NSString initWithData返回null [英] NSString initWithData returns null
问题描述
我通过 NSURLConnection 从网站提取数据，并在 NSMutableData 的实例中保存接收的数据。在 connectionDidFinishLoading 委托方法中，数据将转换为一个字符串，并调用NSString的适当方法：
NSString * result = [[NSString alloc] initWithData：data
encoding：NSUTF8StringEncoding]
b $ b
结果字符串为null。如果我使用 NSASCIIStringEncoding ，但是，我获得适当的字符串，虽然unicode字符乱码如预期。服务器的 Content-Type 标头不是指定UTF-8编码，但我已尝试过一些类似情况的不同网站，有字符串转换发生只是罚款。它似乎问题只与给定的Web服务有关，但我没有线索为什么。
在一个侧面的注意，是从一个API良好实践拉网页和数据
非常感谢！
你说它绝对是UTF-8，但没有Content-Type头，你真的不知道。 （即使你有一个标题说，它仍然可能是错误的。）
我的猜测是，你的数据通常是ASCII，它总是作为UTF -8，但您有时试图解析实际在ISO 8859-1或Windows代码页1252中编码的数据。这些数据通常大多是ASCII，但是有些字节在0-127范围之外ASCII定义。 UTF-8将期望这样的字节形成在指定的范围序列内的代码单元序列，但是在其他编码中，任何字节，无论其值如何，都是其自身的完整字符。尝试将非ASCII非UTF-8数据解释为UTF-8将几乎总是得到错误的结果（错误的字符）或根本没有结果（不能解码;解码器返回 nil ），因为数据从来没有以UTF-8编码。
你应该首先尝试UTF-8，如果失败， 8859-1。如果您允许用户检索任何网页，您应该允许他们更改您用于解码数据的编码，以防他们发现它实际上是8859-9或codepage-1252或其他一些8位编码。 / p>
如果您要从特定服务器下载数据，特别是如果您对该服务器上运行的内容有影响，则应该提供准确的内容类型头文件和/或修复任何错误导致它提供不在UTF-8中的文本。
I am pulling data from a website via NSURLConnection and stashing the received data away in an instance of NSMutableData. In the connectionDidFinishLoading delegate method the data is convert into a string with a call to NSString's appropriate method:
NSString *result = [[NSString alloc] initWithData:data
encoding:NSUTF8StringEncoding]
The resulting string turns out to be a null. If I use the NSASCIIStringEncoding, however, I do obtain the appropriate string, albeit with unicode characters garbled up as expected. The server's Content-Type header does not specify the UTF-8 encoding, but I have attempted a number of different websites with a similar scenario, and there string conversion happens just fine. It seems like the problem only pertains to the given web service but I have no clue why.
On a side note, is pulling web pages and data from an API good practice, i.e. buffering the data, converting into a string, and manipulating the string afterwards?
Much appreciated!
You say that it "is definitely UTF-8", but without a Content-Type header, you don't really know that. (And even if you did have a header saying that, it could still be wrong.)
My guess is that your data is usually ASCII, which always parses correctly as UTF-8, but you sometimes are trying to parse data that's actually encoded in ISO 8859-1 or Windows codepage 1252. Such data will generally be mostly ASCII, but with some bytes outside the 0–127 range ASCII defines. UTF-8 would expect such bytes to form a sequence of code units within a specified sequence of ranges, but in other encodings, any byte, regardless of value, is a complete character on its own. Trying to interpret non-ASCII non-UTF-8 data as UTF-8 will almost always get you either wrong results (wrong characters) or no results at all (cannot decode; decoder returns nil), because the data was never encoded in UTF-8 in the first place.
You should try UTF-8 first, and if it fails, use ISO 8859-1. If you're letting the user retrieve any web page, you should let them change the encoding you use to decode the data, in case they discover that it was actually 8859-9 or codepage-1252 or some other 8-bit encoding.
If you're downloading the data from a specific server, and especially if you have influence on what runs on that server, you should make it serve up an accurate Content-Type header and/or fix whatever bug is causing it to serve up text that isn't in UTF-8.
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