Adopting Modern Objective-C

[說明：這是Apple的文檔。 本文介紹了現代OC的幾個特性： instanceType（替代方法的返回id類型）, 屬性property,?枚舉Enumeration Macros, ARC,對象初始化宏?NS_DESIGNATED_INITIALIZER?macro]

Over the years, the Objective-C language has grown and evolved. Although the core concepts and practices remain the same, parts of the language have been through significant changes and improvements. These modernizations improve type safety, memory management, performance, and other aspects of Objective-C, making it easier for you to write correct code. It’s important to adopt these changes in your existing and future code to help it become more consistent, readable, and resilient.

Xcode provides a tool to help make some of these structural changes for you. But before you use this tool, you want to understand what changes it will offer to make to your code, and why. This document highlights some of the most significant and useful modernizations to adopt in your codebase.

instancetype

Use the instancetype keyword as the return type of methods that return an instance of the class they are called on (or a subclass of that class). These methods include alloc, init, and class factory methods.

Using instancetype instead of id in appropriate places improves type safety in your Objective-C code. For example, consider the following code:

@interface MyObject : NSObject

+ (instancetype)factoryMethodA;

+ (id)factoryMethodB;

@end

?

@implementation MyObject

+ (instancetype)factoryMethodA { return [[[self class] alloc] init]; }

+ (id)factoryMethodB { return [[[self class] alloc] init]; }

@end

?

void doSomething() {

? ? NSUInteger x, y;

?

? ? x = [[MyObject factoryMethodA] count]; // Return type of +factoryMethodA is taken to be "MyObject *"

? ? y = [[MyObject factoryMethodB] count]; // Return type of +factoryMethodB is "id"

}

Because of the instancetype return type of +factoryMethodA, the type of that message expression is MyObject *. Since MyObject doesn’t have a -count method, the compiler gives a warning about the x line:

main.m: ’MyObject’ may not respond to ‘count’

However, because of the id return type in +factoryMethodB, the compiler can give no warning about the y line. Since an object of type id can be any class, and since a method called -count exists somewhere on some class, to the compiler it’s possible that the return value of +factoryMethodB implements the method.

To make sure instancetype factory methods have the right subclassing behavior, be sure to use [self class] when allocating the class rather than referring directly to the class name. Following this convention ensures that the compiler will infer subclass types correctly. For example, consider trying to do this with a subclass of MyObject from the previous example:

@interface MyObjectSubclass : MyObject

@end

?

void doSomethingElse() {

? ? ? ? NSString *aString = [MyObjectSubclass factoryMethodA];

}

The compiler gives the following warning about this code:

main.m: Incompatible pointer types initializing ’NSString *’ with an expression of type ’MyObjectSubclass *’

In the example, the +factoryMethodA message send returns an object of type MyObjectSubclass, which is the type of the receiver. The compiler appropriately determines that the return type of +factoryMethodA should be of the subclass MyObjectSubclass, not of the superclass in which the factory method was declared.

How to Adopt

In your code, replace occurrences of id as a return value with instancetype where appropriate. This is typically the case for init methods and class factory methods. Even though the compiler automatically converts methods that begin with “alloc,” “init,” or “new” and have a return type of id to return instancetype, it doesn’t convert other methods. Objective-C convention is to write instancetype explicitly for all methods.

Note that you should replace id with instancetype for return values only, not elsewhere in your code. Unlike id, the instancetype keyword can be used only as the result type in a method declaration.

For example:

@interface MyObject

- (id)myFactoryMethod;

@end

should become:

@interface MyObject

- (instancetype)myFactoryMethod;

@end

Alternatively, you can use the modern Objective-C converter in Xcode to make this change to your code automatically. For more information, see Refactoring Your Code Using Xcode.

Properties

An Objective-C property is a public or private method declared with the @property syntax.

@property (readonly, getter=isBlue) BOOL blue;

Properties capture the state of an object. They reflect the object’s intrinsic attributes and relationships to other objects. Properties provide a safe, convenient way to interact with these attributes without having to write a set of custom accessor methods (although properties do allow custom getters and setters, if desired).

Using properties instead of instance variables in as many places as possible provides many benefits:

• Autosynthesized getters and setters. When you declare a property, by default getter and setter methods are created for you.
• Better declaration of intent of a set of methods. Because of accessor method naming conventions, it’s clear exactly what the getter and setter are doing.
• Property keywords that express additional information about behavior. Properties provide the potential for declaration of attributes like assign (vs copy), weak, atomic (vs nonatomic), and so on.

