This is more of a design best practice question:
When you are designing the structure of lets say a location based app. The location Manager is obviously an important instance and should be given easy access for other objects.
Should you have it as a property of appDelegate? or a singleton on its own?
Under what scenario would you prefer one over the other?
I understand both would work, but I want to make sure I'm doing things the right way, not just hacking everything together.
Your inputs are much appreciated!
Neither.
Pass in a location manger object via a custom init method or property where ever you need it.
This will conform with the SOLID principals S, O & D (single responsible, open-close, dependency inversion).
Also testing with mocks will be possible more easily.
The worst choice IMO is to store things in the app delegate. See What describes the Application Delegate best? How does it fit into the whole concept? for much more on that. In short, the app delegate is the Application Delegate. It is not "the Application Dumping Ground for Globalish Things."
Singletons are a long-established approach in ObjC, via the shared... pattern. After decades of popularity, and extensive use within the core Cocoa frameworks (NSUserDefaults, NSNotificationCenter, NSApplication, NSFontManager, NSWorkspace, UIDevice, etc. etc. etc.), they have in recent years fallen into some disregard in favor of other techniques, particularly "dependency injection," which is just to say "assigning to a property."
After years of using singletons in ObjC, I am coming around to the DI way of thinking. The improvements in testability and the improved clarity of dependencies are quite nice. That said, sometimes DI can lead to awkward transitions, particularly when dealing with storyboards.
So, I would say:
When practical, just construct objects and assign them to properties on the objets that need them.
If you have a lot to pass around, consider collecting them into a single "configuration" object and pass that around (but this hurts modularity somewhat).
If DI creates chaos (particularly if it leads to a lot of passing objects to things just so they can pass the object on to something else), or if it forces a lot of storyboard segue code that you could otherwise avoid, singletons are a well-established and well-respected pattern in Cocoa and are not wrong. They are a Cocoa Core Competency.
If you ever find yourself calling (MyAppDelegate *)[[UIApplication sharedApplication] delegate], then you're doing something wrong.
Make another singleton for location management. Single responsibility is a first principle of SOLID.
Think about do you really need it in AppDelegate ?
For Location Manager for example, no you don't. You better keep the location manager with all its related methods in a separate class to keep the singularity principle as #vladimir said, and to be able to reuse it later.
AppDelegate is responsible for handling what happens when the app launch and/or going to the background, initializing core data, registering to push notification and other 3rd party libraries like parse,...
Adding other things to appDelegate will make it grow larger by time and it would be very hard to maintain.
When to add things that don't belong to AppDelegate to it? I think when the app is small, you know it won't scale up, and you are required to favor time over clean code.
Check the AppDelegate responsibilities , and this answer by Matt
You can create multiple instances of CLLocationManager and use them wherever you need them.
If you create one instance and try to share it then you'll have trouble trying to forward the delegate methods around or try to re-implement them as notifications leading to a big mess.
Related
When I first began working on a long iOS project, my first, I had need for a small class that was readily accessible throughout the project. At first I passed it around as a property and it became way too much of a headache.
So, even though I've seen posts here advising against it, I created that class in the AppDelegate and access it as needed through a pointer to the AppDelegate. Then, I would revisit it later.
So, it is now later, I'm approaching release of the project, and I want to deal with this issue. It works just fine as it is but if it is lousy practice I want to fix it before release.
I don't understand why it is lousy practice. Sometimes you just need a class that is readily available and these seems like as good a way as any to get it. But there could be some downside I'm not understanding.
Assuming I need this class instance (it is very lightweight but heavily used) to be accessible throughout the project (probably about 50 VCs, total), what would be a good alternative to just referencing it via the App Delegate?
TIA for comments. I hope it doesn't start a war.
There's no hard-fast rule for for this and hundreds of opinions to be had. Here's my take on it.
I don't understand why it is lousy practice. Sometimes you just need a class that is readily available and these seems like as good a way as any to get it. But there could be some downside I'm not understanding.
