ios - 设置 NSMutableURLRequest header 和添加一个 header 之间的区别

Question

我想知道设置 header 值和将 header 值添加到 NSMutableURLRequest 之间的区别是什么。听起来很明显，但是，例如，您不能每次都使用 addValue 吗？设置不存在的 header 会引发错误吗？当请求中已存在 header 时添加 header 会覆盖现有值吗？

例子

let request.NSMutableURLRequest(URL: NSURL(string: "someURL")!) 
request.addValue("application/json", forHTTPHeaderField: "Content-Type")
...

Answer 1

我觉得苹果官方文档里的讨论很清楚:

addValue

This method provides the ability to add values to header fields incrementally. If a value was previously set for the specified field, the supplied value is appended to the existing value using the appropriate field delimiter. In the case of HTTP, the delimiter is a comma.

setValue

The new value for the header field. Any existing value for the field is replaced by the new value.

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setValue 替换。 addValue 附加一个分隔符