WritingExpressionEvaluators.md

Writing expression evaluators

Expression evaluators are classes which implement the interface IEvaluatesExpression. You are encouraged to read the API documentation for this interface.

No need to deal with expression prefixes

TALES expression evaluators operate upon a prefix-basis; the prefix is used to determine which evaluator implementation should receive the expression. When the implementation of IEvaluatesExpression receives the evaluation request, the prefix which lead to that implementation class will have already been removed from the expression.

For example, if you are creating a new ruby: expression type, you will register your evaluator implementation with the ruby prefix string. Once a Ruby expression reaches your evaluator, the ruby: prefix will have already been removed from the expression string.

Registering evaluators with their prefixes

The actual registration of an evaluator implementation class and the association of that implementation type with a prefix is performed during dependency injection set-up. Typically, a new expression evaluator will add an extension method for IBuildsHostingEnvironment which is used to add its services to DI and also register the implementation with its prefix. That extension method should register the implementation type and prefix like so:

builder.ServiceRegistry.ExpresionEvaluatorTypes
  .Add("myprefix", typeof(MyEvaluatorType));

Where:

• builder is the IBuildsHostingEnvironment
• "myprefix" is the prefix for your expression type
• MyEvaluatorType is the concrete class which implements IEvaluatesExpression

Commonly-used services

Because ZptSharp is based upon dependency injection, any class may constructor-inject other service interfaces freely.

• Inject an IEvaluatesExpression in order to be able to evaluate nested expressions
• Inject IEvaluatesDomValueExpression to evaluate expressions which return content specifically for the DOM
• Add your own services to dependency injection and inject them, as required