iOS Engineer

A learning resource for iPhone and iPad developers. By Gaurav Sharma www.gauravsharma.com

[iosengineering]

UITableView Editing

Set the editing property on UITableView to YES to enable editing mode:

To enable this for the user, we add a headerView at the top of the table and add buttons to it to Add New Item and Enable/Disable Editing:

@interface MyViewController : UITableViewController {     IBOutlet UIView *headerView; } - (UIView *)headerView; - (IBAction)addNewItem:(id)sender; - (IBAction)enableDisableEditing:(id)sender; @end

File > New > New File > iOS > User Interface, and choose Empty. Name this file "HeaderView". Change the File Owner object of this XIB file to MyViewController above.

Drag a new UIView on to the empty canvas. Drag 2 buttons on to the UIView, labelling them "Add" and "Edit". Connect these to relevant IBActions on the File Owner (MyViewController).

In the implementation file:

- (UIView *)headerView {     if (!headerView)     {         [[NSBundle mainBundle] loadNibNamed:@"HeaderView" owner:self options:nil];     }     return headerView; }

The first time headerView is called on MyViewController, it will load the HeaderView XIB file created above. The Add and Edit buttons inside will send messages to ItemsViewController when pressed.

Implement these methods in the implementation file:

- (UIView *)tableView: (UITableView *)tableView                          viewForHeaderInSection:(NSInteger)section {     return [self headerView]; } - (CGFloat)tableView:(UITableView *)tableView                       heightForHeaderInSection:(NSInteger)section {     return [[self headerView] bounds].size.height; }

These are both required for a header view.

UITableView sends messages to its delegate ItemsViewController. As soon as headerView is needed it will be loaded from the XIB file.

To load a XIB file manually, call loadNibNamed:owner:options.

Implement the methods for the buttons:

- (IBAction)toggleEditingMode:(id)sender {     if ([self isEditing])     {         [sender setTitle:@"Edit" forState:UIControlStateNormal];         [self setEditing:NO animated:YES];     }     else     {         [sender setTitle:@"Done" forState:UIControlStateNormal];         [self setEditing:YES animated:YES];     } }

Adding Rows

- (IBAction)addNewItem: (id)sender {     Record *newRecord = [recordStore createItem];     int lastRow = [[recordStore allItems] indexOfObject:newRecord];

NSIndexPath *path = [NSIndexPath indexPathForRow:lastRowinSection:0];

    [[self tableView] insertRowsAtIndexPaths: [NSArray arrayWithObject:path] withRowAnimation:UITableViewRowAnimationTop]; }

Removing Rows

To delete a cell, remove it from the UITableView and then from the model.

In the interface for our Record object:

- (void)removeItem:(Record *)record;

Then in the item store:

- (void)removeItem:(Record *)record {     [allItems removeObjectByIdenticalTo:record]; }

Then in MyViewController:

- (void)tableView:(UITableView *)tableView            commitEditingStyle:(UITableViewCellEditingStyle)editingStyle            rowRowAtIndexPath:(NSIndexPath *)indexPath {     if (editingStyle == UITableViewCellEditingStyleDelete)     {         NSArray records = [ItemStore allItems];         Record *record = [records objectAtIndex:[indexPath row]];         [ItemStore removeItem:record];         [tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath]         withRowAnimation:UITableViewRowAnimationFade];     } }

Moving Rows

Declare and implement the following method in the item store:

- (void)moveItemAtIndex:(int)from                   toIndex:(int)to {     if (from == to) { return; }     Record *record = [allItems objectAtIndex:from];     [allItems removeObjectAtIndex:from];     [allItems insertObject:record atIndex:to]; }

Then in MyViewController:

- (void)tableView:(UITableView *)tableView         moveRowAtIndexPath:(NSIndexPath *)sourceIndexPath         toIndexPath:(NSIndexPath *)destinationIndexPath {     [ItemStore moveItemAtIndex:[sourceIndexPath row]                      toIndex:[destinationIndexPath row]]; } }

[iosengineering]

Reusing UITableViewCells

In tableView:cellForRowAtIndexPath:

UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:@"UITableViewCell"];

//only create a cell if a suitable one doesn't already exist if (!cell) {     cell = [[UITableViewCell alloc]             initWithStyle:UITableViewCellStyleDefault            reuseIdentifier:@"UITableViewCell"]; }

This is faster, as it avoids creating a separate UITableViewCell for every object, even for ones which are not being shown.

