001// Copyright 2006-2014 The Apache Software Foundation
002//
003// Licensed under the Apache License, Version 2.0 (the "License");
004// you may not use this file except in compliance with the License.
005// You may obtain a copy of the License at
006//
007//     http://www.apache.org/licenses/LICENSE-2.0
008//
009// Unless required by applicable law or agreed to in writing, software
010// distributed under the License is distributed on an "AS IS" BASIS,
011// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
012// See the License for the specific language governing permissions and
013// limitations under the License.
014
015package org.apache.tapestry5.services;
016
017import java.util.NoSuchElementException;
018
019/**
020 * Provides access to environment objects, which are almost always provided to enclosed components by enclosing
021 * components. Environmental services are a form of very late binding.
022 * <p/>
023 * The Environment acts like a collection of stacks. Each stack contains environmental objects of a given type. Most
024 * often, a stack has zero or one elements, but on occasion, a particular component will push an override onto the stack
025 * for the benefit of the components it encloses.
026 *
027 * @see org.apache.tapestry5.annotations.Environmental
028 * @see org.apache.tapestry5.services.EnvironmentalShadowBuilder
029 */
030public interface Environment
031{
032    /**
033     * Peeks at the current top of the indicated stack.
034     *
035     * @param <T>  the type of environmental object
036     * @param type class used to select the object
037     * @return the current object of that type, or null if no service of that type has been added
038     */
039    <T> T peek(Class<T> type);
040
041    /**
042     * Peeks at the current top of the indicated stack (which must have a non-null value).
043     *
044     * @param <T>  the type of environmental object
045     * @param type class used to select the object
046     * @return the current object of the specified type
047     * @throws RuntimeException if no service of that type has been added
048     */
049    <T> T peekRequired(Class<T> type);
050
051    /**
052     * Removes and returns the top environmental object of the selected type.
053     *
054     * @param <T>  the type of environmental object
055     * @param type class used to select the object
056     * @return the object just removed
057     * @throws NoSuchElementException if the environmental stack (for the specified type) is empty
058     */
059    <T> T pop(Class<T> type);
060
061    /**
062     * Pushes a new service onto the stack. The old service at the top of the stack is returned (it may be null).
063     *
064     * @param <T>      the type of environmental object
065     * @param type     class used to select the object
066     * @param instance the service object
067     * @return the previous top service
068     */
069    <T> T push(Class<T> type, T instance);
070
071    /**
072     * Hides all current environment values, making the Environment object appear empty, until
073     * a call to {@link #decloak()}} restores the original state.
074     *
075     * @since 5.3
076     * @deprecated Deprecated in 5.4 with no replacement; not longer used by Tapestry.
077     * @see org.apache.tapestry5.TapestryConstants#RESPONSE_RENDERER
078     */
079    void cloak();
080
081    /**
082     * Restores state previously hidden by {@link #cloak()}}.
083     *
084     * @since 5.3
085     * @deprecated Deprecated in 5.4 with no replacement; not longer used by Tapestry.
086     * @see org.apache.tapestry5.TapestryConstants#RESPONSE_RENDERER
087     */
088    void decloak();
089}