Canceling a Coroutine Simplified ❌

Just like making a function main safe, you are also responsible for making it cancelable. It won’t do it itself.

In lifecycle dependent environment like Android, you should try to make each suspend function cancelable. Because any job can be cancelled at any time.

Why is that important?

Allow me to explain via an example, suppose you wrote a nice-shinny main-safe function that compresses the image to show it to user.

Take this code snippet as an example:

class BitmapCompressor(
    private val context: Context
) {

    suspend fun compressImage(
        contentUri: Uri,
        compressionThreshold: Long
    ): Bitmap? {
        return withContext(Dispatchers.IO) {
            val inputBytes = context
                .contentResolver
                .openInputStream(contentUri)?.use { inputStream ->
                    inputStream.readBytes()
                } ?: return@withContext null


            withContext(Dispatchers.Default) {
                val bitmap = BitmapFactory.decodeByteArray(inputBytes, 0, inputBytes.size)


                var outputBytes: ByteArray
                var quality = 100
                do {
                    ByteArrayOutputStream().use { outputStream ->
                        bitmap.compress(Bitmap.CompressFormat.JPEG, quality, outputStream)
                        outputBytes = outputStream.toByteArray()
                        quality -= (quality * 0.1).roundToInt()
                    }
                } while (outputBytes.size > compressionThreshold && quality > 5)


                BitmapFactory.decodeByteArray(outputBytes, 0, outputBytes.size).also{
                  println("Compression Finished")
                }
            }
        }
    }
}

Don’t get overwhelmed, it’s just a function that compresses the image to a provided compressionThreshold

Now, suppose user opened the screen, which had a viewModel which triggered this function, so that a compressed image can be shown.

Before the function could finish compressing the image, user navigated back, at this point, there is no use of the compressed image. So ideally the function should cancel it’s execution.

But if you actually implement it, you will see it won’t cancel itself, it will execute all the way to last statement which prints “Compression Finished”

Don’t believe me?

Check out this manual cancelation

class MainActivity(): AppCompactActivity(){

  override onCreate(..){
  //some other stuff

    lifecycleScope.launch{
        val job = launch{
          BitmapCompressor(this@MainActivity).compressImage(
          //pass URI of the image & threshold
          )
          println("Inner Job Finished")
        }

        delay(1000)
        println("Canceling the job")
        job.cancel()

    }


  }


}

The compression takes around 5 seconds, on my testing device when provided a good size image. And in code, I am canceling the coroutine after one second.
This is my output:

Canceling the job
Compression Finished

It did not print Canceling the job because the job was canceled BUT strangely enough, it printed Compression Finished

BUT WHY??

Check it out here