You're right to feel this way and yes sometimes you need a class readily available with your data. It's lousy because it is not how AppDelegate should be used. In an ideal world it shouldn't contain lots of unrelated state-data and should serve a singular purpose: delegating system calls to your app. The trick is where do you put your model so that everyone can have access to it?
Assuming I need this class instance (it is very lightweight but heavily used) to be accessible throughout the project (probably about 50 VCs, total), what would be a good alternative to just referencing it via the App Delegate?
Put your state in a class using the singleton pattern. It will ensure that only one copy of it is ever created and all your classes should be able to access it. This is a well accepted pattern in the iOS SDK (NSUserDefaults, UIApplication, etc..). Just beware of this class growing too large or doing too many things. Try your best to keep it simple and focused and the Object-Oriented Police will leave you alone.
Other resources and opinions worth considering:
http://www.cocoawithlove.com/2008/11/singletons-appdelegates-and-top-level.html
Is it a bad practice to use your AppDelegate as your Singleton?
Is it good practice to use AppDelegate for data manipulation and Handling?
I'm working on an iOS app where we need different binaries for each customer based on their needs. A customer may want to change all the colors, icons and texts. We can do that through white labeling process. The problem here, though, is when they ask for different behavior, for instance, removing login screen and making it optional to login.
I thought we can use dependency injections and use different handlers for each customer if needed. For instance, we can have LoginHandler1 and LoginHandler2, both implementing ILoginHandler and inherit from UIViewController.
However, use of dependency injection is costly, it slows down the app because resolving is expensive comparing to normal instantiation.
The other way is to define all these behaviors in the app and enable/disable them in a plist file. like "is login optional? yes/no"
Any suggestions?
Thanks
You should create the entire object graph up-front, in the composition root. Object creation, and constructor injection, should not take much time at all as long as your constructors are not doing any actual work.
That being said, there are times when creating the entire object graph at the start of the application may take longer than is acceptable. In those cases, you can use lazy-loading to defer the costly initialization until later - while still creating the objects in the composition root.
Mark Seemann describes this approach in more detail here: Compose object graphs with confidence.
I thought we can use dependency injections and use different handlers for each customer if needed.
You thought right. Flexibility is one of the main reasons people use DI.
However, use of dependency injection is costly, it slows down the app because resolving is expensive comparing to normal instantiation.
It really doesn't cost that much at all. Have you tried it yourself? Unless the object in question (i.e. the object being injected) is very expensive to instantiate, you have no real reason to stay away from DI and Inversion Of Control. Also, as #Lilshieste noted above, creating the object graph up front (see AppDelegate) will probably make this even less a problem.
A good way of doing that is described here:
http://cocoapatterns.com/passing-data-between-view-controllers/ and here http://cocoapatterns.com/ios-view-controller-transitions-mediator-pattern/
The other way is to define all these behaviors in the app and enable/disable them in a plist file. like "is login optional? yes/no"
While less "elegant", this solution is a pretty useful one, especially if the project is not really big in terms of number of classes and VCs. It is also the easiest one to implement if the app code is already laid out and introducing major design changes would ask for lots of refactoring.
Always take action based on the task at hand, there is rarely if ever a single solution to a software design problem.
It seems to satisfy the three requirements here: On Design Patterns: When to use the Singleton?
I need it to exist only once.
I need to access it from all over the source base.
It handles concurrent access, i.e. Locks for writes, but can handle concurrent reads.
Hi all,
I've been reading a lot of no doubt intelligent educated and wise gems of advice that Singletons are 'Evil' and singletons are anti patterns or just plain bad news.
With the argument that logging makes sense but not much else.
Just interested to know if the case of essentially a persistent data store context makes sense for a singleton, i.e. Reading/Writing from disk to memory and referencing an object graph.
