i a newbie for iphone development. I got some questions here.
I know IB is a convinience tool for UI desgin and you also can do most things programmatically. I am just wondering, when should I create an interface controll without IB and why so. I am trying to form a good habit for this. Thank you very much. A friend told me that when efficiency should be considered for the application, then i should create interface controller programmatically, any other cases?
I am studying Learn Objective c on Mac now. It says that "Apple suggests you avoid using autorelease on your own code". So, does it mean I cannot use "autorelease" or just i should avoid using it. For example, can i use following code in my own code for iphone development?
#Interface Test {
A* a;
}
#Implementation {
a = [[[A alloc]init]autorelease];
}
Thank you for your time to read this. I am looking forward to answers :D.
Sometimes creating custom controls will require you to build them programmatically.
No, it's not that you can not use autorealease, it's just that using it adds to the burden of memory management. But in some cases you probably don't have an option, such as when you have a method that returns a temporary object. Using retain/release method to manually control object lifespan is the recommended way of doing things in iPhone development. Please see Autorelease pool for more details.
See Matt Gallagher's blog post for a discussion of efficiency. Simply put, one method is probably no more efficient than the other. This depends on your specific application, of course. Generally speaking, do what is most comfortable in designing your user interface. Make optimizations as needed.
1) If you are doing basic views that interface building has the widgets ready to go for, then use interface builder, it will make your life easier. Plus if you are just starting out it will let you get some sample code out the door faster. I'm not a huge fan of interface builder, but if you have to maintain code, you'll come across it so good thing to get familiar with it.
2) I don't think that autorelease is a bad thing. If you are writing single threaded code there is not as much to worry about. However, the thing that can come back to bite you is that you don't actually know when things will be released. So if you have programmed poorly, and try to reference an object that you have autoreleased later in code then you may get inconsistent behavior. I autorelease, but I also am very good about retain/release in other parts of my code that is passed these objects.
Related
I'm making a calculator app to learn Objective-C and maybe improve my OO design skills a bit. In an attempt to do things more MVClike, i have separated the actual do-the-calculator-stuff code from the view controller. For every action, pretty much all the view controller does is tell the "model" to do the operation meant for that action.
Thing is, that gives me a bunch of methods that do basically nothing but forward the action to the model, like this:
- (IBAction)clearAll:(id)sender {
[self.model clearAll];
}
- (IBAction)clearDisplay:(id)sender {
[self.model clearDisplay];
}
- (IBAction)clearMemory:(id)sender {
[self.model clearMemory];
}
- (IBAction)storeMemory:(id)sender {
[self.model storeMemory];
}
- (IBAction)addMemory:(id)sender {
[self.model addMemory];
}
- (IBAction) subtractMemory:(id)sender {
[self.model subtractFromMemory];
}
- (IBAction)recallMemory:(id)sender {
[self.model recallMemory];
}
Objective-C so far seems outrageously flexible with dynamically forwarding messages, and these methods are alike enough to look rather easily automated away. Do they really have to be there? Or is there a less repetitive way to tell the controller to just pass certain messages through to the model (ideally, while stripping off the sender arg)?
I've been looking a bit and trying some stuff with selectors and NSInvocation, but it seems like that'd mess with Interface Builder by taking away all the (IBAction) markers that let me hook up buttons to actions. (I'd prefer if the view didn't have to know or care that the controller's just forwarding to the model in these cases.)
So, is there a less repetitive and/or hacky way? Or is it not worth the trouble? (Or is it a bad idea in the first place? Or is this trying to make the model do too much? Or...)
You can do what Gabriele suggested and it is certainly an example of how dynamic ObjC can be, but you are likely better off avoiding it. As Gabriele said, you'd better know exactly what you are doing and definitely not to overuse such feature. And that often indicates that such feature is likely more trouble than it is worth.
The reality is that your calculator application is a quite contrived for the purposes of driving home the separation inherent to the Model-View-Controller pattern. It is a learning app, as you state.
