Effective Objective-C [上]

网上看到的 http://esoftmobile.com/2013/08/10/effective-objective-c/

本文是针对《Effective Objective-C》一书的代码解读，笔者并没有看过原书，只是通过阅读该书的代码，并结合相应的主题，来臆测作者可能要表达的内容并用自己的语言来描述出来。

Chapter 1: Accustoming Yourself to Objective-C

Item 1: Familiarize Yourself with Objective-C's Roots

Item 2: Minimize Importing Headers in Headers

减少头文件中引入（#import）文件的数量，大部分情况下我们应该在头文件中用@class申明要引用的类，在实现文件中涉及到对该类的操作时才#import。

Item 3: Prefer Literal Syntax over the Equivalent Methods

尽可能使用对象字面量的形式来创建或操作基础对象（NSString、NSNumber、NSArray、NSDictionary等），这种形式不仅使用方便，代码看起来也更清晰。

NSString *someString = @"Effective Objective-C";

NSNumber *someNumber = @1;
NSNumber *intNumber = @10;
NSNumber *floatNumber = @2.5f
NSNumber *doubleNumber = @3.14159;
NSNumber *boolNumber = @YES;
NSNumber *charNumber = @'a';

NSArray *array = @[ object1, object2, object3 ];
NSDictionary *dict = @{ @"name": @"TracyYih",
                        @"blog": @"http://esoftmobile.com"
                      };
                      
NSMutableArray *mutableArray = [@[ object1, object2 ] mutableCopy];
mutableArray[2] = object3;

NSMutableDictionary *mutableDict = [@{ @"name": @"TracyYih" } mutableCopy];
mutableDict[@"age"] = @25;

Item 4: Prefer Typed Constants to Preprocessor #define

尽量用类型化的常量来代替使用#define定义的常量。

//In head file.
extern const NSTimeInterval EOCAnimatedViewAnimationDuration;
extern NSString *const EOCLoginManagerDidLoginNotification;

//In implmentation file.
const NSTimeInterval EOCAnimatedViewAnimationDuration = 0.3;
NSString *const EOCLoginManagerDidLoginNotification = @"EOCLoginManagerDidLoginNotification";

Item 5: Use Enumerations for States, Options, and Status Codes

使用枚举来表示各种状态，选项。其中申明枚举类型推荐使用Apple的NS_ENUM和NS_OPTIONS。

typedef NS_ENUM(NSUInteger, EOCConnectionState) {
    EOCConnectionStateDisconnected,
    EOCConnectionStateConnecting,
    EOCConnectionStateConnected,
};

switch (_currentState) {
    case EOCConnectionStateDisconnected:
        break;
    case EOCConnectionStateConnecting:
        break;
    case EOCConnectionStateConnected:
        break;
}

使用枚举表示状态使代码的可读性大大增强，对比下面的代码你就知道了：

switch (_currentState) {
	case 0:
		break;
	case 1:
		break;
	case 2:
		break;
}

Chapter 2: Objects, Messaging, and the Runtime

Item 6: Understand Properties

《Ry’s Objective-C Tutorial》# Properties

Item 7: Access Instance Variables Primarily Directly When Accessing Them Internally

Item 8: Understand Object Equality

在自定义类中，我们可以通过实现-(BOOL)isEqual:(id)object;和-(NSUInteger)hash;两个方法来对该类的对象进行比较。

Item 9: Use the Class Cluster Pattern to Hide Implementation Detail

必要时通过工厂模式来隐藏一些实现的细节。

@implementation Employee
+ (Employee *)employeeWithType:(EmployeeType)type {
    switch (type) {
        case EmployeeTypeDeveloper:
            return [[EmployeeDeveloper alloc] init];
            break;
        case EmployeeTypeDesigner:
            return [[EmployeeDesigner alloc] init];
            break;
        case EmployeeTypeFinance:
            return [[EmployeeFinance alloc] init];
            break;
    }
}
@end

Item 10: Use Associated Objects to Attach Custom Data to Existing Classes

代码内容与该主题看不出来关联，先说主题吧，应该是必要时使用objc_setAssociatedObject， objc_getAssociatedObject两个方法将一些自定义的数据与已有的类相关联，通常在分类（Category）中添加属性时会用到，后面会涉及到。

代码部分是可以通过函数指针来延迟函数的绑定。

void printHello() {
   printf("Hello, world!\n");
}
void printGoodbye() {
    printf("Goodbye, world!\n");
} 

void doTheThing(int type) { void (*fnc)(); //here. if (type == 0) { fnc = printHello; } else { fnc = printGoodbye; }
    fnc();
    return 0;
}

