## 用于检查 spec 是否符合当前 Source 要求  
require 'cocoapods-core/source/acceptor'  
## 记录本地 source 的集合  
require 'cocoapods-core/source/aggregate'  
## 用于校验 source 的错误和警告  
require 'cocoapods-core/source/health_reporter'  
## source 管理器  
require 'cocoapods-core/source/manager'  
## source 元数据  
require 'cocoapods-core/source/metadata'  

module Pod  
  class Source  
    ## 仓库默认的 Git 分支  
    DEFAULT_SPECS_BRANCH = 'master'.freeze  
    ## 记录仓库的元数据  
    attr_reader :metadata  
    ## 记录仓库的本地地址  
    attr_reader :repo  
    ## repo 仓库地址 ~/.cocoapods/repos/{repo_name}  
    def initialize(repo)  
      @repo = Pathname(repo).expand_path  
      @versions_by_name = {}  
      refresh_metadata  
    end  
    ## 读取 Git 仓库中的 remote url 或 .git 目录  
    def url  
      @url ||= begin  
        remote = repo_git(%w(config --get remote.origin.url))  
        if !remote.empty?  
          remote  
        elsif (repo + '.git').exist?  
          "file://#{repo}/.git"  
        end  
      end  
    end  

    def type  
      git? ? 'git' : 'file system'  
    end  
    ## ...  
  end  
end

def refresh_metadata  
  @metadata = Metadata.from_file(metadata_path)  
end  

def metadata_path  
  repo + 'CocoaPods-version.yml'  
end

autoload :Digest, 'digest/md5'  
require 'active_support/hash_with_indifferent_access'  
require 'active_support/core_ext/hash/indifferent_access'  

module Pod  
  class Source  
    class Metadata  
      ## 最低可支持的 CocoaPods 版本，对应字段 `min`  
      attr_reader :minimum_cocoapods_version  
      ## 最高可支持的 CocoaPods 版本，对应字段 `max`  
      attr_reader :maximum_cocoapods_version  
      ## 最新 CocoaPods 版本，对应字段 `last`  
      attr_reader :latest_cocoapods_version  
      ## 规定截取的关键字段的前缀长度和数量  
      attr_reader :prefix_lengths  
      ## 可兼容的 CocoaPods 最新版本  
      attr_reader :last_compatible_versions  
      ## ...  
    end  
  end  
end

---  
min: 1.0.0  
last: 1.10.0.beta.1  
prefix_lengths:  
- 1  
- 1  
- 1

def path_fragment(pod_name, version = nil)  
  prefixes = if prefix_lengths.empty?  
               []  
             else  
               hashed = Digest::MD5.hexdigest(pod_name)  
               prefix_lengths.map do |length|  
                 hashed.slice!(0, length)  
               end  
             end  
  prefixes.concat([pod_name, version]).compact  
end

$ Digest::MD5.hexdigest('AFNetworking')  
"a75d452377f3996bdc4b623a5df25820"

def versions(name)  
  return nil unless specs_dir  
  raise ArgumentError, 'No name' unless name  
  pod_dir = pod_path(name)  
  return unless pod_dir.exist?  
  @versions_by_name[name] ||= pod_dir.children.map do |v|  
    basename = v.basename.to_s  
    begin  
      Version.new(basename) if v.directory? && basename[0, 1] != '.'  
    rescue ArgumentError  
    raise Informative, 'An unexpected version directory ...'  
    end  
  end.compact.sort.reverse  
end

class Version < Pod::Vendor::Gem::Version  
  METADATA_PATTERN = '(\+[0-9a-zA-Z\-\.]+)'  
  VERSION_PATTERN = "[0-9]+(\\.[0-9a-zA-Z\\-]+)*#{METADATA_PATTERN}?"  
  ## ...  
end

3.0.0  
3.0.0-beta.1  
3.0.0-beta.2  
3.0.0-beta.3  
3.0.1

require 'cocoapods-core/source'  
## ...  
module Pod  
  class CDNSource < Source  
    def initialize(repo)  
      ## 标记是否正在同步文件  
      @check_existing_files_for_update = false  
      ## 记录时间用于对比下载文件的新旧程度，以确认是否需要更新保存所下的资源  
      @startup_time = Time.new  
      ## 缓存查询过的 PodSpec 资源  
      @version_arrays_by_fragment_by_name = {}  
      super(repo)  
    end  

    def url  
      @url ||= File.read(repo.join('.url')).chomp.chomp('/') + '/'  
    end  

    def type  
      'CDN'  
    end  
    ## ...  
  end  
end

module Pod  
  class TrunkSource < CDNSource  
    ## 新版落盘后仓库名称  
    TRUNK_REPO_NAME = 'trunk'.freeze  

