android - Kotlin coroutines `runBlocking`

Question

I am learning Kotlin coroutines. I've read that runBlocking is the way to bridge synchronous and asynchronous code. But what is the performance gain if the runBlocking stops the UI thread? For example, I need to query a database in Android:

    val result: Int
    get() = runBlocking { queryDatabase().await() }

private fun queryDatabase(): Deferred<Int> {
    return async {
        var cursor: Cursor? = null
        var queryResult: Int = 0
        val sqlQuery = "SELECT COUNT(ID) FROM TABLE..."
        try {
            cursor = getHelper().readableDatabase.query(sqlQuery)
            cursor?.moveToFirst()
            queryResult = cursor?.getInt(0) ?: 0
        } catch (e: Exception) {
            Log.e(TAG, e.localizedMessage)
        } finally {
            cursor?.close()
        }
        return@async queryResult
    }
}

Querying the database would stop the main thread, so it seems that it would take the same amount of time as synchronous code? Please correct me if I am missing something.

Answer 1

runBlocking is the way to bridge synchronous and asynchronous code

I keep bumping into this phrase and it's very misleading.

runBlocking is almost never a tool you use in production. It undoes the asynchronous, non-blocking nature of coroutines. You can use it if you happen to already have some coroutine-based code that you want to use in a context where coroutines provide no value: in blocking calls. One typical use is JUnit testing, where the test method must just sit and wait for the coroutine to complete.

You can also use it while playing around with coroutines, inside your main method.

The misuse of runBlocking has become so widespread that the Kotlin team actually tried to add a fail-fast check which would immediately crash your code if you call it on the UI thread. By the time they did this, it was already breaking so much code that they had to remove it.