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4.6 Property Lists

Property lists (plists) are a feature of OPENSTEP and OS X that crop up in a lot of places. The simplicity of the form and its utility have caused a number of other systems to implement support for property lists.

The original NeXT property lists were a simple ASCII format with single characters representing the borders of different types of collection. This had a few limitations. It was difficult to extend, and relatively difficult to embed in other formats. To address these problems, OS X added an XML format for property lists. This can store a few more formats than the original format and, with proper namespacing, can be embedded in any XML format.

Unfortunately, XML is a very verbose format. Parsing it is relatively expensive and storing it requires a lot of space. To address this, Apple added a third format, which is a proprietary (and undocumented) binary encoding. This encoding is very fast to parse and very dense.

The NeXT format is largely deprecated on OS X, although a few command-line tools (such as the defaults utility) still use it since it is the most human-readable. The XML format is primarily used for interchange and the binary format is used for local storage. Table 4.1 shows a comparison of the XML and OpenStep property list formats. In particular, you will see how much more verbose arrays and dictionaries are in the new XML format.

It is worth noting that GNUstep has extended the OpenStep format to allow NSValue and NSDate objects to be stored, making GNUstep OpenStep-style property lists as expressive as XML ones, at the cost of interoperability.

The plutil tool can convert to the XML and binary formats. It can also convert from the original NeXT format, but not to it. This is due to the fact that this format is less expressive than the new versions—for example, it cannot store NSDate objects—and so the tool cannot guarantee that property lists can be safely converted. As well as converting, the -lint option to the tool causes it to check a plist file and report errors. This is very useful if you ever need to edit a property list by hand.

Table 4.1. The data types that can be stored in OpenStep (NeXT) and XML (Apple) property lists.

Type

Cocoa Class

NeXT

XML

String

NSString

"a string"

<string>a string</string>

Boolean

NSNumber

N/A

<true /> or <false />

Integer

NSNumber

12

<integer>12</integer>

Floating Point

NSNumber

12.42

<real>12.42</real>

Date

NSDate

N/A

<date>2009-01-07T13:39Z</date>

Binary data

NSData

<666f6f>

<data>fooZm9v</data>

Arrays

NSArray

( "a" )

<array>
  <string>a</string>
</array>

Dictionaries

NSDictionary

{"key" = "value";}

<dict>
 <key>
  <string>key</string>
 </key>
 <value>
  <string>value</string>
 </value>
</dict>

4.6.1 Serialization

The main use for property lists is to store collections of data in a format that can be easily written out and read back in. Table 4.1 showed the Cocoa objects that correspond to various elements in a property list, but because only a relatively small set of general formats of data are supported, Cocoa is not the only system that can handle plists.

The Core Foundation library also supports all of the data types, and so does CFLite. This means that property lists can be used by simple C applications without requiring all of Cocoa or even Core Foundation. There are other libraries available for parsing property lists. NetBSD's proplib is a BSD-licensed C library for handling XML property lists without any Apple code.

This means that Cocoa can be used to create configuration or simple data files in a format that is easy to parse and usable by non-Cocoa apps using a variety of toolkits. A lot of core configuration information for OS X is stored in property lists and accessed long before any Cocoa applications start.

The Cocoa collections can be written to property list files directly and read from them with a single call. Listing 4.14 shows an example of this. The program creates a simple array, writes it to a file, and reads it back into a new array. Logging the two arrays shows that they are equivalent.

Listing 4.14. Storing an array in a property list. [from: examples/PropertyList/plist.m]

 1| #import <Foundation/Foundation.h>
 2|
 3| int main(void)
 4| {
 5|     [NSAutoreleasePool new];
 6|     NSArray *a = [NSArray arrayWithObjects:@"this", @"is", @"an", @"array",
            nil];
 7|     [a writeToFile:@"array.plist" atomically:NO];
 8|     NSArray *b = [NSArray arrayWithContentsOfFile:@"array.plist"];
 9|     NSLog(@"a:_%@", a);
10|     NSLog(@"b:_%@", b);
11|     return 0;
12| }

When we run this program, we can verify that it works correctly:

$ gcc -framework Foundation plist.m && ./a.out
2009-01-07 19:13:15.299 a.out[34155:10b] a: (
    this,
    is,
    an,
    array
)
2009-01-07 19:13:15.300 a.out[34155:10b] b: (
    this,
    is,
    an,
    array
)

Although this simple example just contains strings, the same code will work on an array containing any of the types that can be stored in a property list.

This basic functionality is enough for a lot of uses, but for more advanced cases the NSPropertyListSerialization class is helpful. This provides the ability to validate property lists, and to load and store them from NSData objects in memory, rather than from files. The plutil utility mentioned earlier is a very simple wrapper around this class.

To create a property list with NSPropertyListSerialization, you would use this method:

+ (NSData *)dataFromPropertyList: (id)plist
                          format: (NSPropertyListFormat)format
                errorDescription: (NSString**)errorString;

The plist object is an object that will be turned into property list form. The format can be either NSPropertyListXMLFormat_v1_0 for the XML format, or NSPropertyListBinaryFormat_v1_0 for binary property lists. There is also NSPropertyListOpenStepFormat defined by the NSPropertyListFormat enumeration, but this is only valid for reading OpenStep property lists—OS X no longer has the capability to write them. The final parameter is a pointer to a string that will be used to return an error message.