    TRUNK_REPO_URL = 'https://cdn.cocoapods.org/'.freeze  

    def url  
      @url ||= TRUNK_REPO_URL  
      super  
    end  
  end  
end

module Pod  
  class Source  
    class Manager  
      attr_reader :repos_dir  

      def initialize(repos_dir)  
        @repos_dir = Pathname(repos_dir).expand_path  
      end  

      def source_repos  
        return [] unless repos_dir.exist?  
        repos_dir.children.select(&:directory?).sort_by { |d| d.basename.to_s.downcase }  
      end  

      def aggregate  
        aggregate_with_repos(source_repos)  
      end  

      def aggregate_with_repos(repos)  
        sources = repos.map { |path| source_from_path(path) }  
        @aggregates_by_repos ||= {}  
        @aggregates_by_repos[repos] ||= Source::Aggregate.new(sources)  
      end  

      def all  
        aggregate.sources  
      end  
      ## ...  
    end  
  end  
end

def source_from_path(path)  
  @sources_by_path ||= Hash.new do |hash, key|  
    hash[key] = case  
                when key.basename.to_s == Pod::TrunkSource::TRUNK_REPO_NAME  
                  TrunkSource.new(key)  
                when (key + '.url').exist?  
                  CDNSource.new(key)  
                else  
                  Source.new(key)  
                end  
  end  
  @sources_by_path[path]  
end

module Pod  
  class Source  
    class Aggregate  
      attr_reader :sources  

      def initialize(sources)  
        raise "Cannot initialize an aggregate with a nil source: (#{sources})" if sources.include?(nil)  
        @sources = sources  
      end  
      ## 查询依赖对应的 specs  
      def search(dependency) ... end  

      ## 查询某个 pod 以发布的 specs  
      def search_by_name(query, full_text_search = false) ... end  

      ## ...  
  end  
end

source 'https://github.com/CocoaPods/Specs.git'  
source 'https://github.com/artsy/Specs.git'

## lib/cocoapods/installer/analyzer.rb  

def sources  
  @sources ||= begin  
    ## 省略获取 podfile、plugins、dependencies 的 source url ...  
    sources = ...  

    result = sources.uniq.map do |source_url|  
      sources_manager.find_or_create_source_with_url(source_url)  
    end  
    unless plugin_sources.empty?  
      result.insert(0, *plugin_sources)  
      plugin_sources.each do |source|  
        sources_manager.add_source(source)  
      end  
    end  
    result  
  end  
end

## lib/cocoapods/sources_manager.rb  

module Pod  
  class Source  
    class Manager  

      def find_or_create_source_with_url(url)  
        source_with_url(url) || create_source_with_url(url)  
      end  

      def create_source_with_url(url)  
        name = name_for_url(url)  
        is_cdn = cdn_url?(url)  
    ## ...  
        begin  
          if is_cdn  
            Command::Repo::AddCDN.parse([name, url]).run  
          else  
            Command::Repo::Add.parse([name, url]).run  
          end  
        rescue Informative => e  
          raise Informative, ## ...  
        ensure  
          UI.title_level = previous_title_level  
        end  
        source = source_with_url(url)  
        raise "Unable to create a source with URL #{url}" unless source  
        source  
      end  
      ## ...  
    end  
  end  
end

## lib/cocoapods-core/source/manager.rb  

def source_with_url(url)  
  url = canonic_url(url)  
  url = 'https://github.com/cocoapods/specs' if url =~ %r{github.com[:/]+cocoapods/specs}  
  all.find do |source|  
    source.url && canonic_url(source.url) == url  
  end  
end  

def canonic_url(url)  
  url.downcase.gsub(/\.git$/, '').gsub(%r{\/$}, '')  
end

## lib/cocoapods-core/source/aggregate.rb  

def search(dependency)  
  found_sources = sources.select { |s| s.search(dependency) }  
  unless found_sources.empty?  
    Specification::Set.new(dependency.root_name, found_sources)  
  end  
end

## lib/cocoapods-core/cdn_source.rb  

def search(query)  
  unless specs_dir  
    raise Informative, "Unable to find a source named: `#{name}`"  
  end  
  if query.is_a?(Dependency)  
    query = query.root_name  
  end  

  fragment = pod_shard_fragment(query)  
  ensure_versions_file_loaded(fragment)  
  version_arrays_by_name = @version_arrays_by_fragment_by_name[fragment] || {}  

  found = version_arrays_by_name[query].nil? ? nil : query  

  if found  
    set = set(query)  
    set if set.specification_name == query  
  end  
end

