I have implemented MPPlayableContentDataSource protocol. It has a method
- (void)beginLoadingChildItemsAtIndexPath:(NSIndexPath *)indexPath completionHandler:(void(^)(NSError * __nullable))completionHandler;
head unit calls this method each time when some container was shown on screen or user was tapped on some container.
How can I recognize who initialize call of the method? (user by tap or head unit)
First off, why would the difference (if there was any) matter? If that delegate method is being called, then you need to load the child items – period. That said, the head unit never requests your items without a user interaction of some kind.
When the user taps on a container, the child items for that index path are requested. Depending on whether you have a synchronous or asynchronous interface, you can then implement the respective delegate methods.
Related
Here's the scenario:
-A UIViewController (A) is pushed onto the navigation stack
-On viewDidLoad an async GET is called using AFNetworking (a singleton AFHTTPClient shared throughout the application) to populate various user elements on the view (say a UILabel).
-The user presses the back button before the request returns
-Assume other active view controllers may be making requests so you can't cancel all open operations
So question #1 is, should you track the open requests made by UIViewController A and cancel the outstanding ones when the user leaves that view, or should you let them finish up and ignore them? Since AFNetworking uses blocks, the user elements being updated are retained inside the block and therefore won't cause a crash when the success/fail block is executed after the view has been popped. However the downside to ignoring them seems to be unnecessary network traffic.
Question #2 is, where would you execute the code to cancel the operations made by UIViewController A? viewDidDisappear doesn't seem right because the user may have gone forward (pushed a new view onto the stack) instead of back (popped the current view), in which case you don't want to cancel the open requests because the user may come back to the current view and it won't load again. However, I don't think dealloc or viewDidUnload will be called while the request is executing since the block will keep a retain on the user elements so I don't think it can go there.
Would appreciate thoughts on this. What do you think is best practice?
Generally speaking, you don't really need to cancel requests when a user leaves a view controller. In terms of memory management, a reference to block self will prevent any crashes caused by sending messages to deallocated instances, so no worries there.
As far as user experience, I would say that you shouldn't really worry about it until it's a problem (we developers have a knack for guessing completely wrong on what will be slow in our applications). If you are making large GET requests, though, and it's creating noticeable sluggishness, my suggestion would be to have the controller do HTTPClient -cancelAllHTTPOperationsWithMethod:path: in -viewDidUnload: (any other callback would be premature).
Maybe you could have a singleton which manages all the network stuff, and just set its delegate to the current vc (in viewDidLoad) so you get any incoming data, and send it a cancel message when the vc disappears (or else let a different vc become its delegate). Or the singleton could keep the data for access by any vc at some later stage. I tend not to put async code into my VCs for this reason.
What does it actually mean to set the delegate of a textfield?
For example: txtField.delegate = self
"In short, that you are receiving calls from the txtField. You are setting the object 'self' as the delegate for txtField."
"That means that your 'txtField' will receive events from itself
These two answers essentially mean the same thing. But seemingly contradictory. But the first makes more sense to me. I can see why a beginner gets confused, I've been there!
Basically one is the caller one is the receiver Think of it as a chef in a kitchen call his assistant to cut up some onions. In this particular case, txtField is the chef, "self" is the assistant. txtField orders self "Do this, this and this!" Like it or not the assistant has to oblige cuz he has wife and kids to feed. :)
It means that self will be the recipient of certain method calls that are made in response to actions on the text field.
In short, that you are receiving calls from the txtField. You are setting the object 'self' as the delegate for txtField.
Delegating is a programming pattern that is widely used in Objective-C.
The basic idea is let an object delegate some tasks to another object. For example, your UITextField object delegate some tasks to your view controller. In this case, your UITextField object becomes a delegating object, and the view controller the delegate of the UITextField object. The delegating object sends certain messages to its delegate in order to get necessary information, or to notify certain events, etc.
That means that your 'txtField' will receive events from itself (kind of a weird example, maybe a larger source code section could be provided?)
For some of its methods, the textfield (any object in a class using the delegation pattern) is going to try to call some other object to so that that object can customize some of the textfield's behaviors. The object that the textfield will try call is called it's delegate. The delegate is initially set to nil, so, by default, no customization happens.
If a class has a line of code like: textfield.delegate = self; then it says that this object in this class wants to get called to handle the textfield's customization for certain of the textfield's defined delegate methods.
It means the actual class where 'txtField.delegate =self' is called will receive callsbacks from events. This is often a convenient way to do things.
Alright, so here is what I am trying to do. I have a UITextField. When I single tap it, I want to call one of my methods. When I double tap (tap it twice with 1 finger) it, I want to edit the text field (as though I had single tapped it on a normal UITextField.)
I am not sure how to go about this. I was thinking of sub-classing UITextField and trying to intercept the touch event as it is sent, but I can't figure out which methods to override to do this. Which methods should I override and how?
Or, if there is a better way to do this, let me know! I'm all ears, and not sure how to proceed.