In reality, no application is ever that simple. You will rarely, if ever, have a field of buttons where that the control layer blindly forwards said functionality on to the model.
Instead, there will be all manners of business logic in that control layer that will may do everything from automating various actions to validation (potentially by querying the model) to updating UI state in response to various actions.
Likely this code will be present from very early in the project, thus that generic forwarding mechanism will quickly become completely unused.
As well, such forwarding mechanisms become funnels full of pain when it comes to debugging. you no longer have a concrete spot to drop a breakpoint, but now have to add conditions. Nor do you have an easy means of finding all the places that might invoke or implement a particular method. As well, it makes following the control flow more difficult.
If you do find yourself with lots of repetitive boiler-plate code, it is more of a sign that your architecture is likely flawed than a sign that you need to inject a spiffy dynamic mechanism to reduce the repetitiveness.
As well, if you were to continue to flesh out your calculator app, how much of your coding time would have been spent doing those repetitive methods vs. all other features in your app? Likely, very very little and, because of their simplicity and convenience to debugging, it is unlikely that said repetitive methods are ever going to incur any significant maintenance cost whereas a spiffy-dynamic bit of trickery (which is very cool and I encourage you to explore that in other contexts) is pretty much guaranteed to require a "Huh. What was I thinking here?!" moment later on.
You can use the dynamic features of the language.
From the Objective-C Runtime Programming documentation
When an object can’t respond to a message because it doesn’t have a method matching the selector in the message, the runtime system informs the object by sending it a forwardInvocation.
So in your case you can implement the forward invocation method as follows
- (void)forwardInvocation:(NSInvocation *)anInvocation {
if ([self.model respondsToSelector:[anInvocation selector]])
[anInvocation invokeWithTarget:self.model];
else
[super forwardInvocation:anInvocation];
}
Note
You have also have to uniform your methods signatures. Either remove the sender parameter or add it to the model's method, otherwise respondsToSelector will return NO and the method won't be called.
In this case forwardInvocation will act as a dispatcher and it will try to send every message not implemented by your controller to the self.model object. If this is not responding to a selector it will call super, very likely resulting in an unrecognized selector exception.
I personally find it very elegant, even though you'd better know exactly what you are doing and definitely not to overuse such feature.
Question
We're developing a custom EventEmitter inspired message system in Objective-C. For listeners to provide callbacks, should we require blocks or selectors and why?
Which would you rather use, as a developer consuming a third party library? Which seems most in line with Apple's trajectory, guidelines and practices?
Background
We're developing a brand new iOS SDK in Objective-C which other third parties will use to embed functionality into their app. A big part of our SDK will require the communication of events to listeners.
There are five patterns I know of for doing callbacks in Objective-C, three of which don't fit:
NSNotificationCenter - can't use because it doesn't guarantee the order observers will be notified and because there's no way for observers to prevent other observers from receiving the event (like stopPropagation() would in JavaScript).
Key-Value Observing - doesn't seem like a good architectural fit since what we really have is message passing, not always "state" bound.
Delegates and Data Sources - in our case, there usually will be many listeners, not a single one which could rightly be called the delegate.
And two of which that are contenders:
Selectors - under this model, callers provide a selector and a target which are collectively invoked to handle an event.
Blocks - introduced in iOS 4, blocks allow functionality to be passed around without being bound to an object like the observer/selector pattern.
This may seem like an esoteric opinion question, but I feel there is an objective "right" answer that I am simply too inexperienced in Objective-C to determine. If there's a better StackExchange site for this question, please help me by moving it there.
UPDATE #1 — April 2013
We chose blocks as the means of specifying callbacks for our event handlers. We're largely happy with this choice and don't plan to remove block-based listener support. It did have two notable drawbacks: memory management and design impedance.
Memory Management
Blocks are most easily used on the stack. Creating long-lived blocks by copying them onto the heap introduces interesting memory management issues.
