What kind of change does this PR introduce?

Fixes #2537
Allow registering custom services before calling InitializeReactiveUI by adding HasRegistration check.

This is mostly to allow cleaner substitution of custom services when using dependency inversion. You wouldn't need to unregister or have a redundant service registration anymore.

What is the current behavior?

Registers a service, without checking if a registration already exists.

What is the new behavior?

Now it checks, if a service with a certain type is already registered.

With this approach, one would register a custom service before the initialization call (like you currently do to override e.g. ILogger in InitializeSplat). Autofac DI is used in the example:

// Register your overrides first
builder.RegisterInstance(new NullLogger()).As<ILogger>();
builder.RegisterType<MyCustomViewLocator>().As<IViewLocator>().SingleInstance();
// etc.

// Since I am using AutofacDependencyResolver, I create it at this point
var autofacResolver = builder.UseAutofacDependencyResolver();
// Register the resolver in Autofac so it can be later resolved
builder.RegisterInstance(autofacResolver);

// Then call InitializeReactiveUI, which would ignore already registered IViewLocator
autofacResolver.InitializeReactiveUI();

This means there aren't any redundant registrations, and no potential unexpected side-effects from that.

What might this PR break?
Nothing should brake.

Please check if the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information: