Composer系列—install命令

InstallCommand.php

从composer命令的执行过程我们知道最终程序会走到InstallCommand的execute方法：

在方法的执行过程中，首先根据默认配置和输入参数设置了命令，获取到composer实例，通过工厂方法创建了一个Installer实例，这个实例的初始化过程把很多组件关联在了一起。

    protected function execute(InputInterface $input, OutputInterface $output)    {        $io = $this->getIO();        if ($args = $input->getArgument('packages')) {            $io->writeError('<error>Invalid argument '.implode(' ', $args).'. Use "composer require '.implode(' ', $args).'" instead to add packages to your composer.json.</error>');            return 1;        }        // 检查配置是否禁用plugins        if ($input->getOption('no-custom-installers')) {            $io->writeError('<warning>You are using the deprecated option "no-custom-installers". Use "no-plugins" instead.</warning>');            $input->setOption('no-plugins', true);        }        // 检查配置是否安装dev包        if ($input->getOption('dev')) {            $io->writeError('<warning>You are using the deprecated option "dev". Dev packages are installed by default now.</warning>');        }        // 获取composer实例        $composer = $this->getComposer(true, $input->getOption('no-plugins'));        //获取composer的下载管理器 包括git hg svn file zip等        $composer->getDownloadManager()->setOutputProgress(!$input->getOption('no-progress'));        // 触发事件        $commandEvent = new CommandEvent(PluginEvents::COMMAND, 'install', $input, $output);        $composer->getEventDispatcher()->dispatch($commandEvent->getName(), $commandEvent);        // 初始化安装器实例 安装器把io 和 Repository仓库 Downloader Packages等组件关联在一起        $install = Installer::create($io, $composer);        $preferSource = false;        $preferDist = false;        // 获取默认的配置信息        $config = $composer->getConfig();        switch ($config->get('preferred-install')) {            case 'source':                $preferSource = true;                break;            case 'dist':                $preferDist = true;                break;            case 'auto':            default:                // noop                break;        }        if ($input->getOption('prefer-source') || $input->getOption('prefer-dist')) {            $preferSource = $input->getOption('prefer-source');            $preferDist = $input->getOption('prefer-dist');        }        //        $optimize = $input->getOption('optimize-autoloader') || $config->get('optimize-autoloader');        $authoritative = $input->getOption('classmap-authoritative') || $config->get('classmap-authoritative');        // 安装器设置执行的各种参数 和require update 参数不同        $install            ->setDryRun($input->getOption('dry-run'))            ->setVerbose($input->getOption('verbose'))            ->setPreferSource($preferSource)            ->setPreferDist($preferDist)            ->setDevMode(!$input->getOption('no-dev'))            ->setDumpAutoloader(!$input->getOption('no-autoloader'))            ->setRunScripts(!$input->getOption('no-scripts'))            ->setSkipSuggest($input->getOption('no-suggest'))            ->setOptimizeAutoloader($optimize)            ->setClassMapAuthoritative($authoritative)            ->setIgnorePlatformRequirements($input->getOption('ignore-platform-reqs'))        ;        if ($input->getOption('no-plugins')) {            $install->disablePlugins();        }        // 最终执行到安装器的入口 执行安装过程        return $install->run();    }

Installer.php

文件有关键的一步，禁用垃圾回收器 gc_disable()

• 由于PHP的GC是基于引用计数的，为了能够回收循环引用的对象，会在refcount减少但不到0的时候，试图检测并回收循环引用的孤岛对象，但当有效对象的数量及互相引用较大（比如composer中代表包、版本和互相的依赖关系）的时候，这种搜索的开销就会变得非常巨大，造成大量的CPU计算。

• gc_disable 不是完全关闭 gc ，而是关闭检查循环引用计数。

• composer 的这个地方是在进行依赖包检查，对于安装了大量包的项目来说，这是比较耗时、耗内存的操作，并且这部分代码（依据设计/依据提交者的看法）无需考虑循环引用计数问题。

