But what about handling those navigation-related actions? If they are handled by controllers, all of them will need to do it - that's potentially tons of code duplication and just a waste of time. With router this issue simply doesn't exist - we can just reuse it along with some common Handler. View model should not be the data source, but it should rather expose data needed for cell configuration - this is especially useful for testing complicated tableViews and collectionViews.
You should also create data sources as separate objects - they can be easily reused in future and there is no harm in doing this right away. Data that is being used to populate certain view, for example UserTableViewCell, should be wrapped into a single structure - like UserCellConfiguration.
This structure is just a thin layer between actual data and its transformation that is used to fill all the labels, imageViews and so on. It makes the distinction between actual model and view easier.
Use dependency injection - it enables usage of mock objects in your unit tests, making them easy to write. View model should not import UIKit - this isn't something that is crucial, but if you keep this in mind, it will help you with keeping the separation between UI and logic layer.
Apart from introducing routers, I have also been using one more component in my app - ControllersBuilder. ControllersBuilder is just a simple structure capable of creating all controllers in the entire application. The important thing to note here - it's better to have builder methods return plain UIViewControllers, rather than an instance of the specific subclass. Same goes for router - it is better to have it hold a reference to UIViewController.
It is not mandatory by any means, however, this is the way I have been doing it so far and it helps me with keeping those objects as dumb as possible. One of the biggest benefits of router and DI introduction is how it improves unit tests.
I would like to show you how does one of them look like in my project:. Router is just a protocol injected to view model and thanks to the dependency injection we can also mock service responses, making our tests extremely easy to write and read - test above is not an exception, it is just a part of MVVM-R's reality.
This is simply brilliant - thanks to DI and ControllersBuilder, we can snapshot test every single screen in app, in just a few lines of code. I have been using this architecture to a great extent in production-ready application for the past five months and I am really satisfied with how it improved the quality of my work:.
Ideally View will only contain subscriptions to LiveData. Compared to MVP it can significantly decrease logic contained in View. However, any newer pattern needs some time for trial and error and finding the best approach in your project. Avoid those mistakes that we made to avoid the headache of refactoring of your freshly created project. You can try avoiding using RX libraries and return LiveData objects from the data source and use Transformations in the Interactor layer.
Happy coding! Sign in. Submit Archive About Events droidcon. Anastasia Finogenova Follow. Thanks to Simon Percic. Android Engineer Facebook.
ProAndroidDev Follow. Written by Anastasia Finogenova Follow. More From Medium. Understanding Scoped Storage in Android. Sanjay Prajapati in Mindful Engineering. Carlos Daniel. SingleLiveEvent to help you work with LiveData and events.
Carlos Daniel in ProAndroidDev. John Papa because it's so much more focused with less cruft. We have really enjoyed using MVVM light on my current project. Jeremy - thanks, that's very helpful - it's the latter - do something, then fire a command. What you suggest is quite close to what I have ended up with. I'll have to revisit. Rui - I assume you mean you are using an event aggregator? You wouldn't want the view to know the concrete type of the ViewModel would you?
I like how it is lightweight. As you say, it would be nice to have a few MVVM helper behaviours baked into the framework to stop us all reinventing the wheel badly.
I too love the elegance of it, but it's difficult to get others on-board when I haven't really used simpler methods like direct text binding. To be honest, despite the fact that testability is big in the industry, I haven't really worked in many places where they didn't see it to be a waste of time.
But the big selling point for me is dividing the code into manageable areas. The xaml and code behind only worries about user-interaction while the viewmodel is concerned with building collections, communicating with the host, etc. I think the interesting part of your question is whether it's a pain or not for small straight forward applications.
Gustavo - good point. Over time, I have no doubt that I will build up enough useful helper classes, techniques and behaviours to make MVVM really quick and productive for small projects. But it does take a while to get to that point. The only problem I see with using MVVM on large-scale projects is that on the user control level it becomes burdensome to maintain the pattern, which means collapsing the separation between business logic ViewModel and UI View.
We've gone back and forth between keeping "control-specific" business logic in the ViewModel and having a lot of repeated code, and putting said logic in the usercontrol itself. There is also a particular issue with the DataGrid control in Silverlight, in that none of the properties on the DataGridColumns most importantly, Header and Visibility can be bound, meaning that the ViewModel has to interact directly with the View if any of these properties need to be dynamic.
Also, like Jeremy said, we've been able to have a designer work in parallel with development without any conflicts for the most part. I think most people are taking the "ViewModel" name to literally. I tend to see the VM more as the glue between the view and the model whereas the VM only has an abstracted knowledge of the view if any. Usually the pattern we use is to have a presenter as the datacontext which then exposes the view models for data binding. If the presenter needs to interact with the view then the view becomes an interface which gets passed into the presenter.
Also published on Medium. Email: jaim sharpfivesoftware. A bit of background Once a fading pattern used in. Extract ViewModel???? Good code MVVM gives the illusion of improving your architecture without actually improving anything.
The real problem The real problem is Poorly Defined Classes. The prime example in iOS development is Controllers Soroush Khanlou does a good writeup of why they are bad here. Some other types of classes that exist in many apps are Validator StringFormatter NetworkClient Repository These are common operations. Let the business needs drive your architecture While most apps reuse many common components, each app IS a special snowflake.
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