Skip to main content

How to produce the SavedStateHandle in your ViewModel using AssistedInject


One of my previous article Dagger Multibinding Saved My Time "claimed" to have found the right practice for providing ViewModels without producing a ViewModel Factory for each ViewModel. Apparently, I was wrong.  There are 2 main problems with that approach:

1 - I might forget to add a ViewModel in the Map graph.
2 - With the new SavedStateHandle which stays uniqely in each ViewModel I can't use a generic ViewModel Factory.  (you can check the implementation in the article provided above).

So,  what does AssistedInject do to help solve this case?

Check this example below:


Dagger doesn't know how to create  a SavedStateHandle. And since the SavedStateHandle cannot be used as a Singleton, we want a different state for each of our fragments. In order to achieve this, let's allow the AssistedInject library create a unique Factory for us. It might require a little more work on configurations, but after the set up, it's just kindergarten game for you.

Instead of  annotating constructor with @Inject , we annotate it with @AssistedInject:


The constructor parameters which cannot be evaluated by Dagger, should be marked with @Assisted. Now, we should create the Factory. With AssistedInject it's pretty easy. You just define an interface and annotate it with the @AssistedInject.Factory annotation and it is going to have a method with parameters that Dagger doesn't know how to create, and will return the current ViewModel:


Perhaps someone would ask why not having different scopes for each ViewModel in order to provide that unknown parameter uniquely in each ViewModel? In that case, you will have a component for each fragment, a module and you should not forget to perform DI in all creation state of each fragment. So the short answer is: Don't do it.

One more step. We need to somehow connect the strings here. How does Dagger provide dependencies that don't know how to create? It will ask the component if there is any of the modules providing the dependencies. But there isn't any module, or is there 👀 ? Yes, the AssistedInject has another small step to conclude:


In this way, the our ViewModel Factories will be auto generated. We just have to call them from the Singleton component:


That's it.

Since the SavedStateViewModel Factory is now the default ViewModel Factory, to provide a SavedStateHandle, we can't extend our ViewModelProvider.Factory. Instead, we need to extend our AbstractSavedStateViewModelFactory:


And with the help of Kotlins' delegation, we achieve something like this in our fragment:


And that's it.

Credits:
Zhuinden
It's complicated, but it doesn't have to be: a Dagger journey by Fred Porciúncula, Blinkist EN

Popular posts from this blog

Modularizing your Android app, breaking the monolith (Part 1)

Inspired by a Martin Fowlers post about Micro Frontends, I decided to break my monolithic app into a modular app. I tried to read a little more about breaking monolithic apps in Android, and as far as I got, I felt confident to share my experience with you. This will be some series of blog posts where we actually try to break a simple app into a modularized Android app.

Note: You should know that I am no expert in this, so if there are false statements or mistakes please feel free to criticize, for the sake of a better development. 

What do you benefit from this approach:
Well, people are moving pretty fast nowadays and delivery is required faster and faster. So, in order to achieve this, modularising Android apps is really necessary.You can share features across different apps. Independent teams and less problems per each.Conditional features update.Quicker debugging and fixing.A feature delay doesn't delay the whole app. As per writing tests, there is not too much difference about…

What I learned from Kotlin Flow API

I used to check the docs and just read a lot about flows but didn't implement anything until yesterday. However, the API tasted really cool (even though some operations are still in Experimental state).Prerequisites: If you don't know RxJava it's fine. But a RxJava recognizer would read this faster.Cold vs Hot streamsWell, I really struggled with this concept because it is a little bit tricky. The main difference between cold and hot happened to be pretty simple: Hot streams produce when you don't care while in cold streams, if you don't collect() (or RxJava-s equivalent subscribe()) the stream won't be activated at all. So, Flows are what we call cold streams. Removing the subscriber will not produce data at all, making the Flows one of the most sophisticated asynchronous stream API ever (in the JVM world). I tried to make a illustration of hot and cold streams: Since I mentioned the word asynchronous this implies that they do support coroutines also. Flows vs…

From Gson to Moshi, what I learned

There is no doubt that people are getting away from GSON and I agree with those reasons too. The only advantage GSON has over other parsing libraries is that it takes a really short amount of time to set up. Furthermore, the most important thing is that Moshi is embracing Kotlin support.

First let's implement the dependency:
implementation("com.squareup.moshi:moshi:1.8.0") It's not a struggle to migrate to Moshi. It's really Gson look-a-like. The only thing to do is annotate the object with @field:Json instead of @SerializedName (which is Gsons way for JS representation):

data class User( //GSON way @SerializedName("name") val name: String, @SerializedName("user_name") val userName: String, @SerializedName("last_name") val lastName: String, @SerializedName("email") val email: String ) data class User( //Moshi way @field:Json(name = "name") val name: String, @field:Json(name = "user_name…