Effective Objective-C [下]

Chapter 6: Blocks and Grand Central Dispatch

Item 37: Understand Blocks

《Ry’s Objective-C Tutorial》# Blocks

Item 38: Create typedefs for Common Block Types

当我们程序中要使用一些具有共性的Block时（返回值类型、参数个数和类型相同），我们可以给这种Block定义一个类型：

typedef NSComparisonResult (^NSComparator)(id obj1, id obj2);
//...
- (NSArray *)sortedArrayUsingComparator:(NSComparator)cmptr;
- (void)sortUsingComparator:(NSComparator)cmptr;
//...

// Simplified with typedef
typedef void(^EOCCompletionHandler)(NSData *data, NSError *error);
- (void)startWithCompletionHandler:(EOCCompletionHandler)completion;

国内比较有名的开源框架BeeFramework中就大量应用到Block，并通过类型定义的Block作为属性，实现类似于很多脚本语言方法调用：self.HTTP_GET(URL).PARAM(postDict);， 笔者之前在TouchXML基础上封装了一层W3C标准DOM API时也尝试过这种实现，最后在Objective-C中可以直接这样调用：document.getElementById(@"xxx").setAttribute(@"class", @"xxx"); 是不是有点写JS的赶脚。

Item 39: Use Handler Blocks to Reduce Code Separation

当我们要执行一个异步操作，比如异步请求时，通常需要在操作（或请求）完成后将结果返回，在Objective-C中一般有两种实现方式：代理和Block回调。

代理使用起来比较麻烦，有定义协议，申明代理方法，代理回调、设置代理、实现代理方法等一些列流程，而使用Block回调要简洁得多，我们通常可以申明一个Block类型的属性，在异步操作执行完后调用一下该Block。

//CXMLHttpRequest.h
typedef void (^CXMLHttpRequestCallbackBlock) (CXMLHttpRequest *request);
@interface CXMLHttpRequest : NSObject
//...
@property (nonatomic, copy) CXMLHttpRequestCallbackBlock        onreadystatechange;
//...
@end

//CXMLHttpRequest.m
//call when request state changed.
_onreadystatechange(self);

//User CXMLHttpRequest
CXMLHttpRequest *request = [CXMLHttpRequest new];
request.onreadystatechange = ^(CXMLHttpRequest *req) {
    if (req.state == 4 && req.statusCode == 200) {
        //get req.responseText.
    }
};
//...

推荐项目：BlocksKit。

Item 40: Avoid Retain Cycles Introduced by Blocks Referencing the Object Owning Them

由于Block会强引用里面出现的对象，如果Block中使用成员变量，则self本身会被Block强引用，所以稍不注意就会出现Retain Cycle。所以通常避免的方法是在Block中引用对象的值而非对象本身，在非ARC下，可以使用__block关键字来申明需要在Block中引用的对象，这样该对象就不会被Block retain，然后在Block结束时将引用对象设为nil：

MyViewController * __block myController = [[MyViewController alloc] init…];
// ...
myController.completionHandler =  ^(NSInteger result) {
    [myController dismissViewControllerAnimated:YES completion:nil];
    myController = nil;
};

在ARC模式下，则也可以用__weak（iOS5.0一下版本用__unsafe_unretained）关键字申明一个弱引用对象：

MyViewController *__weak weakSelf = self;
self.completionHandler = ^(NSData *data) {
    //...
    [weakSelf clearUp];
};

Item 41: Prefer Dispatch Queues to Locks for Synchronization

在多线程环境下，为了保证某些资源操作的可控性，需要给一些方法加锁，保证同时只响应一个对象的调用，通常可以用@synchronized()和NSLock：

// @synchronized block
- (void)synchronisedMethod {
    @synchronized(self) {
        // Safe
    }
}

// NSLock
_lock = [[NSLock alloc] init];

