Value Converters: writing a custom value converter

Value converters are methods that you can use to bind data to views, but in the format that the views natively don't understand. For example, one of the converters in the library converts a Boolean into a View.VISIBILITY, which then enables you to bind a Boolean directly to the Visibility property of a View. It is used like this: {Visibility @= ToVisibility(ViewVisible)}, where ToVisibility is the converter's name and the ViewVisible is the name of the bound data or method (with added get prefix, so it becomes getViewVisible).

Let's examine the anatomy of a Value Converter.

First, it needs to implement a solutions.alterego.androidbound.converters.interfaces.IValueConverter interface, that has only two methods:

Object convert(Object value, Class<?> targetType, Object param, Locale locale);

Object convertBack(Object value, Class<?> targetType, Object param, Locale locale);

Converters that bind only in the direction of a View use the convert method, and binding in the direction from the View to the Object/ViewModel uses convertBack.

Parameters are always the same, and beyond the obvious ones - value for the bound data and targetType for the View class - there's also a param, which is an optional parameter that can be passed to the converter. For example, ToVisibility converter supports the invert parameter,that inverts the boolean value so that true sets the view visibility to View.GONE instead of View.VISIBLE. There's also the locale parameter that you can use if you need the Locale to perform your conversions.

Second, your custom converter also needs to be registered with the ViewBinder so that it knows about it. That's easily done through registerConverter(String name, IValueConverter converter) method, that takes a name - with which you will call the converter from XML - and your custom converter reference.