How I made my own LiveData!

When I first used LiveData, I was just amazed by it.
Now I can get a callback whenever anyone changes the LiveData.
Also, the callback will only be received only if the UI is in an active state.

This is extremely useful for an app that shows UI based on a data model.
Using LiveData, I can observe the data model and update the UI whenever its value changes.

So, basically…

All this was so interesting that I decided to make my very own LiveData.
In this article, I'll show you how I made my own LiveData, what was my approach, and what were my learnings.

Let's start with the data holder part first.

Here's a basic implementation of LiveData that will hold/wrap our data.

Now, it's time to make our LiveData observable.

According to this definition, our LiveData is the subject.
So our LiveData should maintain a list of observers and all these observers should be notified whenever the value of our LiveData changes.

Here's the implementation of LiveData that is observable.

As you can see, now our LiveData contains an ArrayList of the observers that are attached to it.

For example, if we make a LiveData of an integer like this…

Then our observer's lambda expression would be like this…

To add and remove these observers from the list we have two methods observe and removeObserver respectively. Both of these methods take the observer as an argument of function type (T?) -> Unit.

The main magic is in the setValue method.

Here, after setting the value we are looping through all the observers that are attached and invoking them with the updated value. This way all the observers will receive the updated value and they can handle the new value appropriately.

Now, our LiveData is ready to be observed.

This is how we can use it in our apps…

But! This version of LiveData has some problems.

Basically, our LiveData is not lifecycle aware.

Now, it’s time to make our LiveData lifecycle aware.

So by passing the LifecycleOwner to our LiveData, we can easily know and observe the lifecycle of the observer.

Here’s the implementation of LiveData which accepts LifecycleOwner and only notifies the observers only if they are in an active state.

As you can see, now our LiveData contains a HashMap of observers and owners associated with them.

Also when looping through all the observers, we are checking its owner's current state and invoking them only if they are in an active state.

Now, our observers will receive the updated value only if they are in an active state.

But! What about removing observers from the LiveData when their UI components are destroyed? And what about passing the current value to the observers immediately after they are attached to the LiveData?

For that, we need to monitor the lifecycle associated with the observers.

Then this LifecycleObserver can be attached to a LifecycleOwner like this…

Here’s the implementation of LiveData which is lifecycle aware.

As you can see, now the observe method is initializing and attaching the LiveDataLifecycleObserver to the owner.

And, the LiveDataLifecycleObserver is having 3 annotated methods.

onStarted and onResumed will notify the observer with the updated value whenever the UI component comes to an active state.

onDestroyed will remove the observer from the LiveData once the UI component is destroyed.

Now, our LiveData is finally complete and ready to use!

Making my own LiveData was a fun and interesting exercise.
I hope this article helped you in understanding the internals of LiveData.

The full source code can be found here!

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