Blocks which make calls to methods on the containing object implicitly boost self's reference count. Suppose you have a setter for the name property of your class, if you call name = #"foo" inside a block, the compiler treats this as [self setName:#"foo"] and retains self so that it won't be deallocated while the block is still around.
Implementing an EventEmitter means having long-lived blocks. To prevent the implicit retain, the user of the emitter needs to create a __block reference to self outside of the block, ex:
__block *YourClass this = self;
[emitter on:#"eventName" callBlock:...
[this setName:#"foo"];...
}];
The only problem with this approach is that this may be deallocated before the handler is invoked. So users must unregister their listeners when being deallocated.
Design Impedance
Experienced Objective-C developers expect to interact with libraries using familiar patterns. Delegates are a tremendously familiar pattern, and so canonical developers expect to use it.
Fortunately, the delegate pattern and block-based listeners are not mutually exclusive. Although our emitter must be able to be handle listeners from many places (having a single delegate won't work) we could still expose an interface which would allow developers to interact with the emitter as though their class was the delegate.
We haven't implemented this yet, but we probably will based on requests from users.
UPDATE #2 — October 2013
I'm no longer working on the project that spawned this question, having quite happily returned to my native land of JavaScript.
The smart developers who took over this project decided correctly to retire our custom block-based EventEmitter entirely.
The upcoming release has switched to ReactiveCocoa.
This gives them a higher level signaling pattern than our EventEmitter library previously afforded, and allows them to encapsulate state inside of signal handlers better than our block-based event handlers or class-level methods did.
Personally, I hate using delegates. Because of how objective-C is structured, It really clutters code up If I have to create a separate object / add a protocol just to be notified of one of your events, and I have to implement 5/6. For this reason, I prefer blocks.
While they (blocks) do have their disadvantages (e.x. memory management can be tricky). They are easily extendable, simple to implement, and just make sense in most situations.
While apple's design structures may use the sender-delegate method, this is only for backwards compatibility. More recent Apple APIs have been using blocks (e.x. CoreData), because they are the future of objective-c. While they can clutter code when used overboard, it also allows for simpler 'anonymous delegates', which is not possible in objective C.
In the end though, it really boils down to this:
Are you willing to abandon some older, more dated platforms in exchange for using blocks vs. a delegate? One major advantage of a delegate is that it is guaranteed to work in any version of the objc-runtime, whereas blocks are a more recent addition to the language.
As far as NSNotificationCenter/KVO is concerned, they are both useful, and have their purposes, but as a delegate, they are not intended to be used. Neither can send a result back to the sender, and for some situations, that is vital (-webView:shouldLoadRequest: for example).
I think the right thing to do is to implement both, use it as a client, and see what feels most natural. There are advantages to both approaches, and it really depends on the context and how you expect the SDK to be used.
The primary advantage of selectors is simple memory management--as long as the client registers and unregisters correctly, it doesn't need to worry about memory leaks. With blocks, memory management can get complex, depending on what the client does inside the block. It's also easier to unit test the callback method. Blocks can certainly be written to be testable, but it's not common practice from what I've seen.
The primary advantage of blocks is flexibility--the client can easily reference local variables without making them ivars.
So I think it just depends on the use case--there is no "objective right answer" to such a general design question.
Great writeup!
Coming from writing lots of JavaScript, event-driven programming feels way cleaner than having delegates back and forth, in my personal opinion.
Regarding the memory-managing aspect of listeners, my attempt at solving this (drawing heavily from Mike Ash's MAKVONotificationCenter), swizzles both the caller and emitter's dealloc implementation (as seen here) in order to safely remove listeners in both ways.
I'm not entirely sure how safe this approach is, but the idea is to try it 'til it breaks.
A thing about a library is, that you can only to some extend anticipate, how it will be used. so you need to provide a solution, that is as simple and open as possible — and familiar to the users.