- (void)synchronisedMethod {
    [_lock lock];
    // Safe
    [_lock unlock];
}

我们还可以使用dispatch queue来保证同步操作，首先创建一个dispatch queue，然后将同步操作在该queue中执行：

// Using GCD queue for synchronisation
_syncQueue = dispatch_queue_create("com.effectiveobjectivec.syncQueue", NULL);

// …

- (NSString*)someString {
    __block NSString *localSomeString;
    dispatch_sync(_syncQueue, ^{
        localSomeString = _someString;
    });
    return localSomeString;
}

- (void)setSomeString:(NSString*)someString {
    dispatch_sync(_syncQueue, ^{
        _someString = someString;
    });
}

Item 42: Prefer GCD to performSelector and Friends

不在使用GCD时，如果一项任务需要分别在主线程和非主线程中执行，我们需要通过performSelector方法来改变执行的线程，我们还不得不把任务分解成不同的方法，某些方法内的代码在主线程执行，某些在非主线执行：

- (void)pulldown {
    _indicator.hidden = NO;
    [_indicator startAnimating];
    [self performSelectorInBackground:@selector(download) withObject:nil];
}

- (void)download {
    NSURL *URL = [NSURL URLWithString:@"http://xxx."];
    NSString *data = [NSString stringWithContentsOfURL:URL encoding:NSUTF8StringEncoding error:nil];
    if (data) {
        [self performSelectorOnMainThread:@selector(reloadData:) withObject:data waitUntilDone:NO];
    }
}

- (void)reloadData {
    [_indicator stopAnimating];
    _indicator.hidden = YES;
    //refresh view with data.
}

而如果使用GCD，所有的操作就要简洁很多：

- (void)pulldown {
    _indicator.hidden = NO;
    [_indicator startAnimating];
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSURL *URL = [NSURL URLWithString:@"http://xxx"];
        NSString *data = [NSString stringWithContentsOfURL:URL encoding:NSUTF8StringEncoding error:nil];
        if (data) {
            dispatch_async(dispatch_get_main_queue(), ^{
                [_indicator stopAnimating];
                _indicator.hidden = YES;
                //refresh view with data.
            });
        }
    };
}

Item 43: Know When to Use GCD and When to Use Operation Queues

Item 44: Use Dispatch Groups to Take Advantage of Platform Scaling

很多情况下我们使用GCD来执行一些异步操作，但是异步操作就存在一个返回顺序问题，如我们需要异步下载3个数据，只有当3个数据都下载完成后才刷新视图，而3个异步下载返回顺序是未知的，这是我们可以使用dispatch group来管理这三个任务：

dispatch_group_t group = dispatch_group_create();
dispatch_group_async(group, dispatch_get_global_queue(0,0), ^{
    //下载数据1
});
dispatch_group_async(group, dispatch_get_global_queue(0,0), ^{
    //下载数据2
});
dispatch_group_async(group, dispatch_get_global_queue(0,0), ^{
    //下载数据3
});

dispatch_group_notify(group, dispatch_get_main_queue(), ^{
    //刷新视图
 });

其实熟悉JS或者说熟悉Node.js的人都了解，异步编程下的协同问题一直是比较受关注的话题，其中 Node大牛 @朴灵的EventProxy，个人感觉和dispatch group有异曲同工之妙：

var ep = EventProxy.create("template", "data", "l10n", function (template, data, l10n) {
  _.template(template, data, l10n);
});

$.get("template", function (template) {
  // something
  ep.emit("template", template);
});
$.get("data", function (data) {
  // something
  ep.emit("data", data);
});
$.get("l10n", function (l10n) {
  // something
  ep.emit("l10n", l10n);
});

Item 45: Use dispatch_once for Thread-Safe Single-Time Code Execution

// `dispatch_once' singleton initialisation
+ (id)sharedInstance {
    static EOCClass *sharedInstance = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedInstance = [[self alloc] init];
    });
    return sharedInstance;
}