Item 11: Understand the Role of objc_msgSend

该主题没有对应代码，详见 《Objective-C Runtime Programming Guide》。

Item 12: Understand Message Forwarding

必要时我们可以通过实现+ (BOOL)resolveClassMethod:(SEL)sel和+ (BOOL)resolveInstanceMethod:(SEL)sel方法来动态的为选择器（selector）提供对应的实现（implementation）。

+ (BOOL)resolveInstanceMethod:(SEL)selector {
    NSString *selectorString = NSStringFromSelector(selector);
    if (/* selector is from a @dynamic property */) {
        if ([selectorString hasPrefix:@"set"]) {
            class_addMethod(self, selector, (IMP)autoDictionarySetter, "v@:@");
        } else {
            class_addMethod(self, selector, (IMP)autoDictionaryGetter, "@@:");
        }
        return YES;
    }
    return [super resolveInstanceMethod:selector];
}

Item 13: Consider Method Swizzling to Debug Opaque Methods

我们可以通过runtime提供的method_exchangeImplementations方法来交换两个方法的实现。

// Exchanging methods
Method originalMethod = 
    class_getInstanceMethod([NSString class], 
                            @selector(lowercaseString));
Method swappedMethod = 
    class_getInstanceMethod([NSString class], 
                            @selector(uppercaseString));
method_exchangeImplementations(originalMethod, swappedMethod);

这里例子比较有意思，实现了以上代码后，你再使用-(NSString *)lowercaseString;和-(NSString *)uppercaseString;时得到的结果和你预期的相反。

Item 14: Understand What a Class Object Is

借助于Objective-C强大的runtime系统，我们可以在代码中判断一个对象是属于什么类型。
isMemberOfClass：判断一个对象是否为某类的实例。
isKindOfClass：判断一个对象是否为某类或该类的子类的实例。

// Class hierarchy checking
NSMutableDictionary *dict = [NSMutableDictionary new];
[dict isMemberOfClass:[NSDictionary class]]; ///< NO
[dict isMemberOfClass:[NSMutableDictionary class]]; ///< YES
[dict isKindOfClass:[NSDictionary class]]; ///< YES
[dict isKindOfClass:[NSArray class]]; ///< NO

Chapter 3: Interface and API Design

Item 15: Use Prefix Names to Avoid Namespace Clashes

在类名（Class）、协议名(Protocal)、分类名(Category)等加上自己的前缀避免与其他库或代码发生命名冲突。

Item 16: Have a Designated Initializer

初始化方法是一个类的入口，所以我们需要精心的设计（其实每个方法都得用心设计），我个人习惯初始化方法中一般不会超过两个参数，尽量让初始化方法更简单，同时我们也需要照顾到继承来的初始化方法：-(id)init; 和 -(id)initWithCode:。

// Designated initialiser
- (id)initWithWidth:(float)width 
          andHeight:(float)height
{
    if ((self = [super init])) {
        _width = width;
        _height = height;
    }
    return self;
}

// Super-class’s designated initialiser
- (id)init {
    return [self initWithWidth:5.0f andHeight:10.0f];
}

// Initialiser from NSCoding
- (id)initWithCoder:(NSCoder*)decoder {
    // Call through to super’s designated initialiser
    if ((self = [super init])) {
        _width = [decoder decodeFloatForKey:@"width"];
        _height = [decoder decodeFloatForKey:@"height"];
    }
    return self;
}

Item 17: Implement the description Method

我们可以在自己的类中实现description方法，返回关于该对象关键信息，这样在打印（Log）该对象时可以看到更多信息，否则默认就是该对象的类名和地址。

@implementation EOCPerson
...
// Description method for EOCPerson
- (NSString*)description {
    return [NSString stringWithFormat:@"<%@: %p, \"%@ %@\">", 
            [self class], self, _firstName, _lastName];
}
...
@end

Item 18: Prefer Immutable Objects

很多情况下我们申明一个属性只是为了让外部能够获取一些信息（get），并不需要对这些信息作修改（set），所以这种情况下最好不要让外部能够修改，我们可以在申明该属性时加上readonly。
或者我们还一可以在实现文件中申明“私有”的成员变量，并开放一个方法来获取该变量的一些信息。

Item 19: Use Clear and Consistent Naming

该部分应该讲的Objective-C编码规范，这里推荐Apple的《Coding Guidelines for Cocoa》。

Item 20: Prefix Private Method Names

该部分建议给“私有方法”(只在当前类的实现文件中使用的方法)加上前缀以便和其他方法区分开，这里建议的命名形式为：- (void)_privateMethod;，即加上下杠符_。

