Elements for an iOS Backend

A Possible way for Client
Backend
lcerveau@nephorider.com

• This talk follows the one about API Design and
starts from its principle
• Although it is mainly focused on iOS (and code
example will be), principles can be applied to
other environment. In particular for modern JS
framework like Angular (and the creation of a
service)
• Conﬁguration: a client talks to a server
Foreword

!
• Make sure the client can talk to the server and
fetch/display data
• Why the world fetch data each time????? We
should start faster
• Did you think about Facebook login?
• Ofﬂine usage is a must
The marketing requirement road

!
• Client side is usually determined on a “pane”
basis. In iOS wording: “view controllers”
• At ﬁrst fetch is done on a per pane basis with
simple network calls
• When it evolves it is good to have a backend
managing all
Consequences

Architecture evolution
Pane1 Pane2 Pane1 Pane2
Backend

Backend Roles
!
• User management : switch, registration
• Network state management : online/ofﬂine
• Data fetch in the context of one user
• Communication backend frontend

Global Application
!
• May be good to have a special objects
managing the navigation (Pane Manager)
• This one can be triggered from anywhere (e.g
also with an application special URL)
• Let the view controllers have a view controller
logic

Reminder: API prerequisite
!
• Each returned object is having
• an unique identiﬁer : uuid
• an self describing type : __class_name

Talk & code
!
• Follows Obj-C/Apple conventions
• Use MM as an example preﬁx
• For now, no sample code available, contact me
for further questions
• Example uses only Apple API calls, no third
parties components (but they may be worth be
looked at)

Storage
!
• At ﬁrst separate data for each user. Do not try to
optimize by saying some data is common
• Store in “Application Support” or “Documents”
folder, Complement with a storage in Cache for
data that can be lost, with same structure

One user - one provider
• Let’s deﬁne an object “doing all for a user” as a
MMDataProvider. An entry point to manage all
data for a user
• Let’s deﬁne an object managing
MMDataProvider as MMDataProviderManager.
It holds the main server endpoint
• The manager will also be responsible for user
switch as well as network state listening
(SCNetworkReachability). If the app features
elements like location services, they should be
here also

As objects
MMDataProviderManager
Reachability
Location
Manager
Dictionary :
userID/
MMDataProvider
@property (nonatomic,strong) NSString *currentUserUUID;
- (MMDataProvider *)providerForUser:(NSString *)userUUID;
Main access through
Social
“engines”:FB,
Google+,etc…

Session management
• Users will be created as session are started and
linked to possible already existing storage
• The MMDataProviderManager is the only one
storing the last save user, which can be read at start
for direct login
• Special userID can be deﬁned to keep the front end
code light : kMMCurrentUser, kMMLastUser,
kMMAnonymousUser….
• The manager will be the main point to manage
session and internally ask each provider to start/
stop itself

Registration
• As no user/provider exists before registration, the
manager is the one handling the process
• In terms of implementation, one must take care of
possible “network cookies” side effect.
• Usually multiple registration methods should exists :
login/password, token, Facebook, Anonymous (boils
down to one user with a device linked UUID)

A note about Facebook login
• The iOS Facebook SDK is easy to put in place but
usually stores its data inside the preferences
• It may be necessary to push tokens to the server.
This should be done by subclassing the
FBSessionTokenCachingStrategy that will read and
write data to a server
• Development tokens behaves differently than
production ones

MMDataProvider
Manager
- (BOOL)registerUserWithMethod:(MMRegistrationMethod)method
parameters:(NSDictionary *)parameterDictionary
Social Engine
Platform
Registration
StartSessionFor
UserID
Finalize creation of
MMDataProvider

StartSessionFor
UserID
userID == kMMLastUser?
Current User
Provider Stop
Session
Read Last User
in defaults
New User
Provider start
Session
Bail
Found
Not Found

Local and remote
• There may be differences in local objects than
remote one. Runtime versus Persistent
• As a consequence thinking about “let’s sync
everything” should be done in a cautious way
• Remote __class_name and uuid will drive
instantiations

Base class: MMBaseObject
• Holds as class variables the matching between
local class and server __class_name
• Useful to have additionally a local object type as int
for fast comparison
• Default implementation method may do nothing, or
even be forbidden (use of abstract class). For
exemple local storage in a DB
• At the base level : handling of UUID, present ﬁelds,
instantiation with JSON Data, storage creation