And if not, how do people normally tackle this issue, I don't currently have any problem with it, I know it's created only once, it's fast and the accessor logic is in one place. Meaning it takes me one line of code to do anything data model related.
Which leaves the only argument that it's bad for testing, in that it's a hard coded production implementation to data, but couldn't I just write a method swizzle through a category or in the test code to create the test version of the singleton?
And the final argument from DI testers, is that it is a hard coded implementation, rather than simply an interface to something, which I do get but I don't really have a major drive to use a DI framework given that I can use protocols for implementation, and use separate init methods for setting up an objects state in testing. There's only ever going to be two types of state for the singleton, or realistically one type...production.
Making it (in my opinion) much easier to read and faster to develop.
Change my view SO?
Yup, for some singletons are Evil. For the new developers who has little MRC knowledge and more ARC it sounds scary because they need to mess with memory,volatile,synchronize etc.
However it is not against to use them, they indeed has their own purpose to use with some are below.
when sharing large data models like arrays and dictionaries etc between multiple screens (VC's) we can't prefer storing them in UserDefaults (because userdefaults is permanent storage and storing such large entries make app start lazily) instead singletons are best since they stay only current app context and restarting app creates new one.
when we need a stable db connection to be accessible allover the app without messing up with connecting and closing in every business classes we can go for it.
when we wanted an ability to app for theming itself dynamically we would need to create a singleton class which holds all the color,image instances etc. and use that instance in application VC/Views etc so no code duplication and re-processing theme occurs in all places.
You dont have to change your view but tweak it a bit to get some positive intention towards singletons.
Hoping this clears it out, thanks
I commonly create apps that require storage of data, and this data is used across the entire program. It's not much, though, so I usually use NSUserDefaults to load/save this data. However, the saving/loading code, along with packing/unpacking the data, takes up space, and I thought moving this code to reusable methods inside a global singleton would be a good idea. It seems to have worked well.
Even so, I've read a lot lately on the evils of singletons and global objects, and I've started to have second thoughts. People often say that the use of singletons is almost always an indications of poor design. For the most part, I'd disagree (I think simple uses like this are a good design pattern), but I'm certainly no expert on the matter.
So, is using singletons even in a simple way like this bad? If so, what's the better alternative?
I definitely don't agree that singletons are evil. They are sometimes overused perhaps but on some occasions are just perfect for the job. In some applications it makes sense to have some kind of general data manager. The singleton pattern is used extensively in the SDK itself (app delegates, shared managers, default centers and so on). Most often these are not "pure" singletons, as in you can access a shared instance but can also create new instances if necessary.
The question you need to ask yourself is whether it would be useful to have access to a single instance of a data manager from anywhere at anytime, if it isn't then a singleton is probably not needed. If you are going to be using singletons in multi-threaded environments however, you do need to worry about race conditions (can one thread modify a resource while another is accessing it), the documentation has good explanations on how best to achieve this in Cocoa.
Let me try to explain with an example - Am using some code from a game I wrote. Let's say you have a GameMap class and a Tile class. The GameMap represents a 2 dimension grid of Tile objects.
GameMap *gameMap = [[GameMap alloc] init];
NSArray *theTiles = gameMap.tiles;
The instance of the GameMap owns the grid of tiles, and creates the tiles when the game map is created. No singleton needed.
You may say "but I only have one GameMap at a time, what's the big deal?". What about loading saved games, or loading new levels? Well that becomes as easy as:
// In whatever class object owns the game map
self.gameMap = [[GameMap alloc] initWithSaveData:saveData];
In conclusion, create an instance of a class that has code to manage other instances of things. Keep as little global as possible and your code will be more scalable and maintainable.
Just curious why ManagedObjectContexts should be passed to UIViewControllers when they are created, rather than just grabbing them from a UIApplicationDelegate?
The docs say that this makes your applications more rigid, but I am failing to see the nuances of when to use which pattern.
Thanks!
