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UIViewAnimationWithBlocks使用block,动画结束后不需要使用回调方法，相比UIViewAnimation 方式要简洁很多

- (void)setSelectedVeg:(id)sender

{    

    [selectedVegetableIcon setAlpha:0.0];

    [UIView animateWithDuration:0.4

                     animations: ^{

                         float angle = [self spinnerAngleForVegetable:sender];

                         [vegetableSpinner setTransform:CGAffineTransformMakeRotation(angle)];

                     } 

                     completion:^(BOOL finished) {

                         [selectedVegetableIcon setAlpha:1.0];

                     }];

}

以上代码来自WWDC2010 iPlant PlantCareViem.m

UIViewAnimation style Animation

- (void)setSelectedVeg:(id)sender

{    

    [selectedVegetableIcon setAlpha:0.0];

[UIView beginAnimations:@"setSelectedVeg" context:nil];

float angle = [self spinnerAngleForVegetable:sender];

[vegetableSpinner setTransform:CGAffineTransformMakeRotation(angle)];

[UIView setAnimationDuration:0.4];

[UIView setAnimationDelegate:self];

[UIView setAnimationDidStopSelector:@selector(done)];

[UIView commitAnimations];

}

-(void)done

{

[selectedVegetableIcon setAlpha:1.0];

}

traceOperation 方法跟踪 PageLoadOperation（NSOperation 子类）

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {    

    // Override point for customization after app launch 

for (NSString *urlString in urlArray) 

{

        NSURL *url = [NSURL URLWithString:urlString];

        PageLoadOperation *plo = [[PageLoadOperation alloc] initWithURL:url];

        [queue addOperation:plo];

[self traceOperation:plo];

        [plo release];

    }

    [window addSubview:viewController.view];

    [window makeKeyAndVisible];

return YES;

}

traceOperation 方法

KeyPath 官方文档说明 http://developer.apple.com/mac/library/documentation/cocoa/reference/NSOperation_class/Reference/Reference.html#//apple_ref/doc/c_ref/NSOperation

KVO-Compliant Properties

The NSOperation class is key-value coding (KVC) and key-value observing (KVO) compliant for several of its properties. As needed, you can observe these properties to control other parts of your application. The properties you can observe include the following:

• isCancelled - read-only property

• isConcurrent - read-only property

• isExecuting - read-only property

• isFinished - read-only property

• isReady - read-only property

• dependencies - read-only property

• queuePriority - readable and writable property

• completionBlock - readable and writable property (Mac OS X only)

-(void)traceOperation:(NSOperation*)obj

{

[obj addObserver:self

  forKeyPath:@"isExecuting"

  options:0

  context:NULL];

[obj addObserver:self

  forKeyPath:@"isFinished"

  options:0

  context:NULL];

[obj addObserver:self

  forKeyPath:@"isReady"

options:0

context:NULL];

[obj addObserver:self

  forKeyPath:@"isCancelled"

options:0

context:NULL];

}