The issue is that by default most of these methods leave characters such as & = ? within a URL, as they are strictly speaking valid. The problem is that these characters have special meanings in a GET request, and will more than likely make your request invalid.
也就是说，你提供的 URL 字符串 里面可能包含某些字符，比如‘$‘ ‘&’ ‘？’...等，这些字符在 URL 语法中是具有特殊语法含义的，
比如 URL ：http://www.baidu.com/s?wd=%BD%AA%C3%C8%D1%BF&rsv_bp=0&rsv_spt=3&inputT=3512
中 的 & 起到分割作用 等等，如果 你提供的URL 本身就含有 这些字符，就需要把这些字符 转化为 “%+ASCII” 形式，以免造成冲突。
这就引入：CFURLCreateStringByAddingPercentEscapes 函数。
该函数将 将要添加到URL的字符串进行特殊处理，如果这些字符串含有 &， ？ 这些特殊字符，用“%+ASCII” 代替之。
CFURLCreateStringByAddingPercentEscapes( kCFAllocatorDefault, (CFStringRef)parameter, NULL,
CFSTR(":/?#[]@!$&’()*+,;="), kCFStringEncodingUTF8 )；
// 确定 parameter 字符串中含有:/?#[]@!$&’()*+,;= 这些字符时候，这些字符需要被转化，以免与语法冲突，其中空格是默认被转化的，所以没有列出来
例如： 建立一个 NSURL 的 category
+ (NSURL *)URLWithBaseString:(NSString *)baseString parameters:(NSDictionary *)parameters{ NSMutableString *urlString =[NSMutableString string]; //The URL starts with the base string[urlString appendString:baseString]; [urlString appendString:baseString]; NSString *escapedString; NSInteger keyIndex = 0; for (id key in parameters) { //First Parameter needs to be prefixed with a ? and any other parameter needs to be prefixed with an & if(keyIndex ==0) { CFStringRef encodedCFString = CFURLCreateStringByAddingPercentEscapes(kCFAllocatorDefault, (__bridge CFStringRef)[parameters valueForKey:key],nil,CFSTR("?!@#$^&%*+,:;='/"`<>()[]{}///| "), kCFStringEncodingUTF8);
escapedString = [[NSString alloc] initWithString:(__bridge_transfer NSString*) encodedCFString];
[urlString appendFormat:@"?%@=%@",key,escapedString]; }else{CFStringRef encodedCFString = CFURLCreateStringByAddingPercentEscapes(kCFAllocatorDefault, (__bridge CFStringRef)[parameters valueForKey:key],nil,CFSTR("?!@#$^&%*+,:;='/"`<>()[]{}///| "), kCFStringEncodingUTF8);
escapedString = [[NSString alloc] initWithString:(__bridge_transfer NSString*) encodedCFString];
[urlString appendFormat:@"&%@=%@",key,escapedString];
}
keyIndex++;
}
return [NSURL URLWithString:urlString];
}
@end
调用测试：
NSString * baseString = @"http://twitter.com/statuses/update.xml"; NSDictionary*dictionary=[NSDictionary dictionaryWithObjectsAndKeys:@"This is my status",@"status",@"meng ya", @"meyers",nil]; NSURL * url = [NSURL URLWithBaseString:baseString parameters:dictionary]; NSLog(@"the url : %@", url);
输出：
the url : http://twitter.com/statuses/update.xml?status=This%20is%20my%20status&meyers=meng%20ya
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