对象间的通讯

- (void)downloadObjectsAtPath:(NSString *)path
                   completion:(void(^)(NSArray *objects, NSError *error))completion;

• 通常这成功处理和失败处理会共享一些代码（比如让一个进度条或者提示消失）；
• Apple 也是这样做的，与平台一致能够带来一些潜在的好处；
• block 通常会有多行代码，如果不作为最后一个参数放在后面的话，会打破调用点；
• 使用多个 block 作为参数可能会让调用看起来显得很笨拙，并且增加了复杂性。

• 若 objects 不为 nil，则 error 必须为 nil
• 若 objects 为 nil，则 error 必须不为 nil

- (void)downloadObjectsAtPath:(NSString *)path
                   completion:(void(^)(NSArray *objects, NSError *error))completion {
    if (objects) {
        // do something with the data
    }
    else {
        // some error occurred, 'error' variable should not be nil by contract
    }
}

• block 是在栈上创建的
• block 可以复制到堆上
• Block会捕获栈上的变量(或指针)，将其复制为自己私有的const(变量)。
• (如果在Block中修改Block块外的)栈上的变量和指针，那么这些变量和指针必须用__block关键字申明(译者注：否则就会跟上面的情况一样只是捕获他们的瞬时值)。

// 下面代码是译者加的
CGFloat blockInt = 10;
void (^playblock)(void) = ^{
    NSLog(@"blockInt = %zd", blockInt);
};
blockInt++;
playblock();
...
// 结果为:blockInt = 10

嗨，这个指针或者原始的类型依赖它们在的栈。请用一个栈上的新变量来引用它。我是说，请对它进行双重解引用，不要 retain 它。 谢谢，哥们。

• 例子:

  • __weak __typeof(self) weakSelf = self;
    [self executeBlock:^(NSData *data, NSError *error) {
        [weakSelf doSomethingWithData:data];
    }];
    

• 不要这样:

  • [self executeBlock:^(NSData *data, NSError *error) {
        [self doSomethingWithData:data];
    }];
    

• 多个语句的例子:

  • __weak __typeof(self)weakSelf = self;
    [self executeBlock:^(NSData *data, NSError *error) {
        __strong __typeof(weakSelf) strongSelf = weakSelf;
        if (strongSelf) {
            [strongSelf doSomethingWithData:data];
            [strongSelf doSomethingWithData:data];
        }
    }];
    

• 不要这样:

  • __weak __typeof(self)weakSelf = self;
    [self executeBlock:^(NSData *data, NSError *error) {
        [weakSelf doSomethingWithData:data];
        [weakSelf doSomethingWithData:data];
    }];
    

__weak __typeof(self)weakSelf = self;
__strong __typeof(weakSelf)strongSelf = weakSelf;

1. 直接在 block 里面使用关键词 self
2. 在 block 外定义一个 __weak 的 引用到 self，并且在 block 里面使用这个弱引用
3. 在 block 外定义一个 __weak 的 引用到 self，并在在 block 内部通过这个弱引用定义一个 __strong 的引用。

dispatch_block_t completionBlock = ^{
    NSLog(@"%@", self);
}
MyViewController *myController = [[MyViewController alloc] init...];
[self presentViewController:myController
                   animated:YES
                 completion:completionHandler];

self.completionHandler = ^{
    NSLog(@"%@", self);
}
MyViewController *myController = [[MyViewController alloc] init...];
[self presentViewController:myController
                   animated:YES
                 completion:self.completionHandler];

Capturing 'self' strongly in this block is likely to lead to a retain cycle （在 block 里面发现了 `self` 的强引用，可能会导致循环引用）

__weak typeof(self) weakSelf = self;
self.completionHandler = ^{
    NSLog(@"%@", weakSelf);
};
MyViewController *myController = [[MyViewController alloc] init...];
[self presentViewController:myController
                   animated:YES
                 completion:self.completionHandler];

MyViewController *myController = [[MyViewController alloc] init...];
// ...
MyViewController * __weak weakMyController = myController;
myController.completionHandler = ^(NSInteger result) {
    MyViewController *strongMyController = weakMyController;
    if (strongMyController) {
        // ...
        [strongMyController dismissViewControllerAnimated:YES completion:nil];
        // ...
    }
    else {
        // Probably nothing...
    }
};

__weak typeof(self) weakSelf = self;
dispatch_block_t block = ^{
    [weakSelf doSomething]; // weakSelf != nil
    // preemption, weakSelf turned nil
    [weakSelf doSomethingElse]; // weakSelf == nil
};

__weak typeof(self) weakSelf = self;
myObj.myBlock = ^{
    __strong typeof(self) strongSelf = weakSelf;
    if (strongSelf) {
      [strongSelf doSomething]; // strongSelf != nil
      // preemption, strongSelf still not nil（抢占的时候，strongSelf 还是非 nil 的)
      [strongSelf doSomethingElse]; // strongSelf != nil
    }
    else {
        // Probably nothing...
        return;
    }
};

Dereferencing a __weak pointer is not allowed due to possible null value caused by race condition, assign it to a strong variable first. (对一个 __weak 指针的解引用不允许的，因为可能在竞态条件里面变成 null, 所以先把他定义成 strong 的属性)

__weak typeof(self) weakSelf = self;
myObj.myBlock = ^{
    id localVal = weakSelf->someIVar;
};

• 方案 1 : 只能在 block 不是作为一个 property 的时候使用，否则会导致 retain cycle。
• 方案 2 : 当 block 被声明为一个 property 的时候使用。
• 方案 3 : 和并发执行有关。当涉及异步的服务的时候，block 可以在之后被执行，并且不会发生关于 self 是否存在的问题。

