ios-programming-the-big-nerd-ranch-guide-4th-edition.markdown

iOS Programming: The Big Nerd Ranch Guide, 4/e

by Christian Keur, Aaron Hillegass, and Joe Conway

Chapter 1: A Simple iOS Application

• The lefthand side of Xcode shows the navigator area.
• XIB stands for XML Interface Builder. It is pronounced "zib". You create and configure objects, and then save them into an archive, or the XIB file.
• In Interface Builder, the utility area is to the right of the editor. It is divided into the inspector and library sections.
• XIB files are complied into NIB files that are smaller and easier for the application to parse. That is copied into the application's bundle.
• A connection allows two objects to communicate. There are two kinds: An outlet points to an object, while an action defines a callback.
• Select the File's Owner object, and then the connections inspector to view all wired outlets and actions.
• The App Delegate manages the single top-level UIWindow. Assign its window.rootViewController to specify the starting view controller.
• The App ID in you provisioning profile must match the bundle identifier of your application. But a development profile can use an App ID of * and match any bundle identifier.
• The provisioning profile refers to a developer certificate, a single App ID, and a list of device IDs that the application can be installed on.
• The developer certificate is on your Mac's keychain. The provisioning profile belongs to your development device and your computer.
• When deploying to the App Store, an application must have an icon for every device class on which it can run.
• The Images.xcassets file contains an AppIcon section where you can place the various icons for your application.
• A good launch image is a content-less screenshot of the application. Although not required, you can provide one for each class of device.

Chapter 2: Objective-C

• In Objective-C, we typically put an underscore at the beginning of an instance variable, or ivar, name.
• A method is a chunk of code to be executed, while a message is the act of acting a class or object to execute a method.
• A message sent to nil is ignored. So if your program is not doing anything when you expect something, an unexpected nil is usually the culprit.
• Perform fast iteration over an array while attempting to add or remove objects will cause an exception to be thrown.
• In Objective-C, the name of a getter method is just the name of the instance variable that it returns.
• Use accessor methods to access instance variables, even inside a class. Do not refer to them by name, which is prefaced with an underscore.
• All initializers of a class call its designated initializer, which in turn calls the designated initializer of the superclass.
• The instancetype keyword can only be used for return types, and matches the return type of the receiver. init methods always return instancetype.
• In Objective-C, class methods that return an object of their type (like stringWithFormat) are called convenience methods.
• You can use array[index] as shorthand for objectAtIndex:, insertObject:atIndex:, and replaceObjectAtIndex:withObject:.
• Objective-C has no notion of namespaces. Prefix class names with three letters to keep them distinct. Two-letter prefixes are reserved by Apple.
• Precompiled header files saves us from parsing and compiling standard headers repeatedly, but Apple will eventually replace them with the @import directive.

Chapter 3: Managing Memory with ARC

• A variable that does not take ownership of an object is called a weak reference. This helps avoid a strong reference cycle, or retain cycle.
• To decide what reference in a cycle should be weak, look for a parent-child relationship. A parent should own its child, but a child should never own its parent.
• Preface the instance variable declaration with __weak to convert it to a weak reference.
• A weak reference knows when the object that it points to is destroyed, and responds by setting itself to nil. This avoids a dangling pointer.
• When using @property, the name of the generated instance variable is prefixed by an underscore.
• By default, properties are declared atomic, readwrite, and strong. We explicitly declare the strong attribute, in case a new default attribute value is chosen later.
• When a property points to an instance of a class that has a mutable subclass, you should set its memory management attribute to copy.
• To prevent needless copying, immutable classes implement copy to quietly return a pointer to the original and immutable object.
• If you implement a custom setter and a custom getter on a readwrite property, or a custom getter on a readonly property, then you must declare your own instance variable.
• You can use this technique to define accessor methods that are not backed by any instance variable, or backed by some property of a contained object.
• ARC was born when the Clang static analyzer became so good that it could insert all the retain and release messages automatically.
• Inside an @autoreleasepool directive, any newly instantiated object returned from a method that does not have alloc or copy in its name is put into an autorelease pool.