AFNetworking/0.10.0/0.10.1/.../4.0.1  
AppseeAnalytics/2.4.7/2.4.8/2.4.8.0/...  
DynamsoftBarcodeReader/7.1.0/...  
...

def ensure_versions_file_loaded(fragment)  
  return if !@version_arrays_by_fragment_by_name[fragment].nil? && !@check_existing_files_for_update  

  index_file_name = index_file_name_for_fragment(fragment)  
  download_file(index_file_name)  
  versions_raw = local_file(index_file_name, &:to_a).map(&:chomp)  
  @version_arrays_by_fragment_by_name[fragment] = versions_raw.reduce({}) do |hash, row|  
    row = row.split('/')  
    pod = row.shift  
    versions = row  

    hash[pod] = versions  
    hash  
  end  
end  

def index_file_name_for_fragment(fragment)  
  fragment_joined = fragment.join('_')  
  fragment_joined = '_' + fragment_joined unless fragment.empty?  
  "all_pods_versions#{fragment_joined}.txt"  
end

module Pod  
  class Specification  
    class Set  
      attr_reader :name  
      attr_reader :sources  

      def specification_name  
        versions_by_source.each do |source, versions|  
          next unless version = versions.first  
          return source.specification(name, version).name  
        end  
        nil  
      end  

      def versions_by_source  
        @versions_by_source ||= sources.each_with_object({}) do |source, result|  
          result[source] = source.versions(name)  
        end  
      end  
      ## ...  
    end  
  end  
end

$ pod search `QUERY`

## lib/cocoapods/installer/analyzer.rb  

def update_repositories  
  sources.each do |source|  
    if source.updateable?  
      sources_manager.update(source.name, true)  
    else  
      UI.message "Skipping ..."  
    end  
  end  
  @specs_updated = true  
end

## lib/cocoapods/sources_managers.rb  

def update(source_name = nil, show_output = false)  
  if source_name  
    sources = [updateable_source_named(source_name)]  
  else  
    sources = updateable_sources  
  end  

  changed_spec_paths = {}  

  ## Do not perform an update if the repos dir has not been setup yet.  
  return unless repos_dir.exist?  

  File.open("#{repos_dir}/Spec_Lock", File::CREAT) do |f|  
    f.flock(File::LOCK_EX)  
    sources.each do |source|  
      UI.section "Updating spec repo `#{source.name}`" do  
        changed_source_paths = source.update(show_output)  
        changed_spec_paths[source] = changed_source_paths if changed_source_paths.count > 0  
        source.verify_compatibility!  
      end  
    end  
  end  
  update_search_index_if_needed_in_background(changed_spec_paths)  
end

def update(show_output)  
  return [] if unchanged_github_repo?  
  prev_commit_hash = git_commit_hash  
  update_git_repo(show_output)  
  @versions_by_name.clear  
  refresh_metadata  
  if version = metadata.last_compatible_version(Version.new(CORE_VERSION))  
    tag = "v#{version}"  
    CoreUI.warn "Using the ..."  
    repo_git(['checkout', tag])  
  end  
  diff_until_commit_hash(prev_commit_hash)  
end

def update(_show_output)  
  @check_existing_files_for_update = true  
  begin  
    preheat_existing_files  
  ensure  
    @check_existing_files_for_update = false  
  end  
  []  
end  

def preheat_existing_files  
  files_to_update = files_definitely_to_update + deprecated_local_podspecs - ['deprecated_podspecs.txt']  

  concurrent_requests_catching_errors do  
    loaders = files_to_update.map do |file|  
      download_file_async(file)  
    end  
    Promises.zip_futures_on(HYDRA_EXECUTOR, *loaders).wait!  
  end  
end

