Can you provide examples of how constructor injection is used in MvvmCross ViewModels

1. Basic Constructor Injection

In MvvmCross, when you create a ViewModel, you can define its constructor to accept interfaces or services that will be injected by the IoC container. For example:

csharp
public class MyViewModel : MvxViewModel
{
    private readonly IMvxJsonConverter _jsonConverter;
    private readonly IMvxGeoLocationWatcher _locationWatcher;

    public MyViewModel(IMvxJsonConverter jsonConverter, IMvxGeoLocationWatcher locationWatcher)
    {
        _jsonConverter = jsonConverter;
        _locationWatcher = locationWatcher;
    }

    // ViewModel logic here
}

In this example, `IMvxJsonConverter` and `IMvxGeoLocationWatcher` are interfaces that need to be registered with concrete implementations in the MvvmCross IoC container. When `MyViewModel` is instantiated, MvvmCross will automatically resolve these dependencies using the registered implementations.

2. Registering Dependencies

To use constructor injection, you must register the interfaces with their concrete implementations in the IoC container. This is typically done in the `Setup.cs` file of your MvvmCross application:

csharp
public class Setup : MvxSetup
{
    public override void InitializeFirstChance()
    {
        Mvx.IoCProvider.RegisterType();
        Mvx.IoCProvider.RegisterType();
    }
}

Here, `JsonConverter` and `GeoLocationWatcher` are concrete classes implementing `IMvxJsonConverter` and `IMvxGeoLocationWatcher`, respectively.

3. Platform-Specific Implementations

One of the powerful features of MvvmCross is its ability to handle platform-specific implementations. For example, you might have different implementations of `IMvxGeoLocationWatcher` for iOS and Android. MvvmCross allows you to register these platform-specific implementations, which will be resolved based on the current platform:

csharp
// In iOS-specific setup
Mvx.IoCProvider.RegisterType();

// In Android-specific setup
Mvx.IoCProvider.RegisterType();

4. Chaining Dependencies

Constructor injection also supports chaining dependencies, where a service depends on other services. For instance:

csharp
public interface ITaxCalculator
{
    double TaxDueFor(int customerId);
}

public class TaxCalculator : ITaxCalculator
{
    private readonly ICustomerRepository _customerRepository;
    private readonly IForeignExchange _foreignExchange;
    private readonly ITaxRuleList _taxRuleList;

    public TaxCalculator(ICustomerRepository customerRepository, IForeignExchange foreignExchange, ITaxRuleList taxRuleList)
    {
        _customerRepository = customerRepository;
        _foreignExchange = foreignExchange;
        _taxRuleList = taxRuleList;
    }

    public double TaxDueFor(int customerId)
    {
        // Implementation
    }
}

You would register `TaxCalculator` as follows:

csharp
Mvx.IoCProvider.RegisterType();

When resolving `ITaxCalculator`, MvvmCross will automatically resolve its dependencies (`ICustomerRepository`, `IForeignExchange`, and `ITaxRuleList`) if they are registered.

5. ViewModel Lifecycle and Navigation Parameters

In MvvmCross, navigation parameters are passed to the `Init` method of the ViewModel rather than the constructor. This allows the constructor to be used solely for dependency injection:

csharp
public class DetailViewModel : MvxViewModel
{
    private readonly IMvxJsonConverter _jsonConverter;

    public DetailViewModel(IMvxJsonConverter jsonConverter)
    {
        _jsonConverter = jsonConverter;
    }

    public void Init(string first, string second, int answer)
    {
        // Handle navigation parameters here
    }
}

You can navigate to this ViewModel with parameters like this:

csharp
ShowViewModel(new { First = "Hello", Second = "World", Answer = 42 });

This approach keeps the constructor clean for dependency injection while allowing flexible handling of navigation parameters.