Chapter 4: Views and the View Hierarchy

• Each view draws itself by rendering itself to a layer, which is an instance of CALayer. These layers are then composited together on the screen.
• The x and y of CGPoint and width and height of CGRect are specified in points, not pixels, so they are consistent across different resolutions.
• A pixel is equal to half a point on Retina, and equal to one point on non-retina. When printing to paper, an inch is 72 points long.
• The drawRect: method renders the view onto its layer. UIView subclasses override this method to perform custom drawing.
• The bounds rectangle is in the view's own coordinate system, for drawing itself. The frame rectangle is in the superview's coordinate system, for positioning the subview.
• Instances of UIBezierPath define and draw lines and curves that you can use to make shapes.
• To set the color drawn by the stroke method of UIBezierPath, send the message setStroke of a UIColor instance.
• CGContextRef is the graphics context, which holds the drawing properties (like the pen color and line thickness) and the memory that is being drawn upon.
• The system creates a CGContextRef instance before calling drawRect:, and composites that instance after the method completes executing.
• The current context is an application-wide pointer that is assigned just before drawRect: is called. You can retrieve it by calling UIGraphicsGetCurrentContext().
• A Core Graphics type ending in Ref is a pointer typedef. If you create such an object with a function that has Create or Copy in the name, then you must pass it to the matching Release function.
• Some Core Graphic features cannot be "unset." Instead, you must pass currentContext to CGContextSaveGState beforehand, and then to CGContextRestoreGState afterward.

Chapter 5: Views: Redrawing and UIScrollView

• When the user touches a view, it is sent the message touchesBegan:withEvent:.
• After the run loop dispatches an event, it invokes drawRect: on all dirty views. Send the message setNeedsDisplay to a view to mark it as dirty.
• If you call setNeedsDisplayInRect: instead, that CGRect method is passed to drawRect:, which you can use to optimize re-drawing. But most developers don't bother with this.
• The contentSize of a UIScrollView is the size of the area that it can be used to view. This is usually the size of its subview.
• After adding multiple subviews to a UIScrollView, you can set its pagingEnabled property to page between them, which snaps its viewing port to a subview.

Chapter 6: View Controllers

• A view controller's view is not created until it needs to appear on screen.
• When a view controller initializes its view hierarchy from a NIB file, you do not override loadView. The default implementation loads the NIB file.
• Declare outlets as weak. This ensures that, when memory is low, destroying view also destroys its subviews and avoids memory leaks.
• When a view controller is contained by a UITabBarController, its tabBarItem property appears in the tab bar for that purpose.
• Calling init calls the designated initializer initWithNibName:bundle:. It looks for a NIB with the class name in the main bundle, which it specifies by passing nil as the bundle.
• To preserve the benefits of lazy loading, you should never access the view property of a view controller in initWithNibName:bundle:.
• Destroying and reloading the view of a view controller due to memory constraints is not the typical behavior on newer devices.
• When calling valueForKey:, key-value coding will first look for a corresponding getter method, followed by an instance variable.
• Retina display is 640x1136 pixels on a 4" display, and 640x960 pixels on a 3.5" display, while earlier devices have a display of 320x480 pixels.
• By appending the suffix @2x to a higher resolution image, method imageNamed: of UIImage loads the file that is appropriate for the particular device.

