blocks|key|1191902|text|在模块中，self本身就是模块类。举个例子|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1191903|puts+self|code-block|syntax|javascript|1191904|会返回瑞克所以，|1191905|extend+self|1191906|基本上是让Rake中定义的实例方法对它可用，所以你可以这样做|1191907|Rake.run_tests|1191908|entityMap^0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|S|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|U|8|@]|9|@]|A|$]]|$1|M|3|N|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

In a module, self is the module class itself. So for example

<pre><code>puts self
</code></pre>

will return Rake
so, 

<pre><code>extend self
</code></pre>

basically makes the instance methods defined in Rake available to it, so you can do 

<pre><code>Rake.run_tests
</code></pre>

blocks|key|1191940|text|这是一种将实例方法转换为类方法的便捷方法。但您也可以将其用作more+efficient+singleton。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1191941|entityMap|0|LINK|mutability|MUTABLE|url|https://web.archive.org/web/20180727051657/http://ozmm.org/posts/singin_singletons.html^0|U|O|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@$A|M|B|N|1|O]]|C|$]]|$1|D|3|-4|5|6|7|P|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]]]

It is a convenient way to make instance methods into class methods. But you can also use it as a <a href="https://web.archive.org/web/20180727051657/http://ozmm.org/posts/singin_singletons.html" rel="nofollow noreferrer">more efficient singleton</a>.

blocks|key|1407391|text|对我来说，将extend看作单例类(也称为元或本征类)中的include总是很有帮助的。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1407392|您可能知道在singleton类中定义的方法基本上都是类方法：|1407393|module+A
++class+<<+self
++++def+x
++++++puts+'x'
++++end
++end
end

A.x+#=>+'x'|code-block|syntax|javascript|1407394|现在我们知道了，extend将include单例类内部模块中的方法，从而将它们公开为类方法：|1407395|module+A
++class+<<+self
++++include+A

++++def+x
++++++puts+'x'
++++end
++end

++def+y
++++puts+'y'
++end
end

A.x+#=>+'x'
A.y+#=>+'y'|1407396|entityMap^0|6|6|T|7|0|0|0|8|6|F|7|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]|$9|V|A|W|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|X|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|Y|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|Z|8|@$9|10|A|11|B|C]|$9|12|A|13|B|C]]|D|@]|E|$]]|$1|O|3|P|5|J|7|14|8|@]|D|@]|E|$K|L]]|$1|Q|3|-4|5|6|7|15|8|@]|D|@]|E|$]]]|R|$]]

For me it always helps to think of <code>extend</code> as <code>include</code> inside the singleton class (also known as meta or eigen class).

You probably know that methods defined inside the singleton class are basically class methods:

<pre><code>module A
 class &lt;&lt; self
 def x
 puts 'x'
 end
 end
end

A.x #=&gt; 'x'
</code></pre>

Now that we know that, <code>extend</code> will <code>include</code> the methods in the module inside the singleton class and thus expose them as class methods:

<pre><code>module A
 class &lt;&lt; self
 include A

 def x
 puts 'x'
 end
 end

 def y
 puts 'y'
 end
end

A.x #=&gt; 'x'
A.y #=&gt; 'y'
</code></pre>

blocks|key|1192057|text|extend+self包含所有现有的实例方法作为模块方法。这相当于说extend+Rake。此外，Rake是Module类的对象。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1192058|另一种实现等效行为的方法是：|1192059|module+Rake
++include+Test::Unit::Assertions

++def+run_tests+#+etc.
++end

end+

Rake.extend(Rake)|code-block|syntax|javascript|1192060|这可以用来定义具有私有方法的自包含模块。|1192061|entityMap^0|0|B|Y|B|1D|4|1I|6|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@$9|R|A|S|B|C]|$9|T|A|U|B|C]|$9|V|A|W|B|C]|$9|X|A|Y|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|Z|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|10|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|11|8|@]|D|@]|E|$]]|$1|O|3|-4|5|6|7|12|8|@]|D|@]|E|$]]]|P|$]]

<code>extend self</code> includes all the existing instance methods as module methods. This is equivalent to saying <code>extend Rake</code>. Also <code>Rake</code> is an object of class <code>Module</code>.