$ pod repo update `[NAME]`

## lib/cocoapods/command/repo/update.rb  

module Pod  
  class Command  
    class Repo < Command  
      class Update < Repo  
        def run  
          show_output = !config.silent?  
          config.sources_manager.update(@name, show_output)  
          exclude_repos_dir_from_backup  
        end  
        ## ...  
      end  
    end  
  end  
end

module Molinillo  
  ## 解析状态  
  ResolutionState = Struct.new(  
    ## [String] 当前需求名称  
    :name,  
    ## [Array<Object>] 未处理的需求  
    :requirements,  
    ## [DependencyGraph] 依赖关系图  
    :activated,  
    ## [Object] 当前需求  
    :requirement,  
    ## [Object] 满足当前需求的可能性  
    :possibilities,  
    ## [Integer] 解析深度  
    :depth,  
    ## [Hash] 未解决的冲突，以需求名为 key  
    :conflicts,  
    ## [Array<UnwindDetails>] 记录着未处理过的需要用于回溯的信息  
    :unused_unwind_options  
  )  

  class ResolutionState  
    def self.empty  
      new(nil, [], DependencyGraph.new, nil, nil, 0, {}, [])  
    end  
  end  

  ## 记录一组需求和满足当前需求的可能性  
  class DependencyState < ResolutionState  
  ## 通过不断 pop 过滤包含的可能性，找出最符合需求的解  
    def pop_possibility_state  
      PossibilityState.new(  
        name,  
        requirements.dup,  
        activated,  
        requirement,  
        [possibilities.pop],  
        depth + 1,  
        conflicts.dup,  
        unused_unwind_options.dup  
      ).tap do |state|  
        state.activated.tag(state)  
      end  
    end  
  end  

  ## 仅包含一个满足需求的可能性  
  class PossibilityState < ResolutionState  
  end  
end

## lib/molinillo/resolution.rb  

def resolve  
  ## 1. 初始化 timer 统计耗时初始位置打点  
  ## 2. 内部会调用 push_initial_state 初始化 DependencyState 压栈  
  ## 3. 初始化 DependencyGraph 实例  
  start_resolution  

  while state  
    break if !state.requirement && state.requirements.empty?  
    ## 输出一个进度占位  
    indicate_progress  
    if state.respond_to?(:pop_possibility_state) ## DependencyState  
      ## 调试日志入口  
      ## 如果环境变量 MOLINILLO_DEBUG 是非 nil 就输出 log  
      ## 这里的调试日志有助于排查 Pod 组件的依赖问题  
      debug(depth) { "Creating possibility state for #{requirement} (#{possibilities.count} remaining)" }  
      state.pop_possibility_state.tap do |s|  
        if s  
          states.push(s)  
          activated.tag(s)  
        end  
      end  
    end  
    ## 处理栈顶 Possibility State  
    process_topmost_state  
  end  
 ## 遍历 Dependency Graph   
  resolve_activated_specs  
ensure  
  end_resolution  
end

target 'Example' do  
  pod 'C', :path => '../'  
  pod 'B', :path => '../'    
  pod 'A', :path => '../’  
end

[  
   #<struct Molinillo::DependencyState name="C", requirements=[B, A], ...>,   
   #<struct Molinillo::PossibilityState name="C", requirements=[B, A], ...>  
   #<struct Molinillo::DependencyState name="B", requirements=[A], ...>  
   #<struct Molinillo::PossibilityState name="B", requirements=[A], ...>  
   ## 省略了 C、C、A、A...  
   #<struct Molinillo::DependencyState name="B", requirements=[], ...,   
   #<struct Molinillo::PossibilityState name="B", requirements=[], ...,   
   #<struct Molinillo::DependencyState name="", requirements=[], ...,   
]

module Molinillo  

  class DependencyGraph  
    ## 有向边  
    Edge = Struct.new(:origin, :destination, :requirement)  
    ## @return [{String => Vertex}] 用字典保存顶点, key 为顶点名称(即 requirement.name)  
    attr_reader :vertices  
    ## @return [Log] 操作日志  
    attr_reader :log  
  ...  
end

module Molinillo  
  class DependencyGraph  
    class Vertex  
      attr_accessor :name  
      ## @return [Object] 顶点的元数据，reqiuremnt 对应的 possiblity  
      attr_accessor :payload  
      ## @return [Array<Object>] 需要依赖该顶点可能性能的需求  
      attr_reader :explicit_requirements  
      ## @return [Boolean] 是否为根结点  
      attr_accessor :root  
      ## @return [Array<Edge>] 出度 {Edge#origin}  
      attr_accessor :outgoing_edges  
      ## @return [Array<Edge>] 入度 {Edge#destination}  
      attr_accessor :incoming_edges  
      ## @return [Array<Vertex>] 入度的起点  
      def predecessors  
        incoming_edges.map(&:origin)  
      end  
      ## @return [Array<Vertex>] 出度的终点  
      def successors  
        outgoing_edges.map(&:destination)  
      end  
      ...  
    end  
  end  
end

