介绍一下Mojolicious的DOM选择器Mojo::DOM和它的Mojo::UserAgent（比较Web::Scraper）

最近正好又需要做页面分析，以前全是用AnyEvent::HTTP和Web::Scraper。这次试了试Mojo::DOM和Mojo::UserAgent。
先说结论，我的试用结论是：如果程序不和web沾边，只是个页面分析或文件处理程序，那还是前者好。否则的话可以考虑Mojo.

先说Mojo::DOM和Mojo::UserAgent的优点：
Mojo::DOM做的这个dom选择器在一些时候是非常方便的
读入HTML以后可以精确定位需要的元素或是用回调的方式遍历。

1. my $dom = Mojo::DOM->new($html_string);
2. $dom->find(‘p[id]‘)->each(sub { say shift->{id} });

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在
配合Mojo::UserAgent使用的时候就更方便了。Mojo::UserAgent有丰富的功能，但如果你不想用那些，你可以就把它当成一个
wget（http client）用。它不但支持同步get也支持非阻塞get网页。而且和Mojo::DOM整合的很好。比如：

1. my $ua = Mojo::UserAgent->new;
2. my $title = $tx->res->dom->at(‘head title‘)->text;

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当把这一切放到Mojolicious web框架里的时候就更美好了，因为都是一个作者写的，整合性就非常好。以前要兴师动众的工作现在2，3行代码就完成了。

以上看着都很美好了，我说些在我看来的缺点。
1. 不支持XPATH。
我很熟悉XPATH，但很不幸，不支持XPATH。虽然很多东西都可以用mojo的方式实现，但我还是能说出一些我常用但没实现的东西。并且我猜测因为
此，效率也会差很多。由于Web::Scraper是用xpath，并且可以用XML::LibXML来解析html/xml，XML::LibXML是
目前所有DOM方式中最快的（libxml2 >
expat）。所以我认为一个纯perl写的非xpath方式的DOM选择器的效率是不足以做大规模数据分析的。（仅是猜测）

2. 可能是我的使用习惯，页面复杂的时候我还是更喜欢用Web::Scraper
用过Web::Scraper的人都知道，你需要先用xpath写一个符合某类页面的统一规则，然后用这一整套规则去分析一类页面。页面信息复杂的时候这
一套规则可能几十甚至上百行。而用Mojo::DOM就只能用好多find->each和perl回调函数裹在一起，不方便调试，写页面分析规则的
人还必须得会perl。

3. 没法用Coro::rouse_cb和Coro::rouse_wait了。

1. my $coro = async {
2. http_get "http://www.example.com/", Coro::rouse_cb;
3. my ($data, $header) = Coro::rouse_wait;
4. print Dumper $header;
5. };

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上面的这个可以。下面的这个就不行了。

1. my $coro = async {
2. my $ua = Mojo::UserAgent->new;
3. $ua->get(‘http://www.example.com/‘ => Coro::rouse_cb);
4. my ($ua2, $tx) = Coro::rouse_wait;
5. my $title = $tx->res->dom->at(‘head title‘)->text;
6. print "$title\n";
7. };

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