Item 37: Understand Blocks
《Ry’s Objective-C Tutorial》# Blocksjavascript
Item 38: Create typedefs for Common Block Types
当咱们程序中要使用一些具备共性的Block时（返回值类型、参数个数和类型相同），咱们能够给这种Block定义一个类型：html
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，并经过类型定义的Block做为属性，实现相似于不少脚本语言方法调用：self.HTTP_GET(URL).PARAM(postDict);， 笔者以前在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中能够直接这样调用：document.getElementById(@"xxx").setAttribute(@"class", @"xxx"); 是否是有点写JS的赶脚。java
Item 39: Use Handler Blocks to Reduce Code Separation
当咱们要执行一个异步操做，好比异步请求时，一般须要在操做（或请求）完成后将结果返回，在Objective-C中通常有两种实现方式：代理和Block回调。ios
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，咱们一般能够申明一个Block类型的属性，在异步操做执行完后调用一下该Block。git
//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。github
Item 40: Avoid Retain Cycles Introduced by Blocks Referencing the Object Owning Them
因为Block会强引用里面出现的对象，若是Block中使用成员变量，则self自己会被Block强引用，因此稍不注意就会出现Retain Cycle。因此一般避免的方法是在Block中引用对象的值而非对象自己，在非ARC下，可使用__block关键字来申明须要在Block中引用的对象，这样该对象就不会被Block retain，而后在Block结束时将引用对象设为nil：objective-c
MyViewController * __block myController = [[MyViewController alloc] init…];
// ...
myController.completionHandler = ^(NSInteger result) {
[myController dismissViewControllerAnimated:YES completion:nil];
myController = nil;
};
在ARC模式下，则也能够用__weak（iOS5.0一下版本用__unsafe_unretained）关键字申明一个弱引用对象：编程
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做的可控性，须要给一些方法加锁，保证同时只响应一个对象的调用，一般能够用@synchronized()和NSLock：多线程
// @synchronized block
- (void)synchronisedMethod {
@synchronized(self) {
// Safe
}
}
// NSLock
_lock = [[NSLock alloc] init];
- (void)synchronisedMethod {
[_lock lock];
// Safe
[_lock unlock];
}
咱们还可使用dispatch queue来保证同步操做，首先建立一个dispatch queue，而后将同步操做在该queue中执行：app
// 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，若是一项任务须要分别在主线程和非主线程中执行，咱们须要经过performSelector方法来改变执行的线程，咱们还不得不把任务分解成不一样的方法，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.
}
而若是使用GCD，全部的操做就要简洁不少：
- (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
不少状况下咱们使用GCD来执行一些异步操做，可是异步操做就存在一个返回顺序问题，如咱们须要异步下载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，这是咱们可使用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，我的感受和dispatch group有殊途同归之妙：
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，因此不要在Target的dealloc中销毁（invalidate）NSTimer对象，由于Timer和Target之间已经造成了Retain cycle，须要在dealloc前就破坏这个Retain cycle。
咱们能够对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，所有的代码都过了一遍了，网友@Alfred_Kwong说原书不少内容没有在代码中体现，建议仍是读一读原书。其实也是，即便原书全部的内容在代码中都有体现，我也不可能两篇博文就把全部东西总结出来。我更多的是经过该书的52个主题，结合代码，本身对Objective-C内容进行一遍梳理，因此不要由于我这两篇文章来决定你该不应买本书看看，我不想作推销，更不想黑。