使程序更健壮 对NString 为 nil null 的判断

不要对nsstring length == 0 并不了事nsstring 为nil ，例如
nsstring *tmp =@""; 在很多网络请求的应用中 对nil进行判断是十分必要的 不然程序很容易莫名其妙的崩溃 下面附上 正确代码
+ (BOOL ) stringIsEmpty:(NSString *) aString {
if ((NSNull *) aString == [NSNull null]) {
return YES;
}
if (aString == nil) {
return YES;
} else if ([aString length] == 0) {
return YES;
} else {
aString = [aString stringByTrimmingCharactersInSet: [NSCharacterSet whitespaceAndNewlineCharacterSet]];
if ([aString length] == 0) {
return YES;
}
}
return NO;
}


+ (BOOL ) stringIsEmpty:(NSString *) aString shouldCleanWhiteSpace:(BOOL)cleanWhileSpace {
if ((NSNull *) aString == [NSNull null]) {
return YES;
}
if (aString == nil) {
return YES;
} else if ([aString length] == 0) {
return YES;
}
if (cleanWhileSpace) {
aString = [aString stringByTrimmingCharactersInSet: [NSCharacterSet whitespaceAndNewlineCharacterSet]];
if ([aString length] == 0) {
return YES;
}
}
return NO;
}