Decorator pattern is a design pattern belonging to the structural group. It provides the ability to extend an object by “attaching” a new variable or method to an object. Object is oblivious to the fact that it is being “decorated” with new methods and variables. This design pattern is suitable for systems that need to expand continuously. Decorator idea is relatively close to inheritance and you can easily understand this design pattern if you understand the idea of inheritance.
In this lesson we will look at the idea, design, and implementation of this pattern in detail.
What is the Decorator Pattern?
The Decorator design pattern helps to add new states (i.e. member variables) and operations (methods) to an existing object.
Do you sound familiar? Add new variables and methods to an existing object. So that’s not inheritance? In fact, you can think of Decorator as a type of “inheritance”, because their ideas are quite close. However, Decorator “inheritance” is done at the object level, rather than the class level. Because of its object-level implementation, Decorator “inheritance” occurs at runtime, rather than at compile time type definition.
Let’s take another example of an image displayed on the screen. Programs that support image viewing/processing can incorporate many decorative elements such as borders, tags. These decorative elements appear above the image but do not affect the original image file. The combination of the original image and the decorations creates a new object. You see this composite image.
The Decorator pattern is exactly the same idea. Each decorative element for the original image is an independent object. And there are also countless ways to decorate the photo. From one original photo you can create a lot of pictures with different decorations. You can even combine decorations to create unique photos.
In summary, the Decorator pattern has the following characteristics:
- Can add (dynamically) new members (decorators) to the object;
- The original Object does not change and knows nothing about what is added to it;
- So you don’t need to build a giant class with everything inside;
- Decorative objects are independent of each other and can be combined.
Thus, in general, Decorator is close to inheritance. So why not always use inheritance. If a class is developed by someone else and marked sealed, you cannot inherit from that class anymore. At this point, if you want to expand it, think of Decorator. If you need to extend the object at runtime, you need Decorator.
In addition, Decorator has a variety of combinations to extend the object that if you use inheritance, the result will be a lot of subclasses that make it difficult to maintain the code later.
Design Decorator Pattern
Below is a UML design diagram of the Decorator design pattern. If you don’t remember the symbols, reread the UML section of the Design Pattern overview lesson.
The components of this design are as follows:
Component: the original class. You can think of it as the original image, or the parent class in inheritance.
Decorator: decoration class. You can think of it as a subclass (in inheritance), or a decorated image.
Both Component and Decorator must implement a common interface IComponent.
addedState and addedBehavior are Decorator’s own variables and methods that add to the original object (of the Component). It’s easy to imagine it as private member variables and methods of subclasses.
Operation: decorator not only complements but can also replace the method of the original class. If there are methods to replace, it must be specified in the common interface between the Component and the Decorator. It’s easy to imagine it’s like overriding methods in inheritance.
Between Decorator and IComponent there is also an aggregate relationship (has-a), expressed through a private component variable of type IComponent inside the Decorator. It is very close to the idea of creating an inheritance tree of object-oriented programming. Because of this relationship, an extended object can be extended by another decorator creating an entire “inheritance tree” in the style of Decorator. This is like a redecorated photo that can continue to be decorated further.
Implementing Decorator Pattern
Once you understand the idea of the Decorator pattern, it’s time to do a basic illustration.
using System;
using static System.Console;
namespace P01_Concept
{
interface IComponent
{
string Operation();
}
class Component : IComponent
{
public string Operation()
{
return “Hello world! This is the original object”;
}
}
class DecoratorA : IComponent
{
private readonly IComponent _component;
public DecoratorA(IComponent component)
{
_component = component;
}
// consider “inherit” this method from the original object
// if you want you can “simulate overriding” by changing this method content
public string Operation()
{
return _component.Operation();
}
//add this method to the original object
public string AddedBehavior()
{
return “This is the A Decorator object”;
}
}
class DecoratorB : IComponent
{
private readonly IComponent _component;
public DecoratorB(IComponent component)
{
_component = component;
}
// simulate overriding Operation
public string Operation()
{
var s = _component.Operation();
return $”{s}.But I was ‘overrode'”;
}
}
class Program
{
static void Main(string[] args)
{
Title = “DECORATOR DESIGN PATTERN”;
IComponent orgComponent = new Component();
DecoratorA aComponent = new DecoratorA(orgComponent);
DecoratorB bComponent = new DecoratorB(orgComponent);
DecoratorA abComponent = new DecoratorA(bComponent);
ForegroundColor = ConsoleColor.Green;
WriteLine($”Original object: {orgComponent.Operation()}”);
ForegroundColor = ConsoleColor.Yellow;
WriteLine($”A Decorator object: {aComponent.Operation()}. {aComponent.AddedBehavior()}”);
ForegroundColor = ConsoleColor.Cyan;
WriteLine($”B Decorator object: {bComponent.Operation()}”);
ForegroundColor = ConsoleColor.Magenta;
WriteLine($”AB Decorator object: {abComponent.Operation()}”);
ReadKey();
}
}
}
In this example reuse the correct names in the UML design diagram. It also illustrates the correlation between Decorator and inheritance.
