英文原文  http://weblog.bignerdranch.com/?p=215

当你做一个软件的动画时，你会花费大量时间在控制动画的代码上。通常，视图控制器的代码会花费大量时间在检测动画各图层的完成情况上。动画多了以后，用这种方法就非常麻烦。你必须为每个动画里的元素设置代号，并防止他们播放完了就消失。为此你不得不写一大堆重复的代码。

要是像控制UI那样控制动画该多简单，每个图层的动画完成时，都会触发此前预设的行为。这样，你就控制一段动画完成以后继续播放动画或执行代码。
怎么实现呢？看看“CALayer”子类，图层和动画间有很好的对应关系。下面是这个子类的代码：

BNRActionLayer.h:

#import <Foundation/Foundation.h>
#import <QuartzCore/QuartzCore.h>

@interface BNRActionLayer : CALayer {
    NSMutableDictionary *targetActionPairs;
}
- (void)addTarget:(id)t action:(SEL)a forKey:(NSString *)k;
- (void)removeTarget:(id)t action:(SEL)a forKey:(NSString *)k;
- (NSArray *)actionsForTarget:(id)t forKey:(NSString *)k;
@end

BNRActionLayer.m:

#import "BNRActionLayer.h"

// Declare a private class to keep track of target-action pairs
@interface BNRActionLayerTargetActionPair : NSObject
{
    id target;
    SEL action;
}
@property (nonatomic, assign) id target;
@property (nonatomic, assign) SEL action;
@end

@implementation BNRActionLayerTargetActionPair
@synthesize target, action;
@end

@interface BNRActionLayer (Private)
- (NSMutableArray *)pairsForKey:(NSString *)k;
@end

@implementation BNRActionLayer
- (void)animationDidStop:(CAAnimation *)theAnimation finished:(BOOL)flag
{
    for(NSString *observedKey in targetActionPairs) {
        if([self animationForKey:observedKey] == theAnimation) {
            NSArray *pairs = [self pairsForKey:observedKey];
            [self removeAnimationForKey:observedKey];
            for(BNRActionLayerTargetActionPair *pair in pairs) {
                if(flag)
                    [[pair target] performSelector:[pair action]
                                        withObject:self];
            }
        }
    }
}
- (void)addAnimation:(CAAnimation *)theAnimation forKey:(NSString *)key
{
    NSArray *targetActionsForThisKey = [targetActionPairs objectForKey:key];
    if([targetActionsForThisKey count] > 0) {
        [theAnimation setRemovedOnCompletion:NO];
        [theAnimation setDelegate:self];
    }
    [super addAnimation:theAnimation forKey:key];
}
- (void)addTarget:(id)t action:(SEL)a forKey:(NSString *)k
{
    if(!targetActionPairs)
        targetActionPairs = [[NSMutableDictionary alloc] init];

    NSMutableArray *pairsForKey = [self pairsForKey:k];
    if(!pairsForKey) {
        pairsForKey = [NSMutableArray array];
        [targetActionPairs setObject:pairsForKey forKey:k];
    }

    for(BNRActionLayerTargetActionPair *pair in pairsForKey) {
        if([pair target] == t && [pair action] == a)
        return;
    }

    BNRActionLayerTargetActionPair *newPair =
        [[BNRActionLayerTargetActionPair alloc] init];
    [newPair setTarget:t];
    [newPair setAction:a];
    [pairsForKey addObject:newPair];
    [newPair release];
}
- (void)removeTarget:(id)t action:(SEL)a forKey:(NSString *)k
{
    NSMutableArray *pairsForKey = [self pairsForKey:k];
    if(!pairsForKey)
		return;

    BNRActionLayerTargetActionPair *removablePair = nil;
    for(BNRActionLayerTargetActionPair *pair in pairsForKey) {
        if([pair target] == t && [pair action] == a) {
            removablePair = pair;
            break;
        }
    }
    [pairsForKey removeObject:removablePair];
}
- (NSMutableArray *)pairsForKey:(NSString *)k
{
    return [targetActionPairs objectForKey:k];
}
- (NSArray *)actionsForTarget:(id)t forKey:(NSString *)k
{
    NSMutableArray *list = [NSMutableArray array];
    NSMutableArray *pairsForKey = [self pairsForKey:k];
    for(BNRActionLayerTargetActionPair *pair in pairsForKey) {
        if([pair target] == t)
            [list addObject:NSStringFromSelector([pair action])];
    }
    return [NSArray arrayWithArray:list];
}
- (void)dealloc
{
    [targetActionPairs release];
    [super dealloc];
}
@end

怎么使用呢？确保图层使用的类型为BNRActionLayer

@implementation MyView
+ (Class)layerClass
{
    return [BNRActionLayer class];
}
@end

当你创建 MyView 实例时, 可以为他添加目标行为.

- (void)applicationDidFinishLaunching:(UIApplication *)app
{
    MyView *v = [[[MyView alloc] initWithFrame:someRect] autorelease];
    [window addSubview:v];
    [(BNRActionLayer *)[v layer] addTarget:self
                                    action:@selector(viewDidFadeIn:)
                                    forKey:@"Fade In"];

    [window makeKeyAndVisible];

}

然后需要实现viewDidFadeIn，假设一种视角，使得这一切是可触的

- (void)viewDidFadeIn:(BNRActionLayer *)layer
{
    // We can operate on the layer here, do some controllery stuff, and we
    // can also get the this layers owning view. An implicit layer's delegate
    // is always its view (on the iPhone).
    MyView *v = [layer delegate];
    [v setUserInteractionEnabled:YES];
}

那如何创建一个播放完成后能发送信息的动画呢？用同样的方法创建一个动画

- (void)activateView:(MyView *)v
{
    CABasicAnimation *a = [CABasicAnimation animationWithKeyPath:@"opacity"];
    [a setToValue:[NSNumber numberWithFloat:1]];
    [a setDuration:1];

    // The key here matches the key the target-action pair was added for
    [[v layer] addAnimation:a forKey:@"Fade In"];
}

很酷吧，声明：我用了几天没发现问题。当然，这并不意味着代码是完美的，只是给您提个醒。