This method is quite unusual in taking a pointer to a string as a parameter for returning an error. This is due to its age. It was introduced with OS X 10.2. Prior to this, exceptions were always used for returning errors and methods that could soft-fail returned a BOOL. With 10.2.7 (or earlier versions with Safari installed), Apple introduced the NSError class. A pointer to a pointer to an instance of this class is often passed in as a final argument, and set to non-nil on return, but this method was written just a few months too early to take advantage of it.

For deserializing property lists, the converse method is

+ (id)propertyListFromData: (NSData*)data
          mutabilityOption: (NSPropertyListMutabilityOptions)opt
                    format: (NSPropertyListFormat*)format
          errorDescription: (NSString**)errorString

Most of the parameters are the same here. The format is now an output parameter, which is set to the format of the property list, which is autodetected. The new parameter, opt, defines whether deserialized objects should be mutable. Property lists do not store mutability options—an NSString and an NSMutableString will be stored in the same way—so you must specify this when deserializing. You can define whether everything (NSPropertyListMutableContainersAndLeaves), only containers (NSPropertyListMutableContainers), or nothing (NSPropertyListImmutable) should be mutable.

4.6.2 User Defaults

One of the problems on any system is how to store preferences for an application. Numerous solutions have been suggested for this problem, from individual configuration files to a centralized registry. OS X picks a path somewhere in the middle. Each application has a property list file containing a dictionary associated with it. This is accessed via the NSUserDefaults class, which also handles notifying parts of an application of changes to individual keys.

This combines most of the benefits of both approaches. You can treat user defaults as a system-maintained database. The defaults command-line tool can browse and modify the defaults for any application. Since they are just property lists, you can also modify them outside of the defaults system and delete them for unwanted applications.

To get a user defaults object for your application, you do

[NSUserDefaults standardUserDefaults];

This returns a singleton object. You can call this as many times as you want from your application and still be using the same object. It maintains a copy of the defaults in memory and periodically synchronizes it with the copy stored on disk. Alternatively, you can explicitly synchronize it by sending a -synchronize message.

The shared defaults object is similar to an NSMutableDictionary. It has a setObject:forKey: and an objectForKey: method, which set and get objects, respectively. There are also some convenience methods, like boolForKey: that fetches the boxed value, unboxes it, and returns a BOOL.

Since user defaults supports key-value coding (KVC), you can also use the standard KVC methods to access defaults. In particular, this includes the valueForKeyPath: method. This is very useful when you have a set of attributes stored in a dictionary in defaults. You can get at a nested key with a single call:

[[NSUserDefaults standardUserDefaults] valueForKeyPath: @"dict.key"];

As long as the user defaults system contains a dictionary called "dict" that contains a key called "key," this will return the corresponding value. You can use this for even deeper-nested dictionaries using a longer key path. Unfortunately, using the corresponding setValue:forKeyPath: method will cause a run-time exception.

There are a few difficulties with mutable collections in defaults. This is not directly supported, and so you must do it by creating a new copy of the collection and inserting this back into defaults. This typically involves the following steps:

NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
NSMutableDictionary *d = [[defaults dictionaryForKey: aKey] mutableCopy];
// Some operations on d
[defaults setObject: d forKey: aKey];

This is somewhat clumsy, and so it is common to wrap it up in a method. In particular, you should remember that categories allow you to add methods to NSUserDefaults. If your application stores a dictionary of sounds, then you might consider adding a -setSound:forAction: or similar method that sets an entry in the sounds dictionary in defaults.

User defaults only supports storing objects that can be saved in property lists. A notable exception to this is the NSColor object. Apple suggests adding a category on NSUserDefaults for storing these, using the NSArchiver mechanism.

NSArchiver allows objects that support creating a serialized form of objects that implement the NSCoding mechanism. If the object you want to store in defaults implements these, then NSArchiver can turn it into a NSData instance and NSUnarchiver can restore it. This provides a mechanism for storing the object in defaults, since the defaults system can handle NSData instances.

Often, a more lightweight approach is possible. Some objects, like NSURL can easily be stored as strings. Listing 4.15 shows a simple category for storing URLs as strings in defaults. If your object already implements -stringValue and -initWithString: methods, then you might find this mechanism simpler than implementing NSCoding.

Listing 4.15. A category for storing URLs in defaults. [from: examples/URLDefaults/urldefaults.m]

 3| @implementation NSUserDefaults (NSURL)
 4| - (void) setURL: (NSURL*)aURL forKey: (NSString*)aKey
 5| {
 6|     [self setObject: [aURL absoluteString] forKey: aKey];
 7| }
 8| - (NSURL*) URLForKey: (NSString*)aKey
 9| {
10|     return [NSURL URLWithString: [self stringForKey: aKey]];
11| }
12| @end

Either mechanism can be used for your own objects, depending on their complexity.

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