Item 21: Understand the Objective-C Error Model

在Objective-C中，错误处理可以有两种形式：NSException 和 NSError 。

// Throwing exception
id someResource = …;
if ( /* check for error */ ) {
    @throw [NSException exceptionWithName:@"ExceptionName"
                                   reason:@"There was an error"
                                 userInfo:nil];
}
[someResource doSomething];
[someResource release];

// Returning the error
- (BOOL)doSomethingError:(NSError**)error {
    // Do something
    
    NSError *returnError = nil;
    if (/* there was an error */) {
        if (error) {
            *error = [NSError errorWithDomain:domain
                                         code:code
                                     userInfo:userInfo];
        }
        return YES;
    } else {
        return NO;
    }
}

其实在Objective-C中后一种更常见，我们可以结合前面提到的使用枚举类表示一些错误码类型。

typedef NS_ENUM(NSUInteger, EOCError) {
    EOCErrorUnknown               = −1,
    EOCErrorInternalInconsistency = 100,
    EOCErrorGeneralFault          = 105,
    EOCErrorBadInput              = 500,
};

Item 22: Understand the NSCopying Protocol

我们知道大部分系统的类（UI & NS）可以调用-(id)copy;方法来获得该对象的一份拷贝，如果是自定义的类我们也想使用该方法，必须遵循NSCopying协议，并实现-(id)copyWithZone:(NSZone *)zone);方法。

//Support the NSCopying protocol.
@interface EOCPerson : NSObject <NSCopying>
@end

@implementation EOCPerson
// NSCopying implementation
- (id)copyWithZone:(NSZone*)zone {
    Person *copy = [[[self class] allocWithZone:zone] 
                    initWithFirstName:_firstName 
                          andLastName:_lastName];
    return copy;
}
@end

我们也可以在该方法中控制是深拷贝还是浅拷贝，区别就是是否将当前对象的所有信息（所有成员变量）赋给拷贝后的对象。

Chapter 4: Protocols and Categories

Item 23: Use Delegate and Data Source Protocols for Interobject Communication

《Ry’s Objective-C Tutorial》# Protocols

Item 24: Use Categories to Break Class Implementations into Manageable Segments

当我们要实现一个功能丰富的类时，我们可以使用分类(Category)将该类分割成相对独立一些的块，这样代码结构会比所有东西都放在一起实现要清晰的多。

//RenderObject.h
@class CXMLNode;
@interface RenderObject : NSObject
@property (nonatomic, copy, readonly) NSString *name;
@property (nonatomic, weak, readonly) CXMLNode *node;
@property (nonatomic, strong, readonly) UIView *view;
@property (nonatomic, strong, readonly) NSDictionary *style;
- (instancetype)initWithNode:(CXMLNode *)node style:(NSDictionary *)style;
//...
@end

//RenderObject+RenderTree.h
@interface RenderObject (RenderTree)
@property (nonatomic, weak, readonly) RenderObject *parent;
@property (nonatomic, weak, readonly) RenderObject *firstChild;
@property (nonatomic, weak, readonly) RenderObject *lastChild;
@property (nonatomic, weak, readonly) RenderObject *nextSibling;
@property (nonatomic, weak, readonly) RenderObject *previousSibling;
@end

//RenderObject+Layout.h
@interface RenderObject (Layout)
- (void)layout;
- (void)loadView;
- (void)paint;
//...
@end

以上代码并非该书所附代码，为笔者开发的一商业浏览器项目代码。

Item 25: Always Prefix Category Names on Third-Party Classes

这个感觉与之前（Item 15）内容相似，给自己创建的所有分类（不管是基于Cocoa类还是第三方类）加上自己的前缀。

// Namespacing the category
@interface NSString (ABC_HTTP)

// Encode a string with URL encoding
- (NSString*)abc_urlEncodedString;

// Decode a URL encoded string
- (NSString*)abc_urlDecodedString;

@end

Item 26: Avoid Properties in Categories

Objective-C分类中是不允许增加成员变量的（Instance variables may not be placed in categories），我们可以通过运行时函数objc_setAssociatedObject 和 objc_getAssociatedObject 来让分类支持保存和获取一些数据，从而支持属性。

//EOCPerson+FriendShip.h
@interface EOCPerson (FriendShip)
@property (nonatomic, strong) NSArray *friends;
@end

