I'm working with some code that I need to refactor. A view controller is acting as a container for two other view controllers, and will swap between them, as shown in the code below.
This may not be the best design. Swapping the view controllers in this way might not be required. I understand that. However, as I work with this code I want to further understand what happens with the addChildViewController call. I haven't been able to find the answer in Apple's docs or in related questions, here (probably an indication that the design needs to change).
Specifically - how does the container view controller handle a situation where it is asked to add a child view controller, which it has already added? Does it recognise that it has already added that view controller object?
E.g. if the code below is inside a method - and that method is called twice...
[self addChildViewController:viewControllerB];
[self.view addSubview:viewControllerB.view];
[viewControllerB didMoveToParentViewController:self];
[viewControllerA willMoveToParentViewController:nil];
[viewControllerA.view removeFromSuperview];
[viewControllerA removeFromParentViewController];
Thanks,
Gavin
In general, their guidelines for view controller "containment", when one contains another, should be followed to determine whether you will need to implement containment.
In particular, worrying about adding the same child view controller twice is like worrying about presenting the same view controller twice. If you've really thought things through, you shouldn't need to face that problem. Your hunch is correct.
I agree that Apple's docs should be more up-front about what happens with weird parameters or when called out of sequence, but it may also be a case of not wanting to tie themselves to an error-correcting design that will cause trouble down the road. When you work out a design that doesn't ever call these methods in the wrong way, you solve the problem correctly and make yourself independent of whatever error correction they may or may not have - even more important if you consider that, since it's not documented, that error correction may work differently in the future, breaking your app.
Going even a bit further, you'll notice that Apple's container view controllers can't get in an invalid state (at least not easily with public API). With a UITabViewController, switching from one view controller to another is an atomic operation and the tab view controller at any point in time knows exactly what's going on. The most it ever has to do is remove the active one and show the new one. The only time where it blows everything out of the water is when you tell it "you should blow everything out of the water and start using these view controllers instead".
Coding for anything else, like removing all views or all view controllers no matter what may in some cases seem expedient or robust, but it's quite the opposite since in effect one end of your code doesn't trust the other end of your code to keep its part of the deal. In any situation where that actually helps you, it means that you've let people add view controllers willy-nilly without the control that you should desire, and in that case, that's the problem you should fix.
Related
I would like to add a file to my project, who's sole purpose would be to hold an array. I would then #import this file wherever I need to add/get from the array.
My problem is that when I create a new file (I'm using Xcode 4), I'm not sure what type of template to choose, and then what subclass to choose.
My reason for doing all of this is because I have a SplitView-Based app, and one of the views has a textfield, where I am trying to output data. My problem is that whenever I switch to a different view and then switch back, only the most recent entry is there. I am not 100% why that is but I suspect it is because when I switch to a different view, the current view is forgotten about, along with the variables in it.
This is not a good way to do it. There are many ways to do what you want: prepareForSegue: if you are using storyboards, delegation, instantiating your viewcontroller in code and setting a property in the header-file..those are just a few ways.
The way you are proposing is a slippery slope to bad Objective-C code and is only going to cause you more headaches in the future. Take the time to learn to do it right.
Check out this to get you thinking in the right direction.
How you save your data doesn't appear to be your problem. Take a look at the MVC design pattern and how view controllers implement it. They often rely on a dataSource protocol, which links the data from a "Model" to your "View" in a logical way to achieve your intended purpose.
The view controller should then be able to assign a delegate (usually itself (self) to keep the view populated with the correct data, whether the view gets unloaded or not.
If your view controller doesn't refer to a data source or a corresponding protocol, it would still be worth your time to see how you might take advantage of that design pattern. It will pay off in the long run to know this.
Instead of saving variables to a text file, you should consider using NSUserdefaults instead.
But I don't think that's the real solution to your problem, just wanted you know that there are other ways than saving stuff to a text file.
I have a custom graphic that is to be displayed to a user when an event occurs. The graphic needs to be displayed on whichever viewController is currently being displayed to the user.
The way i have programmed it so far is by adding to ALL viewcontrtollers:
1) the .h file for the custom graphic class
2) an observer for the NSNotification event that is raised
3) the method which actually draws the graphic.
This doesnt feel like a very efficient way of doing things and i was wondering if anyone has a better way of doing things?
To me it sounds like you've done it in a fairly sane way. The only other way I can think is to just add the graphic to the window which would then overlay on the current view controller and you'd only need to have one object listening for the notification. You could use the app delegate for instance. But then you would have to worry about rotation of the screen yourself when adding the graphic over the top.
What you are doing is correct .. The only thing you can improve is to mauve the drawing graphics part to the custom graphic class.. (if you are not already doing so...
just Make a UIViewController variable as a member variable to the graphics class..and then set it up to the current view displaying..after you receive the notifications..and the class will itself draw the code based on the ViewController you set it up to
The reason it doesn't feel efficient is that you're duplicating a lot of code. That's more work at the outset, and it creates a maintenance headache. You should be taking advantage of the inheritance that's built into object oriented languages, including Objective-C.
If you want all your view controllers to share some behavior, then implement that behavior in a common superclass. Derive all your other view controllers from that superclass, and they'll all automatically get the desired behavior. Your superclass's initializer can take care of registering the view controller for the notification(s) that you care about, and -dealloc can unregister it. This way, you don't have to clutter up each view controller with the same repeated code, and if you want to change the code you only have to do it in one place.