Application of the Decorator Pattern
The Decorator design pattern has a number of practical applications.
The first is in graphics, video, audio. For example, so that streaming video can be compressed in different ratios; Audio can offer multiple conversion services at the same time.
Second, use it in some I/O APIs. For example, the .NET Stream class string (Stream => FileStream, MemoryStream, NetworkStream => BinaryWriter/Reader, StreamWriter/Reader) uses Decorator. If you’ve ever worked with one of the above stream types, you’ll probably notice: (1) adapters extend the backing store stream but don’t inherit from it; (2) in an object, for example a NetworkStream or a StreamWriter, there is an object of Stream; (3) stream types are mutually extensible. Recall the design model of Decorator to see if there are any similarities.
Third, browsers and mobile apps use this template to create the right interface for each type of screen size.
- In short, in the following situations you should think of Decorator:
- Want to extend sealed class;
- Extend or change objects “dynamically” at runtime without touching the original object;
- Do not want to create subclasses (avoid using inheritance).
Benefits of using the decorator pattern layouts
When it comes to software development, there are few things more valuable than code flexibility and extensibility. That’s where the decorator pattern comes in. By extending objects without inheritance, the decorator pattern allows developers to modify an object’s behavior or add new features without having to modify the object’s underlying code. This approach offers a range of benefits that can make your code more robust and maintainable.
Maintaining the Single Responsibility Principle (SRP)
One of the main benefits of using decorator pattern layouts is that it enables developers to maintain the Single Responsibility Principle (SRP). This principle states that a class or module should have only one reason to change. By breaking down functionality into smaller, more focused classes, you can ensure that each class has a single responsibility. Decorator pattern layouts help you achieve this by allowing you to add new functionality without having to modify the existing code.
Improved code flexibility and extensibility
Another benefit of using decorator pattern layouts is that it makes your code more flexible and extensible. With the decorator pattern, you can add new functionality to your codebase by creating new decorators that wrap around existing objects. This means you can easily modify the behavior of an object by simply adding or removing decorators. As a result, your code becomes more flexible and easier to extend.
Simplification of code maintenance and management
Using decorator pattern layouts can also simplify code maintenance and management. When you need to modify an object’s behavior or add new features, you don’t have to modify the object’s underlying code. Instead, you can simply create a new decorator and apply it to the object. This makes it easier to manage your codebase because you don’t have to worry about breaking existing functionality when making changes.
In addition, using decorator pattern layouts can make it easier to reuse code components across multiple projects. Because decorators can be used to modify existing objects, you can create a library of decorators that can be used in different contexts. This can save you time and effort when developing new projects because you can leverage existing code components.
Use cases of decorator pattern layouts
The decorator pattern is a powerful tool for building flexible and extensible software systems. With its ability to modify an object’s behavior without changing its underlying code, decorator pattern layouts can be used in a variety of contexts to enhance application functionality, simplify code maintenance, and promote code reuse. Here are some examples of how you can use decorator pattern layouts in your projects.
Modifying layout design without modifying the underlying code
When building a user interface, you may need to modify the layout design to accommodate different use cases. For example, you may need to add a new button to a form or rearrange the elements on a page. Traditionally, making such changes would require modifying the underlying code. However, with decorator pattern layouts, you can modify the layout design without changing the underlying code.
By creating a layout object and applying decorators to it, you can modify the object’s behavior to achieve the desired layout design. For example, you can create a decorator that adds a new button to the form or rearranges the elements on the page. This approach allows you to modify the layout design without having to change the underlying code, making it easier to maintain and update the application over time.
Creating reusable code components
Another use case for decorator pattern layouts is creating reusable code components. By defining an object and applying decorators to it, you can create a set of building blocks that can be used in different contexts. For example, you can create a set of decorators that modify the behavior of a database query object. These decorators can be used in different applications to modify the behavior of the query object without having to write new code.
