Is NSNotificationCenter is part of UITextView? - ios

Question

Sorry for the basic Question, I'm fairly new to programming and trying to understand something in the code apple suggester for a certain solution to something I wanted to preform.
I created a simple notes app, very basic, currently I have:
1. CreateNote view controller
2. NotesList table view controller
So I wanted to add a behaviour when a note is being created and a user types below the keyboard so the text resized so the user can still see what he types and the text is not going behind the keyboard.
So I add some lines of code suggested by apple to accomplish that.
In the viewWillAppear called a method on NSNotificationCenter and I could not understand where is an NSNotificationCenter object is declared...?
So this is my CreateNote view controller(Please help me understand why they could preform this call):
#import "NMCreateNotesViewController.h"
#interface NMCreateNotesViewController () <UITextViewDelegate>
#property (weak, nonatomic) IBOutlet UIBarButtonItem *createButton;
#property (weak, nonatomic) IBOutlet UITextView *textField;
#end
#implementation NMCreateNotesViewController
- (void)viewWillAppear:(BOOL)animated
{
[super viewWillAppear:animated];
// listen for keyboard hide/show notifications so we can properly adjust the table's height
[[NSNotificationCenter defaultCenter] addObserver:self
selector:#selector(keyboardWillShow:)
name:UIKeyboardWillShowNotification
object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self
selector:#selector(keyboardWillHide:)
name:UIKeyboardWillHideNotification
object:nil];
}
#pragma mark - Notifications
- (void)adjustViewForKeyboardReveal:(BOOL)showKeyboard notificationInfo:(NSDictionary *)notificationInfo
{
// the keyboard is showing so resize the table's height
CGRect keyboardRect = [[notificationInfo objectForKey:UIKeyboardFrameEndUserInfoKey] CGRectValue];
NSTimeInterval animationDuration =
[[notificationInfo objectForKey:UIKeyboardAnimationDurationUserInfoKey] doubleValue];
CGRect frame = self.textField.frame;
// the keyboard rect's width and height are reversed in landscape
NSInteger adjustDelta = UIInterfaceOrientationIsPortrait(self.interfaceOrientation) ? CGRectGetHeight(keyboardRect) : CGRectGetWidth(keyboardRect);
if (showKeyboard)
frame.size.height -= adjustDelta;
else
frame.size.height += adjustDelta;
[UIView beginAnimations:#"ResizeForKeyboard" context:nil];
[UIView setAnimationDuration:animationDuration];
self.textField.frame = frame;
[UIView commitAnimations];
}
- (void)keyboardWillShow:(NSNotification *)aNotification
{
[self adjustViewForKeyboardReveal:YES notificationInfo:[aNotification userInfo]];
}
- (void)keyboardWillHide:(NSNotification *)aNotification
{
[self adjustViewForKeyboardReveal:NO notificationInfo:[aNotification userInfo]];
}
- (void) prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
{
if (sender != self.createButton) return;
if (self.textField.text.length > 0) {
self.note = [[NMNote alloc] init];
self.note.content = self.textField.text;
}
}
- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
if (self) {
// Custom initialization
}
return self;
}
- (void)viewDidLoad
{
[super viewDidLoad];
// Do any additional setup after loading the view.
}
- (void)didReceiveMemoryWarning
{
[super didReceiveMemoryWarning];
// Dispose of any resources that can be recreated.
}
#end

Answer 1

Is NSNotificationCenter is part of UITextView?
No it is not. NSNotificationCenter is - as it's name says - a notification center. Objects can subscribe to notifications and post notifications with NSNotificationCenter to handle and notify of certain events.
They are using NSNotificationCenter to have the viewcontroller subscribe to UIKeyboardWillHideNotification and UIKeyboardWillShowNotification events.
Take a look at this one:
[[NSNotificationCenter defaultCenter] addObserver:self
selector:#selector(keyboardWillShow:)
name:UIKeyboardWillShowNotification
object:nil];
NSNotificationCenter is designed to be used as a singleton ( I believe this is the correct term, correct me if I'm wrong ) so we access the NSNotificationCenter for this app's process by calling the class method defaultCenter. it adds observer 'self' ( which in this case is an instance of the view controller) and basically instructs it to send the message keyboardWillShow to the observer when a Notification under the name of UIKeyboardWillShowNotification is fired.
What object fires the UIKeyboardWillShowNotification? Well it's not a UITextView, this notification name is actually defined in UIWindow.h so it probably came from there, which in turn probably was invoked from UIKeyboard which is not a public API as far as I know.

Answer 2

NSNotificationCenter is a class. In Objective-C, classes are declared in header files; this one is in NSNotification.h. (Try pressing Command-Shift-O and typing "NSNotificationCenter" to find this yourself.) When you want to use a class, you #import the header file that the class is declared in; this makes the compiler read that header file first and make all the classes (and other globals) available to your code to use.
This would be a huge pain, though, since a typical Cocoa app uses zillions of classes and other globals from Apple's libraries. So, instead, you just need to #import <Foundation/Foundation.h> which is a header file that just includes a bunch of other header files, including NSNotification.h. (The import statement for Foundation is probably in your own header file, or something else like #import <UIKit/UIKit.h> which also will include foundation and ultimately NSNotification.h.)
One final detail is that there's is probably a "prefix" header in your project which includes UIKit.h in all of your files automatically, so anything declared in there is always available to your code.

Answer 3

NSNotificationCenter is a class in Foundation.

Answer 4

NSNotificationCenter doesn't need to be declared and stored in a variable, its just a call, to explain it further, think of NSNotificationCenter as a tackboard where things get posted, you add a note to that backboard by creating the NSNotificationCenter, and you assingn observers to look at that board, and do something when a note is added.

Answer 5

The NSNotificationCenter object being used in this case is a Singleton. What you need to know is that sending the message defaultCenter to the NSNotificationCenter class object always returns the same NSNotificationCenter object.
Here's what the call to default center might look like
+ (NSNotificationCenter*)defaultCenter
{
static dispatch_once_t once;
static NSNotificationCenter* sharedInstance;
dispatch_once(&once, ^{
sharedInstance = [[self alloc] init];
/*
set up properties, etc
*/
});
return sharedInstance;
}