Objective-C implementation
/* Registration of MMXXX class at load time */
+ (void)load
{
[MMBaseObject registerClass:NSStringFromClass([self
class]) forType:kMMObjectTypeUser JSONClassName:@"user"
persistentDBType:@"USER"];;
}
/* Main object instantiation entry point */
[MMBaseObject createMMObjectsFromJSONResult:tmpJSON
parsedTypes:&tmpbjectTypes context:(void *)context];
!
/* Abstract method for Storage creation */
+ (char *)persistentSQLCreateStatement;

Objective-C implementation
/* To be implemented by subclass */
- (id)initWithJSONContent:(id) JSONContent;
!
/* To be implemented by subclass */
- (void)updateWithJSONContent:(id) JSONContent;
!
/* Write to SQL Database */
- (BOOL)writeToDatabaseWithHandle:(sqlite3 *)dbHandle;
!
/* remove to SQL Database */
- (BOOL)removeFromDataBaseWithHandle:(sqlite3 *)dbHandle;
!
/* Create with init dictionary SQL Database */
- (id)initWithDatabaseInformation:(NSDictionary *)information;

Collections
• An additionnel object should exist storing list of
items. We call it a collection, it is purely local
• Will be useful for handling of slices
• In addition to its UUID it should have a secondary
identiﬁer, describing what it is linked too (e.g a slice
endpoint, an HTTP request)
• It should be able to hold multiple orders, which may
be more or less complete
• It should be KVO/KVC compliant

Parsing
• Having declared a base class, parsing can be
generic
• The parser is called with the result of every request
• A context should be provided to the parser. For
example if a sliced endpoint is queried, this can be
the collection class in order to enhance it
• The parser itself is recursive.
• It can contain a preparing phase to “ﬁx/enhance/
modify” objects from coming from the backend

Parsing implementation
/* Entry point for JSON parsing and MMObject instantiations */
+ (void)createMMObjectsFromJSONResult:(id)jsonResult parsedTypes:
(MMObjectType *)parsedTypes contextProvider:(MMDataProvider
*)provider contextTask:(MMHTTPTask*)task parsingContext:(void
*)context
{
MMObjectType allParsedType =
_ParseAPIObjectWithExecutionBlock(jsonResult, provider, task);
if (parsedTypes) { *parsedTypes = allParsedType; }
return ;
}

if ([inputObj isKindOfClass:[NSArray class]]) {
[inputObj enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
MMObjectType tmpObjectType = _ParseAPIObjectWithExecutionBlock(obj, provider, task);
result |= tmpObjectType;
}];
} else if ([inputObj isKindOfClass:[NSDictionary class]]) {
NSDictionary *tmpDictionary = (NSDictionary *)inputObj;
NSString *objectAPIType = tmpDictionary[@"__class_name"];
NSString *objectUUID = tmpDictionary[@"uuid"] ;
if (objectUUID) {
MMBaseObject *tmpObject = nil;
BOOL objectIsHere = [provider.dataStore containsObjectWithUUID:objectUUID];
if (objectIsHere) {
tmpObject = [provider.dataStore objectWithUUID :objectUUID];
[tmpObject updateWithJSONContent:tmpDictionary];
result |= tmpObject.type;
} else {
if (!objectAPIType) return result;
tmpObject = nil;
NSString *objectClass = [MMBaseObject classNameForStringAPIType:objectAPIType];
if (!objectClass) return result;
tmpObject = [[NSClassFromString(objectClass) alloc] initWithJSONContent:tmpDictionary];
result |= tmpObject.type;
[provider.dataStore addObject:tmpObject replace:NO];
}
[tmpDictionary enumerateKeysAndObjectsUsingBlock:^(id key, id obj, BOOL *stop) {
if([obj isKindOfClass:[NSArray class]] || [obj isKindOfClass:[NSDictionary class]]) {
MMObjectType tmpObjectType = _ParseAPIObjectWithExecutionBlock(obj,provider, task);
result |= tmpObjectType;
}
}];
} else { //this is a slice
if (tmpDictionary[@"data"] && tmpDictionary[@"limit"] && tmpDictionary[@"offset"]){
_ParseAPIObjectWithExecutionBlock(tmpDictionary[@"data"],provider, task);
}
}
}
return result;