Enhancing application functionality with minimal code changes
Finally, decorator pattern layouts can be used to enhance application functionality with minimal code changes. By creating a set of decorators that modify the behavior of an object, you can add new features to the application without having to modify the underlying code. For example, you can create a decorator that adds logging functionality to a database query object. This allows you to track the queries being executed without having to modify the underlying code.
In addition, using decorator pattern layouts can help you avoid code duplication. Instead of writing new code for each feature, you can create a set of decorators that modify the behavior of existing objects. This can save you time and effort when developing new features and reduce the risk of introducing bugs into the system.
Potential drawbacks and limitations of decorator pattern layouts
While the decorator pattern offers numerous benefits, there are also some potential drawbacks and limitations to consider when using this design pattern. Here are some of the main challenges you may encounter when working with decorator pattern layouts.
Complexity in understanding and implementing the decorator pattern
One of the main challenges of using decorator pattern layouts is that it can be complex to understand and implement. The pattern requires a solid understanding of object-oriented programming principles and can be difficult to grasp for developers who are new to these concepts. In addition, implementing decorator pattern layouts can be challenging because it requires a high degree of precision and attention to detail.
Increased code complexity and potential performance issues
Another potential drawback of using decorator pattern layouts is that they can increase code complexity and potentially lead to performance issues. Each decorator adds additional code to the system, which can make the overall codebase more complex and harder to maintain. In addition, adding too many decorators can potentially slow down the application, leading to performance issues.
Possible difficulty in identifying the appropriate decorator pattern layout
Finally, it can be challenging to identify the appropriate decorator pattern layout for a given use case. There are many different types of decorators that can be applied to objects, and selecting the right one can be a challenge. Additionally, using too many decorators can make the code harder to read and understand, making it difficult to identify which decorators are responsible for which behavior.
Best practices for using decorator pattern layouts
While the decorator pattern can be a powerful tool for building flexible and extensible software systems, it’s important to use it correctly in order to avoid potential issues. Here are some best practices to keep in mind when using decorator pattern layouts.
Keep decorators small and focused
One of the most important best practices for using decorator pattern layouts is to keep your decorators small and focused. Each decorator should modify a specific aspect of the object’s behavior rather than trying to modify multiple aspects at once. This makes it easier to maintain and update the decorators over time.
Use an interface to define the object being decorated
Another best practice is to use an interface to define the object being decorated. This allows you to easily swap out different implementations of the object without having to modify the decorators. It also makes it easier to test the decorators in isolation.
Avoid circular dependencies between decorators
Circular dependencies between decorators can cause issues with the order in which the decorators are applied. To avoid this, make sure that each decorator only depends on the object being decorated and not on other decorators. If you do need to have dependencies between decorators, use dependency injection to manage them.
Use dependency injection to make it easier to apply decorators
Finally, using dependency injection can make it easier to apply decorators to objects. By injecting the object being decorated and its decorators as dependencies, you can easily modify the behavior of the object at runtime. This can make your code more flexible and easier to maintain.
Future of the decorator pattern
The decorator pattern has been a popular design pattern in object-oriented programming for many years. As software development continues to evolve, the decorator pattern is likely to remain a valuable tool for building flexible and extensible systems. Here are some potential directions for the future of the decorator pattern.
Use cases for the decorator pattern in emerging technologies such as AI and machine learning
As AI and machine learning continue to gain popularity in various industries, there may be new use cases for the decorator pattern in these fields. For example, decorators could be used to modify the behavior of AI models at runtime, allowing them to adapt to new data and circumstances. Similarly, decorators could be used to modify the output of machine learning models to improve their accuracy and effectiveness.
Possible modifications to the decorator pattern to address its limitations
While the decorator pattern offers many benefits, it also has some limitations, as discussed earlier. One possible direction for the future of the decorator pattern is to modify the pattern to address these limitations. For example, new variants of the decorator pattern could be developed that address issues with code complexity and performance. Additionally, new tools and frameworks could be developed to make it easier to use the decorator pattern in real-world projects.
Importance of staying up-to-date with the latest developments in software design patterns
Finally, it’s important to stay up-to-date with the latest developments in software design patterns, including the decorator pattern. As new technologies and programming languages emerge, new design patterns may be developed to address new challenges and opportunities. By staying informed about the latest trends and developments in software design, you can continue to improve your skills as a developer and build more robust and maintainable software systems.
Conclusion
In this lesson you became familiar with the Decorator design pattern. This is a pretty simple design pattern to implement. Decorator has many practical applications and you will probably need Decorator as an alternative to inheritance. Don’t forget to follow our blog for other useful programming lessons.