Imagine that I ask you to do some task, like painting a room. If I just tell you "go paint a room," you'll need to ask me a lot of questions, like:
Which room?
Where's the paint?
Where are the brushes?
Should I use a dropcloth?
In short, you won't be able to complete the task without help from me. If you have to depend on me every time, you won't be a very flexible painter. One way to deal with that problem is for me to give you all the stuff you need at the outset. Instead of "go paint a room," I'll say "please paint room number 348 using this bucket of paint and this brush, and don't bother with a dropcloth." Now, you've got everything you need, and you can get right to work with no further help from me. You're a much more flexible worker because you no longer depend on me.
The same thing applies to view controllers (and objects generally); it's better to give them everything they need than to have them depend on a particular object like the app delegate. It's true not just for managed object contexts, but for any information they need to do their job.
This is mainly because you want to use dependency injection with your UIViewControllers instead of just grabbing everything from UIApplication, this keeps your delegate clean instead of full of reference hacks.
This is also to keep with the MVC pattern:
Model
View Controller (Only for view logic)
Controller (For coordinating between the view and the model)
I tend not to agree with this pattern.
First of all I try to treat Core Data as an implementation detail, and as any implementation detail it should be hidden behind a good facade. The facade is the interfaces I expose for my model objects. For example if I have two model objects; Cource and Student, any cource can have a number of students. I do not want to let the controller take upon the duty to setup predicates and sort descriptors, and jump through all Core Data hoops just to get a list of students for a particular class. There is a perfectly valid way to expose this in the model:
#interface Cource (StudentAccess)
-(NSArray*)studentsStortedByName;
#end
Then implement the ugly stuff once and for all in the Model class. Hiding all the complex details of Core Data, and no need to pass around managed object contexts. But how would I find the sources, it has to start somewhere right? Yes, it does but you need not expose it to the controller. Adding methods such as these are perfectly reasonable as well:
#interface Cource (CourceAccess)
+(Cource*)caurceByID:(NSString*)courceID;
+(NSArray*)allCources;
+(NSArray*)courcesHeldByTeacher:(Teacher*)teacher;
#end
This also helps in minimizing dependencies between controllers. And reducing he dependencies between the model and controller. Assuming I have a CourceViewController and a StudenViewController is I did not hide the Core Data details behind a facade and wanted to pass around the managed object context as well, then I would end up with a designated initializer like this:
-(id)initWithManagedObjectContext:(NSManagedObjectContext*)moc
student:(Student*)student;
Whereas with good a good facade I end up with this:
-(id)initWithStudent:(Student*)student;
Minimizing dependencies behind facades, in favor of dependency injection also makes it much easier to change the internal implementations. Passing around the managed object context encourages each controller to implement their own logic for basic stuff. Take for example studentsSortedByName method. At first it might be sorter by last/first name, if later changed to last/first name sort you would have to go to each and every controller that has sorted students and make the change. Where a good facade method requires you to change in one method, and all controller automagically get the update for free.
The Apple Docs try to foster the most widely applicable and sustainable design patterns.
Dependency injection is preferred because it allows for the most flexible, expandable, reusable and maintainable design.
As apps grow in complexity, using a quasi-singleton like parking the context in the app delegate breaks down. In more complex apps, you may have multiple context tied to multiple stores. You might want the same view-controller/view pair to display data from different context at different times or you may end up with multiple context on different threads/operations. You can't pile all those context up in the app delegate.
If you have a simple app with a single context then using the quasi-singleton with the app delegate can work well. I've used it on several smaller apps in the past without immediate issue but I did hit scalability problems on a couple of apps when the apps grew overtime.
Which pattern to use depends on your shipping constraints and you best guesses about of the evolution app over its entire lifecycle. If its a small one shot app, then the app delegate quasi-singleton will work fine. If the app is more complex, might grow more complex or might spawn other related apps that will reuse existing components, then dependency injection is the way to go.