Item 46: Avoid dispatch_get_current_queue

Chapter 7: The System Frameworks

Item 47: Familiarize Yourself with the System Frameworks

《iOS Technology Overview》# Cocoa Touch Frameworks

Item 48: Prefer Block Enumeration to for Loops

// Block enumeration
NSArray *anArray = /* … */;
[anArray enumerateObjectsUsingBlock:^(id object, NSUInteger idx, BOOL *stop){
    // Do something with `object’
    if (shouldStop) {
        *stop = YES;
    }
}];

NSDictionary *aDictionary = /* … */;
[aDictionary enumerateKeysAndObjectsUsingBlock:^(id key, id object, NSUInteger idx, BOOL *stop){
    // Do something with `key’ and `object’
    if (shouldStop) {
        *stop = YES;
    }
}];

NSSet *aSet = /* … */;
[aSet enumerateObjectsUsingBlock:^(id object, NSUInteger idx, BOOL *stop){
    // Do something with `object’
    if (shouldStop) {
        *stop = YES;
    }
}];

Item 49: Use Toll-Free Bridging for Collections with Custom Memory-Management Semantics

// No-ops for non-retaining objects.
static const void* EOCRetainNoOp(CFAllocatorRef allocator, const void *value) { return value; }
static void EOCReleaseNoOp(CFAllocatorRef allocator, const void *value) { }


NSMutableArray* EOCNonRetainArray(){
    CFArrayCallBacks callbacks = kCFTypeArrayCallBacks;
    callbacks.retain = EOCRetainNoOp;
    callbacks.release = EOCReleaseNoOp;
    return (NSMutableArray *)CFArrayCreateMutable(nil, 0, &callbacks);
}


NSMutableDictionary* EOCNonRetainDictionary(){
    CFDictionaryKeyCallBacks keyCallbacks = kCFTypeDictionaryKeyCallBacks;
    CFDictionaryValueCallBacks callbacks = kCFTypeDictionaryValueCallBacks;
    callbacks.retain = EOCRetainNoOp;
    callbacks.release = EOCReleaseNoOp;
    return (NSMutableDictionary *)CFDictionaryCreateMutable(nil, 0, &keyCallbacks, &callbacks);
}

Item 50: Use NSCache Instead of NSDictionary for Caches

Item 51: Keep initialize and load Implementations Lean

+ (void)load;

Invoked whenever a class or category is added to the Objective-C runtime; implement this method to perform class-specific behavior upon loading.

+ (void)initialize;

Initializes the receiver before it’s used (before it receives its first message).

Item 52: Remember that NSTimer Retains Its Target

NSTimer会对retain它的Target，所以不要在Target的dealloc中销毁（invalidate）NSTimer对象，因为Timer和Target之间已经形成了Retain cycle，需要在dealloc前就破坏这个Retain cycle。

我们可以对NSTimer拓展，让它支持调用Block方法：

// Block support for NSTimer
#import <Foundation/Foundation.h>

@interface NSTimer (EOCBlocksSupport)

+ (void)eoc_scheduledTimerWithTimeInterval:(NSTimeInterval)interval
                                 block:(void(^)())block
                               repeats:(BOOL)repeats;

@end

@implementation NSTimer (EOCBlocksSupport)

+ (void)eoc_scheduledTimerWithTimeInterval:(NSTimeInterval)interval
                                 block:(void(^)())block
                               repeats:(BOOL)repeats
{
    return [self scheduledTimerWithTimeInterval:interval
                                         target:self
                           selector:@selector(eoc_blockInvoke:)
                                       userInfo:[block copy]
                                        repeats:repeats];
}

+ (void)eoc_blockInvoke:(NSTimer*)timer {
    void (^block)() = timer.userInfo;
    if (block) {
        block();
    }
}
@end

总结

到这里，全部的代码都过了一遍了，网友@Alfred_Kwong说原书很多内容没有在代码中体现，建议还是读一读原书。其实也是，即使原书所有的内容在代码中都有体现，我也不可能两篇博文就把所有东西总结出来。我更多的是通过该书的52个主题，结合代码，自己对Objective-C内容进行一遍梳理，所以不要因为我这两篇文章来决定你该不该买本书看看，我不想做推销，更不想黑。