[iosengineering]

UITableViewCells and UITableViewDataSource

A UITableViewDataSource must implement:

tableView:numberOfRowsInSection:

tableView:cellForRowAtIndexPath:

Each row in a UITableView is a UITableViewCell. Each UITableViewCell has a contentView, which holds the content for the cell.

Each TableViewCell has a UITableViewCellStyle which determines the composition of subviews within the contentView.

To map cells to items:

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {     UITableViewCell *cell = [[UITableViewCell alloc]       initWithStyle:UITableViewCellStyleDefault        reuseIdentifier:@"UITableViewCell"];     Record *record = [[itemStore allItems]                                     objectAtIndex:[indexPath row]];     [[cell textLabel] setText:[p description]];

    return cell; } - (NSInteger)tableView(UITableView *)tableView     numberOfRowsInSection:(NSInteger)section {     return [[itemStore allItems] count]; }

itemStore here is an object that holds an NSMutableArray, allItems. This data source will typically exist on the UITableViewController backing the UITableView.

[iosengineering]

static

Items can be prefixed with the static keyword:

static int sharedNumber = 42;

A static variable does not live on the stack, and is not destroyed when a containing method returns. It is declared only once (subsequent declarations are ignored), and it is never destroyed.

[iosengineering]

UITableView

A UITableView shows a single column of data, with 1 or more rows:

A UITableView is best accompanied by a UIViewController to handle its appearance on the screen.

A UITableView takes a data source, conforming to UITableViewDataSource, from which to display data.

A UITableView takes a delegate, conforming to UITableViewDelegate, to inform it of events involving itself.

An associated UITableViewController normally acts as view controller, data source, and delegate.

UITableViewController has a view, which is an instance of UITableView. When a UITableViewController creates its view, the dataSource and delegate variables of the UITableView are automatically set to point to the UITableViewController.

The designated initializer of UITableViewController is initWithStyle:, which can be either UITableViewStylePlain or UITableViewStyleGrouped (where the top and bottom rows have rounded corners).

loadView on UITableViewController creates and loads an empty UITableView if none exists.

[iosengineering]

UIDevice Notifications

UIDevice can post these notifications:

UIDeviceByOrientationDidChangeNotification UIDeviceBatteryStateDidChangeNotification UIDeviceBatteryLevelDidChangeNotification UIDeviceProximityStateDidChangeNotification

Set this up in the App Delegate, application:didFinishLaunchingWithOptions:

UIDevice *device = [UIDevice currentDevice];

[device beginGeneratingDeviceOrientationNotifications];

NSNotificationCenter *center = [NSNotificationCenter defaultCenter];

[center addObserver:self             selector:@selector(orientationChanged:)             name:UIDeviceOrientationDidChangeNotification             object:device];

- (void)orientationChanged:(NSNotification *)n {     NSLog(@"orientationChanged: %d", [[n object] orientation]); }

[iosengineering]

NSNotificationCenter and NSNotification

Every app has an instance of NSNotificationCenter.

Objects can register with NSNotificationCenter as an observer.

Objects can post notifications to NSNotificatonCenter.

When NSNotificationCenter receives a notification of a matching kind, it will forward these on to the appropriate registered objects.

Notification instances are of type NSNotification.

To register a notification:

NSNotificationCenter *center = [NSNotificationCenter defaultCenter]; [center addObserver:self              selector:@selector(openDoor:)              name:@"Knock"              object:nil];

In this example, the openDoor message is sent to the current object (self) when a notification with name "Knock" is posted, by any object (signified by the last argument being nil).

The following method is triggered when the "Knock" notification arrives:

- (void)openDoor:(NSNotification *)n {     id poster = [n object];     NSString *name = [n name];     NSDictionary *extraInformation = [n userInfo]; }

userInfo is a dictionary used to pass additional information.

Example notification:

NSDictionary *info = [NSDictionary dictionaryWithObject:@"Fast" forKey:@"Speed]; NSNotification *notification = [NSNotificationnotificationWithName:@"Knock" object:self userInfo:info]; [[NSNotificationCenter defaultCenter] postNotification:notification];

Remember to unregister registered objects if no longer in use:

- (void)dealloc {     [[NSNotificationCenter defaultCenter] removeObserver:self]; }

[iosengineering]

UIApplication

int main(int argc, char *argv[]) {     @autoreleasepool     {         return UIApplicationMain(argc, argv, nil, NSStringFromClass([MyAppDelegate class]));     } }

UIApplicationMain creates an instance of a class called UIApplication.

Every app has a UIApplication instance, which maintains the application's run loop, which executes indefinitely, until the application terminates.