I'm writing a program for iPhone that will first let the user take a photo, then will dynamically retrieve a colour of the place where the user taps on the image, and draw a rectangle of that colour. I have two relevant classes for this: AppViewController and AppView. The former contains all the UI elements and IBActions, the latter the position of last tap, the touches-handling methods and the drawRect (and a static method to get colour data at a given coords of an image).
What I wanted to do is to put the touch-handling (calling drawRect in touchesMoved/Ended) and the drawRect in the AppViewController. That doesn't work, since that class doesn't inherit from UIView, but from UIViewController. What's the correct way to do that?
Another way to phrase that: How to constantly change something (well, constantly as long as the user is swiping across the screen) in a class that doesn't support touch-detection methods?
(This probably doesn't explain it well. Please ask clarifying questions).
I think the delegate pattern might be helpful to you in this situation. You could call your delegate's shouldUpdateRectangle selector in touchesMoved/Ended.
There is not really a correct way to move the view's behaviors into a view controller, since that is not the way the classes are meant to be used. You should probably look at what's driving you to try to subvert the framework's design this way, because that is likely going to be easier to fix.
It's not uncommon, though, for a view to call out to a supporting class for help in this stuff. You could certainly have your view's drawRect: call methods in the view controller, though I would be careful about mixing their concerns too much, because it could get hard to figure out who's responsible for what.
My app requires an interface that has many buttons, text fields and matrixes. And they need to change from time to time. Right now I do this by having all elements in IB already and hiding/showing/moving them when needed. What would others recommend? Should I do that? Should I use an NSTabView? NSView? Should create the elements programatically? If so, what if I have an element that is already created that I need again without changes? It would be a waste of releasing it and creating it again.
Any help would be greatly appreciated.
In my opinion, it's better to create interfaces programmatically if you have to animate views around a lot. If it's just a matter of hiding/unhiding them, IB works great, but if you need re-layout or create unknown numbers of views dynamically it's not worth trying to make it all work with nib files.
As for general advice:
Create subclasses (from UIView or UIControl or one of their subclasses) for every kind of element you're going to use. It's tempting to piece together composite views from your UIViewController, but you'll really be much better off creating real classes.
Study the standard Cocoa view classes, and try to create similar API:s in your own controls and views.
Put as much data (sub-element positioning etc) into a plist, so that you can easily change it from one centralized place instead of having to dig around in the code.
If you are often creating several dozen short-lived views, it's worth keeping them in a pool and reusing them. But if it's just a few labels being added and removed intermittently I wouldn't worry too much about it. As usual: don't optimize too early.
Your current approach sounds fine. If you're showing/hiding them but otherwise they remain unchanged, why go through the trouble of creating them with code, when your XIB keeps a "freeze-dried" copy of exactly what you need already?
As long as you're keeping them within logical groups, you can just move/swap/show/hide the group's container (like NSBox or an NSView). If you have a LOT of logical groups, which aren't always shown every session, you can separate them out into their own XIBs and only load them when they're needed, to save launch time and memory.
If you use NSViewController, it's even better because you can make clean breaks for each logical group. Load the panel as the view and the view controller will keep outlets/actions and has a one-to-one relationship with a xib.
MVC/OOP design patterns say you don't set a property, per se, you ask an object to set its property. Similarly, in Cocoa you don't tell an object when to draw itself. Your object's code has detailed HOW it will draw itself so we trust the frameworks to decide when (for the most part) it should draw.
But, when it comes to animation in Cocoa (specifically Cocoa-Touch) it seems that we now must take control of when the object draws itself from within the objects view controller. I can't send a message to a UIView subclass asking it to change some value and then leave it alone knowing it will slowly (duration = X) animate itself to a new position, alpha, rotation, etc. depending on the property changes. Or can I?
Basically, I'm looking for a way to set the property and then walk away. Instead, it seems, I need to wrap the code that calls the object asking it to change its property with an animation block of some sort "[UIView beginAnimations:nil context:NULL]; ... [UIView commitAnimations];"
I'm ending up with lots and lots of animation blocks in my view controllers and none in my view objects...I guess I'm just looking for someone to verify that this is how things are done and I'm not overlooking something. I haven't gotten much farther than the UIView animations within Cocoa-Touch, so maybe that's my problem and it's time to dig deeper?!?
You are correct that UIView does not animate its property changes by default the way CALayer does, but I don't think this indicates a break in MVC. It is appropriate for a Controller to instruct a View in how it should transform. That is the role of a Controller class as surely as it is appropriate for the Controller to know the correct frame for the View and even manage layout. I agree that it's a little weird that you call -beginAnimations:context: on the UIView class rather than on an instance, but in practice it does actually work much better that way since you may want to animate many views together.
That said, if you had a UIView subclass that managed the layout of its subviews, there would be nothing wrong with allowing that UIView to manage the animation rather than relying on a UIViewController to do it. So this is something that could go either place, but in practice it generally goes in the Controller as you've discovered.
I am using "MVC" here in the typical Cocoa sense. You're correct that this might not be appropriate in a SmallTalk program, but then SmallTalk Controllers have a much more limited role (management of user input events). Cocoa significantly expands the role of Controllers in MVC and I think it's an improvement, even if it means there are now some functions that could go in either the Controller or the View (and this is one of them).