## frozen_string_literal: true  

module Molinillo  
  class DependencyGraph  
    class Log  
      def initialize  
        @current_action = @first_action = nil  
      end  

      def pop!(graph)  
        return unless action = @current_action  
        unless @current_action = action.previous  
          @first_action = nil  
        end  
        action.down(graph)  
        action  
      end  

      ## 回撤到指定的操作节点  
      def rewind_to(graph, tag)  
        loop do  
          action = pop!(graph)  
          raise "No tag #{tag.inspect} found" unless action  
          break if action.class.action_name == :tag && action.tag == tag  
        end  
      end  

      private  

      ## 插入操作节点  
      def push_action(graph, action)  

        action.previous = @current_action  
        @current_action.next = action if @current_action  
        @current_action = action  
        @first_action ||= action  
        action.up(graph)  
      end  
      ...  
    end  
  end  
end

## frozen_string_literal: true  

module Molinillo  
  class DependencyGraph  

    class Action  
      ## @return [Symbol] action 名称  
      def self.action_name  
        raise 'Abstract'  
      end  

      ## 对图执行正向操作  
      def up(graph)  
        raise 'Abstract'  
      end  

      ## 撤销对图的操作  
      def down(graph)  
        raise 'Abstract'  
      end  

      ## @return [Action,Nil] 前序节点  
      attr_accessor :previous  
      ## @return [Action,Nil] 后序节点  
      attr_accessor :next  
    end  
  end  
end

## frozen_string_literal: true  

require_relative 'action'  
module Molinillo  
  class DependencyGraph  
  ## @!visibility private  
    class AddVertex < Action ## :nodoc:  
      def self.action_name  
        :add_vertex  
      end  

      ## 操作添加顶点  
      def up(graph)  
        if existing = graph.vertices[name]  
          @existing_payload = existing.payload  
          @existing_root = existing.root  
        end  
        vertex = existing || Vertex.new(name, payload)  
        graph.vertices[vertex.name] = vertex  
        vertex.payload ||= payload  
        vertex.root ||= root  
        vertex  
      end  

      ## 删除顶点  
      def down(graph)  
        if defined?(@existing_payload)  
          vertex = graph.vertices[name]  
          vertex.payload = @existing_payload  
          vertex.root = @existing_root  
        else  
          graph.vertices.delete(name)  
        end  
      end  

      ## @return [String] 顶点名称 (或者说依赖名称)  
      attr_reader :name  
      ## @return [Object] 顶点元数据  
      attr_reader :payload  
      ## @return [Boolean] 是否为根  
      attr_reader :root  
  ...  
    end  
  end  
end

def tag(graph, tag)  
  push_action(graph, Tag.new(tag))  
end  

def add_vertex(graph, name, payload, root)  
  push_action(graph, AddVertex.new(name, payload, root))  
end  

def detach_vertex_named(graph, name)  
  push_action(graph, DetachVertexNamed.new(name))  
end  

def add_edge_no_circular(graph, origin, destination, requirement)  
  push_action(graph, AddEdgeNoCircular.new(origin, destination, requirement))  
end  

def delete_edge(graph, origin_name, destination_name, requirement)  
  push_action(graph, DeleteEdge.new(origin_name, destination_name, requirement))  
end  

def set_payload(graph, name, payload)  
  push_action(graph, SetPayload.new(name, payload))  
end

module Molinillo  
  class DependencyGraph  
    def add_vertex(name, payload, root = false)  
      log.add_vertex(self, name, payload, root)  
    end  
    ...  
  end  
end

## frozen_string_literal: true  
module Molinillo  
  class DependencyGraph  
    ## @!visibility private       
    class Tag < Action  

      def up(graph)  
      end  

      def down(graph)  
      end  

      attr_reader :tag  

      def initialize(tag)  
        @tag = tag  
      end  
    end  
  end  
end

def possibility_satisfies_requirements?(possibility, requirements)  
  name = name_for(possibility)  

  activated.tag(:swap)  
  activated.set_payload(name, possibility) if activated.vertex_named(name)  
  satisfied = requirements.all? { |r| requirement_satisfied_by?(r, activated, possibility) }  
  activated.rewind_to(:swap)  

  satisfied  
end

possibility: #<Pod::Specification name="Alamofire" version="4.0.0">  
requirements: [  
 <Pod::Dependency name=Alamofire requirements=~> 3.0 source=nil external_source=nil>,   <Pod::Dependency name=Alamofire requirements=~> 4.0 source=nil external_source=nil>  
]

