// Copyright 2015 The Bazel Authors. All rights reserved.
//
// 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.
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package com.google.devtools.build.lib.packages;
/**
* A class of aspects.
*
* <p>An aspect allows a rule to create actions in its dependencies, without their knowledge.
* It can be viewed as the ability to attach shadow targets to transitive dependencies or a way
* to run visitations of certain parts of the transitive closure of a rule in such a way that can
* be cached (even partially) and reused between different configured targets requiring the same
* aspect. Some examples where aspects are useful:
*
* <ul>
* <li>Converting the .jar files in the transitive closure of an Android binary to dexes</li>
* <li>Emitting Java sources for a <code>proto_library</code> and the messages it depends
* on</li>
* <li>Collecting all the dependencies of a rule to make sure that it does not contain a
* forbidden one</li>
* </ul>
*
* <p>When a configured target requests that an aspect be attached to one of its dependencies,
* the {@link com.google.devtools.build.lib.analysis.TransitiveInfoProvider}s generated by that
* aspects are merged with those of the actual dependency, that is,
* {@link com.google.devtools.build.lib.analysis.RuleContext#getPrerequisite(
* String, com.google.devtools.build.lib.analysis.RuleConfiguredTarget.Mode)} will
* contain the transitive info providers produced both by the dependency and the aspects that are
* attached to it.
*
* <p>Configured targets can specify which aspects should be attached to some of their dependencies
* by specifying this in their {@link com.google.devtools.build.lib.analysis.RuleDefinition}: each
* attribute can have a list of aspects to be applied to the rules in that attribute and each
* aspect can specify which {@link com.google.devtools.build.lib.analysis.TransitiveInfoProvider}s
* it needs on a rule so that it can do meaningful work (for example, dexing only makes sense for
* configured targets that produce Java code).
*
* <p>Aspects can be defined natively, in Java ({@link NativeAspectClass})
* or in Skylark ({@link SkylarkAspectClass}).
*
* Bazel propagates aspects through a multistage process. The general pipeline is as follows:
*
* <pre>
* {@link AspectClass}
* |
* V
* {@code AspectDescriptor} <- {@link AspectParameters}
* \
* V
* {@link Aspect} <- {@link AspectDefinition} (might require loading Skylark files)
* |
* V
* {@code ConfiguredAspect} <- {@code ConfiguredTarget}
* </pre>
*
* <ul>
* <li>{@link AspectClass} is a moniker for "user" definition of the aspect, be it
* a native aspect or a Skylark aspect. It contains either a reference to
* the native class implementing the aspect or the location of the Skylark definition
* of the aspect in the source tree, i.e. label of .bzl file + symbol name.
* </li>
* <li>{@link AspectParameters} is a (key,value) pair list that can be used to
* parameterize aspect classes</li>
* <li>{@link AspectDescriptor} is a pair
* of {@code AspectClass} and {@link AspectParameters}. It uniquely identifies
* the aspect and can be used in SkyKeys.
* </li>
* <li>{@link AspectDefinition} is a class encapsulating the aspect definition (what
* attributes aspoect has, and along which dependencies does it propagate.
* </li>
* <li>{@link Aspect} is a fully instantiated instance of an Aspect after it is loaded.
* Getting an {@code Aspect} from {@code AspectDescriptor} for Skylark aspects
* requires adding a Skyframe dependency.
* </li>
* <li>{@link com.google.devtools.build.lib.analysis.ConfiguredAspect} represents a result
* of application of an {@link Aspect} to a given
* {@link com.google.devtools.build.lib.analysis.ConfiguredTarget}.
* </li>
* </ul>
*
* {@link AspectDescriptor}, or in general, a tuple
* of ({@link AspectClass}, {@link AspectParameters}) is an identifier that should be
* used in SkyKeys or in other contexts that need equality for aspects.
* See also {@link com.google.devtools.build.lib.skyframe.AspectFunction} for details
* on Skyframe treatment of Aspects.
*
* @see com.google.devtools.build.lib.rules.RuleConfiguredTargetFactory
* @see com.google.devtools.build.lib.skyframe.AspectFunction
*/
public interface AspectClass {
/**
* Returns an aspect name.
*/
String getName();
}