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath;
- (void)tableView:(UITableView *)tableView didHighlightRowAtIndexPath:(NSIndexPath *)indexPath;

- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath;
- (BOOL)tableView:(UITableView *)tableView canPerformAction:(SEL)action forRowAtIndexPath:(NSIndexPath *)indexPath withSender:(id)sender;

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath;
- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView;

• 委托模式(delegate pattern)：事件发生的时候，委托者需要通知代理者。
• 数据源模式(datasource pattern): 委托者需要从数据源对象拉取数据。

@class ZOCSignUpViewController;
@protocol ZOCSignUpViewControllerDelegate <NSObject>
- (void)signUpViewControllerDidPressSignUpButton:(ZOCSignUpViewController *)controller;
@end
@protocol ZOCSignUpViewControllerDataSource <NSObject>
- (ZOCUserCredentials *)credentialsForSignUpViewController:(ZOCSignUpViewController *)controller;
@end
@interface ZOCSignUpViewController : UIViewController
@property (nonatomic, weak) id<ZOCSignUpViewControllerDelegate> delegate;
@property (nonatomic, weak) id<ZOCSignUpViewControllerDataSource> dataSource;
@end

- (void)calculatorDidCalculateValue:(CGFloat)value;

@protocol ZOCSignUpViewControllerDelegate <NSObject>
@required
- (void)signUpViewController:(ZOCSignUpViewController *)controller didProvideSignUpInfo:(NSDictionary *)dict;
@optional
- (void)signUpViewControllerDidPressSignUpButton:(ZOCSignUpViewController *)controller;
@end

if ([self.delegate respondsToSelector:@selector(signUpViewControllerDidPressSignUpButton:)]) {
    [self.delegate signUpViewControllerDidPressSignUpButton:self];
}

- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath {
    CGFloat retVal = 0;
    if ([super respondsToSelector:@selector(tableView:heightForRowAtIndexPath:)]) {
        retVal = [super tableView:self.tableView heightForRowAtIndexPath:indexPath];
    }
    return retVal + 10.0f;
}

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[UIViewControllerB tableView:heightForRowAtIndexPath:]: unrecognized selector sent to instance 0x8d82820'
*** 由于未捕获异常 `NSInvalidArgumentException(无效的参数异常)`导致应用终止，理由是：向实例 ox8d82820 发送了无法识别的 selector `- [UIViewControllerB tableView:heightForRowAtIndexPath:]`

- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath {
    CGFloat retVal = 0;
    if ([[UIViewControllerA class] instancesRespondToSelector:@selector(tableView:heightForRowAtIndexPath:)]) {
        retVal = [super tableView:self.tableView heightForRowAtIndexPath:indexPath];
    }
    return retVal + 10.0f;
}

@interface ZOCWeakObject : NSObject
@property (nonatomic, readonly, weak) id object; 
// 译者注：这里原文并没有很好地实践自己在本书之前章节所讨论的关于property属性修饰符的
// 人体工程学法则: 从左到右： 原子性 ===》 读写权限 (别名) ===》 内存管理权限符
+ (instancetype)weakObjectWithObject:(id)object;
- (instancetype)initWithObject:(id)object;
@end

@interface ZOCWeakObject ()
@property (nonatomic, weak) id object;
@end
@implementation ZOCWeakObject
+ (instancetype)weakObjectWithObject:(id)object {
    return [[[self class] alloc] initWithObject:object];
}
- (instancetype)initWithObject:(id)object {
    if ((self = [super init])) {
        _object = object;
    }
    return self;
}
- (BOOL)isEqual:(id)object {
    if (self == object) {
        return YES;
    }
    if (![object isKindOfClass:[object class]]) {
        return NO;
    }
    return [self isEqualToWeakObject:(ZOCWeakObject *)object];
}
- (BOOL)isEqualToWeakObject:(ZOCWeakObject *)object {
    if (!object) {
        return NO;
    }
    BOOL objectsMatch = [self.object isEqual:object.object];
    return objectsMatch;
}
- (NSUInteger)hash {
    return [self.object hash];
}
@end

@protocol ZOCServiceDelegate <NSObject>
@optional
- (void)generalService:(ZOCGeneralService *)service didRetrieveEntries:(NSArray *)entries;
@end
@interface ZOCGeneralService : NSObject
- (void)registerDelegate:(id<ZOCServiceDelegate>)delegate;
- (void)deregisterDelegate:(id<ZOCServiceDelegate>)delegate;
@end
@interface ZOCGeneralService ()
@property (nonatomic, strong) NSMutableSet *delegates;
@end

@implementation ZOCGeneralService
- (void)registerDelegate:(id<ZOCServiceDelegate>)delegate {
    if ([delegate conformsToProtocol:@protocol(ZOCServiceDelegate)]) {
        [self.delegates addObject:[[ZOCWeakObject alloc] initWithObject:delegate]];
    }
}
- (void)deregisterDelegate:(id<ZOCServiceDelegate>)delegate {
    if ([delegate conformsToProtocol:@protocol(ZOCServiceDelegate)]) {
        [self.delegates removeObject:[[ZOCWeakObject alloc] initWithObject:delegate]];
    }
}
- (void)_notifyDelegates {
    ...
    for (ZOCWeakObject *object in self.delegates) {
        if (object.object) {
            if ([object.object respondsToSelector:@selector(generalService:didRetrieveEntries:)]) {
                [object.object generalService:self didRetrieveEntries:entries];
            }
        }
    }
}
@end