# configs详解——之field
`field`定义一个抽取项, 一个`field`可以定义下面这些东西
#### `name`
> 给此项数据起个变量名\
> 变量名中不能包含.\
> 如果抓取到的数据想要以文章或者问答的形式发布到网站(WeCenter,\
> WordPress, Discuz!等), field的命名请参考两个完整demo中的命名, 否则无法发布成功
**String类型 不能为空**
举个栗子:
给`field`起了个名字叫`content`
```
array(
'name' => "content",
'selector' => "//*[@id='single-next-link']"
)
```
#### `selector`
> 定义抽取规则, 默认使用xpath\
> 如果使用其他类型的, 需要指定selector\_type
**String类型 不能为空**
举个栗子:\
使用xpath来抽取糗事百科的笑话内容，selector的值就是内容的xpath
```
array(
'name' => "content",
'selector' => "//*[@id='single-next-link']"
)
```
#### `selector_type`
抽取规则的类型
> 目前可用[xpath](http://www.w3school.com.cn/xpath/index.asp), [jsonpath](http://www.cnblogs.com/draem0507/p/5111002.html), [regex](http://www.runoob.com/regexp/regexp-tutorial.html)\
> 默认`xpath`
**枚举类型**
栗子1:\
selector默认使用xpath
```
array(
'name' => "content",
'selector' => "//*[@id='single-next-link']" // xpath抽取规则
)
```
栗子2:\
使用正则表达式来抽取数据
```
array(
'name' => "content",
'selector_type' => 'regex',
'selector' => '#([^/]+)#i' // regex抽取规则
)
```
#### `required`
> 定义该`field`的值是否必须, 默认false\
> 赋值为true的话, 如果该`field`没有抽取到内容, 该field对应的整条数据都将被丢弃
**布尔类型**
举个栗子:
```
array(
'name' => "content",
'selector' => "//*[@id='single-next-link']",
'required' => true
)
```
#### `repeated`
> 定义该`field`抽取到的内容是否是有多项, 默认`false`\
> 赋值为true的话, 无论该`field`是否真的是有多项, 抽取到的结果都是数组结构
**布尔类型**
举个栗子:\
爬取的网页中包含多条评论，所以抽取评论的时候要将repeated赋值为true
```
array(
'name' => "comments",
'selector' => "//*[@id='zh-single-question-page']//a[contains(@class,'zm-item-tag')]",
'repeated' => true
)
```
#### `children`
> 为此`field`定义子项\
> 子项的定义仍然是一个`fields`数组\
> 没错, 这是一个树形结构
**数组类型**
举个栗子:\
抓取糗事百科的评论，每个评论爬取了内容，点赞数
```
array(
'name' => "article_comments",
'selector' => "//div[contains(@class,'comments-wrap')]",
'children' => array(
array(
'name' => "replay",
'selector' => "//div[contains(@class,'replay')]",
'repeated' => true,
),
array(
'name' => "report",
'selector' => "//div[contains(@class,'report')]",
'repeated' => true,
)
)
)
```
#### `source_type`
该field的数据源, 默认从当前的网页中抽取数据\
选择`attached_url`可以发起一个新的请求, 然后从请求返回的数据中抽取\
选择`url_context`可以从当前网页的url附加数据（点此查看“url附加数据”实例解析）中抽取
**枚举类型**
#### `attached_url`
当source\_type设置为`attached_url`时, 定义新请求的url
**String类型**
举个栗子:\
当爬取的网页中某些内容需要异步加载请求时，就需要使用attached\_url，比如，抓取知乎回答中的评论部分，就是通过AJAX异步请求的数据
```
array(
'name' => "comment_id",
'selector' => "//div/@data-aid",
),
array(
'name' => "comments",
'source_type' => 'attached_url',
// "comments"是从发送"attached_url"这个异步请求返回的数据中抽取的
// "attachedUrl"支持引用上下文中的抓取到的"field", 这里就引用了上面抓取的"comment_id"
'attached_url' => "https://www.zhihu.com/r/answers/{comment_id}/comments",
'selector_type' => 'jsonpath'
'selector' => "$.data",
'repeated => true,
'children' => array(
...
)
}
```
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