//EOCPerson+FriendShip.m
static const char* kFriendsPropertyKey = "kFriendsPropertyKey";
@implementation EOCPerson (Friendship)
- (NSArray*)friends {
    return objc_getAssociatedObject(self, kFriendsPropertyKey);
}

- (void)setFriends:(NSArray*)friends {
    objc_setAssociatedObject(self, kFriendsPropertyKey, friends, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}
@end

Item 27: Use the Class-Continuation Category to Hide Implementation Detail

我们可以在实现文件中利用拓展（Class Extension）将不需要外界了解的成员变量移到拓展中，也就是所有我们应该在头文件中申明为@private的成员变量都可以移到拓展中，这样能够保证头文件只出现外界关心的东西。

//EOCClass.h
@class EOCSuperSecretClass

@interface EOCClass : NSObject {
@private
    EOCSuperSecretClass *_secretInstance;
}
@end

其中_secretInstance既不会被子类继承，也不会在类外被访问，就不需要留在头文件中了。

//EOCClass.h
@interface EOCClass : NSObject
@end

// EOCClass.m
#import "EOCClass.h"
#import "EOCSuperSecretClass.h"

@interface EOCClass () {
    EOCSuperSecretClass *_secretInstance;
}
@end

@implementation EOCClass
// Methods here
@end

在新版本编译器中，实现（@implmentation）中也支持申明成员变量了，所以我们还可以这样写：

@implementation EOCClass {
    EOCSuperSecretClass *_secretInstance;
}
// Methods here
@end

Item 28: Use a Protocol to Provide Anonymous Objects

我们可以通过协议来提供匿名对象来调用一些方法或获取一些信息。

// Database connection protocol
@protocol EOCDatabaseConnection
- (void)connect;
- (void)disconnect;
- (BOOL)isConnected;
- (NSArray*)performQuery:(NSString*)query;
@end


// Database manager class
#import <Foundation/Foundation.h>

@protocol EOCDatabaseConnection;

@interface EOCDatabaseManager : NSObject
+ (id)sharedInstance;
- (id<EOCDatabaseConnection>)connectionWithIdentifier:(NSString*)identifier;
@end

这种用法在CoreData中也可以遇到：

// Fetched results controller with section info
NSFetchedResultsController *controller = /* some controller */;
NSUInteger section = /* section index to query */;

NSArray *sections = controller.sections;
id <NSFetchedResultsSectionInfo> sectionInfo = sections[section];
NSUInteger numberOfObjects = sectionInfo.numberOfObjects;

Chapter 5: Memory Management

Item 29: Understand Reference Counting

《Ry’s Objective-C Tutorial》#Memory Management

Item 30: Use ARC to Make Reference Counting Easier

《Ry’s Objective-C Tutorial》#Memory Management

Item 31: Release References and Clean Up Observation State Only in dealloc

在ARC模式下，dealloc方法中一般只应该出现两种操作：释放非Cocoa对象和移除观察者。

// Releasing CF objects and removing observer in `dealloc'
- (void)dealloc {
    CFRelease(coreFoundationObject);
    [[NSNotificationCenter defaultCenter] removeObserver:self];
}

Item 32: Beware of Memory Management with Exception-Safe Code

在MRR（Manual Retain Release）下，需要特别留意异常情况下的内存管理问题。

// @try/@catch block under manual reference counting
@try {
    EOCSomeClass *object = [[EOCSomeClass alloc] init];
    [object doSomethingThatMayThrow];
    [object release];
}
@catch (...) {
    NSLog(@"Whoops, there was an error. Oh well, it wasn’t important.");
}

// Fixing the potential leak
EOCSomeClass *object;
@try {
    object = [[EOCSomeClass alloc] init];
    [object doSomethingThatMayThrow];
}
@catch (...) {
    NSLog(@"Whoops, there was an error. Oh well, it wasn’t important.");
}
@finally {
    [object release];
}

其实同样需要注意的还有在switch-case或if-else条件下，避免return前必要的对象没释放问题。

Item 33: Use Weak References to Avoid Retain Cycles

Item 34: Use Autorelease Pool Blocks to Reduce High-Memory Waterline

// Reducing high memory waterline with appropriately places @autoreleasepool
NSArray *databaseRecords = …;
NSMutableArray *people = [NSMutableArray new];
for (NSDictionary *record in databaseRecords) {
    @autoreleasepool {
        EOCPerson *person = [[EOCPerson alloc] initWithRecord:record];
        [people addObject:person];
    }
}

Item 35: Use Zombies to Help Debug Memory-Management Problems

Item 36: Avoid Using retainCount

// Never do this
while ([object retainCount]) {
    [object release];
}