Property methods follow a simple naming convention. The getter is the name of the property (for example, date), and the setter is the name of the property with the set prefix, written in camel case (for example, setDate). The naming convention for Boolean properties is to declare them with a named getter starting with the word “is”:

@property (readonly, getter=isBlue) BOOL blue;

As a result, all of the following work:

if (color.blue) { }

if (color.isBlue) { }

if ([color isBlue]) { }

When deciding what can be a property, keep in mind that the following are not properties:

• init method
• copy method, mutableCopy method
• A class factory method
• A method that initiates an action and returns a BOOL result
• A method that explicitly changes internal state as a side effect of a getter

Additionally, consider the following set of rules when identifying potential properties in your code:

• A read/write property has two accessor methods. The setter takes one argument and returns nothing, and the getter takes no arguments and returns one value. If you convert this set of methods into a property, tag it with the readwrite keyword.
• A read-only property has a single accessor method, the getter, which takes no arguments and returns one value. If you convert this method into a property, tag it with the readonly keyword.
• The getter should be idempotent (if a getter is called twice, the second call results in the same result as the first). However, it is acceptable for a getter to compute the result each time it’s called.

How to Adopt

Identify a set of methods that qualify to be converted into a property, such as these:

- (NSColor *)backgroundColor;

- (void)setBackgroundColor:(NSColor *)color;

and declare them using the @property syntax with the appropriate keyword(s):

@property (copy) NSColor *backgroundColor;

For information about property keywords and other considerations, see Encapsulating Data in Programming with Objective-C.

Alternatively, you can use the modern Objective-C converter in Xcode to make this change to your code automatically. For more information, see Refactoring Your Code Using Xcode.

Enumeration Macros

The NS_ENUM and NS_OPTIONS macros provide a concise, simple way of defining enumerations and options in C-based languages. These macros improve code completion in Xcode and explicitly specify the type and size of your enumerations and options. Additionally, this syntax declares enums in a way that is evaluated correctly by older compilers, and by newer ones that can interpret the underlying type information.

Use the NS_ENUM macro to define enumerations, a set of values that are mutually exclusive:

typedef NS_ENUM(NSInteger, UITableViewCellStyle) {

? ? ? ? UITableViewCellStyleDefault,

? ? ? ? UITableViewCellStyleValue1,

? ? ? ? UITableViewCellStyleValue2,

? ? ? ? UITableViewCellStyleSubtitle

};

The NS_ENUM macro helps define both the name and type of the enumeration, in this case named UITableViewCellStyle of type NSInteger. The type for enumerations should be NSInteger.

Use the NS_OPTIONS macro to define options, a set of bitmasked values that may be combined together:

typedef NS_OPTIONS(NSUInteger, UIViewAutoresizing) {

? ? ? ? UIViewAutoresizingNone ? ? ? ? ? ? ? ? = 0,

? ? ? ? UIViewAutoresizingFlexibleLeftMargin ? = 1 << 0,

? ? ? ? UIViewAutoresizingFlexibleWidth? ? ? ? = 1 << 1,

? ? ? ? UIViewAutoresizingFlexibleRightMargin? = 1 << 2,

? ? ? ? UIViewAutoresizingFlexibleTopMargin? ? = 1 << 3,

? ? ? ? UIViewAutoresizingFlexibleHeight ? ? ? = 1 << 4,

? ? ? ? UIViewAutoresizingFlexibleBottomMargin = 1 << 5

};

Like enumerations, the NS_OPTIONS macro defines both a name and a type. However, the type for options should usually be NSUInteger.

How to Adopt

Replace your enum declarations, such as this one:

enum {

? ? ? ? UITableViewCellStyleDefault,

? ? ? ? UITableViewCellStyleValue1,

? ? ? ? UITableViewCellStyleValue2,

? ? ? ? UITableViewCellStyleSubtitle

};

typedef NSInteger UITableViewCellStyle;

with the NS_ENUM syntax:

typedef NS_ENUM(NSInteger, UITableViewCellStyle) {

? ? ? ? UITableViewCellStyleDefault,

? ? ? ? UITableViewCellStyleValue1,

? ? ? ? UITableViewCellStyleValue2,

? ? ? ? UITableViewCellStyleSubtitle

};

But when you use enum to define a bitmask, such as here:

enum {

? ? ? ? UIViewAutoresizingNone ? ? ? ? ? ? ? ? = 0,

? ? ? ? UIViewAutoresizingFlexibleLeftMargin ? = 1 << 0,

? ? ? ? UIViewAutoresizingFlexibleWidth? ? ? ? = 1 << 1,

? ? ? ? UIViewAutoresizingFlexibleRightMargin? = 1 << 2,

? ? ? ? UIViewAutoresizingFlexibleTopMargin? ? = 1 << 3,

? ? ? ? UIViewAutoresizingFlexibleHeight ? ? ? = 1 << 4,

? ? ? ? UIViewAutoresizingFlexibleBottomMargin = 1 << 5

};

typedef NSUInteger UIViewAutoresizing;

use the NS_OPTIONS macro.