For me all this fits best to delegation. Although you are right, that it can only have on listener (delegate), this means no limitation, as the user can write a class as delegate, that knows about all desired listeners and informs them. Of course you can provide a registering class. that will call the delegate methods on all registered objects.
Blocks are as good.
what you name selectors is called target/action and simple yet powerful.
KVO seems to be a not optimal solution for me as-well, as it would possibly weaken encapsulation, or lead to a wrog mental model of how using your library's classes.
NSNotifications are nice to inform about certain events, but the users should not be forced to use them, as they are quite informal. and your classes wont be able to know, if there is someone tuned-in.
some useful thoughts on API-Design: http://mattgemmell.com/2012/05/24/api-design/
This is just a theoretical question. It was born from a real problem in my app, but I re-designed the problem out of the application. But the question remains:
If in my app delegate I write my singleton object to disk upon applicationWillTerminate: but also use NSNotificationCenter to call updateSingletonData upon UIApplicationWillTerminateNotification in some view controller, which will happen first? Will my data be written to the singleton, then the singleton be written to disk, then the app terminates? Or will the reverse happen, with the singleton being serialized and then the singleton updated (worse), or will the app just terminate after a certain amount of time if the serialization takes too long (much worse!)?
I guess this shows my lack of understanding of the guts of Springboard... thanks to anyone who can shed some light here.
A couple of things to note here:
Only Apple know the order these will happen in, as they wrote the code that does it.
You shouldn't care about the order these will happen in. If you do care, then you've designed your code badly.
In reality, you could go and check what order the happen in - for your particular device, for your particular iOS version, etc.
But really, you shouldn't care what order they happen in. From the sounds of it, you should be either firing off to the view controller to write the data before saving in applicationWillTerminate:, or letting the view controller handle saving after it's written its data.
This question is old and the response by #mattjgalloway is correct in terms of code quality but, for the sake of the knowledge, I just saw in the docs that the notification is posted after the UIApplicationDelegate method is called (emphasis mine):
After calling this method, the app also posts a UIApplicationWillTerminate notification to give interested objects a chance to respond to the transition.
https://developer.apple.com/reference/uikit/uiapplicationdelegate/1623111-applicationwillterminate
I've got a lot of really huge viewControllers. In fact there's a lot of useless code:
property for each view from InterfaceBuilder
synthesize
and release in dealloc
I'm thinking may be it is possible to have them all (all views that I need) in collection. But I won't have any IBOutlet in this case. how to connect them with IB?
Can you help me? I really don't like when there's 3 places with so similar code=(
You could use an IBOutletCollection, but generally those should be reserved for when you are using very similar objects (e.g. several buttons whose state needs to change together). I would seriously reconsider trying to rewrite your code to use something that could potentially make your job harder down the road.
Honestly though, you shouldn't really worry about the lines of code you need for setting up and tearing down the view. You could just collapse those sections of code if you don't like looking at them. These are going to be standard for Obj-C graphical apps you write. Consider that if someone else needed to pick up work on your application where you left off, it would take longer for them to figure out the IBOutletCollections vs using separate IBOutlets for each piece of the UI.
SO is telling me this question is subjective and likely to be closed. It is indeed subjective, because I'm asking for the opinion of experienced Objective-C developers. Should I post this somewhere else? Please advise.
Fairly new to Objective-C, though fairly confident in the concept of writing OOP code, I've been struggling with the NSNotification vs Delegate dilemma from the start. I've posted a few questions about that subject alone. I do get the gist, I think. Notifications are broadcasted globally, so shouldn't be used for notifying closely related objects. Delegates exist to hand over tasks to other object, that act on behalf of the delegated object. While this can be used for closely related objects, I find the workflow to be verbose (new class, new protocol, etc), and the word "delegation" alone makes me think of armies and bosses and in general makes me feel uneasy.