Chapter 7: Delegation and Text Input

• UIResponder is an abstract class that defines methods for handling events, such as touch events, motion events, and remote control events.
• Touch events are sent directly to the corresponding view. For other event types, UIWindow has a firstResponder pointer that specifies their handler.
• Text fields and text views show and hide the keyboard upon becoming and resigning the first responder. Most views refuse to become the first responder so that they do not steal focus.
• By default, protocol methods are required, but you can precede a list of optional methods with the directive @optional.
• If a method in a protocol is required, then a message will be sent to it without first creating a SEL instance and sending message respondsToSelector:.
• You can declare that an object conforms to a protocol in the private extension of that class, as opposed to in its header file.
• A class that uses a delegate must declare it as a weak reference in order to prevent strong reference cycles.
• The textFieldShouldReturn: method must explicitly dismiss the keyboard by sending the message resignFirstResponder to the text field.
• If your application is throwing exceptions and you're not sure why, adding an exception breakpoint will help you pinpoint what's going on.
• The UIApplicationMain method creates an instance of UIApplication, which maintains the run loop.
• Next, UIApplicationMain creates an instance of the delegate class, assigns it to delegate of the UIApplication instance, and sends a message to application:didFinishLaunchingWithOptions:.

Chapter 8: UITableView and UITableViewController

• The designated initializer of UITableViewController is initWithStyle:.
• To return a read-only view of an NSMutableArray, define a private NSMutableArray property, and then define an NSArray property with the readonly attribute that returns it.
• The contentView of a UITableViewCell contains the three subviews textLabel, detailTextLabel, and imageView.
• Call the registerClass:forCellReuseIdentifier: of UITableView to specify which kind of cell it should instantiate if there are no cells with a given identifier in the reuse pool.
• To define a placeholder in a code snippet from the library, place its name between <# and #>.

Chapter 9: Editing UITableView

• The strong attribute must be used with properties that reference top-level objects in the XIB file.
• To resize an empty view in Interface Builder, go to the Simulated Metrics section, and select None for the Size option.
• To load a NIB file manually, send loadNibNamed:owner:options: to NSBundle, where the owner is substituted for the File's Owner placeholder.
• A UITableViewController automatically sets the editing property of its table view to match its own editing property.
• To add a cell, a button above the cells of the table view is usually for adding a record for which there is a detail view.
• To add a cell, a cell with a green plus sign is usually for adding a new field to a record.
• The view and tableView properties of UITableViewController refer to the same view.
• When you implement tableView:commitEditingStyle:forRowAtIndexPath:, swipe-to-delete also works automatically.
• In the call to tableView:moveRowAtIndexPath:toIndexPath:, the row of the new path is that after the item has been removed from its existing path.
• In editing mode, the UITableView will not display the reordering controls if its data source does not implement tableView:moveRowAtIndexPath:toIndexPath:.

Chapter 10: UINavigationController

• The view of a UINavigationController always has two subviews: a UINavigationBar and the view belonging to the topViewController.
• A UINavigationController will automatically resize the view of a UIViewController on its stack so that it fits below the navigation bar.
• Do not position subviews near the top of a XIB file. A view will extend beneath either a UINavigationBar or beneath a UITabBar.
• If you display an .m file next to its XIB file using the assistant editor, you can Control-drag views from the XIB to the .m file to automatically create outlets.
• The shortcut Command-T opens up a new tab, while Command-Shift-} and Command-Shift-{ cycle through the tabs.
• When the message endEditing: is sent to a view, if it or any of its subviews is the first responder, it will resign its first responder status and dismiss the keyboard.
• Every UIViewController has a navigationItem property that supplies the navigation bar with the content that it needs to draw.
• For the titleView of a UINavigationItem, you can either use a basic string as the title or specify a UIView instance.
• UIViewController has an editButtonItem property that is of type UIBarButtonItem. When pressed, it sends the message setEditing:animated: to the controller.