• 因为占用内存太大，并且频繁触发 gc ，导致效率降低。

整个的运行流程：

• 1.判断是不是更新操作，不是的话读取lock文件。
• 2.最终会获取到各个仓库Repository（Repository中可能包括PlatformRepository InstalledArray InstalledFilesystem - ComposerRepository NpmRepository BowerRepository）
• 3.把Repository都加入到一个Pool池子中，$this->package从rootPackage（要安装依赖的项目）开始找白名单中的Requires和DevRequires
• 4.然后初始化请求队列
• 5.如果是update调到6步否则跳7步
• 6.从远程获取Requires和DevRequires并加入请求队列。
• 7.从本地locked文件获取加入请求队列。
• 8.解决依赖（返回operations） //过程较复杂
• 9.处理devPackages
• 10.遍历operations获取推荐的包和请求的包信息，获取安装管理器执行安装操作
• 11.处理packageUrls，为后续写入lock文件
• 12.安装管理器通知安装器，处理信息后写入lock文件。
• 13.最后生成autoload.php psr4 installed.json等文件，执行脚本，生成binaries文件，重新启用gc。

    /**     * Run installation (or update)     *     * @throws \Exception     * @return int        0 on success or a positive error code on failure     */    public function run()    {        // Disable GC to save CPU cycles, as the dependency solver can create hundreds of thousands        // of PHP objects, the GC can spend quite some time walking the tree of references looking        // for stuff to collect while there is nothing to collect. This slows things down dramatically        // and turning it off results in much better performance. Do not try this at home however.        // 禁用GC 大约效率提升50% 原因下面会详细介绍        gc_collect_cycles();        gc_disable();        // Force update if there is no lock file present        // 这里判断是否更新        if (!$this->update && !$this->locker->isLocked()) {            $this->update = true;        }        // 模拟运行        if ($this->dryRun) {            $this->verbose = true;            $this->runScripts = false;            // NoopInstaller 不会安装任何package            $this->installationManager->addInstaller(new NoopInstaller);            // 模拟运行的话 会伪造本地仓库            $this->mockLocalRepositories($this->repositoryManager);        }        // 检查配置的脚本 在这时触发 composer.json文件中配置的多个脚本        if ($this->runScripts) {            // dispatch pre event            $eventName = $this->update ? ScriptEvents::PRE_UPDATE_CMD : ScriptEvents::PRE_INSTALL_CMD;            $this->eventDispatcher->dispatchScript($eventName, $this->devMode);        }        $this->downloadManager->setPreferSource($this->preferSource);        $this->downloadManager->setPreferDist($this->preferDist);        // create installed repo, this contains all local packages + platform packages (php & extensions)        // 这时候获取已安装的仓库        $localRepo = $this->repositoryManager->getLocalRepository();        if ($this->update) {            $platformOverrides = $this->config->get('platform') ?: array();        } else {            $platformOverrides = $this->locker->getPlatformOverrides();        }        $platformRepo = new PlatformRepository(array(), $platformOverrides);        $installedRepo = $this->createInstalledRepo($localRepo, $platformRepo);        $aliases = $this->getRootAliases();        $this->aliasPlatformPackages($platformRepo, $aliases);        if (!$this->suggestedPackagesReporter) {            $this->suggestedPackagesReporter = new SuggestedPackagesReporter($this->io);        }        try {            // 调用doInstall 将结果 $res = $result[0];$devPackages = $result[1];            list($res, $devPackages) = $this->doInstall($localRepo, $installedRepo, $platformRepo, $aliases);            if ($res !== 0) {                return $res;            }        } catch (\Exception $e) {            if (!$this->dryRun) {                $this->installationManager->notifyInstalls($this->io);            }            throw $e;        }        if (!$this->dryRun) {            $this->installationManager->notifyInstalls($this->io);        }        // output suggestions if we're in dev mode        if ($this->devMode && !$this->skipSuggest) {            $this->suggestedPackagesReporter->output($installedRepo);        }        # Find abandoned packages and warn user        foreach ($localRepo->getPackages() as $package) {            if (!$package instanceof CompletePackage || !$package->isAbandoned()) {                continue;            }            $replacement = (is_string($package->getReplacementPackage()))                ? 'Use ' . $package->getReplacementPackage() . ' instead'                : 'No replacement was suggested';            $this->io->writeError(                sprintf(                    "<warning>Package %s is abandoned, you should avoid using it. %s.</warning>",                    $package->getPrettyName(),                    $replacement                )            );        }        // 不是模拟运行时 写入lock        if (!$this->dryRun) {            // write lock            if ($this->update) {                $localRepo->reload();                $platformReqs = $this->extractPlatformRequirements($this->package->getRequires());                $platformDevReqs = $this->extractPlatformRequirements($this->package->getDevRequires());                // 把包信息更新到lock文件                $updatedLock = $this->locker->setLockData(                    array_diff($localRepo->getCanonicalPackages(), $devPackages),                    $devPackages,                    $platformReqs,                    $platformDevReqs,                    $aliases,                    $this->package->getMinimumStability(),                    $this->package->getStabilityFlags(),                    $this->preferStable || $this->package->getPreferStable(),                    $this->preferLowest,                    $this->config->get('platform') ?: array()                );                if ($updatedLock) {                    $this->io->writeError('<info>Writing lock file</info>');                }            }            if ($this->dumpAutoloader) {                // write autoloader                if ($this->optimizeAutoloader) {                    $this->io->writeError('<info>Generating optimized autoload files</info>');                } else {                    $this->io->writeError('<info>Generating autoload files</info>');                }                // 写入autoload                $this->autoloadGenerator->setDevMode($this->devMode);                $this->autoloadGenerator->setClassMapAuthoritative($this->classMapAuthoritative);                // 执行script                $this->autoloadGenerator->setRunScripts($this->runScripts);                // 写入psr4 autoload 等文件                $this->autoloadGenerator->dump($this->config, $localRepo, $this->package, $this->installationManager, 'composer', $this->optimizeAutoloader);            }            if ($this->runScripts) {                // dispatch post event                // 监听者执行事件                $eventName = $this->update ? ScriptEvents::POST_UPDATE_CMD : ScriptEvents::POST_INSTALL_CMD;                $this->eventDispatcher->dispatchScript($eventName, $this->devMode);            }            // force binaries re-generation in case they are missing            // 重新生成bin文件            foreach ($localRepo->getPackages() as $package) {                $this->installationManager->ensureBinariesPresence($package);            }            $vendorDir = $this->config->get('vendor-dir');            if (is_dir($vendorDir)) {                // suppress errors as this fails sometimes on OSX for no apparent reason                // see https://github.com/composer/composer/issues/4070#issuecomment-129792748                @touch($vendorDir);            }        }        // re-enable GC except on HHVM which triggers a warning here        if (!defined('HHVM_VERSION')) {            // 安装完成重新启用gc            gc_enable();        }        return 0;    }    /**     * @param  RepositoryInterface $localRepo     * @param  RepositoryInterface $installedRepo     * @param  PlatformRepository  $platformRepo     * @param  array               $aliases     * @return array [int, PackageInterfaces[]|null] with the exit code and an array of dev packages on update, or null on install     */    protected function doInstall($localRepo, $installedRepo, $platformRepo, $aliases)    {        // init vars        $lockedRepository = null;        $repositories = null;        // initialize locked repo if we are installing from lock or in a partial update        // and a lock file is present as we need to force install non-whitelisted lock file        // packages in that case        // 如果不更新 就从本地的composer.lock文件获取包信息        if (!$this->update || (!empty($this->updateWhitelist) && $this->locker->isLocked())) {            try {                // 从lockfile中获取                $lockedRepository = $this->locker->getLockedRepository($this->devMode);            } catch (\RuntimeException $e) {                // if there are dev requires, then we really can not install                if ($this->package->getDevRequires()) {                    throw $e;                }                // no require-dev in composer.json and the lock file was created with no dev info, so skip them                $lockedRepository = $this->locker->getLockedRepository();            }        }        // 白名单更新列表 中的依赖 composer update symfony/console        $this->whitelistUpdateDependencies(            $localRepo,            $this->package->getRequires(),            $this->package->getDevRequires()        );        $this->io->writeError('<info>Loading composer repositories with package information</info>');        // 创建默认的协议 prefer        $policy = $this->createPolicy();        // creating repository pool        // 创建一个大池子        $pool = $this->createPool($this->update ? null : $lockedRepository);        $pool->addRepository($installedRepo, $aliases);        // Repository中可能包括PlatformRepository InstalledArray InstalledFilesystem        // ComposerRepository NpmRepository BowerRepository        if ($this->update) {            $repositories = $this->repositoryManager->getRepositories();            foreach ($repositories as $repository) {                $pool->addRepository($repository, $aliases);            }        }        // Add the locked repository after the others in case we are doing a        // partial update so missing packages can be found there still.        // For installs from lock it's the only one added so it is first        // 添加locked的仓库 原因如上 在其他都添加完成之后再添加locked仓库是因为在我们只更新部分包时 未命中的包依然能被找到        if ($lockedRepository) {            $pool->addRepository($lockedRepository, $aliases);        }        // creating requirements request        $request = $this->createRequest($this->package, $platformRepo);        if ($this->update) {            // remove unstable packages from the localRepo if they don't match the current stability settings            $removedUnstablePackages = array();            foreach ($localRepo->getPackages() as $package) {                if (                    !$pool->isPackageAcceptable($package->getNames(), $package->getStability())                    && $this->installationManager->isPackageInstalled($localRepo, $package)                ) {                    $removedUnstablePackages[$package->getName()] = true;                    $request->remove($package->getName(), new Constraint('=', $package->getVersion()));                }            }            $this->io->writeError('<info>Updating dependencies'.