The document discusses various methods for persistent storage in iOS, including NSUserDefaults for simple preferences, archiving objects to files by conforming to the NSCoding protocol, writing directly to files using NSData, and using SQLite for relational databases. NSUserDefaults provides a simple mechanism to save and load primitive data types. Archiving allows saving object graphs to files by implementing encode and decode methods. Files can also be read and written using NSData methods. The document also covers the application sandbox structure.

1. iOS:
Persistant
Storage
Jussi
Pohjolainen

2. Overview
• iOS
provides
many
ways
of
saving
and
loading
data
• NSUserDefaults
– Simple
mechanism
for
preferences
• Archiving
– Save
and
load
data
model
object
a<ributes
from
file
system
• Wri;ng
to
File
System
– Easy
mechanism
for
reading
and
wri@ng
bytes
• SQLite
– Rela@onal
database

3. NSUserDefaults
• Default
database
is
created
automa@cally
for
each
user
• Saves
and
loads
data
to
database
• At
run@me
caches
informa@on
to
avoid
having
to
open
connec@on
to
the
database
– synchronize
method
will
save
cache
to
database
– synchronize
method
is
invoked
at
periodic
intervals
• Methods
for
saving
and
loading
NSData,
NSString,
NSNumber,
NSDate,
NSArray
and
NSDic@onary
– Any
other
object;
archive
it
to
NSData

4. Example
NSUserDefaults:
Saving
NSUserDefaults *defaults = [NSUserDefaults
standardUserDefaults];
[defaults setObject:firstName forKey:@"firstName"];
[defaults setObject:lastName forKey:@"lastname"];
[defaults synchronize];

5. Example
NSUserDefaults:
Loading
NSUserDefaults *defaults =
[NSUserDefaults standardUserDefaults];
NSString *firstName =
[defaults objectForKey:@"firstName"];
NSString *lastName =
[defaults objectForKey:@"lastname"];

6. Archiving
• NSUserDefaults
is
good
for
storing
simple
data
• If
the
applica@on
model
is
complex,
archiving
is
a
good
solu@on
• Archiving?
– Saving
objects
(=instance
variables)
to
file
system
• Classes
that
are
archived
must
– Conform
NSCoding
protocol
– Implement
two
methods:
encodeWithCoder
and
initWithCoder

7. Example
@interface Person : NSObject <NSCoding>
{
NSString* name;
int id;
}
- (void) encodeWithCoder:(NSCoder* ) aCoder;
- (void) initWithCoder:(NSCoder* ) aDeCoder;
* * *
- (void) encodeWithCoder:(NSCoder* ) aCoder
{
[aCoder encodeObject: name forKey:@"personName"];
[aCoder encodeInt: id forKey:@"personId"];
}
- (void) initWithCoder:(NSCoder* ) aDeCoder
{
self = [super init];
if(self) {
name = [aDecoder decodeObjectForKey:@"personName"];
id = [aDecoder decodeIntForKey:@"personId"];
}
return self;
}

8. About
Applica@on
Sandbox
• Every
iOS
app
has
its
own
applica@on
sandbox
• Sandbox
is
a
directory
on
the
filesystem
that
is
barricaded
from
the
rest
of
the
file
system
• Sandbox
directory
structure
– Library/Preferences
• Preferences
files.
Backed
up.
– Tmp/
• Temp
data
storage
on
run@me.
NSTemporaryDirectory
will
give
you
a
path
to
this
dir
– Documents/
• Write
data
for
permanent
storage.
Backed
up.
– Library/Caches
• Same
than
Documents
except
it’s
not
backed
up.
Example:
fetch
large
data
from
web
server
and
store
it
here.

9. Archiving
to
Documents/
dir
// Searches file system for a path that meets the criteria given by the
// arguments. On iOS the last two arguments are always the same! (Mac OS X has several
// more options)
//
// Returns array of strings, in iOS only one string in the array.
NSArray *documentDirectories =
NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserFomainMask, YES);
// Fetch the only document directory found in the array
NSString *documentDirectory =
[documentDirectories objectAtIndex:0];
// Add file name to the end of the array
NSString *path = [documentDirectory stringByAppendingPathComponent:@"file.archive"];

10. NSKeyedArchiver
and
NSKeyedUnarchiver
• NSKeyedArchiver
provides
a
way
to
encode
objects
into
a
file.
[NSKeyedArchiver archiveRootObject: someObject
toFile: path];
• NSKeyedUnarchiver
decodes
the
objects
from
file
someObject = [NSKeyedUnArchiver
unarchiveObjectWithFile: path];

11. Reading
and
Wri@ng
Files
• NSData
and
NSString
provides
easy
access
for
reading
and
wri@ng
bytes.
• NSDic@onary
has
also
methods
for
saving
and
storing
the
dic@onary
to
property
list
(xml
file)
• Also
standard
file
I/O
func@ons
from
C
library
available
– fopen,
fread,
fwrite

12. Wri@ng
to
File
System
using
NSData
• NSData
provides
convinient
methods
for
reading
and
wri@ng
to
file
system
– - writeToFile:atomically
– + dataWithContentsOfFile
• Example
– [someData writeToFile:path atomically:YES]
– NSData* data = [NSData dataWithContestOfFile:
path];