Chapter 11: Camera

• The contentMode property of UIImageView determines where to position and how to resize the content of its frame.
• You cannot use UIImagePickerControllerSourceTypeCamera on devices without cameras, and must send isSourceTypeAvailable: to UIImagePickerController first.
• To populate the photo library in the simulator, open Safari in the simulator, navigate to a page with an image, and save the image.
• Objects of type NSUUID represent a UUID and are generated using the time, a counter, and a hardware identifier, which is usually the MAC address from the WiFi device.
• While a UIBarButtonItem only sends its target an action message when it is pressed, UIControl can send messages in response to a variety of events.
• With the #pragma mark directive, you can specify just a divider with -, a label, or both by prefacing the label with -.
• Just like the class method isSourceTypeAvailable: returns whether a device has a camera, availableMediaTypesForSourceType: returns whether it can capture video.
• After the user takes a video, imagePickerController:didFinishPickingMediaWithInfo: is passed the path to the video in a temporary directory.

Chapter 12: Touch Events and UIResponder

• If a finger moves outside the frame of the UIView that it began on, that view still receives touchesMoved:withEvent: and touchesEnded:withEvent messages.
• While the methods of UIResponder accept a set of UITouch instances, there are usually multiple responder methods with one or more touches.
• Once a touch begins, a UIResponder will ignore subsequent touches unless its multipleTouchesEnabled property is set.
• You can simulate touching with multiple fingers in the simulator by holding down the Option key as you drag.
• The valueWithNonretainedObject: method of NSValue is useful if you want to add an object to a collection without creating a strong reference to it.
• This method also allows objects that do not implement NSCopying to be used as keys in dictionaries.
• Every UIResponder, such as UIView, UIViewController, UIWindow, and UIApplication, has a pointer called nextResponder, forming a responder chain.
• By default, methods touchesBegin:withEvent:, touchesMoved:withEvent:, and touchesEnded:withEvent simply delegate to nextResponder.
• The CGRectContainsPoint method returns whether a given point is in a CGRect variable.

Chapter 13: UIGestureRecognizer and UIMenuController

• A UIGestureRecognizer intercepts touches destined for a view, and so this view may not respond to typical UIResponder messages.
• Setting the delaysTouchesBegan property delays sending touchesBegan:withEvent: to its view if it is still possible for the gesture to be recognized.
• The method requireGestureRecognizerToFail: allows you to require a double-tap recognizer to fail before invoking a single-tap recognizer.
• The locationInView: method of UIGestureRecognizer returns the coordinate where the gesture occurred in the coordinate system of its view argument.
• For a UIMenuController to appear, a view that responds to at least one action message of its menu items must be a first responder of the window.
• If you have a custom view that needs to become the first responder, you must implement canBecomeFirstResponder to return YES.
• A long press gesture begins when the user holds a touch for 0.5 seconds, and ends when the touch ends.
• A gesture recognizer will send gestureRecognizer:shouldRecognizeSimultaneouslyWithGestureRecognizer: to its delegate if another gesture recognizer has recognized its gesture, too.
• The translationInView: method of UIPanGestureRecognizer returns a CGPoint containing how far the pan has moved in the coordinate system of its view argument.
• Pass CGPointZero to setTranslation:inView: of UIPanGestureRecognizer at the end of its action method to report the change in translation between action method calls.
• If a UIResponder implements a method in UIResponderStandardEditActions, it enables the corresponding default menu item in an unmodified menu controller.
• A UIGestureRecognizer for a discrete gesture like tap only enters the recognized state; it does not transition through the begin, change, and end states.
• A gesture recognizer can enter the fail state, such as when no amount of finger contortion can make the particular gesture, given their current position.

Chapter 14: Debugging Tools

• The debug gauges in Xcode are based on the hardware running the simulator, which is undoubtedly more powerful than the iOS device.
• In the Memory Report, the Memory graph scales so that peak usage represents 100%.
• In the Allocations instrument, the # Overall column shows you how many instances of a class have been allocated -- even if they have since been deallocated.
• When you select a method, the percentage on each line of the method specifies the amount of memory that line allocates compared to the other lines.
• The Generation Analysis category, also called Heapshot Analysis, is very useful for inspecting what objects get allocated for a specific event.
• By choosing Invert Call Tree in the Time Profiler, you can see how much time is spent in each method call.
• If a system call takes a lot of time, you can change it so that the time spent in that method is attributed to the method that called it.
• The mach_msg_trap() method is where the main thread sits when waiting for input, and so it is not a bad thing to spend time in this function.
• CGPoint and CGRect are structures instead of Objective-C classes so they do not incur the overhead of sending messages for simple operations.
• A project has at least one target. You build and run the target, which produces a product, such as the application, a library, or a unit test bundle.
• Under Build Settings, setting Analyze During 'Build' will automatically run the static analyzer every time you build your application.

