gradle-master/subprojects/core-api/src/main/java/org/gradle/api/provider/Provider.java
/*
 * Copyright 2017 the original author or authors.
 *
 * 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.gradle.api.provider;

import org.gradle.api.NonExtensible;
import org.gradle.api.Transformer;
import org.gradle.internal.HasInternalProtocol;

import javax.annotation.Nullable;
import java.util.concurrent.Callable;
import java.util.function.BiFunction;

/**
 * A container object that provides a value of a specific type. The value can be retrieved using one of the query methods
 * such as {@link #get()} or {@link #getOrNull()}.
 *
 * <p>
 * A provider may not always have a value available, for example when the value may not yet be known but will be known
 * at some point in the future. When a value is not available, {@link #isPresent()} returns {@code false} and retrieving
 * the value will fail with an exception.
 * </p>
 *
 * <p>
 * A provider may not always provide the same value. Although there are no methods on this interface to change the value,
 * the provider implementation may be mutable or use values from some changing source. A provider may also provide a value
 * that is mutable and that changes over time.
 * </p>
 *
 * <p>
 * A provider may represent a task output. Such a provider carries information about the task producing its value. When
 * this provider is attached to an input of another task, Gradle will automatically determine the task dependencies based
 * on this connection.
 * </p>
 *
 * <p>
 * A typical use of a provider is to pass values from one Gradle model element to another, e.g. from a project extension
 * to a task, or between tasks. Providers also allow expensive computations to be deferred until their value is actually
 * needed, usually at task execution time.
 * </p>
 *
 * <p>
 * There are a number of ways to create a {@link Provider} instance. Some common methods:
 * </p>
 *
 * <ul>
 *     <li>A number of Gradle types, such as {@link Property}, extend {@link Provider} and can be used directly as a provider.</li>
 *     <li>Calling {@link #map(Transformer)} to create a new provider from an existing provider.</li>
 *     <li>Using the return value of {@link org.gradle.api.tasks.TaskContainer#register(String)}, which is a provider that represents the task instance.</li>
 *     <li>Using the methods on {@link org.gradle.api.file.Directory} and {@link org.gradle.api.file.DirectoryProperty} to produce file providers.</li>
 *     <li>By calling {@link ProviderFactory#provider(Callable)} or {@link org.gradle.api.Project#provider(Callable)} to create a new provider from a {@link Callable}.</li>
 * </ul>
 *
 * <p>
 * For a provider whose value can be mutated, see {@link Property} and the methods on {@link org.gradle.api.model.ObjectFactory}.
 * </p>
 *
 * <p>
 * <b>Note:</b> This interface is not intended for implementation by build script or plugin authors.
 * </p>
 *
 * @param <T> Type of value represented by provider
 * @since 4.0
 */
@HasInternalProtocol
@NonExtensible
public interface Provider<T> {

    /**
     * Returns the value of this provider if it has a value present, otherwise throws {@code java.lang.IllegalStateException}.
     *
     * @return the current value of this provider.
     * @throws IllegalStateException if there is no value present
     */
    T get();

    /**
     * Returns the value of this provider if it has a value present. Returns {@code null} a value is not available.
     *
     * @return the value or {@code null}
     */
    @Nullable
    T getOrNull();

    /**
     * Returns the value of this provider if it has a value present. Returns the given default value if a value is
     * not available.
     *
     * @return the value or the default value.
     * @since 4.3
     */
    T getOrElse(T defaultValue);

    /**
     * Returns a new {@link Provider} whose value is the value of this provider transformed using the given function.
     *
     * <p>
     * The resulting provider will be live, so that each time it is queried, it queries the original (this) provider
     * and applies the transformation to the result. Whenever the original provider has no value, the new provider
     * will also have no value and the transformation will not be called.
     * </p>
     *
     * <p>
     * When this provider represents a task or the output of a task, the new provider will be considered an output
     * of the task and will carry dependency information that Gradle can use to automatically attach task dependencies
     * to tasks that use the new provider for input values.
     * </p>
     *
     * @param transformer The transformer to apply to values. May return {@code null}, in which case the provider will have no value.
     * @since 4.3
     */
    <S> Provider<S> map(Transformer<? extends @org.jetbrains.annotations.Nullable S, ? super T> transformer);

