1. Delegation
Object Messaging

2. What is Delegation?
Delegation is a design pattern in object-oriented
programming where an object, instead of
performing one of its stated tasks, delegates that
task to an associated helper object. It passes the
buck, so to speak (technically, an Inversion of
Responsibility).
The helper object is called the delegate. (source:
Wikepedia)

3. Examples: AppDelegate
• UIApplicationDelegate
- (BOOL)application:(UIApplication
*)application
didFinishLaunchingWithOptions:
(NSDictionary *)launchOptions
- (void)applicationWillTerminate:
(UIApplication *)application

4. Examples: UITableView
• UITableViewDelegate
- UITableViewCell *)tableView:
(UITableView *)tableView
cellForRowAtIndexPath:(NSIndexPath
*)indexPath
- ...tableView:didDeselectRowAtIndexPath:

5. What Problem Does
Delegation Solve?
Object A Object B

6. Write to File
Object A Object B
(Equipment) (User Interface)
somefile.txt

7. Define Pointers
Object A Object B
(Equipment) (User Interface)
ObjectB *myObject;
myObject = [self getSomeData:data];

8. App
Delegate
Define Property
Object A Object B
(Equipment) (User Interface)
AppDelegate *d = [[UIApplication sharedApplication] delegate];
d.someString = ...

9. App
Delegate
Define Delegate
Object A Object B
(Equipment) (User Interface)
• Create Delegate Object • Helper App
• Defines Protocols • Delegate of Object A
- List of methods • Implements Protocol

10. Demo
• Build a Program using Delegation