Chapter 15: Introduction to Auto Layout

• A single application that runs natively on both the iPhone and the iPad is called a universal application.
• The iPhone 4S has 320x480 points, the iPhone 5 and later has 320x568 points, and all iPads have 768x1024 points.
• Constraints determine the layout attributes like the size, margins, center, and baseline of the alignment rectangle constructed by Auto Layout.
• The nearest neighbor of a view is the closest sibling view in a specified direction, or its superview if no such sibling view exists.
• Apple recommends that you add constraints using Interface Builder whenever possible, instead of defining them programmatically.
• Constraint problems include missing constraints, conflicting constraints, and constraints not matching a view's size and position on the canvas.
• By Control-dragging from one view to another on the canvas, you can specify a constraint between those views.
• Constraints have a priority value ranging from 1 to 1000, where 1000 is a required constraint. This is their default value.
• To remedy an ambiguous layout, which occurs when there is more than one way to fulfill a set of constraints, add another constraint.
• To display an alternate layout resulting from ambiguous constraints, send the message exerciseAmbiguityInLayout to each subview.
• A misplaced view problem is when a view's frame in a XIB does not match its constraints.
• For a string representation of the view hierarchy, with views having ambiguous layout tagged, set a breakpoint and then enter into the debugger po [[ UIWindow keyWindow] _autolayoutTrace].
• If you need completely different views depending on the device, create one XIB file with a ~iphone suffix, and another with an ~ipad suffix.

Chapter 16: Auto Layout: Programmatic Constraints

• If creating and constraining an additional view to add to a view hierarchy that was created by loading a NIB file, override method viewDidLoad.
• Without setting translatesAutoresizingMaskIntoConstraints to NO, iOS creates constraints from the mask, which may conflict with other constraints.
• In the Visual Format Language, or VFL, the dash by itself sets the spacing to the standard number of points between views, which is 8.
• You should call addConstraints: on the closest common ancestor of all the views that are affected by the constraint.
• Unlike other constraints, intrinsic content size constraints have a content hugging priority and a content compression resistance priority.
• A content hugging priority of 1000 means that the view should never be allowed to grow larger than its intrinsic content size.
• A content compression resistance priority of 1000 means that the view should never be allowed to be smaller than its intrinsic content size.
• Before Auto Layout, each view had a resizing mask that constrained the relationship between a view and its superview.
• When debugging constraints, a constraint from the resizing mask has the type NSAutoResizingMaskLayoutConstraint instead of NSLayoutConstraint.

Chapter 17: Autorotation, Popover Controllers, and Modal View Controllers

• Device orientation represents the physical orientation of the device. You can access it through the orientation property of UIDevice.
• The interface orientation is a property of the running application, and is determined by the location of the Home button.
• For the interface orientation to change, both the rootViewController of the application and the application itself must allow the new orientation.
• To match convention, a view controller on the iPad allows all four orientations, while one on the iPhone allows any orientation other than upside-down.
• A UITabViewController only supports an orientation if the view controllers for each of its tabs support it.
• The statusBarOrientation method of the UIApplication instance returns the interface orientation.
• Trying to instantiate UIPopoverController on anything but an iPad will throw an exception.
• Explicitly calling dismissPopoverAnimated: on a UIPopoverController will not send popoverControllerDidDismissPopover: to its delegate.
• The presentingViewController property of UIViewController allows a modally-presented view controller to communicate with the view controller that presented it.
• When a modally-presented view controller uses the UIModalPresentationFormSheet style, the presenting view controller is not sent viewWillAppear: or viewDidAppear:.
• The two relationships between view controllers are parent-child relationships and presenting-presenter relationships.
• UINavigationController, UITabBarController, and UISplitViewController are all view controller containers, where each has a viewControllers property.
• A view controller container subclasses UIViewController and selectively adds the views of viewControllers as subviews of its own view.
• The parentViewController property of UIViewController refers to the closest view controller ancestor in a family, or tree of view controller containers.
• With a modally-presented view controller, presentingViewController and presentedViewController are valid for each view controller in each family, and always refer to the oldest ancestor in the other family.

