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You can now review the differences across the comparison table below.

Decoratorstructural
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Observerbehavioral
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Template Methodbehavioral
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Criteria	structuralDecoratorView pattern	behavioralObserverView pattern	behavioralTemplate MethodView pattern
Basics
Category	structural	behavioral	behavioral
Summary	Wrap an object with other objects that implement the same contract so responsibilities can be layered dynamically.	—	—
Intent	Wrap an object with other objects that implement the same contract so responsibilities can be layered dynamically.	Define a subscription mechanism so multiple observers can react to subject events without the subject knowing concrete subscriber details.	Define the skeleton of an algorithm in a base class and let subclasses override selected steps without changing the overall structure.
Decision factors
Problem	A system needs to add optional behaviors to an object without creating many subclasses for every feature combination.	A system needs to notify multiple dependent objects when state changes occur, but hardcoding those dependencies makes the design rigid and tightly coupled.	Several classes share the same algorithm structure, but some steps need to vary between implementations.
Tradeoffs	Can introduce many small wrapper classes that are harder to trace in debugging. Ordering of decorators matters, so composition needs to be deliberate. Keeps responsibilities modular and easy to combine.	Notification flows can become hard to trace when many observers react to the same event Observers may receive updates they do not need unless subscriptions are carefully designed	Relies on inheritance, which can be less flexible than composition in some designs May create a deep class hierarchy if many variations are needed
Practical fit
Languages	typescript java python angular react react-native csharp dotnet	typescript java python angular react react-native csharp dotnet	typescript java python angular react react-native csharp dotnet
Platforms	frontend backend services	Web Backend Event-driven systems	Web Backend Mobile UI kits Cross-platform systems
Integration notes	Decorators work well for cross-cutting concerns such as logging, retry, caching, compression, and telemetry because each layer keeps the same interface while adding one concern.	Observers work well inside a single application boundary for UI events, domain events, and in-process notifications where subscribers change dynamically.	Template Method is useful when you have repeated workflows with a common sequence of steps, such as processing orders, generating reports, and importing data.