Objective-C-代码块Block回顾

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原作者: [db:作者] 来自: [db:来源] 收藏邀请

OC中的代码块是iOS4.0+ 和Mac OS X 10.6+ 引进的对C语言的扩展，用来实现匿名函数的特性。类似于其他语言脚本语言或者编程语言中的闭包或者是Lambda表达式，可能第一眼看上去很怪异，不过开发的过程中会越来越多的用到Block，还是了解多一点比较好。Block方面的内容也有很多，本文从实际使用的角度大概讲一下Block的基本概念和实践。

首先看一个苹果官网的例子:

int (^Multiply)(int, int) = ^(int num1, int num2) {
    return num1 * num2;
};
int result = Multiply(7, 4); // Result is 28.

　上面的代码定义了一个Block，类似于C#中的委托，int表示返回类型，^是关键标示符，(int，int)是参数类型，不过需要注意的，正常的方式是将返回类型用()包裹，这里是将Block的名称包裹，网上有一个类似的图片，大家可以参考一下:

Block除了能够定义参数列表、返回类型外，还能够获取被定义时的词法范围内的状态（将变量作为参数传递之后，可以修改之后返回）.Block都是一些简短代码片段的封装，适用作工作单元，通常用来做并发任务、遍历、以及回调。下面看一个通知的例子:

- (void)viewDidLoad {
   [super viewDidLoad];
    [[NSNotificationCenter defaultCenter] addObserver:self
        selector:@selector(keyboardWillShow:)
        name:UIKeyboardWillShowNotification object:nil];
}
 
- (void)keyboardWillShow:(NSNotification *)notification {
    // Notification-handling code goes here.
}

　　以上是调用addServer方法的例子，不过也可以这么写:

- (void)viewDidLoad {
    [super viewDidLoad];
    [[NSNotificationCenter defaultCenter] addObserverForName:UIKeyboardWillShowNotification
         object:nil queue:[NSOperationQueue mainQueue] usingBlock:^(NSNotification *notif) {
             // Notification-handling code goes here. 
    }];
}

　苹果官网这个使用Block的例子如果第一次看可能不是那么好理解，如果仔细的看addServerName的定义发现:

- (id <NSObject>)addObserverForName:(NSString *)name object:(id)obj queue:(NSOperationQueue *)queue usingBlock:(void (^)(NSNotification *note))block NS_AVAILABLE(10_6, 4_0);

其中Block参数定义的时候是这样的:

(void (^)(NSNotification *note))block NS_AVAILABLE(10_6, 4_0)

　括号包裹起来的内容，第一个是返回值类型，之后的用用括号包裹^，之后的话才是传递类型，基于以上的理解，可以这么写:

   void(^MyBlock)(NSNotification *)=^(NSNotification *note){
        note=nil;
    };
    [[NSNotificationCenter defaultCenter] addObserverForName:UIKeyboardWillShowNotification
                                                      object:nil queue:[NSOperationQueue mainQueue] usingBlock:MyBlock];

　Block在苹果的API文档中随处看见其身影,常见的情况任务完成时回调处理，消息监听回调处理，错误回调处理，枚举回调，视图动画、变换，排序，比如说在NSDictionary中的方法中:

- (void)enumerateKeysAndObjectsWithOptions:(NSEnumerationOptions)opts usingBlock:(void (^)(id key, id obj, BOOL *stop))block NS_AVAILABLE(10_6, 4_0);

任务完成时候的调用:

- (IBAction)animateView:(id)sender {
    CGRect cacheFrame = self.imageView.frame;
    [UIView animateWithDuration:1.5 animations:^{
        CGRect newFrame = self.imageView.frame;
        newFrame.origin.y = newFrame.origin.y + 150.0;
        self.imageView.frame = newFrame;
        self.imageView.alpha = 0.2;
    }
                     completion:^ (BOOL finished) {
                         if (finished) {
                             // Revert image view to original.
                             self.imageView.frame = cacheFrame;
                             self.imageView.alpha = 1.0;
                         }
    }];
}

　通知机制中的处理:

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification {
    opQ = [[NSOperationQueue alloc] init];
    [[NSNotificationCenter defaultCenter] addObserverForName:@"CustomOperationCompleted"
             object:nil queue:opQ
        usingBlock:^(NSNotification *notif) {
        NSNumber *theNum = [notif.userInfo objectForKey:@"NumberOfItemsProcessed"];
        NSLog(@"Number of items processed: %i", [theNum intValue]);
    }];
}

　枚举数组的时候的调用:

NSString *area = @"Europe";
NSArray *timeZoneNames = [NSTimeZone knownTimeZoneNames];
NSMutableArray *areaArray = [NSMutableArray arrayWithCapacity:1];
NSIndexSet *areaIndexes = [timeZoneNames indexesOfObjectsWithOptions:NSEnumerationConcurrent
                                passingTest:^(id obj, NSUInteger idx, BOOL *stop) {
    NSString  *tmpStr = (NSString *)obj;
    return [tmpStr hasPrefix:area];
}];
 
NSArray *tmpArray = [timeZoneNames objectsAtIndexes:areaIndexes];
[tmpArray enumerateObjectsWithOptions:NSEnumerationConcurrent|NSEnumerationReverse
                           usingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
                               [areaArray addObject:[obj substringFromIndex:[area length]+1]];
}];
NSLog(@"Cities in %@ time zone:%@", area, areaArray);

　也可以写一个Block在截取字符串，做一些自己需要的事情:

NSString *musician = @"Beatles";
NSString *musicDates = [NSString stringWithContentsOfFile:
    @"/usr/share/calendar/calendar.music"
    encoding:NSASCIIStringEncoding error:NULL];
[musicDates enumerateSubstringsInRange:NSMakeRange(0, [musicDates length]-1)
    options:NSStringEnumerationByLines
    usingBlock:^(NSString *substring, NSRange substringRange, NSRange enclosingRange, BOOL *stop) {
           NSRange found = [substring rangeOfString:musician];
           if (found.location != NSNotFound) {
                NSLog(@"%@", substring);
           }
      }];

　　视图动画和转换中的实践:

[UIView animateWithDuration:0.2 animations:^{
        view.alpha = 0.0;
    } completion:^(BOOL finished){
        [view removeFromSuperview];
    }];

　　两个视图之间的动画:

[UIView transitionWithView:containerView duration:0.2
                   options:UIViewAnimationOptionTransitionFlipFromLeft
                animations:^{
                    [fromView removeFromSuperview];
                    [containerView addSubview:toView]
                }
                completion:NULL];

　数组排序的实践:

NSArray *stringsArray = [NSArray arrayWithObjects:
                                 @"string 1",
                                 @"String 21",
                                 @"string 12",
                                 @"String 11",
                                 @"String 02", nil];
static NSStringCompareOptions comparisonOptions = NSCaseInsensitiveSearch | NSNumericSearch |
        NSWidthInsensitiveSearch | NSForcedOrderingSearch;
NSLocale *currentLocale = [NSLocale currentLocale];
NSComparator finderSort = ^(id string1, id string2) {
    NSRange string1Range = NSMakeRange(0, [string1 length]);
    return [string1 compare:string2 options:comparisonOptions range:string1Range locale:currentLocale];
};
NSLog(@"finderSort: %@", [stringsArray sortedArrayUsingComparator:finderSort]);

当然关于Block究竟是如何实现的，内存中是如何定义，如何保存的可以参考:http://blog.csdn.net/jasonblog/article/details/7756763

参考资料:https://developer.apple.com/library/ios/featuredarticles/Short_Practical_Guide_Blocks/