Debugging Kotlin coroutines can sometimes feel like untangling a web of invisible threads - literally. One common frustration developers encounter is seeing the dreaded “optimised out” message when inspecting variables during a suspended coroutine.

This happens because of how Kotlin coroutines are compiled into state machines. Variables that aren’t explicitly needed after a suspension point may be discarded to optimize performance, making them invisible in the debugger.

Fortunately, there’s a straightforward way to address this issue by tweaking your build configuration. In this post, we’ll show you how to dynamically enable debug-friendly settings using the idea.active property, ensuring you can inspect coroutine variables in IntelliJ IDEA and Android Studio without impacting production performance. Let’s dive in!

Why Variables Get “Optimised Out” in Coroutines

In Kotlin coroutines, variables might become “optimised out” due to the way coroutines are implemented. Coroutines rely on a state machine to manage their execution. When a coroutine suspends, its current state is captured and stored, but only the essential variables required for resumption are preserved. Here’s why this can cause variables to be “optimised out”:

  1. State Machine Design: Coroutines are converted into state machines during compilation. Any local variables that are not directly used after the suspension point might be excluded from the state to reduce memory usage.
  2. Coroutine Inlining: If a coroutine function is marked as inline, the compiler aggressively optimizes the code by eliminating variables that appear redundant during the inlining process.
  3. Variable Scope: Variables defined within a coroutine might be scoped to a specific suspend block. Once the block completes or the variable is no longer referenced, it may no longer be part of the coroutine’s captured state.

By understanding these aspects, you can better anticipate which variables might become “optimised out” and adjust your debugging strategy accordingly.

How to Make “Optimised Out” Variables Visible

Debugging coroutines can be tricky, especially when variables are “optimised out.” However, by tweaking your build configuration, you can retain more variables during debugging. Here’s how you can do it using the idea.active property.

1. Enable Coroutine Debugging Dynamically

You can configure your project to enable coroutine debugging only when you’re working in the IntelliJ IDEA or Android Studio environment. This minimizes the impact of debug-friendly settings on your production code.

Here’s an example configuration in your build.gradle.kts:

// Kotlin Multiplatform
kotlin {
    compilerOptions {
        if (System.getProperty("idea.active") == "true") {
            println("Enable coroutine debugging")
            freeCompilerArgs = listOf("-Xdebug")
        }
    }
}

// Kotlin Jvm
kotlin {
    compilerOptions {
        if (System.getProperty("idea.active") == "true") {
            println("Enable coroutine debugging")
            freeCompilerArgs.add("-Xdebug")
        }
    }
}

How This Works

  • System.getProperty("idea.active"): This system property is automatically set to true when the project is opened in IntelliJ IDEA or Android Studio. It allows you to conditionally apply debug-specific configurations, ensuring they are only active in the IDE.
  • -Xdebug: This compiler argument generates debug-friendly bytecode, preserving more information about coroutine variables for inspection during debugging.

This approach dynamically enables debugging support without requiring manual changes for every environment.

2. Why Use idea.active?

Using idea.active provides a clean and efficient way to manage debug configurations:

  • Avoid Performance Overheads: Debug options like -Xdebug can slow down your app. By enabling them only in the IDE, you avoid affecting your production builds.
  • Streamlined Workflow: Automatically enabling debug settings in IntelliJ IDEA and Android Studio saves time and ensures you don’t forget to toggle them during development.
  • Safe Configuration: This method keeps your build script clean and ensures the debug flags are only applied when needed.

Conclusion

Debugging Kotlin coroutines doesn’t have to be a frustrating experience. By understanding how variables are managed in coroutines and leveraging dynamic configurations like idea.active, you can make “optimised out” variables visible when you need them most.

This approach ensures that your debugging setup is efficient, context-aware, and doesn’t compromise the performance of your production builds. With tools like -Xdebug and the advanced debugging capabilities of IntelliJ IDEA or Android Studio, you can confidently trace coroutine states and resolve even the trickiest bugs. Both IDEs offer powerful tools, such as the ability to inspect coroutine stack traces, evaluate expressions at runtime, and analyze the state of suspended coroutines, making your debugging process smoother and more efficient.

So the next time a variable mysteriously disappears in a coroutine, you’ll know exactly how to bring it back.

Happy debugging! 🚀