Item 37: Understand Blocks
《Ry’s Objective-C Tutorial》# Blocksjavascript
Item 38: Create typedefs for Common Block Types
當咱們程序中要使用一些具備共性的Block時（返回值類型、參數個數和類型相同），咱們能夠給這種Block定義一個類型：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;
國內比較有名的開源框架BeeFramework中就大量應用到Block，並經過類型定義的Block做爲屬性，實現相似於不少腳本語言方法調用：self.HTTP_GET(URL).PARAM(postDict);， 筆者以前在TouchXML基礎上封裝了一層W3C標準DOM API時也嘗試過這種實現，最後在Objective-C中能夠直接這樣調用：document.getElementById(@"xxx").setAttribute(@"class", @"xxx"); 是否是有點寫JS的趕腳。java
Item 39: Use Handler Blocks to Reduce Code Separation
當咱們要執行一個異步操做，好比異步請求時，一般須要在操做（或請求）完成後將結果返回，在Objective-C中通常有兩種實現方式：代理和Block回調。ios
代理使用起來比較麻煩，有定義協議，申明代理方法，代理回調、設置代理、實現代理方法等一些列流程，而使用Block回調要簡潔得多，咱們一般能夠申明一個Block類型的屬性，在異步操做執行完後調用一下該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.
}
};
//...
推薦項目：BlocksKit。github
Item 40: Avoid Retain Cycles Introduced by Blocks Referencing the Object Owning Them
因爲Block會強引用裏面出現的對象，若是Block中使用成員變量，則self自己會被Block強引用，因此稍不注意就會出現Retain Cycle。因此一般避免的方法是在Block中引用對象的值而非對象自己，在非ARC下，可使用__block關鍵字來申明須要在Block中引用的對象，這樣該對象就不會被Block retain，而後在Block結束時將引用對象設爲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，則也能夠用__weak（iOS5.0一下版本用__unsafe_unretained）關鍵字申明一個弱引用對象：編程
MyViewController *__weak weakSelf = self;
self.completionHandler = ^(NSData *data) {
//...
[weakSelf clearUp];
};
Item 41: Prefer Dispatch Queues to Locks for Synchronization
在多線程環境下，爲了保證某些資源操做的可控性，須要給一些方法加鎖，保證同時只響應一個對象的調用，一般能夠用@synchronized()和NSLock：多線程
// @synchronized block
- (void)synchronisedMethod {
@synchronized(self) {
// Safe
}
}
// NSLock
_lock = [[NSLock alloc] init];
- (void)synchronisedMethod {
[_lock lock];
// Safe
[_lock unlock];
}
咱們還可使用dispatch queue來保證同步操做，首先建立一個dispatch queue，而後將同步操做在該queue中執行：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時，若是一項任務須要分別在主線程和非主線程中執行，咱們須要經過performSelector方法來改變執行的線程，咱們還不得不把任務分解成不一樣的方法，某些方法內的代碼在主線程執行，某些在非主線執行：
- (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，全部的操做就要簡潔不少：
- (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
不少狀況下咱們使用GCD來執行一些異步操做，可是異步操做就存在一個返回順序問題，如咱們須要異步下載3個數據，只有當3個數據都下載完成後才刷新視圖，而3個異步下載返回順序是未知的，這是咱們可使用dispatch group來管理這三個任務：
dispatch_group_t group = dispatch_group_create();
dispatch_group_async(group, dispatch_get_global_queue(0,0), ^{
//下載數據1
});
dispatch_group_async(group, dispatch_get_global_queue(0,0), ^{
//下載數據2
});
dispatch_group_async(group, dispatch_get_global_queue(0,0), ^{
//下載數據3
});
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
//刷新視圖
});
其實熟悉JS或者說熟悉Node.js的人都瞭解，異步編程下的協同問題一直是比較受關注的話題，其中 Node大牛 @樸靈的EventProxy，我的感受和dispatch group有殊途同歸之妙：
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會對retain它的Target，因此不要在Target的dealloc中銷燬（invalidate）NSTimer對象，由於Timer和Target之間已經造成了Retain cycle，須要在dealloc前就破壞這個Retain cycle。
咱們能夠對NSTimer拓展，讓它支持調用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
總結
到這裏，所有的代碼都過了一遍了，網友@Alfred_Kwong說原書不少內容沒有在代碼中體現，建議仍是讀一讀原書。其實也是，即便原書全部的內容在代碼中都有體現，我也不可能兩篇博文就把全部東西總結出來。我更多的是經過該書的52個主題，結合代碼，本身對Objective-C內容進行一遍梳理，因此不要由於我這兩篇文章來決定你該不應買本書看看，我不想作推銷，更不想黑。