docs/topics/debug-flow-with-idea.md

[//]: # (title: Debug Kotlin Flow using IntelliJ IDEA – tutorial)

This tutorial demonstrates how to create Kotlin Flow and debug it using IntelliJ IDEA.

The tutorial assumes you have prior knowledge of the [coroutines](coroutines-guide.md) and [Kotlin Flow](flow.md#flows) concepts.

## Create a Kotlin flow

Create a Kotlin [flow](https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.flow/flow.html) with a slow emitter and a slow collector:

1. Open a Kotlin project in IntelliJ IDEA. If you don't have a project, [create one](jvm-get-started.md#create-a-project).
2. To use the `kotlinx.coroutines` library in a Gradle project, add the following dependency to `build.gradle(.kts)`:

   <tabs group="build-script">
   <tab title="Kotlin" group-key="kotlin">

   ```kotlin
   dependencies {
       implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:%coroutinesVersion%")
   }
   ```

   </tab>
   <tab title="Groovy" group-key="groovy">

   ```groovy
   dependencies {
       implementation 'org.jetbrains.kotlinx:kotlinx-coroutines-core:%coroutinesVersion%'
   }
   ```

   </tab>
   </tabs>

   For other build systems, see instructions in the [`kotlinx.coroutines` README](https://github.com/Kotlin/kotlinx.coroutines#using-in-your-projects).

3. Open the `Main.kt` file in `src/main/kotlin`.

    The `src` directory contains Kotlin source files and resources. The `Main.kt` file contains sample code that will print `Hello World!`.

4. Create the `simple()` function that returns a flow of three numbers:

    * Use the [`delay()`](https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/delay.html) function to imitate CPU-consuming blocking code. It suspends the coroutine for 100 ms without blocking the thread.
    * Produce the values in the `for` loop using the [`emit()`](https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.flow/-flow-collector/emit.html) function.

    ```kotlin
    import kotlinx.coroutines.*
    import kotlinx.coroutines.flow.*
    import kotlin.system.*

    fun simple(): Flow<Int> = flow {
        for (i in 1..3) {
            delay(100)
            emit(i)
        }
    }
    ```

5. Change the code in the `main()` function:

    * Use the [`runBlocking()`](https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/run-blocking.html) block to wrap a coroutine.
    * Collect the emitted values using the [`collect()`](https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.flow/collect.html) function.
    * Use the [`delay()`](https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/delay.html) function to imitate CPU-consuming code. It suspends the coroutine for 300 ms without blocking the thread.
    * Print the collected value from the flow using the [`println()`](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.io/println.html) function.

    ```kotlin
    fun main() = runBlocking {
        simple()
            .collect { value ->
                delay(300)
                println(value)
            }
    }
    ```

6. Build the code by clicking **Build Project**.

    ![Build an application](flow-build-project.png)

## Debug the coroutine

1. Set a breakpoint at the line where the `emit()` function is called:

    ![Build a console application](flow-breakpoint.png)

2. Run the code in debug mode by clicking **Debug** next to the run configuration at the top of the screen.

    ![Build a console application](flow-debug-project.png)

    The **Debug** tool window appears:
    * The **Frames** tab contains the call stack.
    * The **Variables** tab contains variables in the current context. It tells us that the flow is emitting the first value.
    * The **Coroutines** tab contains information on running or suspended coroutines.

    ![Debug the coroutine](flow-debug-1.png)

3. Resume the debugger session by clicking **Resume Program** in the **Debug** tool window. The program stops at the same breakpoint.

    ![Debug the coroutine](flow-resume-debug.png)

    Now the flow emits the second value.

    ![Debug the coroutine](flow-debug-2.png)

### Optimized-out variables

If you use `suspend` functions, in the debugger, you might see the "was optimized out" text next to a variable's name:

![Variable "a" was optimized out](variable-optimised-out.png)

This text means that the variable's lifetime was decreased, and the variable doesn't exist anymore.
It is difficult to debug code with optimized variables because you don't see their values.
You can disable this behavior with the `-Xdebug` compiler option.

> __Never use this flag in production__: `-Xdebug` can [cause memory leaks](https://youtrack.jetbrains.com/issue/KT-48678/Coroutine-debugger-disable-was-optimised-out-compiler-feature#focus=Comments-27-6015585.0-0).
>
{type="warning"}

## Add a concurrently running coroutine

1. Open the `Main.kt` file in `src/main/kotlin`.

2. Enhance the code to run the emitter and collector concurrently:

    * Add a call to the [`buffer()`](https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.flow/buffer.html) function to run the emitter and collector concurrently. `buffer()` stores emitted values and runs the flow collector in a separate coroutine.

    ```kotlin
    fun main() = runBlocking<Unit> {
        simple()
            .buffer()
            .collect { value ->
                delay(300)
                println(value)
            }
    }
    ```

3. Build the code by clicking **Build Project**.

## Debug a Kotlin flow with two coroutines

1. Set a new breakpoint at `println(value)`.

2. Run the code in debug mode by clicking **Debug** next to the run configuration at the top of the screen.

    ![Build a console application](flow-debug-3.png)

    The **Debug** tool window appears.

    In the **Coroutines** tab, you can see that there are two coroutines running concurrently. The flow collector and emitter run in separate coroutines because of the `buffer()` function.
    The `buffer()` function buffers emitted values from the flow.
    The emitter coroutine has the **RUNNING** status, and the collector coroutine has the **SUSPENDED** status.

3. Resume the debugger session by clicking **Resume Program** in the **Debug** tool window.

    ![Debugging coroutines](flow-debug-4.png)

    Now the collector coroutine has the **RUNNING** status, while the emitter coroutine has the **SUSPENDED** status.

    You can dig deeper into each coroutine to debug your code.