Chapter 18: Saving, Loading, and Application States

• An NSCoder organizes the stream of data that it writes to the filesystem as a collection of key-value pairs.
• Holding down the Option key and clicking on a class name presents a pop-up window with a brief description and links to its header file and its reference.
• Although a XIB file is not a standard archive, archiving is how Interface Builder writes and reads XIB files.
• In the application sandbox, directories Documents/ and Library/Caches contain data that persists between runs, but only the former is synchronized with iTunes.
• Static method archiveRootObject:toFile: of NSKeyedArchiver archives an object implementing NSCoding to the given filename.
• When an overlay to handle an SMS message, push notification, phone call, or alarm appears on top of your application, it is in the inactive state.
• An application spends about ten seconds in the background state before it enters the suspended state.
• The inactive state loses the ability to receive events, the background state loses visibility, and the suspended state loses the ability to execute code.
• The operating system terminates suspended applications as needed when available memory is low; such an application receives no notification of this.
• Transitioning to the background state is a good place to save outstanding changes, because it's the last time it can execute code before entering the suspended state.
• Calling writeToFile:atomically: of NSData with YES writes the data to a temporary file, and then renames that file to the first parameter.
• In addition to self, the implicit variable _cmd is the selector for the current method, and can be translated to a string using NSStringFromSelector.
• The localizedDescription method of NSError returns a human-readable description that is suitable for display to the user.
• Use NSException if the error lies with the programmer, and NSError if a request is reasonable but could not be fulfilled.
• To use writeToFile: and initWithContentsOfFile: with collection objects, they must contain only property list serializable objects, namely NSString, NSNumber, NSDate, NSData, NSArray, and NSDictionary.
• Files within the application bundle are read-only, and you cannot dynamically add files to the application bundle at runtime.

Chapter 19: Subclassing UITableViewCell

• To subclass UITableViewCell, add subviews to its content view, which is resized when the user enters editing mode, for example.
• Function UIGraphicsBeginImageContextWithOptions creates a new CGContextRef that is offscreen, and sets it as the current context.
• Get a UIImage from the current context by calling UIGraphicsGetImageFromCurrentImageContext, and then UIGraphicsEndImageContext to clean up.
• To allow tapping an image to show a full-size image, add a transparent UIControl or UIButton on top of the thumbnail.
• When such a button is tapped, it invokes a method of the UITableViewCell subclass. This should invoke a block assigned by the controller to implement behavior.
• Blocks are created on the stack and not the heap, and so the assignment of a block to a UIView or persisted instance must use the copy attribute.
• Blocks own the objects that they capture from their enclosing scope, which can easily create a strong reference cycle.
• A block should instead capture a weak reference to whatever object owns it. When the block starts executing, assign that to a strong reference to ensure that exists while the block executes.
• A UICollectionViewLayout subclass controls the attributes of each cell, including its position and size, in a UICollectionView.
• A UICollectionViewCell has a content view with no subviews, and so you must subclass it if you are using a UICollectionView.