Another way to achieve equivalent behavior will be:

<pre><code>module Rake
 include Test::Unit::Assertions

 def run_tests # etc.
 end

end 

Rake.extend(Rake)
</code></pre>

This can be used to define self contained modules with private methods.

blocks|key|1407526|text|为了避免链接损坏，由user83510链接的blog+post+of+Chris+Wanstrath被转发到下面(在他的许可下)。尽管如此，没有什么比原版更好，所以只要他的链接继续工作，就一直使用他的链接。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1407527|1407528|→singin‘singletons+2008年11月18日有些东西我就是不明白。比如大卫·鲍伊。或者南半球。但是没有什么比Ruby的Singleton更让我困惑的了。因为真的，这完全没有必要。|1407529|下面是他们希望你对你的代码做的事情：|1407530|require+'net/http'

#+first+you+setup+your+singleton
class+Cheat
++include+Singleton

++def+initialize
++++@host+=+'http://cheat.errtheblog.com/'
++++@http+=+Net::HTTP.start(URI.parse(@host).host)
++end


++def+sheet(name)
++++@http.get("/s/#{name}").body
++end
end

#+then+you+use+it
Cheat.instance.sheet+'migrations'
Cheat.instance.sheet+'yahoo_ceo'|code-block|syntax|javascript|1407531|但这太疯狂了。与权力抗争。|1407532|require+'net/http'

#+here's+how+we+roll
module+Cheat
++extend+self

++def+host
++++@host+%7C%7C=+'http://cheat.errtheblog.com/'
++end

++def+http
++++@http+%7C%7C=+Net::HTTP.start(URI.parse(host).host)
++end

++def+sheet(name)
++++http.get("/s/#{name}").body
++end
end

#+then+you+use+it
Cheat.sheet+'migrations'
Cheat.sheet+'singletons'|1407533|有何不可呢？API更简洁，代码更易于测试、模拟和存根，而且如果需要，仍然可以非常简单地转换为合适的类。|1407534|((版权所有十个chris+wanstrath+))|1407535|entityMap|0|LINK|mutability|MUTABLE|url|http://ozmm.org/posts/singin_singletons.html^0|M|S|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|13|8|@]|9|@$A|14|B|15|1|16]]|C|$]]|$1|D|3|-4|5|6|7|17|8|@]|9|@]|C|$]]|$1|E|3|F|5|6|7|18|8|@]|9|@]|C|$]]|$1|G|3|H|5|6|7|19|8|@]|9|@]|C|$]]|$1|I|3|J|5|K|7|1A|8|@]|9|@]|C|$L|M]]|$1|N|3|O|5|6|7|1B|8|@]|9|@]|C|$]]|$1|P|3|Q|5|K|7|1C|8|@]|9|@]|C|$L|M]]|$1|R|3|S|5|6|7|1D|8|@]|9|@]|C|$]]|$1|T|3|U|5|6|7|1E|8|@]|9|@]|C|$]]|$1|V|3|-4|5|6|7|1F|8|@]|9|@]|C|$]]]|W|$X|$5|Y|Z|10|C|$11|12]]]]

To avoid link rot, the <a href="http://ozmm.org/posts/singin_singletons.html" rel="noreferrer">blog post of Chris Wanstrath</a> linked by user83510 is reposted below (with his permission).
Still, nothing beats an original, so use his link for as long as it continues to work.
<hr />
→ singin' singletons
18 November 2008
There’s stuff I just don’t understand. David Bowie, for instance. Or the Southern Hemisphere. But nothing quite boggles my mind like Ruby’s Singleton. Because really, it’s totally unnecessary.
Here’s what they want you to do with your code:
<pre><code>require 'net/http'

# first you setup your singleton
class Cheat
 include Singleton

 def initialize
 @host = 'http://cheat.errtheblog.com/'
 @http = Net::HTTP.start(URI.parse(@host).host)
 end


 def sheet(name)
 @http.get(&quot;/s/#{name}&quot;).body
 end
end

# then you use it
Cheat.instance.sheet 'migrations'
Cheat.instance.sheet 'yahoo_ceo'
</code></pre>
But that’s crazy. Fight the power.
<pre><code>require 'net/http'

# here's how we roll
module Cheat
 extend self

 def host
 @host ||= 'http://cheat.errtheblog.com/'
 end

 def http
 @http ||= Net::HTTP.start(URI.parse(host).host)
 end

 def sheet(name)
 http.get(&quot;/s/#{name}&quot;).body
 end
end

# then you use it
Cheat.sheet 'migrations'
Cheat.sheet 'singletons'
</code></pre>
Any why not? The API is more concise, the code is easier to test, mock, and stub, and it’s still dead simple to convert into a proper class should the need arise.
(( copyright ought ten chris wanstrath ))

In Ruby, I understand the basic idea of <code>extend</code>. However, what's happening in this segment of code? Specifically, what does <code>extend</code> do? Is it just a convenient way of making the instance methods into class methods? Why would you do it this way rather than specifying class methods from the beginning?

<pre><code>module Rake
 include Test::Unit::Assertions

 def run_tests # etc.
 end

 # what does the next line do?
 extend self
end
</code></pre>

Ruby: extend self

Ruby语言

在Ruby语言中，我理解了extend的基本思想。然而，在这段代码中发生了什么？具体地说，extend做了什么？这只是一种将实例方法转换为类方法的便捷方法吗？为什么要这样做，而不是从一开始就指定类方法？module Rake  include Test::Unit::Assertions  def run_tests ...

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