Where I come from (AS3) there are things called Events. They're halfway between delegates and NSNotifications and pretty much ruled the world of flash notifying, until fairly recently, a Mr. Robert Penner came along and expressed his dissatisfaction with events. He therefore wrote a library that is now widely used in the AS3 community, called Signals. Inspired by C# events and Signals/Slots in Qt, these signals are actually properties of objects, that you access from the outside and add listeners to. There's much more you can do with a signal, but at it's core, that's it.
Because the concept is so humble, I gave it a go and wrote my own signal class in Objective-C. I've gisted Signal.h/.m here.
A way to use this for notifying class A of an event in class B could look like this:
// In class b, assign a Signal instance to a retained property:
self.awesomeThingHappened = [[[Signal alloc] init] autorelease];
// In class a, owner of class b, listen to the signal:
[b.awesomeThingHappened add:self withSelector:#selector(reactToAwesomeThing)];
// And when something actually happens, you dispatch the signal in class b:
[self.awesomeThingHappened dispatch];
// You might even pass along a userInfo dictionary, your selector should match:
[self.awesomeThingHappened dispatchWithUserInfo:userInfo];
I hope it adheres to the right memory management rules, but when the signal deallocs, it should automatically remove all listeners and pass away silently. A signal like this isn't supposed to be a generic replacement of notification and delegation, but there are lot's of close counter situations where I feel a Signal is cleaner than the other two.
My question for stackoverflow is what do you think of a solution like this? Would you instantly erase this from your project if one of your interns puts it in? Would you fire your employee if he already finished his internship? Or is there maybe already something similar yet much grander out there that you'd use instead?
Thanks for your time, EP.
EDIT: Let me give a concrete example of how I used this in an iOS project.
Consider this scenario of four object types with nested ownership. There's a view controller owning a window manager, owning several windows, each owning a view with controls, among which a close button. There's probably a design flaw in here, but that's not the point of the example :P
Now when the close button is tapped, a gesture recognizer fires the first selector in the window object. This needs to notify the window manager that it's closing. The window manager may then decide whether another window appears, or whether the windows stay hidden alltogether, at which point the view controller needs to get a bump to enable scrolling on the main view.
The notifications from window to window manager, and from window manager to view controller are the ones I've now implemented with Signals. This might have been a case of delegation, but for just a 'close' action, it seemed so verbose to create two delegate protocols. On the other hand, because the coupling of these objects is very well defined, it also didn't seem like a case for NSNotifications. There's also not really a value change that I could observe with KVO, because it's just a button tap. Listening to some kind of 'hidden' state would only make me have to reset that flag when reopening a window, which makes it harder to understand and a little error prone.
Alright, after marinating the answers and comments for a bit, I think I have come to a conclusion that the Signal class I borrowed from AS3, has very little reason for existence in Objective-C/Cocoa. There are several patterns in Cocoa that cover the ranges of use that I was thinking of covering with the Signal class. This might seem very trivial to more experienced Cocoa developers, but it for me it was hard to get the spectrum complete.
I've tried to put it down fairly concisely, but please correct me if I have them wrong.
Target-Action
Used only for notifying your application of user interaction (touches, mostly). From what I've seen and read, there's no way to 'borrow' the target-action system for your own use
KVO (key value observing)
Very useful for receiving notifications when values change in accessible objects. Not so useful for notifying specific events that have no value attached to them, like timer events or interface followup events.
NSNotification
Very useful for receiving notifications when values change or other events happen in less-accessible objects. Due to the broadcast nature of the notification center, this is less suitable for cases where objects have a direct reference to another.
Delegation
Takes the most lines of code compared to the other three, but is also most suitable when the other three are not. Use this one when one object should be notified of specific events in the other. Delegates should not be abused for just accessing methods of the owner object. Stick to methods like 'should', 'will' and 'did'.
