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];