Conflict = Struct.new(  
  :requirement,  
  :requirements,  
  :existing,  
  :possibility_set,  
  :locked_requirement,  
  :requirement_trees,  
  :activated_by_name,  
  :underlying_error  
)

## frozen_string_literal: true  

module Molinillo  

  class ResolverError < StandardError; end  

  ## 错误信息："Unable to find a specification for `#{dependency}`"  
  class NoSuchDependencyError < ResolverError ... end  

  ## 错误信息："There is a circular dependency between ..."  
  class CircularDependencyError < ResolverError ... end  

  ## 当出现版本冲突时抛出  
  ## 错误信息："Unable to satisfy the following requirements:\n\n ..."  
  class VersionConflict < ResolverError ... end  
end

def process_topmost_state  
  if possibility  
    attempt_to_activate  
  else  
    create_conflict  
    unwind_for_conflict  
  end  
rescue CircularDependencyError => underlying_error  
  create_conflict(underlying_error)  
  unwind_for_conflict  
end  

def attempt_to_activate  
  debug(depth) { 'Attempting to activate ' + possibility.to_s }  
  existing_vertex = activated.vertex_named(name)  
  if existing_vertex.payload  
    debug(depth) { "Found existing spec (#{existing_vertex.payload})" }  
    attempt_to_filter_existing_spec(existing_vertex)  
  else  
    latest = possibility.latest_version  
    possibility.possibilities.select! do |possibility|  
      requirement_satisfied_by?(requirement, activated, possibility)  
    end  
    if possibility.latest_version.nil?  
      ## ensure there's a possibility for better error messages  
      possibility.possibilities << latest if latest  
      create_conflict  
      unwind_for_conflict  
    else  
      activate_new_spec  
    end  
  end  
end  

def attempt_to_filter_existing_spec(vertex)  
  filtered_set = filtered_possibility_set(vertex)  
  if !filtered_set.possibilities.empty?  
    activated.set_payload(name, filtered_set)  
    new_requirements = requirements.dup  
    push_state_for_requirements(new_requirements, false)  
  else  
    create_conflict  
    debug(depth) { "Unsatisfied by existing spec (#{vertex.payload})" }  
    unwind_for_conflict  
  end  
end

def unwind_for_conflict  
  details_for_unwind = build_details_for_unwind  
  unwind_options = unused_unwind_options  
  debug(depth) { "Unwinding for conflict: #{requirement} to #{details_for_unwind.state_index / 2}" }  
  conflicts.tap do |c|  
    sliced_states = states.slice!((details_for_unwind.state_index + 1)..-1)  
    raise_error_unless_state(c)  
    activated.rewind_to(sliced_states.first || :initial_state) if sliced_states  
    state.conflicts = c  
    state.unused_unwind_options = unwind_options  
    filter_possibilities_after_unwind(details_for_unwind)  
    index = states.size - 1  
    @parents_of.each { |_, a| a.reject! { |i| i >= index } }  
    state.unused_unwind_options.reject! { |uw| uw.state_index >= index }  
  end  
end

UnwindDetails = Struct.new(  
  :state_index,  
  :state_requirement,  
  :requirement_tree,  
  :conflicting_requirements,  
  :requirement_trees,  
  :requirements_unwound_to_instead  
)  

class UnwindDetails  
  include Comparable  
  ...  
end

[  
  [  
    <Pod::Dependency name=A requirements=nil source=nil external_source=nil>,  
    <Pod::Dependency name=Alamofire requirements=~> 3.0 ...>  
  ],[  
    <Pod::Dependency name=B ...>,  
    <Pod::Dependency name=Alamofire requirements=~> 4.0 ...>  
  ]  
]

def build_details_for_unwind  
  current_conflict = conflicts[name]  
  binding_requirements = binding_requirements_for_conflict(current_conflict)  
  unwind_details = unwind_options_for_requirements(binding_requirements)  

  last_detail_for_current_unwind = unwind_details.sort.last  
  current_detail = last_detail_for_current_unwind  

  ## filter & update details options  
  ...  
  current_detail  
end

def unwind_options_for_requirements(binding_requirements)  
  unwind_details = []  

  trees = []  
  binding_requirements.reverse_each do |r|  
    partial_tree = [r]  
    trees << partial_tree  
    unwind_details << UnwindDetails.new(-1, nil, partial_tree, binding_requirements, trees, [])  
    ## 1.1 获取 requirement 对应的 state  
    requirement_state = find_state_for(r)  
    ## 1.2 确认 possibility 存在  
    if conflict_fixing_possibilities?(requirement_state, binding_requirements)  
      ## 1.3 生成 detail 存入 unwind_details  
      unwind_details << UnwindDetails.new(  
        states.index(requirement_state),  
        r,  
        partial_tree,  
        binding_requirements,  
        trees,  
        []  
      )  
    end  

