata at runtime:

import 'dart:mirrors';

@MyAnnotation('Hello')
class MyClass {}

class MyAnnotation {
  final String value;
  const MyAnnotation(this.value);
}

void main() {
  var classMirror = reflectClass(MyClass);
  var metadata = classMirror.metadata.first.reflectee as MyAnnotation;
  print(metadata.value); // Outputs: Hello
}
    

Best Practices

When using reflection in Dart, keep these best practices in mind:

• Use reflection sparingly and only when absolutely necessary.
• Consider alternative design patterns that don't require reflection.
• Be aware of the performance implications, especially in performance-critical code.
• Test thoroughly, as reflection can introduce runtime errors not caught by static analysis.

Conclusion

Dart reflection is a powerful tool for introspection and dynamic behavior. While it has limitations and performance considerations, it can be valuable in specific scenarios. Use it judiciously, and always consider simpler alternatives first.

For more information on related topics, explore Dart Generics and Dart Callable Classes.