API goodies : fields, version
• Use a NSSet to hold and manage present fields
• Define field sets that can be used to find what is
missing
• User server object versioning to avoid unneeded
parsing
• One point to pay attention : date parsing is costly,
use per thread date formatter caching

Offline storage (problems)
• After a few versions it is always cool to have it
• This is an heavy testing field!!!!!
• You can use CoreData but you should never believe
it is simple
• Simple SQLite 3 may be a good compromise
• Great benefits are also in startup times

Abstract or not abstract
• Abstract: the front end simply says “get me those
objects and if not here the are fetched”
• Non abstract: the front end check if there are
needed objects, and if not decide to fetch them
• Non abstract: network calls need to be launched
manually which is a good way of learning an API
I prefer not abstract

Implementation
• One unique interface
/* Main interface to do queries and all */
- (NSString *)launchRequestToEndPointPath:(NSString
*)endPointPath andHTTPMethod:(NSString *)HTTPMethod
useSecureConnection:(BOOL)isSecure inBackground:(BOOL)background
withBody:(NSString *)body preparsingBlock:
(MMPreparseBlock)preparsingBlock completionBlock:
(MMCompletionBlock)completionBlock
• Endpoint path : the API path minus server. Learn
the API!!!
• Use of blocks avoid to spread code in all places

Technology
• iOS 7 has made a lot of network progress. IMHO no
need for a third party library
• Learn NSURLSession!
• Background modes can be difﬁcult. You are usually
not the owner of time. Never try to go against the OS
all is here to be understood. But clearly it takes time

Communication Back Front
• Give a role to different way of communication
• To avoid definitely : NSNotification for everything.
This easily becomes unmanageable (more than 130
notifications)
• Personal rules :
• Notifications are for application important
changes (Network, User session start and stop)
• KVO is king for data transmission. Be careful of
threading
• Use block to mark end of network operation

Upgrade management
• Dedicate one object to version management
• First usage, ﬁrst usage for current version,
• Mange data upgrade in an incremental way

Upgrade management
/* Use the Objective-C runtime */
- (BOOL) runUpgradeScenario
{
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
__block BOOL result = NO;
!
if(NO == self.firstTimeForCurrentVersion && NO == self.firstTime)
return result;
!
!
}
NSMutableDictionary *allUpgrades= [NSMutableDictionary dictionary];
NSMutableDictionary *allStarts= [NSMutableDictionary dictionary];
//Find all upgrade methods
unsigned int outCount;
Method * allMethods = class_copyMethodList([self class], &outCount);
for(unsigned int idx = 0; idx < outCount; idx++) {
Method aMethod = allMethods[idx];
NSString *aMethodName = NSStringFromSelector(method_getName(aMethod));
if([aMethodName hasPrefix:@"_upgradeFrom"]) {
NSString *upgradeVersionString = [aMethodName substringWithRange:NSMakeRange([@"_upgradeFrom" length], 3)];
[allUpgrades setObject:aMethodName forKey:upgradeVersionString];
} else if ([aMethodName hasPrefix:@"_startAt"]) {
NSString *startVersionString = [aMethodName substringWithRange:NSMakeRange([@"_startAt" length], 3)];
[allStarts setObject:aMethodName forKey:startVersionString];
}
}
if(allMethods) free(allMethods);
if(self.firstTime) {
//sort them and perform the most "recent" one
SEL startSelector = NSSelectorFromString([allStarts[[[allStarts keysSortedByValueUsingSelector:@selector(compare:)]lastObject]]]);
[self performSelector:startSelector withObject:nil];
result = YES;
} else if(self.firstTimeForCurrentVersion) {
//Sort them and apply the one that needs to be applied
[[allUpgrades keysSortedByValueUsingSelector:@selector(compare:)] enumerateObjectsUsingBlock:^(NSString *obj, NSUInteger idx, BOOL
*stop) {
if([obj intValue] > _previous3DigitVersion) {
result = YES;
[self performSelector:NSSelectorFromString([allUpgrades objectForKey:obj]) withObject:nil];
}
}];
}
#pragma clang diagnostic pop
return result;

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