V3 - a new ViewModel lifecycle

One of the changes coming in MvvmCross v3 - Hot Tuna - is that the default ViewModel location and construction has been overhauled to provide 3 new features:

• constructor based Dependency Injection
• navigation using Typed navigation classes
• saving and reloading VM state for 'tombstoning'

I'm sorry that these changes are breaking changes for existing v1 and vNext apps.

However, I hope we'll all find these changes very useful in the future.


How ViewModels are Created in v3

The default ViewModelLocator in v3 builds new ViewModel instances using a 4-step process - CIRS:

1. Construction - using IoC for Dependency Injection
2. Init() - initialisation of navigation parameters
3. ReloadState() - rehydration after tombstoning
4. Start() - called when initialisation and rehydration are complete

Construction

As in vNext, you navigate to a ViewModel using arguments like:



In vNext, these navigation parameters were passed to the constructor of the ViewModel.

However, in v3, these navigation parameters are instead passed to the Init() method, and the constructor is now free to be used for Dependency Injection.

This means that, for example, a DetailViewModel constructor might now look like:


This Dependency Injection is, of course, optional - you code can instead continue to use ServiceLocation if you prefer:



Init()

Now that the construction is used for Dependency Injection, the navigation parameters move to a new method - Init()

Init() will always be called after construction and before ReloadState() and Start()

Init() can come in several flavors:.

• individual simply-Typed parameters
• a single Typed parameter object with simply-Typed properties
• as InitFromBundle() with an IMvxBundle parameter - this last flavor is always supported via the IMvxViewModel interface.

You can declare zero or more of each of these types, but generally you will probably only want to use one within your application

So, for example, to support the navigation:


you could implement any of:


or:



or:



Note that multiple calls can be used together if required. This allows for some separation of logic in your code:




ReloadState

If the View/ViewModel is recovering from a Tombstoned state, then ReloadState will be called with the data needed for rehydration.

If there is no saved state then no ReloadState() methods will be called. 

Exactly as with Init(), ReloadState can be called in several different ways.

• individual simply-Typed parameters
• a single Typed parameter object with simply-Typed properties
• as ReloadStateFromBundle() using an IMvxBundle parameter - this last flavor is always supported via the IMvxViewModel interface.

Normally, I'd expect this to be called as:



Aside: where does the SavedState come from?

One of the new ViewModel APIs available in Hot Tuna is a SaveState pattern.

This can be implemented in one of two ways:
- using one or more paremeterless methods that return Typed state objects
- using the override SavedStateToBundle(IMvxBundle bundle)

Using a Typed state object:


Using SavedStateToBundle:




Start()

After all of Construction, Init, and ReloadState is complete, then the Start() method will be called.

This method is simply:





Putting it all together

For a real app, I would expect the navigation, construction and state saving/loading code to actually look like:


and




Oh yuk - I hate it - this CIRS stuff is not for me....

If you don't like this CIRS flow for building your ViewModels, then the good news is that you can easily override the ViewModelLocator within v3, just as you could within vNext :)

You can easily go back to vNext style ViewModel construction - or you can easily invent something new.


Oh yes - I love it - when can I use it?

Soon... very soon ... 

... well, just as soon as I work out some of this MonoTouch->Xamarin.iOS pain :/