Before the run loop begins, the app calls application:didFinishLaunchWithOptions, to create any Views, Controllers, and other objects that need to be ready before it starts.

[iosengineering]

UITabBarController

UITabBarController is the typical tab bar you see at the bottom of an iPhone application:

In your App Delegate file, inside the application:didFinishLaunchingWithOptions method:

UITabBarController *tabBarController = [[UITabBarController alloc] init]; NSArray *viewControllers = [NSArray arrayWithObjects: viewController1, viewController2, viewController3, nil]; [tabBarController setViewControllers:viewControllers]; [[self window] setRootViewController:tabBarController];

To set titles, in each controller representing a tab bar item:

- (id)initWithNibName:(NSString *)nibName bundle:(NSBundle *)bundle {     self = [super initWithNibName:nil bundle:nil];     if (self)     {         UITabBarItem *item = [self tabBarItem];         [item setTitle:@"First"];         UIImage *image = [UIImage imageNamed:@"circle.png"];         [item setImage:image];     }     return self; }

[iosengineering]

UIViewController

A UIViewController controls a given UIView.

A UIViewController has a loadView method that can be overridden to set its UIView:

- (void)loadView {     CGRect frame = [[UIScreen mainScreen] bounds];     MyView *view = [[MyView alloc] initWithFrame:frame];     [self setView:view]; }

Set the controller as the root one in the App Delegate file:

MyViewController *viewController = [[MyViewController alloc] init]; [[self window] setRootViewController:viewController];

The designated initializer of UIViewController is initWithNibName:bundle:. If you pass in nil for both arguments, an XIB file with the same name as the class will be loaded, and its application bundle with be searched.

Typically, the name of a UIViewController will be the same as the name of the XIB file. So you only need to call:

MyViewController *viewController = [[MyViewController] alloc] init];

//This is the same as 

MyViewController *viewController = [[MyViewController] alloc] initWithNibName:nil bundle:nil];

Views are loaded only as needed. If the system is running low on memory and the view is not on the screen, the view controller will destroy the view.

Override viewDidLoad to perform any actions after the view loads:

- (void)viewDidLoad {     [super viewDidLoad];     NSLog("@"Loaded view!"); }

This is only triggered the first time the view loads.

Every UIViewController has a view method like this:

- (UIView *)view {     if ([self isViewLoaded] == NO)     {         [self loadView];         [self viewDidLoad];     }     return view; }

Any subviews referenced in the UIViewController will continue to exist even if the view is destroyed. These references should be destroyed in viewDidUnload:

- (void)viewDidUnload {     [super viewDidUnload];     NSLog(@"Unloading subviews);     myLabel = nil; }

The alternative is to use a weak reference:

@interface MyViewController : UIViewController {     __weak IBOutlet UILabel *myLabel; }

Declare IBOutlets as weak references to prevent memory leaks. Don't do this for top-level objects such as the controller's view, which need to stay in place. 

UIViewController also has the following methods:

viewWillAppear:

viewDidAppear:

viewWillDisappear:

viewDidDisappear:

[iosengineering]

Motion Events

To take action when the device is shaken, implement motionBegan:withEvent on the relevant view:

- (void)motionBegan:(UIEventSubtype)motion                  withEvent:(UIEvent *)event {     if (motion == UIEventSubtypeMotionShake)     {         NSLog(@"Device Shaken!");     } }

[iosengineering]

UIResponder and First Responder

A UIResponder (which can be a UIView, which is a type of UIResponder) can receive events, such as keys being pressed or the device being shaken. It does this by becoming a first responder:

BOOL b = [view becomeFirstResponder]; if(b) {     NSLog(@"Became first responder"); } else {     NSLog(@"Could not become first responder"); }

Most UIResponder objects return NO to becomeFirstResponder, because they already respond to more specific events. For example a UIButton will get a message when it is pressed, regardless of what's chosen as first responder.

first responder, override canBecomeFirstResponder, returning YES.To enable a custom view to become first responder, override canBecomeFirstResponder, returning YES.

[iosengineering]

UIView and UIWindow

A UIView knows how to draw itself on a UIWindow, which represents the app's window - the container for everything you see.

The user interface is composed on many UIViews within UIViews - everything you see on the screen, from buttons to images to controls, are all different kinds of UIView. These normally exist within a hierarchy. For example a panel UIView may contains button UIViews, and buttons UIViews may contain image UIViews.

A UIView handles related events. For example, when you touch a button on the screen, the UIView for that button will handle what to do when this happens.

