Here is a short description how to add validators to Tapestry 5. In this example a validator is added to check whether the contents of a text field can be parsed as an integer. Changes and corrections welcome.

First define a validation class. This validator does not take an argument (as in min=5 e.g.), so the first template parameter is Void. As no conversion to a specific type of the contents is required, use Object as the second parameter.

The message-key is specified in the constructor, and is validate-int. The name of the validator function is int as specified in the 'addValidation' call in the render method.

A sample class is 

public class ValidateInt extends AbstractValidator<Void, Object> {

	public ValidateInt() {
		super(null, Object.class, "validate-int");
	}

	@Override
	public void render(Field field, Void constraintValue,
			MessageFormatter formatter, MarkupWriter writer,
			FormSupport formSupport) {
		formSupport.addValidation(field, "int", formatter.format(field.getLabel()), null);
	}

	@Override
	public void validate(Field field, Void constraintValue,
			MessageFormatter formatter, Object value) throws ValidationException {
		if (value != null) {
			try {
				Integer.parseInt(value.toString());
			} catch (NumberFormatException ex) {
				throw new ValidationException(formatter.format(field.getLabel()));
			}
		}
	}

}

Specify the message in a properties file, e.g. org/example/ValidationMessages.properties 

validate-int=You must provide a whole number for %s.

These need to be registered in AppModule.java for your application as follows:

	public static void contributeFieldValidatorSource(
			MappedConfiguration<String, Validator> configuration) {
		configuration.add("int", new ValidateInt());
	}

	public void contributeValidationMessagesSource(
			OrderedConfiguration<String> configuration) {
		configuration.add("example","org/example/ValidationMessages");
	}

Not done yet, as the javascript to do client-side validation is still required. Create a javascript file validators.js with e.g. 

Tapestry.Validator.int = function(field, message) {
	Tapestry.addValidator(field, true, function(value, event) {
		if (isNaN(Number(value)) || value.indexOf(".")!=-1)
 			event.recordError(message);
	 });
}

This javascript file needs to be included in all pages/components where the new validator is to be used (Is there a better way?)

Once this is done you can add the validator to your text fields as in 

<t:textfield t:id="select" value="selector" validate="required,int,min=0"/>
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