Dart - 40 - Reflection

Reflection in Dart

Reflection is a powerful but advanced feature in the Dart programming language. It allows you to inspect, examine, and manipulate the structure and behavior of objects at runtime. While reflection can be incredibly useful in certain scenarios, it should be used with care due to its potential impact on performance and safety.

Understanding Reflection

Reflection, also known as introspection, is the ability of a program to examine and modify its own structure and behavior. In Dart, it enables you to inspect and manipulate classes, functions, and objects dynamically during runtime.

Reflection Capabilities in Dart

Dart provides several reflection capabilities, including:

Type Inspection: You can determine the type of an object, class, or function at runtime.
Field and Method Access: Reflection allows you to access and modify fields and methods of an object or class dynamically.
Class Instantiation: You can create instances of classes dynamically based on their names.

Using Reflection in Dart

Reflection in Dart is provided through the dart:mirrors library. To use reflection, you need to import this library and enable reflection for your Dart application. Reflection is generally used for debugging, serialization, dependency injection, or creating dynamic systems.

Here’s a basic example of using reflection to inspect an object’s type:


import 'dart:mirrors';

void main() {
    var someValue = 42;
    var mirror = reflect(someValue);

    if (mirror.type.reflectedType == int) {
        print('The variable is of type int.');
    }
}

In this example, we use reflection to determine the type of the variable someValue at runtime. We create a mirror of the object and inspect its type using reflection capabilities.

Dynamic Field and Method Access

Reflection allows you to access and manipulate fields and methods dynamically. This can be useful for scenarios where you need to work with objects that have unknown structures at compile time.

Here’s an example of using reflection to access a field dynamically:


import 'dart:mirrors';

class MyObject {
    String name = 'Dart';
}

void main() {
    var object = MyObject();
    var mirror = reflect(object);

    var fieldName = 'name';
    var fieldMirror = mirror.getField(Symbol(fieldName));

    print('Field value: ${fieldMirror.reflectee}');
}

In this example, we access the field name of the object MyObject dynamically using reflection. We use Symbol() to specify the field name and then retrieve its value.

Class Instantiation with Reflection

Reflection can also be used to create instances of classes dynamically. This can be helpful in situations where you need to instantiate classes based on their names, which can be determined at runtime.

Here’s an example of creating an instance of a class dynamically using reflection:


import 'dart:mirrors';

class Animal {
    String name = '';
    void speak() {
        print('Animal speaks');
    }
}

void main() {
    var className = 'Animal';
    var classSymbol = Symbol(className);
    var classMirror = reflectClass(classSymbol);

    var instance = classMirror.newInstance(Symbol(''), []).reflectee;
    instance.name = 'Dog';
    instance.speak();

    print('Instance type: ${instance.runtimeType}');
}

In this example, we use reflection to create an instance of the Animal class based on the class name specified in the className variable.

Reflection Limitations and Considerations

While reflection offers powerful capabilities, it’s important to be aware of its limitations and potential downsides:

Performance Impact: Reflection can significantly impact the performance of your application, as it often involves additional runtime checks and metadata inspection.
Code Size: The dart:mirrors library adds to the size of your compiled Dart code, potentially making it larger than necessary.
Type Safety: Reflection bypasses some of Dart’s strong type checking, potentially leading to runtime errors that would otherwise be caught at compile time.

Conclusion

Reflection in Dart is a powerful feature that enables you to inspect and manipulate objects, classes, and functions dynamically at runtime. While it can be valuable for certain use cases, it should be used judiciously and with a full understanding of its potential performance and safety implications. Reflection opens up the door to dynamic and flexible programming, but it should be balanced with careful consideration of its impact on your application.

Dart – 40 – Reflection