Understanding the Difference Between Atomic and Nonatomic Attributes in Objective-C

Understanding the Difference Between Atomic and Nonatomic Attributes in Objective-C

22 May 2024 Stephan Petzl Leave a comment Tech-Help

When declaring properties in Objective-C, you may have come across the terms atomic and nonatomic. These attributes play a crucial role in defining the behavior of properties, especially in a multi-threaded environment. This article aims to clarify the differences between these two attributes and help you decide which one to use in your projects.

What Do Atomic and Nonatomic Mean?

In Objective-C, the atomic and nonatomic attributes specify how properties are accessed and modified in a multi-threaded context. Let’s break down their operational differences:


  • Default Behavior: If you don’t specify nonatomic, the property is assumed to be atomic.
  • Thread Safety: Ensures that a whole value is always returned from the getter or set by the setter, regardless of other thread activities.
  • Performance: Slower compared to nonatomic due to the overhead of ensuring thread safety.

An atomic property guarantees that if one thread is executing a setter or getter, another thread cannot access the property until the operation is complete. This ensures data integrity but does not guarantee complete thread safety when multiple dependent properties are involved.


  • Behavior: Does not ensure thread safety. Multiple threads can access the property simultaneously.
  • Performance: Faster than atomic due to the lack of synchronization overhead.

A nonatomic property does not provide any guarantees about the state of the property when accessed by multiple threads. This can lead to unpredictable behavior but offers better performance.

Practical Examples

Consider the following property declarations:

@property (nonatomic, retain) UITextField *userName;
@property (atomic, retain) UITextField *userName;
@property (retain) UITextField *userName;

The second and third declarations are functionally identical because atomic is the default behavior. In contrast, the first declaration explicitly specifies nonatomic, offering faster performance at the expense of thread safety.

When to Use Atomic vs. Nonatomic

Choosing between atomic and nonatomic depends on your specific use case:

  • Use Atomic: When you need to ensure that a property is accessed in a thread-safe manner, and performance is not a critical concern.
  • Use Nonatomic: When performance is crucial, and you can manage thread safety through other means.

Enhancing Your Development Workflow

Managing properties in a multi-threaded environment can be challenging. However, tools like Repeato can significantly ease this process. Repeato is a no-code test automation tool for iOS and Android that helps you create, run, and maintain automated tests for your apps. It’s particularly fast to edit and run tests, leveraging computer vision and AI.

With Repeato, mobile developers can focus on creating great products without spending excessive time on test automation. Its user-friendly interface also allows non-technical colleagues or QA teams to contribute to the testing process, ensuring that your app functions correctly across different scenarios.

For more information, check out our documentation or visit our blog for the latest updates.

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