If you have tried to send any information using a GET web request, you would have come cross an annoying problem, That annoying problem is making sure that the URL is corrently encoded.
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.
比如 URL ：http://www.baidu.com/s?wd=����ѿ&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。
gPercentEscapes 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(":/?#[]@!$&’()*+,;="), // 确定 parameter 字符串中含有:/?#[]@!$&’()*+,;=这些字符时候，这些字符需要被转化，以免与语法冲突。
kCFStringEncodingUTF8
)；
例如： 建立一个 NSURL 的 category
@implementation NSURL (mm)
+ (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) {
escapedString =(NSString*)CFURLCreateStringByAddingPercentEscapes(kCFAllocatorDefault,(CFStringRef)[parameters valueForKey:key], NULL, CFSTR(":/?#[]@!$&’()*+,;="), kCFStringEncodingUTF8);
[urlString appendFormat:@"?%@=%@",key,escapedString];
[escapedString release];
}else{
escapedString =(NSString*)CFURLCreateStringByAddingPercentEscapes(kCFAllocatorDefault,(CFStringRef)[parameters valueForKey:key], NULL, CFSTR(":/?#[]@!$&’()*+,;="), kCFStringEncodingUTF8);
[urlString appendFormat:@"&%@=%@",key,escapedString];
[escapedString release];
}
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 is my status&meyers=meng ya另外就是 CFURLCreateStringByAddingPercentEscapes 将自动将待转化的字符串中的空格 转化为 ： , 即 空格字符的 ASCII，
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