typedef NS_OPTIONS(NSUInteger, UIViewAutoresizing) {

? ? ? ? UIViewAutoresizingNone ? ? ? ? ? ? ? ? = 0,

? ? ? ? UIViewAutoresizingFlexibleLeftMargin ? = 1 << 0,

? ? ? ? UIViewAutoresizingFlexibleWidth? ? ? ? = 1 << 1,

? ? ? ? UIViewAutoresizingFlexibleRightMargin? = 1 << 2,

? ? ? ? UIViewAutoresizingFlexibleTopMargin? ? = 1 << 3,

? ? ? ? UIViewAutoresizingFlexibleHeight ? ? ? = 1 << 4,

? ? ? ? UIViewAutoresizingFlexibleBottomMargin = 1 << 5

};

Alternatively, you can use the modern Objective-C converter in Xcode to make this change to your code automatically. For more information, see Refactoring Your Code Using Xcode.

Object Initialization

In Objective-C, object initialization is based on the notion of a designated initializer, an initializer method that is responsible for calling one of its superclass’s initializers and then initializing its own instance variables. Initializers that are not designated initializers are known as convenience initializers. Convenience initializers typically delegate to another initializer—eventually terminating the chain at a designated initializer—rather than performing initialization themselves.

The designated initializer pattern helps ensure that inherited initializers properly initialize all instance variables. A subclass that needs to perform nontrivial initialization should override all of its superclass’s designated initializers, but it does not need to override the convenience initializers. For more information about initializers, see Object Initialization in Concepts in Objective-C Programming.

To clarify the distinction between designated and designated initializers clear, you can add the NS_DESIGNATED_INITIALIZER macro to any method in the init family, denoting it a designated initializer. Using this macro introduces a few restrictions:

• The implementation of a designated initializer must chain to a superclass init method (with [super init...]) that is a designated initializer for the superclass.
• The implementation of a convenience initializer (an initializer not marked as a designated initializer within a class that has at least one initializer marked as a designated initializer) must delegate to another initializer (with [self init...]).
• If a class provides one or more designated initializers, it must implement all of the designated initializers of its superclass.

If any of these restrictions are violated, you receive warnings from the compiler.

If you use the NS_DESIGNATED_INITIALIZER macro in your class, you need to mark all of your designated initializers with this macro. All other initializers will be considered to be convenience initializers.

How to Adopt

Identify designated initializers in your classes, and tag them with the NS_DESIGNATED_INITIALIZER macro. For example:

- (instancetype)init NS_DESIGNATED_INITIALIZER;

Automatic Reference Counting (ARC)

Automatic Reference Counting (ARC) is a compiler feature that provides automatic memory management of Objective-C objects. Instead of your having to remember when to use retain, release, and autorelease, ARC evaluates the lifetime requirements of your objects and automatically inserts appropriate memory management calls for you at compile time. The compiler also generates appropriate dealloc methods for you.

How to Adopt

Xcode provides a tool that automates the mechanical parts of the ARC conversion (such as removing retain and release calls) and helps you to fix issues that the migrator can’t handle automatically. To use the ARC migration tool, choose Edit > Refactor > Convert to Objective-C ARC. The migration tool converts all files in a project to use ARC.

For more information, see Transitioning to ARC Release Notes.

Refactoring Your Code Using Xcode

Xcode provides a modern Objective-C converter that can assist you during the modernization process. Although the converter helps with the mechanics of identifying and applying potential modernizations, it doesn’t interpret the semantics of your code. For example, it won’t detect that your -toggle method is an action that affects your object’s state, and it will erroneously offer to modernize this action to be a property. Make sure to manually review and confirm any changes the converter offers to make to your code.

Of the previously described modernizations, the converter offers to:

• Change id to instancetype in appropriate places
• Change enum to NS_ENUM or NS_OPTIONS
• Update to the @property syntax

Besides these modernizations, this converter recommends additional changes to your code, including:

• Converting to literals, so a statement like [NSNumber numberWithInt:3] becomes @3.
• Using subscripting, so a statement like [dictionary setObject:@3 forKey:key] becomes dictionary[key] = @3.

To use the modern Objective-C converter, choose Edit > Refactor > Convert to Modern Objective-C Syntax.

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