What are some common scenarios where custom JSON converters are necessary in MvvmCross

1. Handling Complex or Custom Data Types**

In MvvmCross, you might encounter custom data types that don't serialize or deserialize correctly using the default converters. For example, if you have a `PhoneNumber` class, you might want to serialize it as a string in a specific format (e.g., "+1 XXX-XXX-XXXX") and deserialize it back into the `PhoneNumber` object. Custom converters can handle such scenarios by overriding the serialization and deserialization logic.

csharp
public class PhoneNumberConverter : JsonConverter
{
    public override PhoneNumber Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        string phoneNumberString = reader.GetString()!;
        // Logic to parse the string into PhoneNumber object
        return new PhoneNumber(phoneNumberString);
    }

    public override void Write(Utf8JsonWriter writer, PhoneNumber phoneNumber, JsonSerializerOptions options)
    {
        writer.WriteStringValue(phoneNumber.ToString());
    }
}

2. Polymorphic Deserialization**

When dealing with polymorphic objects, where the actual type of the object is determined at runtime, custom converters are crucial. For instance, if you have a base class `Vehicle` and derived classes `Car` and `Truck`, you might need to deserialize a JSON object into the appropriate derived class based on some property in the JSON.

csharp
public class VehicleConverter : JsonConverter
{
    public override Vehicle Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        if (reader.TokenType != JsonTokenType.StartObject)
            throw new JsonException();

        string? type = null;
        while (reader.Read())
        {
            if (reader.TokenType == JsonTokenType.EndObject)
                break;

            if (reader.TokenType == JsonTokenType.PropertyName)
            {
                string? propertyName = reader.GetString();
                reader.Read();

                if (propertyName == "Type")
                {
                    type = reader.GetString();
                }
                else if (type != null)
                {
                    // Deserialize based on type
                    if (type == "Car")
                        return new Car(reader);
                    else if (type == "Truck")
                        return new Truck(reader);
                }
            }
        }

        throw new JsonException("Unable to determine vehicle type.");
    }

    public override void Write(Utf8JsonWriter writer, Vehicle vehicle, JsonSerializerOptions options)
    {
        writer.WriteStartObject();
        writer.WriteString("Type", vehicle.GetType().Name);
        // Write other properties
        writer.WriteEndObject();
    }
}

3. Enum Handling**

Enums are another scenario where custom converters might be necessary. If you want to serialize or deserialize enums in a specific format (e.g., as strings instead of integers), you can create a custom converter to handle this.

csharp
public class EnumConverter : JsonConverter where T : Enum
{
    public override T Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        string? enumString = reader.GetString();
        if (enumString != null)
        {
            return (T)Enum.Parse(typeof(T), enumString);
        }
        throw new JsonException("Invalid enum value.");
    }

    public override void Write(Utf8JsonWriter writer, T enumValue, JsonSerializerOptions options)
    {
        writer.WriteStringValue(enumValue.ToString());
    }
}

4. Date and Time Formats**

Custom converters are often used to handle date and time formats that differ from the standard ISO 8601 format used by most JSON serializers. For example, if your application requires dates to be in "MM/dd/yyyy" format, you can create a converter to handle this.

csharp
public class CustomDateTimeConverter : JsonConverter
{
    public override DateTime Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        string? dateString = reader.GetString();
        return DateTime.ParseExact(dateString, "MM/dd/yyyy", CultureInfo.InvariantCulture);
    }

    public override void Write(Utf8JsonWriter writer, DateTime dateTimeValue, JsonSerializerOptions options)
    {
        writer.WriteStringValue(dateTimeValue.ToString("MM/dd/yyyy", CultureInfo.InvariantCulture));
    }
}

5. Dependency Injection and Customization**

In MvvmCross, dependency injection is used extensively. You can inject custom JSON converters into your view models or services, allowing for easy customization of serialization and deserialization logic across different platforms or scenarios.

csharp
public class MyViewModel : MvxViewModel
{
    public MyViewModel(IMvxJsonConverter jsonConverter)
    {
        // Use the injected converter for serialization/deserialization
    }
}

By using custom JSON converters in these scenarios, you can ensure that your data is correctly serialized and deserialized, even when dealing with complex or custom data types. This flexibility is crucial for maintaining robust and platform-agnostic applications with MvvmCross.