    ## 2. 沿着 requirement 依赖树的父节点获取其 state  
    parent_r = parent_of(r)  
    next if parent_r.nil?  
    partial_tree.unshift(parent_r)  
    requirement_state = find_state_for(parent_r)  
    ## 重复 1.2， 1.3 步骤 ...  

    ## 6. 沿着依赖树，重复上述操作  
    grandparent_r = parent_of(parent_r)  
    until grandparent_r.nil?  
      partial_tree.unshift(grandparent_r)  
      requirement_state = find_state_for(grandparent_r)  
      ## 重复 1.2、1.3 步骤 ...  
      parent_r = grandparent_r  
      grandparent_r = parent_of(parent_r)  
    end  
  end  

  unwind_details  
end

sliced_states = states.slice!((details_for_unwind.state_index + 1)..-1)

module Molinillo  
   module SpecificationProvider  

    def search_for(dependency)  
      []  
    end  

    def dependencies_for(specification)  
      []  
    end  
    ...  
  end  
end

require_relative 'dependency_graph'  

module Molinillo  

  class Resolver  
    require_relative 'resolution'  

    attr_reader :specification_provider  
    attr_reader :resolver_ui  

    def initialize(specification_provider, resolver_ui)  
      @specification_provider = specification_provider  
      @resolver_ui = resolver_ui  
    end  

    def resolve(requested, base = DependencyGraph.new)  
      Resolution.new(specification_provider,  
                     resolver_ui,  
                     requested,  
                     base).  
        resolve  
    end  
  end  
end

## /lib/CocoaPods/resolver.rb  
def resolve  
  dependencies = @podfile_dependency_cache.target_definition_list.flat_map do |target|  
    @podfile_dependency_cache.target_definition_dependencies(target).each do |dep|  
      next unless target.platform  
      @platforms_by_dependency[dep].push(target.platform)  
    end  
  end.uniq  
  @platforms_by_dependency.each_value(&:uniq!)  
  @activated = Molinillo::Resolver.new(self, self).resolve(dependencies, locked_dependencies)  
  resolver_specs_by_target  
rescue Molinillo::ResolverError => e  
  handle_resolver_error(e)  
end

module Molinillo  
  module Delegates  

    module SpecificationProvider  

      def search_for(dependency)  
        with_no_such_dependency_error_handling do  
          specification_provider.search_for(dependency)  
        end  
      end  

      def dependencies_for(specification)  
        with_no_such_dependency_error_handling do  
          specification_provider.dependencies_for(specification)  
        end  
      end  

      ...  

      private  

      def with_no_such_dependency_error_handling  
        yield  
      rescue NoSuchDependencyError => error  
        if state  
          ...  
        end  
        raise  
      end  
    end  
  end  
end

Test.xcodeproj  
├── project.pbxproj  
├── project.xcworkspace  
│   ├── contents.xcworkspacedata  
│   └── xcuserdata  
│       └── edmond.xcuserdatad  
│           └── UserInterfaceState.xcuserstate  
└── xcuserdata  
    └── edmond.xcuserdatad  
        └── xcschemes  
            └── xcschememanagement.plist

Test.xcworkspace  
└── contents.xcworkspacedata

<?xml version="1.0" encoding="UTF-8"?>  
<Workspace  
   version = "1.0">  
   <FileRef  
      location = "group:Test.xcodeproj">  
   </FileRef>  
   <FileRef  
      location = "group:Pods/Pods.xcodeproj">  
   </FileRef>  
</Workspace>

<?xml version="1.0" encoding="UTF-8"?>  
<Workspace  
   version = "1.0">  
   <FileRef  
      location = "self:">  
   </FileRef>  
</Workspace>

Array：  
plist => ( "1", "2", "3" )  
json => [ "1", "2", "3" ]  

Dictionary：  
plist => { "key" = "value"; ... }  
json => { "key" : "value", ... }

plutil -convert xml1 -s -r -o project.pbxproj.xml project.pbxproj

// !$*UTF8*$!  
{  
 archiveVersion = 1;  
 classes = {  
 };  
 objectVersion = 50;  
 objects = {  
  ...  
   };  
 rootObject = 8528A0D92651F281005BEBA5 /* Project object */;  
}

