Why do AppDelegates have an instance variable for the root controller? - ios

This example is taken from The Big Nerd Range iPhone book (page 143/144) - ItemsViewController is a subclass of UITableViewController:
#interface HomepwnerAppDelegate : NSObject <UIApplicationDelegate>
{
UIWindow *window;
ITemsViewController* itemsViewController;
}
....
itemsViewController = [[ItemsViewController alloc] init];
[window setRootViewController: itemsViewController]
My question is why is it necessary to have the iVar itemsViewController, why not just do this instead:
...
window.rootViewController = [[ItemsViewController alloc] init];
I presume the window will destroy its rootViewController when the app exits and thus there's no leaks, and the window will be in existence for the lifetime of the app so I don't understand why this and many other example have a separate iVar for the root controller?
TIA

The biggest advantage is simply that you can access the methods of your view controller without needing to cast over and over again:
[itemsViewController doSomething];
// vs.
[(ItemsViewController *)window.rootViewController doSomething];
Depending on the app you might need to refer to the root view controller frequently from the app delegate, for example when implementing the handlers for entering background/foreground and similar app delegate callbacks.

There is absolutely no need to keep the ivar around if you don't need it.
BTW, you will leak ItemsViewController if you don't autorelease it. (Unless you are using ARC)

The reason is historical, I think. Back when that book was written, the window and root view controller both used to be IBOutlets and were set from a nib file called MainWindow.nib.
Also, UIWindow didn't used to have a rootViewController property to assign the control to (the root view controller.view was just added directly as a subview to window), so if you didn't store it in an ivar then it wouldn't be retained by anything and your app wouldn't work because the root view controller would be released as soon as it was created.
These days however, since iOS4 and now ARC, the base project template has been updated and doesn't even have ivars any more (which are no longer needed). It does still have an #property for the view controller, but it's technically not needed any more either, and your alternative solution of assigning a new controller directly to the window.rootViewCOntroller would work fine.

This is totally stylistic choice. There are other ways to get the convenience accessor. I don't ever create an ivar in my rootViewController never changes. I will usually go for a read only property.
#property (nonatomic, readonly) MyRootViewController *rootViewController;
- (MyRootViewController *)rootViewController {
if ([self.window.rootViewController isKindOfClass:[MyRootViewController class]) {
return (MyRootViewController *)self.window.rootViewController;
}
return nil;
}

Related

How to call function from one viewcontroller to another controller?

SettingsStore.h
#interface SettingsStore : IASKAbstractSettingsStore
{
#public
NSDictionary *dict;
NSDictionary *changedDict;
}
- (void)removeAccount;
#end
menuView.m
-(IBAction)onSignOutClick:(id)sender
{
SettingsStore *foo = [[SettingsStore alloc]init];
[foo removeAccount];
[self.navigationController pushViewController:foo animated:YES];
exit(0);
}
I want to call this removeAccount function from menuView.m. But I am getting error.
How to fix it and call this removeAccount.
There are few mistakes in your Code please find them below.
[foo removeAccount]; Calling this method is correct
[self.navigationController pushViewController:foo animated:YES];
Not correct because SettingsStore is not subclass of
UIViewController only subclass of UIViewController can be pushed to
Navigation controller
exit(0); Calling this method is not
recommended by Apple
You are calling removeAccount correctly from your menuView.m file, but there are several issues with your code:
You are treating foo as though it were a UIViewController, and it's actually a member of the SettingStore class. Does the SettingStore class refer to an actual screen, or is it more a data object (for storing settings?). If it's the latter, you don't want to push it on. You can create it, and use it, but the user doesn't need to see it.
You are calling exit(0); you can remove that line. If you want to remove the menuView.m file from your memory, remove references to it (e.g. from its parent view controller).
The menuView.m file is confusing, as in, is it a view or a viewController. An IBAction I would normally stick in a ViewController file, rather than a view file. Your basic design pattern is MVC (Model / View / Controller). In this case, it seems your SettingStore file is a Model (data), the menuView.m is a View and your code is for the Controller bit.

