gem letter_a

用法和介绍(github)

1. 主要作用：用于给网站用户默认图像，当用户未上传图像的时候，给用户产生默认的图像.
2. 相关方法使用和源码解析
   方法1: setup(&_block),这个方法主要设置生成图片需要的一些值．

LetterAvatar.setup do |config|
    config.fill_color        = 'rgba(255, 255, 255, 1)' # default is 'rgba(255, 255, 255, 0.65)'
    config.cache_base_path   = 'public/system/lets'     # default is 'public/system'
    config.colors_palette    = :iwanthue                # default is :google
    config.weight            = 500                      # default is 300
    config.annotate_position = '-0+10'                  # default is -0+5
end

源码：

module LetterAvatar
    extend LetterAvatar::Configuration
        def self.setup(&_block)
             yield(self)
        end
end

setup方法需要传入一个代码块，同时代码块需要传递一个参数，此处传递的self代表LetterAvatar.
因此代码块内部的设值操作其实是调用了LetterAvatar的类方法．
这些类方法并未在代码中定义而是通过

extend LetterAvatar::Configuration

获取到的．详细可以看LetterAvatar::Configuration定义

module LetterAvatar
  module Configuration
    def cache_base_path
      @cache_base_path
    end
    def cache_base_path=(v)
      @cache_base_path = v
    end

    def fill_color
      @fill_color || Avatar::FILL_COLOR
    end
    def fill_color=(v)
      @fill_color = v
    end

    def colors_palette
      @colors_palette ||= :google
    end
    def colors_palette=(v)
      @colors_palette = v if v.in?(Colors::PALETTES)
    end

    def weight
      @weight ||= 300
    end
    def weight=(v)
      @weight = v
    end

    def annotate_position
      @annotate_position ||= '-0+5'
    end
    def annotate_position=(v)
      @annotate_position = v
    end
  end
end

方法２：generate(username, size)，用于生成图片．主要使用到了imagemagick.根据username首字母和size（默认会生成一张240*240图片）生成图片

module LetterAvatar
  extend LetterAvatar::Configuration
  def self.generate(username, size)
    Avatar.generate(username, size)
  end
end

因此方法的实现代码在Avatar中：

module LetterAvatar
  class Avatar

    class << self
      class Identity
        attr_accessor :color, :letter

        def self.from_username(username)
          identity = new
          identity.color = LetterAvatar::Colors.for(username)
          identity.letter = username[0].upcase

          identity
        end
      end

      def generate(username, size, opts = nil)
        identity = Identity.from_username(username)

        cache = true
        cache = false if opts && opts[:cache] == false

        size = FULLSIZE if size > FULLSIZE
        filename = cached_path(identity, size)

        return filename if cache && File.exist?(filename)

        fullsize = fullsize_path(identity)
        generate_fullsize(identity) if !cache || !File.exist?(fullsize)

        LetterAvatar.resize(fullsize, filename, size, size)
        filename
      end
    end
  end
end

Identify可以认为是一个局部类，此处主要功能是获取生成图片的两个重要元素，color,letter.
获取颜色的代码如下：

module LetterAvatar
  module Colors
    PALETTES = [:google, :iwanthue]

    GOOGLE_COLORS = [
      [226, 95, 81], # A
      [242, 96, 145], # B
      [187, 101, 202], # C
      [149, 114, 207], # D
      [120, 132, 205], # E
      [91, 149, 249], # F
      [72, 194, 249], # G
      [69, 208, 226], # H
      [72, 182, 172], # I
      [82, 188, 137], # J
      [155, 206, 95], # K
      [212, 227, 74], # L
      [254, 218, 16], # M
      [247, 192, 0], # N
      [255, 168, 0], # O
      [255, 138, 96], # P
      [194, 194, 194], # Q
      [143, 164, 175], # R
      [162, 136, 126], # S
      [163, 163, 163], # T
      [175, 181, 226], # U
      [179, 155, 221], # V
      [194, 194, 194], # W
      [124, 222, 235], # X
      [188, 170, 164], # Y
      [173, 214, 125] # Z
    ].freeze

    def self.for(username)
      public_send("with_#{LetterAvatar.colors_palette}", username)
    end

    def self.with_google(username)
      char = username[0].upcase

      if /[A-Z]/ =~ char
        # 65 is 'A' ord
        idx = char.ord - 65
        google[idx]
      elsif /[\d]/ =~ char
        google[char.to_i]
      else
        google[Digest::MD5.hexdigest(username)[0...15].to_i(16) % google.length]
      end
    end

    # Colors form Google Inbox
    # https://inbox.google.com
    def self.google
      GOOGLE_COLORS
    end

  end
end

LetterAvatar::Color通过一个动态方法public_send调用with_google获取首字母对于的数组GOOGLE_COLORS中的颜色值
接着生成图片的路径，因为会默认生成一个240*240的图片，因此调用了fullsize_path方法．

      def cached_path(identity, size)
        dir = "#{cache_path}/#{identity.letter}/#{identity.color.join('_')}"
        FileUtils.mkdir_p(dir)

        "#{dir}/#{size}.png"
      end

      def fullsize_path(identity)
        cached_path(identity, FULLSIZE)
      end

generate_fullsize(identity)　是真正用于产生图片的方法．主要就是使用imagemagick强大的图片裁剪功能．

      def generate_fullsize(identity)
        filename = fullsize_path(identity)

        LetterAvatar.execute(
          %W(
            convert
            -size #{FULLSIZE}x#{FULLSIZE}
            xc:#{to_rgb(identity.color)}
            -pointsize 140
            -font #{FONT_FILENAME}
            -weight #{LetterAvatar.weight}
            -fill '#{LetterAvatar.fill_color}'
            -gravity Center
            -annotate #{LetterAvatar.annotate_position} '#{identity.letter}'
            '#{filename}'
          ).join(' ')
        )
        filename
      end

最后调用此方法对图片进行裁剪，生成用户需要的大小的图片．

LetterAvatar.resize(fullsize, filename, size, size)