Chapter 20: Dynamic Type

• When a font is requested for a given text style, the system will use the preferred text size to return the appropriately configured font for that style.
• To react to users' changes in text size, you must subscribe to UIContentSizeCategoryDidChangeNotification and reassign the preferred fonts.
• For a view to grow larger than its intrinsicContentSize in a given direction, there must be a constraint with a higher priority than that view's Content Hugging Priority.
• For a view to grow smaller than its intrinsicContentSize in a given dimension, there must be a constraint with a higher priority than that view's Compression Resistance Priority.
• Given the preferredContentSize property value of UIApplication, assign the height property of a UITableView the correct row height.
• The awakeFromNib method is called after unarchiving, and is a great place to do any additional UI work that cannot be done within the XIB file.
• Modifying the frame or bounds of a view will only persist until the next time the view is laid out based on the constraints that it has.
• Constraints are instances of NSLayoutConstraint, so just like you can create outlets to views, you can create outlets to constraints.
• A placeholder constraint in Interface Builder is only temporary and is removed at build time.
• Use placeholder constraints if adding a programmatic constraint creates a conflict, but removing the constraint in IB would warn about misplaced views or ambiguous layout.

Chapter 21: Web Services and UIWebView

• NSURLRequest holds all data necessary to communicate with a server, including an NSURL instance, a caching policy, a timeout value, and any additional HTTP data.
• NSURLSessionTask encapsulates the lifetime of a single NSURLRequest, and has methods to cancel, suspend, and resume the request.
• NSURLSession acts as a configurable factory for NSURLSessionTask instances.
• To percent encode and decode a string, use methods stringByAddingPercentEscapesUsingEncoding: and stringByRemovingPercentEncoding.
• A NSURLSessionTask instance is always created in the suspended state, and so calling resume on the task will start the request.
• The main thread is also referred to as the UI thread, as any code that modifies the UI has to run on the main thread.
• By default, NSURLSessionDataTask runs the completion handler on a background thread, but you can transfer control to the main thread by calling dispatch_async with dispatch_get_main_queue().
• To respond to an authentication challenge, an NSURLSessionDataDelegate can return a NSURLCredential containing a username and password.
• NSURLRequest will compute the size in bytes of its body, and automatically add a Content-Length header with this size.

Chapter 22: UISplitViewController

• A UISplitViewController refers to an array of view controllers, but that array can only contain two view controllers.
• The splitViewController message of UIViewController returns the UISplitViewController it is a part of, even if it is not a member of the array of UISplitViewController.
• By supplying a UIBarButtonItem to the delegate of UISplitViewController, tapping this button displays the master view controller in a specialized UIPopoverController.
• If the detail view controller does not specify a title for the UIBarButtonItem, then it won't appear, unless the master view controller specifies a title for its navigationItem.
• This requires placing the detail view controller inside a UINavigationController, unless you instantiate your own UINavigationBar or UIToolbar to hold the UIBarButtonitem.

Chapter 23: Core Data

• In Core Data, a table/class is called an entity, and its columns/properties are called its attributes.
• With a transformable attribute, a supplied NSValueTransformer subclass converts between instances of a class and NSData for storage in Core Data.
• An entity in a to-one relationship has a pointer to the related entity, while an entity with a to-many relationship has an NSSet containing all related entities.
• An NSManagedObject works a bit like a dictionary, holding a key-value pair for every property in the entity.
• If your model objects must do something in addition to holding data, subclass NSManagedObject, and associate that subclass with the entity in your model file.
• Adding a NSManagedObject to the database invokes its awakeFromInsert method, which can contain the logic that is normally found in an initializer.
• An NSPersistentStoreCoordinator uses your model file in the form of an NSManagedObjectModel to interact with a SQLite database.
• Calling save: on an NSManagedObjectContext will update all records in the SQLite database with any changes since the last time it was saved.
• You use an NSPredicate to select which instances an NSFetchRequest should return, but you can also use it with filteredArrayUsingPredicate: of NSArray.
• To persist a reorderable list to Core Data, add an orderingValue of type double to each entity. When inserting an element, assign its orderingValue to the mean of that of its neighbors.