($this->devMode ? ' (including require-dev)' : '').'</info>');            // 添加update-all命令            $request->updateAll();            $links = array_merge($this->package->getRequires(), $this->package->getDevRequires());            // 将request加入job队列            foreach ($links as $link) {                $request->install($link->getTarget(), $link->getConstraint());            }            // if the updateWhitelist is enabled, packages not in it are also fixed            // to the version specified in the lock, or their currently installed version            if ($this->updateWhitelist) {                $currentPackages = $this->getCurrentPackages($installedRepo);                // collect packages to fixate from root requirements as well as installed packages                $candidates = array();                foreach ($links as $link) {                    $candidates[$link->getTarget()] = true;                    $rootRequires[$link->getTarget()] = $link;                }                foreach ($currentPackages as $package) {                    $candidates[$package->getName()] = true;                }                // fix them to the version in lock (or currently installed) if they are not updateable                foreach ($candidates as $candidate => $dummy) {                    foreach ($currentPackages as $curPackage) {                        if ($curPackage->getName() === $candidate) {                            if (!$this->isUpdateable($curPackage) && !isset($removedUnstablePackages[$curPackage->getName()])) {                                $constraint = new Constraint('=', $curPackage->getVersion());                                $description = $this->locker->isLocked() ? '(locked at' : '(installed at';                                $requiredAt = isset($rootRequires[$candidate]) ? ', required as ' . $rootRequires[$candidate]->getPrettyConstraint() : '';                                $constraint->setPrettyString($description . ' ' . $curPackage->getPrettyVersion() . $requiredAt . ')');                                $request->install($curPackage->getName(), $constraint);                            }                            break;                        }                    }                }            }        } else {            // install命令的的执行过程            $this->io->writeError('<info>Installing dependencies'.($this->devMode ? ' (including require-dev)' : '').' from lock file</info>');            if (!$this->locker->isFresh()) {                $this->io->writeError('<warning>Warning: The lock file is not up to date with the latest changes in composer.json. You may be getting outdated dependencies. Run update to update them.</warning>', true, IOInterface::QUIET);            }            foreach ($lockedRepository->getPackages() as $package) {                $version = $package->getVersion();                if (isset($aliases[$package->getName()][$version])) {                    $version = $aliases[$package->getName()][$version]['alias_normalized'];                }                $constraint = new Constraint('=', $version);                $constraint->setPrettyString($package->getPrettyVersion());                // 往请求中添加job                $request->install($package->getName(), $constraint);            }            foreach ($this->locker->getPlatformRequirements($this->devMode) as $link) {                $request->install($link->getTarget(), $link->getConstraint());            }        }        // force dev packages to have the latest links if we update or install from a (potentially new) lock        $this->processDevPackages($localRepo, $pool, $policy, $repositories, $installedRepo, $lockedRepository, 'force-links');        // solve dependencies        $this->eventDispatcher->dispatchInstallerEvent(InstallerEvents::PRE_DEPENDENCIES_SOLVING, $this->devMode, $policy, $pool, $installedRepo, $request);        $solver = new Solver($policy, $pool, $installedRepo, $this->io);        try {            // 很重要的一步 把request->jobs队列 解决依赖            $operations = $solver->solve($request, $this->ignorePlatformReqs);            $this->eventDispatcher->dispatchInstallerEvent(InstallerEvents::POST_DEPENDENCIES_SOLVING, $this->devMode, $policy, $pool, $installedRepo, $request, $operations);        } catch (SolverProblemsException $e) {            $this->io->writeError('<error>Your requirements could not be resolved to an installable set of packages.</error>', true, IOInterface::QUIET);            $this->io->writeError($e->getMessage());            return array(max(1, $e->getCode()), array());        }        $this->io->writeError("Analyzed ".count($pool)." packages to resolve dependencies", true, IOInterface::VERBOSE);        $this->io->writeError("Analyzed ".$solver->getRuleSetSize()." rules to resolve dependencies", true, IOInterface::VERBOSE);        // force dev packages to be updated if we update or install from a (potentially new) lock        $operations = $this->processDevPackages($localRepo, $pool, $policy, $repositories, $installedRepo, $lockedRepository, 'force-updates', $operations);        // execute operations        if (!$operations) {            $this->io->writeError('Nothing to install or update');        }        /**         * Workaround: if your packages depend on plugins, we must be sure         * that those are installed / updated first; else it would lead to packages         * being installed multiple times in different folders, when running Composer         * twice.         */        $operations = $this->movePluginsToFront($operations);        /**         * Removals of packages should be executed before installations in         * case two packages resolve to the same path (due to custom installers)         */        $operations = $this->moveUninstallsToFront($operations);        // extract dev packages and mark them to be skipped if it's a --no-dev install or update        // we also force them to be uninstalled if they are present in the local repo        if ($this->update) {            $devPackages = $this->extractDevPackages($operations, $localRepo, $platformRepo, $aliases);            if (!$this->devMode) {                $operations = $this->filterDevPackageOperations($devPackages, $operations, $localRepo);            }        } else {            $devPackages = null;        }        foreach ($operations as $operation) {            // collect suggestions            if ('install' === $operation->getJobType()) {                // 展示推荐的package                $this->suggestedPackagesReporter->addSuggestionsFromPackage($operation->getPackage());            }            // updating, force dev packages' references if they're in root package refs            if ($this->update) {                $package = null;                if ('update' === $operation->getJobType()) {                    $package = $operation->getTargetPackage();                } elseif ('install' === $operation->getJobType()) {                    $package = $operation->getPackage();                }                if ($package && $package->isDev()) {                    $references = $this->package->getReferences();                    if (isset($references[$package->getName()])) {                        $this->updateInstallReferences($package, $references[$package->getName()]);                    }                }                if ('update' === $operation->getJobType()                    && $operation->getTargetPackage()->isDev()                    && $operation->getTargetPackage()->getVersion() === $operation->getInitialPackage()->getVersion()                    && (!$operation->getTargetPackage()->getSourceReference() || $operation->getTargetPackage()->getSourceReference() === $operation->getInitialPackage()->getSourceReference())                    && (!$operation->getTargetPackage()->getDistReference() || $operation->getTargetPackage()->getDistReference() === $operation->getInitialPackage()->getDistReference())                ) {                    $this->io->writeError('  - Skipping update of '. $operation->getTargetPackage()->getPrettyName().' to the same reference-locked version', true, IOInterface::DEBUG);                    $this->io->writeError('', true, IOInterface::DEBUG);                    continue;                }            }            $event = 'Composer\Installer\PackageEvents::PRE_PACKAGE_'.strtoupper($operation->getJobType());            if (defined($event) && $this->runScripts) {                $this->eventDispatcher->dispatchPackageEvent(constant($event), $this->devMode, $policy, $pool, $installedRepo, $request, $operations, $operation);            }            // output non-alias ops in dry run, output alias ops in debug verbosity            if ($this->dryRun && false === strpos($operation->getJobType(), 'Alias')) {                $this->io->writeError('  - ' . $operation);                $this->io->writeError('');            } elseif ($this->io->isDebug() && false !== strpos($operation->getJobType(), 'Alias')) {                $this->io->writeError('  - ' . $operation);                $this->io->writeError('');            }            $this->installationManager->execute($localRepo, $operation);            // output reasons why the operation was ran, only for install/update operations            if ($this->verbose && $this->io->isVeryVerbose() && in_array($operation->getJobType(), array('install', 'update'))) {                $reason = $operation->getReason();                if ($reason instanceof Rule) {                    switch ($reason->getReason()) {                        case Rule::RULE_JOB_INSTALL:                            $this->io->writeError('    REASON: Required by the root package: '.$reason->getPrettyString($pool));                            $this->io->writeError('');                            break;                        case Rule::RULE_PACKAGE_REQUIRES:                            $this->io->writeError('    REASON: '.$reason->getPrettyString($pool));                            $this->io->writeError('');                            break;                    }                }            }            $event = 'Composer\Installer\PackageEvents::POST_PACKAGE_'.strtoupper($operation->getJobType());            if (defined($event) && $this->runScripts) {                $this->eventDispatcher->dispatchPackageEvent(constant($event), $this->devMode, $policy, $pool, $installedRepo, $request, $operations, $operation);            }            if (!$this->dryRun) {                $localRepo->write();            }        }        if (!$this->dryRun) {            // force source/dist urls to be updated for all packages            // 处理package url等信息 为后续写入lock做准备            $this->processPackageUrls($pool, $policy, $localRepo, $repositories);            $localRepo->write();        }        return array(0, $devPackages);    }