Signal
It was a fun experiment, but I mostly used this for classic delegation situations. I also used it to circumvent linked delegates (c delegate of b, b delegate of a, where a starts the event that should make it to c) without wanting to resort to NSNotification.
I still think there should be a more elegant solution for this edge case, but for now I'll
just stick to the existing frameworks. If anyone has a correction or another notification concept, please let me know. Thanks for your help!
It's an interesting idea, but I guess I don't see what makes it dramatically different from Cocoa's notification center. Compare and contrast:
self.awesomeThingHappened = [[[Signal alloc] init] autorelease]; // Your signals library
// Cocoa notifications (no equivalent code)
[b.awesomeThingHappened add:self withSelector:#selector(reactToAwesomeThing)]; // Your signals library
[[NSNotificationCenter defaultCenter] addObserver:self
selector:#selector(reactToAwesomeThing:)
name:#"AwesomeThingHappened"
object:n]; // Cocoa notifications
[self.awesomeThingHappened dispatch]; // Your signals library
[[NSNotificationCenter defaultCenter] postNotificationName:#"AwesomeThingHappened"
object:self]; // Cocoa notifications
[self.awesomeThingHappened dispatchWithUserInfo:userInfo]; // Your signals library
[[NSNotificationCenter defaultCenter] postNotificationName:#"AwesomeThingHappened"
object:self
userInfo:userInfo]; // Cocoa notifications
So, okay. I don't think you're trying to say that, line-for-line, a Signals library for Cocoa is different; rather, the argument goes that it terms of coupling, it isn't as tight as delegates, but not as loose as notifications. To that end, I guess I wonder how necessary it is? I guess I can see somewhat of a need to say "this object 'A' relies heavily on 'B', but doesn't need to be coupled all that closely", but to be honest, that seems like somewhat rare situation.
At any rate, NSNotificationCenter and its ilk, as well as delegates, are pretty standard in Cocoa apps. I always use the rule of thumb that if you deviate from a standard, even a de facto standard, you should have a good reason. If you have a good reason for using neither NSNotificationCenter nor delegates, then you might have a good reason to use this Signals setup. (And as an aside, I'm hesitant to associate notifications and delegates -- they each have a role and exist for different reasons.)
It's hard to say more without a specific use case. I'm inclined to say, "Hey, it looks cool in a geeky way, but it looks like it fills a role already served by notifications." Do you have any specific use cases you could cite?
What do you think of a solution like this?
I don't really see what the benefit is. To me, it seems like a combination of target/action+notifications (you can have multiple target/actions for a single notification event, but the t/a pair is registered with the object itself as opposed to a global notification center). In fact, it's more like key-value-observing that way, except that KVO is limited to observable properties.
Would you instantly erase this from your project if one of your interns puts it in?
No. It's not bad code. In fact, it seems kinda neat. But I just don't see an obvious benefit to it.
Would you fire your employee if he already finished his internship?
Of course not. You don't fire people for writing good code.
Is there maybe already something similar yet much grander out there that you'd use instead?
If you really wanted to make this neat, change the API to use blocks instead. Then you could do:
[object dispatchOnAwesomeThingHappened:^{
NSLog(#"holy cow, something awesome just happened!");
}];
Again, however, you'd be limited to reacting to stuff that the objects explicitly "dispatch". It would be much neater if you could attach stuff to immediately before and/or after any arbitrary method call. If you're interested in that, then I'd check out Aspect Objective-C on github.
I think that there is a gap between NSNotifications and Delegates. And KVO has the worst API in all of Cocoa.
As for NSNotificationCenter here are some of its problems:
it's prone to typos
it's hard to track observers of a given object, therefore hard to debug
it's very verbose
you can only pass notification data in a dictionary, which means you can't use weak references, or structs unless you wrap them. (see: very verbose)
doesn't play nice with GCD: limited support for queues (only blocks)
So there is definitely a need for something better.
I created my own observable class which I use on every project. Another alternative is to use ReactiveCocoa's RACSignal.