Where to initialize subview?

I am new to iOS development and I am currently reading the book : iOS Programming (Objective C) by Big Nerd Ranch.
I am confused as in where to initialize subviews such as UIButtons, UIImageView while creating views programtically:
Should the intialization be done in the Main UIView i.e in the
initWithFrame method and maintain a additional weak reference to the subview in the UIView.
or
should I do it in the UIViewControllers loadView method and maintain a weak reference to the subview in the uiviewcontroller (Same approach used while creating UIVew using the interface builder).
I have seen both the approaches being used in various stackoverflow posts but no post that explains which approach is the right one.
you can initialize as per your app's requirement. If any view or button or anything is part of initial setup of your app then you should initialize it in viewDidload.
Now, for example there is requirement like user press button and then new view will be created then you can initialize view in button's click method etc.
So, it's depends on your requirement.
Static views which will live from start to and of app should be initialize in viewdidload, because this is the first method getting called of viewcontroller.
hope this will help :)
It dependes on which architecture you are using. Apple raises the flag of Model-View-Controller, but in fact, UIViewControllers are the View.
For Example:
Let's say that you have a pretty LoginViewController. When you instantiate it, you will be doing something like
LoginViewController *loginVC = [[LoginViewController alloc] init];
At this point, no view is loaded. Your ViewController has just executed the init method, nothing else. When the system calls
loginVC.view
the first method to be executed will be
- (void)loadView;
there you should do exactly that, load your view. So, the approach i like is to have an additional LoginView.
- (void)loadView
{
// you should have a property #property (nonatomic, strong) LoginView *loginView;
self.loginView = [[LoginView alloc] init];
self.view = self.loginView;
}
and in the LoginView init method, you should put your code to build up the view.
However, you could eliminate LoginView, and instantiate all your subviews like this:
- (void)loadView
{
self.view = [[UIView alloc] init];
UIButton *button = [[UIButton alloc] initWithTargetBlaBlaBla...];
[self.view addSubview:button];
// add more fancy subviews
}
In my experience, the first approach is much cleaner than the second one. It also makes version control a lot easier (try to merge a xib, I dare you). I always use MyView.m to build the view (a.k.a setup constriants, style) and use MyViewController.m things like animations, lifeCycle. I like to think that MyView.m is the programatic xib, so anything that you can do with xibs, you should me able to do it inside your view.
Hope it helps!!

Communication and space allocation with multiple controllers

iI'm very new in programming with multiple view controllers.
I have a RootViewController, and a FirstViewController. Inside RootViewController:
#property (nonatomic, retain) FirstController * firstController;
- (FirstController *)firstController {
if (!_firstController) {
UIStoryboard *storyboard = self.storyboard;
_firstController = [storyboard instantiateViewControllerWithIdentifier:#"First View"];
}
return _firstController;
}
So, I manage to see the FirstViewController that I've designed in the storyboard.
The problem is this. If I try to run this instruction inside RootViewController:
_firstController.myString = _myString;
myString is obviously the name of a property inside RootViewController and the same inside FirstViewController. Running this instruction doesn't work without allocating memory for _firstController. But, if I allocate memory and initialize _firstController, every adjustments made in the Interface Builder will be vanished.
How can I solve? I'm sure I'm trying to do something very wrong.
Thank you in advance.
You wrote getter - so use it!
self.firstController.myString = _myString;
instead of
_firstController.myString = _myString;
You're trying to access instance directly and of course there are no memory allocated for it because you are using technique called 'lazy loading'(ivar will be allocated when it needed.) By using self.firstController you simply send message [self firstController] which will allocate memory for first controller.

create every time a new uiviewcontroller or reuse the same?