依赖的解决 RuleSetGenerator

 /**     * Creates a new rule for the requirements of a package     *     * This rule is of the form (-A|B|C), where B and C are the providers of     * one requirement of the package A.     *     * @param  PackageInterface $package    The package with a requirement     * @param  array            $providers  The providers of the requirement     * @param  int              $reason     A RULE_* constant describing the     *                                      reason for generating this rule     * @param  mixed            $reasonData Any data, e.g. the requirement name,     *                                      that goes with the reason     * @return Rule             The generated rule or null if tautological     */    protected function createRequireRule(PackageInterface $package, array $providers, $reason, $reasonData = null)    {        $literals = array(-$package->id);        foreach ($providers as $provider) {            // self fulfilling rule?            if ($provider === $package) {                return null;            }            $literals[] = $provider->id;        }        return new Rule($literals, $reason, $reasonData);    }    /**     * Creates a rule to install at least one of a set of packages     *     * The rule is (A|B|C) with A, B and C different packages. If the given     * set of packages is empty an impossible rule is generated.     *     * @param  array $packages The set of packages to choose from     * @param  int   $reason   A RULE_* constant describing the reason for     *                         generating this rule     * @param  array $job      The job this rule was created from     * @return Rule  The generated rule     */    protected function createInstallOneOfRule(array $packages, $reason, $job)    {        $literals = array();        foreach ($packages as $package) {            $literals[] = $package->id;        }        return new Rule($literals, $reason, $job['packageName'], $job);    }    /**     * Creates a rule to remove a package     *     * The rule for a package A is (-A).     *     * @param  PackageInterface $package The package to be removed     * @param  int              $reason  A RULE_* constant describing the     *                                   reason for generating this rule     * @param  array            $job     The job this rule was created from     * @return Rule             The generated rule     */    protected function createRemoveRule(PackageInterface $package, $reason, $job)    {        return new Rule(array(-$package->id), $reason, $job['packageName'], $job);    }    /**     * Creates a rule for two conflicting packages     *     * The rule for conflicting packages A and B is (-A|-B). A is called the issuer     * and B the provider.     *     * @param  PackageInterface $issuer     The package declaring the conflict     * @param  PackageInterface $provider   The package causing the conflict     * @param  int              $reason     A RULE_* constant describing the     *                                      reason for generating this rule     * @param  mixed            $reasonData Any data, e.g. the package name, that     *                                      goes with the reason     * @return Rule             The generated rule     */    protected function createConflictRule(PackageInterface $issuer, PackageInterface $provider, $reason, $reasonData = null)    {        // ignore self conflict        if ($issuer === $provider) {            return null;        }        return new Rule(array(-$issuer->id, -$provider->id), $reason, $reasonData);    }

A依赖的包是B，B就是provider。A B C 表示包的id

- //createRequireRule-A | B | CA ->issue  B C ->providers（提供者）A依赖于B C- //createConflictRule-A | -BA ->issue  B ->providers（提供者）A和B冲突- //createInstallOneOfRuleA | B | CA B C需要安装- //createRemoveRule-AA需要删除

如果出现-A | B -B | A 环形依赖，composer会报环形依赖错误。
在做决定时，-A | B | C，如果A要安装，那么B，C需要已经确定安装。意思就是需要从一个无依赖的成品单元来开始做决定动作。