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

General Context

!

• 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

Reminder: API prerequisite

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)

User management

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

MMDataProviderManager

- (BOOL)registerUserWithMethod:(MMRegistrationMethod)method parameters:(NSDictionary *)parameterDictionary

Social Engine

Platform Registration

StartSessionForUserID

Finalize creation of MMDataProvider

StartSessionForUserID

userID == kMMLastUser?

Current User Provider Stop

Session

Read Last User in defaults

New User Provider start

SessionBail

FoundNot Found

Object Model

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

Network fetch

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

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

In the application

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;

Thank You!