Emulators and Simulators
Mobile apps are fundamentally different from desktop software and require testing in actual mobile environments for context and compatibility.
Emulators and simulators are valuable for early-stage mobile app testing, allowing developers to test without physical devices. While emulators replicate both hardware and software and simulators mimic the OS and app behavior, they cannot fully replace real-device testing. Real-device testing is crucial for ensuring accurate compatibility, performance, and user experience before an app’s release.
Below is an in-depth explanation of emulators, simulators, what kind of testing are they suited for and how they are different from real device testing.
What are Emulators?
Emulators mimic your target device’s hardware and software on your workstation. Android Emulator (by Android Developer Studio) is a popular example.
How the Emulator Works
All computers–including mobile devices–work on an ISA, that is, Instruction Set Architecture. This is a set of instructions written in machine-language that your processor understands. Different processor families (think Intel, AMD, ARM, etc.) have their own instruction set architectures, which they implement in their own ways.
The emulator mimics the target/mobile device processor. Then it translates its ISA into the one used by your computer, through a process called binary translation. This binary (ABI–or Application Binary Interface) can be equipped with a compatible operating system and APIs.
Capabilities:
- Emulators offer virtual device instances with near-native capabilities.
- Extended controls to adjust physical sensors, battery state, geolocation, etc.
Limitations:
- Performance overhead due to binary translation.
- Hardware-assisted virtualization can speed up translation, but it requires matching ISAs (e.g., ARM vs. Intel x86) and hypervisor components.
- Emulators often fail to replicate real-world conditions accurately, leading to unreliable results for Android apps and websites.
What are Simulators?
Simulators allow you to run programs designed for other operating systems. For mobile app testing, simulators are commonly used in the early stages, particularly for iOS apps through XCode’s iPhone and iPad simulators.
Note: There are no ‘Android simulators’, largely because it’s simpler to emulate Android devices with freely available tools like Quick Emulator (QEMU).
How iOS Simulators Work:
- The iOS simulator runs on macOS and mimics iOS to test apps in an iPhone or iPad-like window.
- Allows interaction via keyboard, touchpad, or mouse.
- Faster than Android emulators, as no machine-language translation is required.
Note: There are no true ‘iPhone or iPad emulators’. There are only iOS simulators dressed up as such. This is because Apple uses proprietary chipsets and custom binaries in all its devices. You will need to reverse engineer the Apple device to virtualize it.
Limitations:
- Cannot mimic battery states or cellular interrupts, unlike Android emulators.
- Exclusive to macOS, as it relies on Apple’s proprietary Cocoa API.
- There are no true iOS emulators—simulators only replicate basic functionalities.
- Tools like Cider APK and iEMU attempted to emulate iOS on Android but have limitations and are discontinued.
While mobile simulators may seem tempting for testing, they cannot accurately replicate real user conditions. Tests conducted on simulators are often inconclusive or inaccurate. For reliable results, testing on real iOS devices is essential.
Emulators and Simulators: Key Differences
Now that you have a better understanding of what are emulators and simulators, below is an overview of the key differences between both.
Understanding the differences between them is key to using the right tool for the right testing purpose.
| Feature | Emulators | Simulators |
|---|---|---|
| Purpose | Emulate hardware and software environments. | Mimic the OS and app behavior without replicating hardware. |
| Platform | Emulates both hardware and OS, including I/O, memory, etc. | Abstracts platform differences, mimics OS behavior. |
| Real-Device Hardware Context | Provides a closer imitation of a real device environment with hardware abstraction. | Provides limited hardware context, cannot replicate real device hardware. |
| Performance | Has significant performance overhead due to binary translation. | Faster, as no machine-language translation is involved. |
| Compatibility | Can emulate different OS and devices, including Android. | Limited to iOS/macOS apps, cannot virtualize actual iOS hardware. |
| Use Cases | Used for testing apps in a real device environment (e.g., Android apps). | Used for rapid prototyping, early-stage testing, and running basic unit tests. |
| Hardware Abstraction | Can replicate the entire device architecture and memory. | Only simulates OS and app behavior, not real hardware. |
| Testing Accuracy | Suitable for unit tests but less accurate for performance tests. | Not reliable for accurate testing due to lack of real hardware context. |
| Pre-Release Testing | Provides better accuracy for checking compatibility and functionality. | Not reliable for final testing, as tests may fail on real devices. |
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Emulators and Simulators in Mobile App Testing
Emulators and simulators are widely used in mobile app testing for their ease of access. Both are available within the SDK (Android and XCode), allowing you to test your current project inside a virtual device environment with just a few clicks.
- Rapid Prototyping and Mockups: UI engineers and designers often use online emulators and simulators for quick prototyping and mockups.
- Basic Unit Testing: In production, developers use emulators and simulators to run basic unit tests on app code within a platform and limited device context. This helps reduce the likelihood of major changes after integration. For example, these tests can check if the UI accepts input as expected and how the code interacts with third-party or platform-specific APIs.
- Web Apps Testing: Emulators and simulators are also useful for testing web apps. For example, Chrome Developer Tools provides a mobile browser emulator that offers a device-viewport context. This feature allows you to test your web app’s UI during development with just a few clicks.
By using emulators and simulators, you can quickly assess and prototype app functionalities, making them useful tools in the early stages of development.
Pre-Release Testing: Virtual vs. Real Devices
Compatibility and performance tests on virtual iOS and Android devices often fail to provide conclusive results. Several factors contribute to this limitation:
- iOS Simulator Limitations: The iOS simulator gives unrestricted access to Mac resources, which can cause tests to pass on the simulator but fail on a real iPhone, especially for features involving large media files.
- Speed Variability: The performance of a simulated iOS device varies depending on the available resources on your Mac, leading to inaccurate results.
- Android Virtual Device Limitations: Virtual Android devices, particularly with non-x86 ABIs, run slower than real devices, regardless of your computer’s specs.
- False Positives: False positives from the iOS simulator can lead to errors that only appear on real devices, but CI engines testing on real devices can catch these in time.
- Real-World Issues: Many mobile apps are deleted quickly due to battery drain, memory issues, and poor UI, which can only be detected through real device testing.
Why Real Device Testing is Crucial
Modern product teams perform automated tests on a matrix of real mobile devices to catch UI, system, and performance bugs before release. This helps pinpoint error sources and speeds up debugging.
While mobile simulators and emulators help test code in a mobile context, they cannot fully replicate hardware differences. Due to the fragmented nature of the mobile market, hardware abstraction is impossible. Both Apple and Google emphasize testing apps on real mobile devices before deployment. Avoid relying on online mobile emulators or phone simulators for production testing.
Conclusion
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