53 => 'Xcode 11.4',  
52 => 'Xcode 11.0',  
51 => 'Xcode 10.0',  
50 => 'Xcode 9.3',  
48 => 'Xcode 8.0',  
47 => 'Xcode 6.3',  
46 => 'Xcode 3.2',  
45 => 'Xcode 3.1',

struct globalidentifier {  
    int8_t user;            // encoded username  
    int8_t pid;             // encoded current pid ##  
    int16_t _random;        // encoded random value  
    int32_t _time;          // encoded time value  
    int8_t zero;            // zero  
    int8_t host_shift;      // encoded, shifted hostid  
    int8_t host_h;          // high byte of lower bytes of hostid  
    int8_t host_l;          // low byte of lower bytes of hostid  
} __attribute__((packed));  // packed, so we always have a length of 12 bytes

/* Begin PBXProject section */  
   8528A0D92651F281005BEBA5 /* Project object */ = {  
      isa = PBXProject;  
      ...  
      mainGroup = 8528A0D82651F281005BEBA5;  
      productRefGroup = 8528A0E22651F281005BEBA5 /* Products */;  
      projectDirPath = "";  
      projectRoot = "";  
      targets = (   
         8528A0E02651F281005BEBA5 /* Test */,  
      );  
   };  
/* End PBXProject section */

module Xcodeproj  
  class Project  
    ## object 模块，后面会提到  
    include Object   
    ...  
    ## 序列化时 project 的最低兼容版本, 与 object_version 对应  
    attr_reader :archive_version  
    ## 作用未知  
    attr_reader :classes  
    ## project 最低兼容版本  
    attr_reader :object_version  
    ## project 所包含的 objects，结构为 [Hash{String => AbstractObject}]  
    attr_reader :objects_by_uuid  
    ## project 的根结点，即 PBXProject  
    attr_reader :root_object  
  end  
end

def inspect_targets_to_integrate  
  project = Xcodeproj::Project.open(project_path)  
  ...  
end

## lib/xcproject/Project.rb  

def self.open(path)  
  path = Pathname.pwd + path  
  unless Pathname.new(path).exist?  
    raise "[Xcodeproj] Unable to open `#{path}` because it doesn't exist."  
  end  
  project = new(path, true)  
  project.send(:initialize_from_file)  
  project  
end  

def initialize_from_file  
  pbxproj_path = path + 'project.pbxproj'  
  plist = Plist.read_from_path(pbxproj_path.to_s)  
  root_object.remove_referrer(self) if root_object  
  @root_object     = new_from_plist(plist['rootObject'], plist['objects'], self)  
  @archive_version = plist['archiveVersion']  
  @object_version  = plist['objectVersion']  
  @classes         = plist['classes'] || {}  
  @dirty           = false  

  unless root_object  
    raise "[Xcodeproj] Unable to find a root object in #{pbxproj_path}."  
  end  

  if archive_version.to_i > Constants::LAST_KNOWN_ARCHIVE_VERSION  
    raise '[Xcodeproj] Unknown archive version.'  
  end  

  if object_version.to_i > Constants::LAST_KNOWN_OBJECT_VERSION  
    raise '[Xcodeproj] Unknown object version.'  
  end  

  ## Projects can have product_ref_groups that are not listed in the main_groups["Products"]  
  root_object.product_ref_group ||= root_object.main_group['Products'] || root_object.main_group.new_group('Products')  
end

/* Begin PBXProject section */  
   8528A0D92651F281005BEBA5 /* Project object */ = {  
      isa = PBXProject;  
      ...  
      mainGroup = 8528A0D82651F281005BEBA5;  
      productRefGroup = 8528A0E22651F281005BEBA5 /* Products */;  
      projectDirPath = "";  
      projectRoot = "";  
      targets = (   
         8528A0E02651F281005BEBA5 /* Test */,  
      );  
   };  
/* End PBXProject section */

class AbstractObject  
  ## object 类型  
  attr_reader :isa  
  ## object 唯一标识  
  attr_reader :uuid  
  ## 持有 object 的 project  
  attr_reader :project  

  def initialize(project, uuid)  
    @project = project  
    @uuid = uuid  
    @isa = self.class.isa  
    @referrers = []  
    unless @isa =~ /^(PBX|XC)/  
      raise "[Xcodeproj] Attempt to initialize an abstract class (#{self.class})."  
    end  
  end  
  ...  
end