/*
 * Copyright (C) 2013 Red Hat, Inc. and/or its affiliates.
 *
 * 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.jboss.errai.ui.shared.api.annotations;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

import org.jboss.errai.ui.client.local.spi.TranslationService;

/**
 * <p>
 * An annotation that provides a mechanism for the developer to declare translation strings from
 * within the GWT application code. This extends the Errai i18n support so that translations can be
 * used from Java code (not just from templates).
 * </p>
 * <p>
 * How does it work? I'm glad you asked. The developer must annotate a field which represents the
 * translation key. This key will then map to a value in the translation bundle file. Once the field
 * is annotated appropriately, the developer must directly invoke the {@link TranslationService}'s
 * <code>format</code> method. This method call will perform a lookup in the translation service of
 * the value mapped to the provided key. Note that value substitution is supported via the {N}
 * format. See below for some examples.
 * </p>
 * 
 * <span><b>AppMessages.java</b></span>
 * 
 * <pre>
 * package org.example.ui.client.local;
 * 
 * public class AppMessages {
 *   @TranslationKey(defaultValue = "I guess something happened!")
 *   public static final String CUSTOM_MESSAGE = "app.custom-message";
 *   @TranslationKey(defaultValue = "Hey {0}, I just told you something happened!")
 *   public static final String CUSTOM_MESSAGE_WITH_NAME = "app.custom-message-with-name";
 * }
 * </pre>
 * 
 * <hr/>
 * 
 * <span><b>CustomComponent.java</b></span>
 * 
 * <pre>
 * package org.example.ui.client.local;
 * 
 * @Dependent
 * @Templated
 * public class CustomComponent extends Composite {
 *   @Inject
 *   private TranslationService translationService;
 * 
 *   @Inject
 *   @DataField
 *   private Button someAction;
 * 
 *   @EventHandler("someAction")
 *   private void doLogin(ClickEvent event) {
 *     // do some action that may require a notification sent to the user
 *     String messageToUser = translationService.format(AppMessages.CUSTOM_MESSAGE);
 *     Window.alert(messageToUser);
 *     String username = getCurrentUserName(); // hand wave, hand wave
 *     String messageToUserWithName = translationService.format(AppMessages.CUSTOM_MESSAGE_WITH_NAME, username);
 *     Window.alert(messageToUserWithName);
 *   }
 * }
 * </pre>
 * 
 * @author eric.wittmann@redhat.com
 */
@Documented
@Target({ ElementType.FIELD })
@Retention(RetentionPolicy.RUNTIME)
public @interface TranslationKey {

  /**
   * Indicates the default value to use if no translation is available for the current locale.
   */
  String defaultValue();

}