    /**
     * Returns a new {@link Provider} from the value of this provider transformed using the given function.
     *
     * <p>
     * While very similar in functionality to the regular {@link #map(Transformer) map} operation, this method
     * offers a convenient way of connecting together task inputs and outputs. (For a deeper understanding of
     * the topic see the <a href="https://docs.gradle.org/current/userguide/lazy_configuration.html">Lazy Configuration</a>
     * section of the Gradle manual.)</p>
     *
     * <p>
     * Task inputs and outputs often take the form of {@link Provider providers} or {@link Property properties},
     * the latter being a special case of provider whose value can be changed at will. An example of using
     * {@code flatMap} for connecting such properties would be following:
     * </p>
     *
     * <pre><code>
     * class Producer extends DefaultTask {
     *     {@literal @}OutputFile
     *     abstract RegularFileProperty getOutputFile()
     *
     *     //irrelevant details omitted
     * }
     *
     * class Consumer extends DefaultTask {
     *     {@literal @}InputFile
     *     abstract RegularFileProperty getInputFile()
     *
     *     //irrelevant details omitted
     * }
     *
     * def producer = tasks.register("producer", Producer)
     * def consumer = tasks.register("consumer", Consumer)
     *
     * consumer.configure {
     *     inputFile = producer.flatMap { it.outputFile }
     * }
     * </code></pre>
     *
     * <p>
     * An added benefit of connecting input and output properties like this is that Gradle can automatically
     * detect task dependencies based on such connections. To make this happen at code level, any task
     * details associated with this provider (the one on which {@code flatMap} is being called) are ignored.
     * The new provider will use whatever task details are associated with the return value of the transformation.
     * </p>
     *
     * <p>
     * The new provider returned by {@code flatMap} will be live, so that each time it is queried, it queries
     * this provider and applies the transformation to the result. Whenever this provider has no value, the new
     * provider will also have no value and the transformation will not be called.
     * </p>
     *
     * @param transformer The transformer to apply to values. May return {@code null}, in which case the
     * provider will have no value.
     * @since 5.0
     */
    <S> Provider<S> flatMap(Transformer<? extends @org.jetbrains.annotations.Nullable Provider<? extends S>, ? super T> transformer);

    /**
     * Returns {@code true} if there is a value present, otherwise {@code false}.
     *
     * @return {@code true} if there is a value present, otherwise {@code false}
     */
    boolean isPresent();

    /**
     * Returns a {@link Provider} whose value is the value of this provider, if present, otherwise the
     * given default value.
     *
     * @param value The default value to use when this provider has no value.
     * @since 5.6
     */
    Provider<T> orElse(T value);

    /**
     * Returns a {@link Provider} whose value is the value of this provider, if present, otherwise uses the
     * value from the given provider, if present.
     *
     * @param provider The provider whose value should be used when this provider has no value.
     * @since 5.6
     */
    Provider<T> orElse(Provider<? extends T> provider);

    /**
     * Returns a view of this {@link Provider} which can be safely read at configuration time.
     *
     * @since 6.5
     */
    @Deprecated
    Provider<T> forUseAtConfigurationTime();

    /**
     * Returns a provider which value will be computed by combining this provider value with another
     * provider value using the supplied combiner function.
     *
     * <p>
     * If the supplied providers represents a task or the output of a task, the resulting provider
     * will carry the dependency information.
     * </p>
     *
     * @param right the second provider to combine with
     * @param combiner the combiner of values. May return {@code null}, in which case the provider
     * will have no value.
     * @param <U> the type of the second provider
     * @param <R> the type of the result of the combiner
     * @return a combined provider
     * @since 6.6
     */
    <U, R> Provider<R> zip(Provider<U> right, BiFunction<? super T, ? super U, ? extends @org.jetbrains.annotations.Nullable R> combiner);
}