A UIView can be sent the message setNeedsDisplay when its drawable content changes. This redraws the UIView. This is called when, for example, the text in a UILabel, which is a type of UIView, changes.

[iosengineering]

Core Location and Delegation

Create a new project with a class prefix of "CA".

In the interface file, CAViewController.h:

@interface CAViewController: UIViewController {     CLLocationManager *locationManager }

In the implementation file, CAViewController.m:

locationManager = [[CLLocationManager alloc] init]; [locationManager setDesiredAccuracy:kCLLocationAccuracyBest]; [locationManager startUpdatingLocation];

CLLocationManager can be assigned a delegate property. This is the object to which CLLocationManager sends messages.

A delegate receives all event messages from a particular object.

The delegate need only implement methods that correspond to the events it wants to hear about.

In this case we are setting the delegate to self (a CAViewController instance), and it receives events from CLLocationManager.

Add the following line under the first in CAViewController.m:

[locationManager setDelegate:self];

locationManager now sends locationManager:didUpdateToLocation:fromLocation and other messages to its delegate, CAViewController (referred to as self above).

Therefore we can implement didUpdateToLocation in CAViewController.m:

- (void)locationManager:(CLLocationManager *)manager       didUpdateToLocation:(CLLocation *)newLocation       fromLocation:(CLLocation *)oldLocation {     NSLog(@"%@", newLocation); }

And also:

- (void)locationManager:(CLLocationManager *)manager        didFailWithError:(NSError *)error {     NSLog(@"Could not find location: %@, "error); }

An object can only send its delegate messages from a specific set listed in a protocol. The delegate implements methods from the protocol for the events that it is interested in.

Here is the protocol for CLLocationManagerDelegate:

@protocol CLLocationManagerDelegate <NSObject> @option - (void)locationManager:(CLLocationManager *)manager     didUpdateToLocation:(CLLocation *)newLocation         fromLocation:(CLLocation *)oldLocation; - (void)locationManager:(CLLocationManager *)manager     didFailWithError:(NSError *)error; @end

 <NSObject> at the end of the protocol header indicates that CLLocationManagerDelegate includes all of the methods in the NSObject protocol as well.

The protocol is not a class - it is simply a list of methods.

A class that implements methods from a protocol conforms to it.

By default, protocol methods must be implemented in a conforming class.

Optional methods can be indicated using the @optional directive. Before sending an optional message, the object first asks its delegate if it is okay to send first a respondsToSelector: message. This method checks at runtime whether an object implements a given method.

- (void)finishedFindingLocation:(CLLocation *)newLocation {     if ([[self delegate[ respondsToSelector:@selector(locationManager:didUpdateToLocation:fromLocation:)])     {         [[self delegate] locationManager:self             didUpdateToLocation:newLocation                 fromLocation:oldLocation];     }

If a protocol method is not marked as optional, is not implemented, yet called, an unrecognized selector exception will be thrown.

Explicitly state that a class conforms to a protocol:

@interface CAViewController : UIViewController <CLLocationManagerDelegate>

Typically, controller objects are made delegates. For example in this case CAViewController was made the delegate of CLLocationManager.

[iosengineering]

Copying Objects

To make a copy of an object:

notes2 = [notes copy];

To declare this in a property:

@property (nonatomic, copy) NSString *notes2;

This ensures notes2 will keep its own copy of the string assigned to it.

[iosengineering]

Weak References

Normally, if an object is no longer to referred to, the memory used for that object is released so it can be reused elsewhere.

However, if 2 objects reference each other, and neither is no longer used, they will keep each other in memory, because they refer to each other.

To prevent this, mark one of the object references as weak:

__weak Record *record;

A weak reference allows the reference object to be destroyed if it is not referenced by anything else.

A weak reference is automatically set to nil if the object it points to is destroyed.

[iosengineering]

For Loops

For loops are useful for repeating similar code many times, with one or more variables changing each time:

for (int i = 0; i < 10; i++) {     NSLog(@"%@ ", i) }

We start this loop by stating int i = 0, which provides a starting value for i.

Here, i goes up by +1 each time the code in the loop runs. This is because each time the code in the for loop runs, we increment i with i++.

i < 10 signifies the condition that must be met each time for the code inside the for loop to run. Therefore the loop runs until i hits 10.

It will log the following output:

0 1 2 3 4 5 6 7 8 9

Fast Enumeration

You can iterate quickly over a collection of items using even simpler syntax:

for (CARecord *record in records) {     NSLog(@"%@", record); }