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Week 2 PresentationIntroduction to Ubiquitous Computing
COSC 4355/6355Fall 2015
Ioannis PavlidisAshik Khatri
Dinesh Majeti
Computational Physiology Lab
[email protected]@[email protected]
September 10, 2015
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Evaluation
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Evaluation
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Objectives
Learn about Categories and Protocols
Understand Delegates and Data Sources
Implement Table View.
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Overview
1 Categories
2 Protocols
3 Delegates and Data Sources
4 The Structure of an App
5 References
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Categories
Categories help you to define additional methods of an existing class -even one whose source code is unavailable to you - withoutsubclassing.
The added methods are inherited by subclasses and areindistinguishable at runtime from the original methods of the class.
Categories also help to modularize a class.
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Categories Example
Category named NSString+Utilities.h.
#import <Foundation/Foundation.h>
@interface NSString (Utilities)
- (BOOL)containsOnlyLetters;
@end
Inside NSString+Utilities.m.
#import "NSString+Utilities.h"
@implementation NSString (Utilities)
- (BOOL)containsOnlyLetters
{
// implementation code goes here
}
@end
Can be called as [myString containsOnlyLetters]
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Protocols
A protocol defines a set of behaviors that are expected of an object ina given situation.
”Make sure this object has this particular set of functionality.”
A protocol comes in the form of a programmatic interface, one thatany class may implement.
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Types of Protocols
A formal protocol declares a list of methods that client classes areexpected to implement. Formal protocols are an extension to theObjective-C language.
An informal protocol is a category on NSObject, which implicitlymakes almost all objects adopters of the protocol.
As of Objective-C 2.0, protocols can have optional methods using the@optional keyword.
Until optional protocol methods were introduced in Objective-C 2.0,informal protocols were essential to the way classes implementeddelegation.
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Delegates and Data Sources
Delegates
A delegate is an object that acts on behalf of, or in coordination with,another object when that object encounters an event in a program.
The object that implements the delegate methods has theresponsibility to handle the user interface events.
Example - MKMapViewDelegate responds to events related to map.
Data Sources
A data source is like a delegate except that, instead of beingdelegated control of the user interface, it is delegated control of data.
A data source is an outlet held by UIView objects such as table viewsthat require a source from which to populate their rows of visible data.
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The App Delegate
The UIApplicationDelegate protocol defines methods that are calledby the singleton UIApplication object in response to important eventsin the lifetime of your app.
The app delegate works alongside the app object to ensure your appinteracts properly with the system and with other apps.
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Delegation Messages
Begin with the name of the UIKit object doing the delegating.
Example delegation methods -
// NSApplication
- (BOOL)application:(NSApplication *)sender
openFile:(NSString *)filename;
// UITableView
- (void)tableView:(UITableView *)tableView
didSelectRowAtIndexPath:(NSIndexPath )indexPath
// UIApplication
- (void)applicationDidEnterBackground:
(UIApplication *)application
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The id type
The id type defines a generic object pointer. Enables polymorphism.
The compiler has no information about the actual type of id object.
id object1;
object1 = [[SavingsAccount alloc] init];
[object1 setAccount: 4543455 andBalance: 3010.10];
object1 = [[CustomerInfo alloc] init];
[object1 displayInfo];
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Applications of the id type
The id type is designed to be for any object type, but you can use thecompiler to ensure that only objects that conform to a protocol areassigned to the object.
id <NSCopying> delegate;
Remember that this is a compiler check, so it only works withstatically typed variables and objects.
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Introspection
Introspection refers to the capability of objects to divulge detailsabout themselves as objects at runtime.
Such details include an object’s place in the inheritance tree, whetherit conforms to a specific protocol, and whether it responds to acertain message.
Used judiciously, introspection makes an object-oriented programmore efficient and robust.
Two of the more powerful introspection methods of NSObject arerespondsToSelector: and conformsToProtocol:.
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The Main Function
Hands control off to the UIKit framework.
The UIApplicationMain function handles this process by creating thecore objects of your app, loading your app’s user interface from theavailable storyboard files, calling your custom code so that you have achance to do some initial setup, and putting the app’s run loop inmotion.
#import <UIKit/UIKit.h>
#import "AppDelegate.h"
int main(int argc, char * argv[])
{
@autoreleasepool {
return UIApplicationMain(argc, argv, nil,
NSStringFromClass([AppDelegate class]));
}
}
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The Structure of an App
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The Main Run Loop
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References
Categories 1
Categories 2
Protocols
Protocols vs Categories
Introspection
Working with Objects
App delegate
The App Life Cycle
MVC
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Sending Messages to nil
In Objective-C, it is valid to send a message to nil - it simply has noeffect at runtime.
If the method returns an object, then a message sent to nil returns 0(nil), for example:
Person *motherInLaw = [[aPerson spouse] mother];
If aPerson’s spouse is nil, then mother is sent to nil and themethod returns nil.
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NULL, nil, Nil, NSNull
NULL - (void *)0 - literal null value for C pointers
nil - (id)0 - literal null value for Objective-C objects
Nil - (Class)0 - literal null value for Objective-C classes
NSNull - [NSNull null] - singleton object used to represent null
NSNull: Something for Nothing!
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