IOS4 note 11 (3)
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NSString and Friends
NSNotFound is a constant integer indicating that some requested element was not found.?For example, if you ask for the index of a certain object in an NSArray and the object isn’t?present in the array, the result is NSNotFound.
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An NSString ?can ?be ?searched ?using ?various ?rangeOf... methods, which ?return ?an?NSRange. In addition, NSScanner lets you walk through a string looking for pieces that?fit certain criteria; for example, with NSScanner (and NSCharacterSet) you can skip?past everything in a string that precedes a number and then extract the number. Starting?with ?iOS ?3.2, ?the ?rangeOfString: ?family ?of ?search methods ?can ?specify ?an ?option?NSRegularExpressionSearch, which lets you search using a regular expression; in iOS 4,?regular ?expressions ?are ?fully ?supported ?as ?a ?separate ?class, ?NSRegularExpression?(which uses NSTextCheckingResult to describe match results).
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In this example from one of my apps, the user has tapped a button whose title is something like “5 by 4” or “4 by 3”. I want to know both numbers; one tells me how many?rows the layout is to have, the other how many columns. I use an NSScanner to locate?the two numbers:
NSString* s = [as buttonTitleAtIndex:ix];
NSScanner* sc = [NSScanner scannerWithString:s];
int rows, cols;
[sc scanInt:&rows];
[sc scanUpToCharactersFromSet:[NSCharacterSet decimalDigitCharacterSet]
? ? ? ? ? ? ? ? ? ?? ??intoString:nil];
[sc scanInt:&cols];
If I were writing this same code to run only on iOS 4, I might do the same thing using?a regular expression:
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NSString* s = [as buttonTitleAtIndex:ix];
int rowcol[2]; int* prowcol = rowcol;
NSError* err = nil;
NSRegularExpression* r = [NSRegularExpression regularExpressionWithPattern:@"\\d"
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ???options:0
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?? ? ??error:&err];
// error-checking omitted
for (NSTextCheckingResult* match in [r matchesInString:s
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?? ? ?options:0
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?? ? ? ??range:NSMakeRange(0, [s length])])
? ? *prowcol++ = [[s substringWithRange: [match range]] intValue];
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The syntax seems oddly tortured, though, because we must convert each match from?an NSTextCheckingResult to a range, then to a substring of our original string, and?finally to an integer.
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String?drawing ?in ?a ?graphical ?context ?can ?be ?performed ?simply ?with ?methods provided?through ?the UIStringDrawing ?category on NSString.
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NSDate and Friends
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An ?NSDate ?is ?a ?date ?and ?time, ?represented ?internally ?as ?a ?number ?of ?seconds?(NSTimeInterval) since some reference date. Calling [NSDate date] gives you a date?object for the current date and time; other date operations may involve NSDateComponents and NSCalendar and can be a bit tricky because calendars are complicated (see?the Date and Time Programming Guide).
You will also likely be concerned with dates represented as strings. Creation and parsing?of ?date ?strings ?involves ?NSDateFormatter, ?which ?uses ?a ?format ?string ?similar ?to?NSString’s stringWithFormat. A complication is added by the fact that the exact string?representation of a date component or format can depend upon the user’s locale, consisting of language, region format, and calendar settings. (Actually, locale considerations can also play a role in NSString format strings.)
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In this example from one of my apps, I prepare the content of a UILabel reporting the?date and time when our data was last updated. The app is not localized — the word?“at” appearing ?in ?the string ?is always going ?to be ?in English — so ?I want complete?control of the presentation of the date and time components as well. To get it, I have?to insist upon a particular locale:
NSDateFormatter *df = [[NSDateFormatter alloc] init];
if ([[NSLocale availableLocaleIdentifiers] indexOfObject:@"en_US"] != NSNotFound) {
? ? NSLocale* loc =?[[NSLocale alloc] initWithLocaleIdentifier:@"en_US"];
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? ? [df setLocale:loc]; // force English month name and time zone name if possible
}
[df setDateFormat:@"d MMMM yyyy 'at' h:mm a z"];
NSString* lastUpdated = [df stringFromDate: [NSDate date]];
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NSNumber
An NSNumber is an object that wraps a numeric value (including BOOL).?
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NSValue
NSValue is NSNumber’s superclass. Use it for wrapping nonnumeric C values such as?structs. Convenience methods provided by ?through ?the NSValueUIGeometryExtensions ?category on NSValue ?(see ?the NSValue UIKit Additions Reference) ?allow ?easy?wrapping ?and unwrapping of CGPoint, CGSize, CGRect, CGAffineTransform, ?and?UIEdgeInsets; additional categories allow easy wrapping and unwrapping of CATransform3D, CMTime, CMTimeMapping, and CMTimeRange.You are unlikely to need to store any other kind of C value in an NSValue, but you can?if you need to.
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NSData
NSData is a general sequence of bytes. It is immutable; the mutable version is its subclass NSMutableData.
In practice, NSData tends to arise in two main ways:
1)When downloading data from the Internet. For example, the NSURLConnection?class supplies whatever it retrieves from the Internet as NSData. Transforming it?from there into (let’s say) a string, specifying the correct encoding, would then be?up to you.
2)When storing an object as a file or in user preferences. For example, you can’t store?a UIColor value directly into user preferences. So if the user has made a color choice?and ?you ?need ?to ?save ?it, ?you ?transform ?the ?UIColor ?into ?an ?NSData ?(using?NSKeyedArchiver) and save that:
[[NSUserDefaults standardUserDefaults] registerDefaults:
? ? [NSDictionary dictionaryWithObjectsAndKeys:
? ? ? ? [NSKeyedArchiver archivedDataWithRootObject:[UIColor blueColor]],
? ? ? ? @"myColor",
? ? ? ? nil]];
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