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Object-C-代码块Block回顾
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/
Object-C-代码块Block回顾
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