This would involve several steps:
1.) Add a NSBoolean property that keeps track of whether the user has already tapped the field once recently (you get to decide what recently means here).
2.) Implement the textFieldShouldBeginEditing: method of the delegate assigned to your UITextField. If the user has tapped twice in quick succession (detectable by checking whether or not the boolean property is true), then return YES. If not, call your method, and then return NO.
I am confused about the iPhone responder chain. Specifically, in the iPhone event handling guide http://developer.apple.com/iPhone/library/documentation/iPhone/Conceptual/iPhoneOSProgrammingGuide/EventHandling/EventHandling.html, we have the following:
The first responder is the responder object in an application (usually a UIView object) that is designated to be the first recipient of events other than touch events.
But UIView is a subclass of UIResponder. And the UIResponder class reference says this:
- (BOOL)canBecomeFirstResponder
Return Value
YES if the receiver can become the first responder, NO otherwise.
Discussion
Returns NO by default. If a responder object returns YES from this method, it becomes the first responder and can receive touch events and action messages. Subclasses must override this method to be able to become first responder.
I am confused by the apparent contradiction. Can anyone clear it up for me?
For what it's worth, I did set up a simple view-based application, and call canBecomeFirstResponder and isFirstResponder on its view. Both returned NO.
The nomenclature can be confusing. Instead of "first responder" think of it as "initial event target" i.e. the object that is the first responder becomes the initial target for all events. In some APIs this is also called the "focus" although in the Apple APIs that is usually reserved to describe windows.
At any given time, there is only one first-responder/intial-event-target in the app. Only individual objects/instances can become a first-responder/intial-event-target. Classes can merely define if their instance have the ability to become a first-responder/intial-event-target. A class need only provide the ability to become the app's first-responder/intial-event-target if it make sense to do so. For example, a textfield obviously needs the ability to trap events so that it can use those event to edit itself. By contrast, a static label needs no such capability.
Whether a particular class inherits from NSResonder has no bearing on whether the class (or a specific instance of the class) will let itself be set as the first-responder/intial-event-target. That ability comes solely from the instances' response to the canBecomeFirstResponder message. The same instance can refuse to be the first-responder/intial-event-target under one set of conditions and then allow it later when conditions change. Classes can of course hardwire the status if they wish.
In other words, first-responder/intial-event-target is a status of a particular instance at a particular time. The first-responder/intial-event-target is like a hot potato or a token that is handed off from instance to instance in the UI. Some classes refuse to grab the hot potato at all. Some always do and others grab it sometimes and ignore it others.
What this means is that the basic UIView is not able to become first responder - it doesn't do anything with motion events, editing-menu messages, etc.
Some UIView subclasses (like UITextView) are able to become first responder, and you can write your own UIView subclass that does so too.
In many situations, such as making the keyboard go away when the use clicks done, there are two options: set the text field's delegate to self, and adopt the UITextFieldDelegate protocol, and then use the method
- (BOOL)textFieldShouldReturn:(UITextField *)textField;
to resignFirstResponder and return YES. But you can also
addTarget:self
action:#selector(myMethod:)
forControlEvent:UIControlEventDidEndOnExit];
or something like that, using the did end on exit event, and then in the method, [sender resignFirstResponder]. So what is the best option in situations like these: the delegate, or the event?
The quick rule of thumb is that delegates are supposed to answer the question of "should I?" on behalf of the object they are a delegate for. Events, on the other hand, are broadcast afterward to let listeners know that something has happened.
In your case, while you could call [sender resignFirstResponder] in response to the event, you're mixing metaphors by doing this. Your delegate should have already made the decision to hide the keyboard (or not) and the event being broadcast is merely to let all the other components know that they keyboard hid.
If you are going to be paired with one other class, where the real type of that class may vary, then it makes a lot of sense to formalize that pairing into a protocol and a delegate arrangement.
If the information you want to send is targeted at a broader set of objects, then it starts to make more sense to use notifications - though now you have somewhat obscured what information is being passed by the notification as there's no central definition for what to expect.
Both are about an equal load to work with - with a delegate you have to set yourself and then remember to unset yourself before you are deallocated. You have to do the same thing with notifications, remember to start listening and then unsubscribe before you are deallocated.
Also, you should try as much as possible to make sure you send notifications out on the main thread as the notices get sent on the same thread they started from. Same goes for delegate methods, it's not very kind to call a delegate method from some other mystery thread!
The delegate makes your objects more reusable they are an adapter that lets any object interact with the defined behaviors of that object and use the object. I would say delegates should be adopted by the object responsible for keeping the state of and defining behavior to actions that will occur in the object that it is using. Events should be used for any other objects that are intersted in particular action that the object that has the protocol does (so objects not responsible for keeping the state of the object that defines the protocol).
For example: A view controller using a textfield will adopt its protocol to dismiss the keyboard and any other behaviors that can occur for a textfield, maybe another controller will do some animation when the keyboard is dismissed, so it will register to the textfield as an event in order to receieve the event of the keyboard being dismissed.