There are lots of definitions of the word Model. We will follow this description: domain model.
In practice, model classes in MVO will mostly contain data and/or logic, they will very likely pass off tasks like network or database access to sub layers, making good use of composition. They typically employ dependency injection techniques to do that, preferably via their constructors. In MVO, the state is located in these models and accessible via quick returning getter methods or properties (usually called by thin views), it’s the state of these models that is the main focus for tests.
When the model’s state changes, it’s the model’s responsibility to call notifyObservers() after that change, this will inform any observers that it’s now time to re-query the model for fresh data.
“It’s the model’s responsibility to call notifyObservers()”
An important thing about these models is that none of the code should know anything about View layer classes. The models are concerned with their data, their logic and their state, and that is all. They don’t know or care what interrogates their state via their getter methods - and this makes our Models extremely easy to test.
“The models are concerned with their data, their logic and their state, and that is all”
In the sample apps, the models are all found in the feature package, in a clean architecture app they will typically live in the domain module.
Here’s the code for a very simple model which represents a user’s Wallet
Writing a Basic Model
If you write a good model, using it in the rest of your app should be a piece of cake.
You’ll see that in all the sample apps, the models have been written with the assumption that all the methods are being accessed on a single thread (which for a live app would be the UI thread). Not having to worry about thread safety here is a very big win in terms of code complexity. The models can use threads and coroutines internally of course.
If you need to hop onto another thread for IO or any heavy processing, do it explicitly with something like an AsyncBuilder or launch a coroutine for example, and then pop back on to the UI thread when you are ready to update your state.
pop back on to the UI thread when you are ready to update your state
Check out a [few] [examples] from the sample apps, or if you’re already comfortable writing model code (most of this advice is generic and applies to writing ViewModels too, so it’s all fairly obvious) feel free to skim over the checklist below for a refresher and you should be good to go.
For reference here’s a check list of recommendations for the model classes, as used in fore. Once you’ve had a go at writing one you can come back here to double check you have everything down:
- The model classes should know nothing about android lifecycle methods or View classes, fragments, Composables, or specific Activities
- In fact the less the models knows about Android the better
- Avoid referencing Contexts from your model if you can, although sometimes the design of Android makes this awkward
- Prefer referencing Application over Context or Activity if you have a choice, as that reduces the chance of a memory leak
- Where it’s reasonable, prefer the observer pattern over callbacks / listeners
- In MVO, the model’s current state at any point in time is typically exposed by getters or properties. These are used by the View classes to ensure they are displaying the correct data, and by the test classes to ensure the model is calculating its state correctly.
- The getters must return quickly. Don’t do any complicated processing here, just return data that the model should already have. i.e. front load the processing and do work in setter type methods if necessary, not the getters. You’ll find similar advice applies when using a Compose based UI - the composables must be able to complete quickly.
- You’ll save yourself the need to write a lot of tricky thread safety code by keeping access to your models on the UI thread (or test thread), user interaction like clicks will all come through on the UI thread anyway, even in Compose. Where you need asynchronous operation for db or network access etc, make it explicit and obvious, then return to the UI thread once you are done.
- When any data in your model changes, inside the model code call notifyObservers() after the state has changed.
- The models should make good use of dependency injection (via constructor arguments ideally). Any dependency that is not being injected will be difficult to mock for a test.
- Written in this way, the models will already be testable but it’s worth highlighting testability as a specific goal. The ability to thoroughly test model logic is a key part of reducing unecessary app bugs.
- (For models written in Java specifically), it’s very useful to immediately crash in your model constructor if any caller tries to send you null objects. Your constructor is your public interface and could be used by anyone. You can help other developers out by immediately crashing here rather than sometime later, when the cause might not be so obvious. In the sample Java apps, this is done with the Affirm.notNull() call.