zxing barcode scanner javascript Objective-C for iPhone Developers: A Beginner s Guide in Objective-C

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Objective-C for iPhone Developers: A Beginner s Guide
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Listing 11-8
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Modifying the viewWillAppear method
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- (void) viewWillAppear:(BOOL)animated { [super viewWillAppear:animated]; selffb = [[[FooBar alloc] init] autorelease]; //[fb longMethod]; [fb performSelectorInBackground:@selector(longMethod) withObject:nil]; }
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14 Open FooBarm and modify longMethod so that it posts a notification named
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longMethodDone to the application s notification center (Listing 11-9)
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15 Open SelectorDelayViewControllerm and implement the viewDidLoad method so that
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it registers itself as an observer of the longMethodDone notification (Listing 11-10)
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16 Modify SelectorDelayViewController s dealloc method so that it
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removes itself as an observer from the application s notification center (Listing 11-11)
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17 Build and Run the application and the activity view is removed from the view after the
longMethod finishes
Listing 11-9 Modifying longMethod to send a notification
- (void) longMethod { int x = 0; while (x++ < 5000) { NSLog(@""); } [[NSNotificationCenter defaultCenter] postNotificationName:@"longMethodDone" object:nil]; }
Listing 11-10
Modifying viewDidLoad to register as an observer
- (void) viewDidLoad { [super viewDidLoad]; [[NSNotificationCenter defaultCenter] addObserver:selfmyActivity selector:@selector(stopAnimating) name:@"longMethodDone" object:nil]; }
11:
Selectors and Targets
Listing 11-11
Modifying dealloc to removeObserver
- (void)dealloc { [fb release]; [[NSNotificationCenter defaultCenter] removeObserver:selfmyActivity]; [myActivity release]; [super dealloc]; }
In this example SelectorDelayViewController registers itself as an observer of all longMethodDone notifications When SelectorDelayViewController s view appears, it fully loads the view because it runs the longMethod on the background thread
[fb performSelectorInBackground:@selector(longMethod) withObject:nil];
Notice that the longMethod has no memory management needs It loops through 5000 iterations, logging a period to the debugger console In the meantime, while it is processing, your application is still responsive You could have used the performSelector: withObject:afterDelay: method to achieve the same results
Delegates
Notifications are one way you might alert another object that a particularly long method has completed Another technique, arguably better, is using a delegate How it works is that you have a class adopt a protocol, where the protocol is the delegate Then in the performSelector: method, you pass the delegate as a parameter The method invoked by the selector then sends a message to the delegate when completed Recall how you declare a method that takes a protocol as a parameter
- (void) longMethod: (id<FooBarDelegate>) delegate;
You can invoke a method using the performSelector: method, then pass the delegate as the object parameter Because you know the method adopts the FooBarDelegate protocol, the invoked method can then invoke one of FooBarDelegate s methods, without worrying about the passed object not implementing it The following Try This illustrates
Objective-C for iPhone Developers: A Beginner s Guide
Try This
Using a Selector with a Delay and a Delegate
1 Copy the preceding Try This project to a new location, as you are modifying this project 2 Remove the code related to NSNotification from SelectorDelayView
Controller and FooBar The lines you should find and remove are as follows
[[NSNotificationCenter defaultCenter] postNotificationName:@"longMethodDone" object:nil]; [[NSNotificationCenter defaultCenter] removeObserver:selfmyActivity]; [[NSNotificationCenter defaultCenter] addObserver:selfmyActivity selector:@selector(stopAnimating) name:@"longMethodDone" object:nil];
3 Build and Run the application The activity indicator should never disappear 4 Create a new C header file named FooBarDelegate Make the file a protocol
declaration, as in Listing 11-12
5 Open FooBarh and modify longMethod s signature so that it takes a
FooBarDelegate as a parameter (Listings 11-13 and 11-14)
6 Open SelectorDelayViewControllerh and modify it so that it adopts the
FooBarDelegate protocol (Listing 11-15),
7 Open SelectorDelayViewControllerm and implement the longMethodDone:
in SelectorDelayViewController, as in Listing 11-16; also modify viewWillAppear so that SelectorDelayViewController passes itself as the delegate to longMethod:
8 Build and Run the application The activity indicator disappears and the dictionary
content is logged to the debugger console But there is a problem: you forgot to manage memory using a different autorelease pool for the longMethod: code Remember, it runs in a different thread In longMethod: you used the NSDictionary s convenience constructor, which uses autorelease, and so the memory leaks because there is no pool in place You should also see something similar to the warning in Listing 11-17 in the debugger console
9 To fix this memory leak, you have three main options You could use the
performSelector:withObject:afterDelay: method; create your own NSAutoReleasePool; or change the code in FooBar and SelectorDelayView Controller to explicitly allocate, initialize, and release all objects
11:
Selectors and Targets
10 But in this example, rather than fixing the code s memory handling, simply change
the performSelectorInBackground:withObject method to use performSelector:withObject:afterDelay
//[selffb performSelectorInBackground:@selector(longMethod:) withObject:self]; [selffb performSelector:@selector(longMethod:) withObject:self afterDelay:01];
11 Build and Run the application and there are no warnings Moreover, the end results are
the same visually
Listing 11-12 FooBarDelegateh
@protocol FooBarDelegate - (void) longMethodDone: (NSDictionary *) data; @end
Listing 11-13
FooBarh
#import <Foundation/Foundationh> @protocol FooBarDelegate; @interface FooBar : NSObject { } - (void) longMethod: (id<FooBarDelegate>) delegate; @end
Listing 11-14
FooBarm
#import "FooBarh" #import "FooBarDelegateh" @implementation FooBar - (void) longMethod: (id<FooBarDelegate>) delegate { int x = 0; while (x++ < 5000) { NSLog(@""); } NSDictionary * data = [NSDictionary dictionaryWithObjectsAndKeys:@"5000 loops", @"msg1", @"while loop",@"msg2",nil]; [delegate longMethodDone:data]; } @end
(continued)
Objective-C for iPhone Developers: A Beginner s Guide
Listing 11-15
SelectorDelayViewControllerh
#import <UIKit/UIKith> #import "FooBarDelegateh" @class FooBar; @interface SelectorDelayViewController : UIViewController <FooBarDelegate> { IBOutlet UIActivityIndicatorView * myActivity; FooBar * fb; } @property (nonatomic,retain) UIActivityIndicatorView * myActivity; @property (nonatomic, retain) FooBar * fb; @end
Listing 11-16
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