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Quick introduction to Kotlin coroutines part 2

If you have not read part one of this series, please read it first, it explains the main idea behind Kotlin coroutines.

Error handling

If you read the part one of this series, you may notice that error handling was not covered as a topic at all, so in this post, we will go through techniques you can utilize to handle errors that may occur inside coroutines.


The main idea introduced in part one of this series is the idea of async code written as a synchronous code and if you think about it, how would you handle errors in a sync style code, you catch exceptions. Same idea here, we can surround code that throws an error inside try/catch block, and handle error states in your async code. Let’s see an example of a retrofit network call.
As we can see in the example above, we surrounded the retrofit method execute with try and catch blocks, and two exceptions are caught that execute method can throw, IOException and RuntimeException. Why just these two exceptions? If you look at the documentation for execute method, you can see that execute method can throw only those two exceptions. In case an exception is thrown, the user will be notified, and that part of the code runs on the main thread.
A better way would be to pass in an exception handler that will be called once the launch coroutine encounters and exception.

Async vs launch builder

In part 1, we mainly used launch coroutine builder as our main coroutine block, and one problem with launch builder is the exception handling, if an exception is not caught inside a coroutine, the whole app will crash. Kotlin provides another coroutine builder, Async builder, which behaves differently in few ways. One way in which it differs is exception handling, async block just ignores thrown exceptions if they are not caught resulting in a non app crash scenario. Here is a way to catch an exception with async block


What if you need to cancel a coroutine, for example, the android activity gets paused or destroyed, and the coroutine result is no longer needed?


All coroutine builders return a job, which can be canceled at any point by calling job.cancel(). What is interesting with Kotlin coroutines is the ability to specify a parent job as a context for multiple coroutines, and calling cancel() on the parent coroutine will result in all coroutines being canceled, very convinient. In all examples above you may notice the parentJob passed in in every coroutine, and calling parentJob.cancel() inside onPause() activity method results in all coroutines being cancelled.

Hooking into callback code

The last thing I want to talk about is hooking into code that is async but uses callbacks to handle the suspension of the flow. For example, firebase database uses callbacks to inform the caller an operation has completed. In that case, you need to bridge between a Kotlin coroutine and a callback listener. SuspendCoroutine to the rescue! A special coroutine which will be suspended by default, and you call resume on it inside the callback method. Below is an example from one of my apps that uses google firebase as a backend.
This getBooleanValue can be used inside any coroutine and will behave as any other suspend function, hence creating a bridge between Kotlin coroutine and callback.


I have created a sample app that uses all exception handling cases shown in this blog post on this Github repository.
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blog kotlin

Quick introduction to Kotlin coroutines part 1


So first, let me start this article by saying that I spent few years developing Android apps in Java, and the one thing that I used to do frequently was to separate layers of abstraction with interfaces. And for example in the networking layer, I end up writing lots of interfaces that declare methods, with a listener parameter, and this listener will implement some onSuccess and onFail methods. What this leads to is lots of nested callbacks where I go few layers deep into callbacks and onSuccess and onFail blocks, something like the code below.
Then Kotlin came along, got supported by Google officially and suddenly, everyone went mad, and for a good reason, it is a modern swift like, compiled, language, which is a huge step up from the Java world. And one thing caught my eye when I first read through the Kotlin documentation, Kotlin coroutines. I thought, well this for me, the one thing I can use right now to avoid my callback hell problem. So I jumped straight into it. Here is my journey to understanding what Kotlin coroutines are.

Asynchronous code as a synchronous code

The header above is the essence of what coroutines are, it is a way to declare a block of code in which every line suspends execution of the block, what does that mean? Well, you simply don’t have to provide a callback for long-running functions, you just add that function to a coroutine and the coroutine will suspend its execution until that method returns hence no more callbacks!
Look at the code below:
This is my interface for the data storage contract in Kotlin, no listener interface needed, you will notice the suspend keyword in front of all methods, that is a special Kotlin syntax to indicate this function should be run from a coroutine block or another suspend function, basically anything that involves a network call or a CPU intensive task should be placed inside suspend functions and then consumed from a coroutine block.

Coroutine builders

If we want to call our suspend function, we need to pass it to the coroutine builder, which will do the heavy lifting for us, and ensure that the code inside that builder will run on a thread of our choice, for example, let’s use launch coroutine builder:
That’s it, you simply consume your suspend function inside the launch block and you are good to go.  Every time a suspend function is encountered, the execution of this coroutine block suspends, waiting for the function to return. Any code above our getStringValue method call will be executed first, then our function, then the rest of code below, no callbacks! So on which thread will the above launch block execute? Obviously, we want it to execute on a background thread and we want to update the UI with values from our suspend functions.

Coroutine context

Every coroutine builder can have a context in which to run, the main thread, or some other thread, if no context is specified, like the example above, it’s the CommonPool of threads, meaning it will run on a separate thread from the main thread of the app. So if the code above is running in the background thread, how to update the UI then? To update the UI there is one more step that needs to be done, and its another launch block with a coroutine context specified, like the example below:
Everything inside the second launch will use the main thread event dispatcher, that will post our update on the main thread. What is cool about this is that the second launch block will also wait for the code above to execute and once it does execute, the value variable will be set and we can safely update our UI. Coroutine context is simply a coroutine dispatcher, which controls on what thread the coroutine block is running.

Coroutines are lightweight threads

Coroutines run on threads and can suspend its execution without blocking the thread it runs on, so the thread execution is not tied to the coroutine execution, they operate separately. If you think about it, callbacks are almost the same thing, they suspend execution and continue once the result is available, but have a boilerplate syntax and require you to repeat yourself everytime you want to suspend an execution.

Launch vs Run coroutine builder

One problem with nesting coroutines is that every coroutine block will have its own execution line, meaning the second coroutine block will not suspend the parent coroutine. There is a better way of writing the code above, since getStringValue  function returns a value, we can use a run coroutine builder to return a value from a suspend function, hence we can start on the main thread, and invoke our getStringValue function inside a background thread, the other way around from the example above.
The main difference between launch and run coroutine builder is the return value, launch has none while run has, so in this case, we will assign our variable value to whatever the result value of the run block is.
I specified a dispatcher for run block intentionally since nested coroutines take the parent coroutine context if not specified, and we do not want that in this case, we want the run block to execute on a background thread.
I believe this is all you need to know to start playing around with coroutines, don’t hesitate to open android studio and write some coroutines!

Further reading

Coroutines can do much more if you need it, there are many different coroutine builders, channels, synchronization options etc. The best introduction to coroutines is the official Kotlin coroutines guide on Github, don’t hesitate to read it, you will find lots of gems in there.


To use coroutines in your project, you need a couple of additions to your build grade file, mainly these two lines:
The coroutines-android dependency is for the special UI context that can be passed to coroutine builders that uses the main thread dispatcher to dispatch events on the main thread.
Then you have to tell Gradle that we will use coroutines in the app by adding these lines to the app build Gradle file:

Github sample project: Fibonacci counter

I wrote a simple Android app in Kotlin, that implements a Fibonacci counter with coroutines. Check it out on Github, clone the repository and play with it and most importantly have fun!


This is a part one of two-part series, check out part two of the series.