I want to know which is the better way: Either to create a new uiviewcontroller for instance, if in a view i have to load a search view to search new element on a web server, or load a view that display the result from a request, Which is the better way?
This:
MasterViewController.h
#property (nonatomic, strong) PreviewViewController *reviewViewController;
MasterViewController.m
-(void)openPreviewView
{
if (!self.previewViewController) {
self.previewViewController = [[PreviewViewController alloc] init];
}
[self.navigationController pushViewController:self.previewViewController animated:YES];
}
or this:
MasterViewController.m
-(void)openPreviewView
{
PreviewViewController *previewView = [[PreviewViewController alloc] init];
[self.navigationController pushViewController:previewView animated:YES];
}
I always create each view controller as it is needed. The only time I keep a view controller around is if, after doing some proper performance testing, I find that keeping a specific view controller around is better for the user and for performance. Of course you must deal with memory warnings to properly cleanup any cached view controllers.
Don't worry about performance optimizations too early. Wait until you find, through real testing, that you actually have an issue to worry about.
The first one. iOS devices are resource-constrained, so in the absence of a compelling reason not to, you should always re-use if you can.

Delegate is nil

I have a property on a ViewController which I set from a parent SplitViewController:
Property declaration/synthesization
#interface DownloadRecipesViewController_iPad : DownloadRecipesViewController<PopoverMenuDelegate, RecipeChangeDelegate>{
id <NSObject, PopoverMenuParentDelegate, RecipeChangeDelegate, RecipeDownloadedDelegate> _selectionDelegate;
}
#property (strong) UIBarButtonItem *btnMenu;
#property (strong) id <NSObject, RecipeChangeDelegate, RecipeDownloadedDelegate> selectionDelegate;
#implementation DownloadRecipesViewController_iPad
#synthesize btnMenu;
#synthesize selectionDelegate = _selectionDelegate;
I wire up the delegate in the parent SplitViewVC's viewDidLoad method:
Wiring up the delegate
self.downloadVc = [self.storyboard instantiateViewControllerWithIdentifier:#"RecipeDownload"];
[self.downloadVc setSelectionDelegate:self];
A button in the Child VC calls a method to fire an event up to the parent ViewController, but when this event is called, the delegate is nil and the event isn't fired. I've wracked my brains trying every which way to find out why this happens but I'm at a total loss.
Delegate is nil here (firing the delegate):
-(IBAction)didTapMenu:(id)sender{
if([_selectionDelegate respondsToSelector:#selector(shouldToggleMenu)])
[_selectionDelegate shouldToggleMenu];
}
I've also tried without the backing property but hit the same problem.
Suggestions on how to find this follow. But first, why not save yourself some typing and remove the ivar and remove the #synthesize - its totally unnecessary typing at this time. Also, as a comment said, delegates should almost always be typed as weak.
Suggestions:
1) Write a (temporary) setter for the selectionDelegate, then set a break point where you actually set the value (or after) so you can verify that its getting set, and that nothing else is zeroing it out.
2) Set a breakpoint on the IBAction method, on the line where the if statement is, and when you hit it verify the object is the same one where you set the delegate, what the delegate value is, and then see if the respondsTo method succeeds (use single step).
The way I eventually solved this was:
Create a new delegate which exposes a method: -(void)segueBeginning:(UIStoryboardSegue*)destination
Use this delegate to expose prepareForSegue from the child UINavigationController to the parent SplitViewController
.3. Hook up my child VC in the parent ViewController when prepareForSegue is raised in the child nav controller:
if([destination.destinationViewController isKindOfClass:[DownloadRecipesViewController_iPad class]] && !self.downloadVc){ // download recipes
self.downloadVc = destination.destinationViewController;
self.downloadVc.selectionDelegate = self;
[self.downloadVc setSnapshotChangeDelegate:self];
[self.downloadVc.navigationItem setRightBarButtonItems:[NSArray arrayWithObjects:btnAdd, nil]];
}

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