// Copyright 2006, 2007, 2008 The Apache Software Foundation
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package org.apache.tapestry5;

/**
 * Responsible for validation of a single field.
 *
 * @param <T> the type of the values being validated
 * @see Validator
 * @see org.apache.tapestry5.services.FieldValidatorDefaultSource
 */
public interface FieldValidator<T>
{
    /**
     * Invoked after the client-submitted value has been {@link Translator translated} to check that the value conforms
     * to expectations (often, in terms of minimum or maximum value). If and only if the value is approved by all
     * Validators is the value applied by the field.
     *
     * @param value the translated value supplied by the user
     * @throws ValidationException if the value violates the constraint
     */
    void validate(T value) throws ValidationException;

    /**
     * Invokes {@link Validator#render(Field, Object, org.apache.tapestry5.ioc.MessageFormatter, MarkupWriter,
     * org.apache.tapestry5.services.FormSupport)}. This is called at a point "inside" the tag, so that additional
     * attributes may be added.  In many cases, the underlying {@link org.apache.tapestry5.Validator} may write
     * client-side JavaScript to enforce the constraint as well.
     *
     * @param writer markup writer to direct output to.
     * @see org.apache.tapestry5.MarkupWriter#attributes(Object[])
     */
    void render(MarkupWriter writer);

    /**
     * Returns true if any underlying {@link org.apache.tapestry5.Validator} returns true from {@link
     * org.apache.tapestry5.Validator#isRequired()}.
     *
     * @return true if the field is required   (a non-blank value is expected)
     */
    boolean isRequired();
}
