Adopting Modern Objective-C
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.
多年來，Objective-C逐漸發展及演變。雖然核心概念和做法保持一致，部分的語言已經通過顯著的變化和改進。這些現代化(modernizations)改進了類型安全，記憶體管理，性能和Objective-C的其他方面，使您更輕鬆地編寫出正確的代碼。採用這些變化到你既有或未來的程式碼是非常重要的，讓它變得更加一致，可讀性，彈性。
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.
@interface MyObject
- (instancetype)myFactoryMethod;
@end
個人理解是
alloc,
init, and
class factory methods 都從回傳
id 改成回傳
instancetype
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.
為了確保instancetype工廠方法有正確的子類的行為，一定要使用[self class] when allocating the class rather than referring directly to the class name.
+ (instancetype)factoryMethodA { return [[[self class] alloc] init]; }
Properties
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 information about behavior. Properties provide the potential for declaration of attributes like assign (vs copy), weak, atomic (vs nonatomic), and so on.
使用property而不是實體變數，可以提供許多好處在許多方面
1. 自動合成getter & setter
2. 更明確的方法意圖：因為存取方法已有它的命名慣例，因此它可以很明確的說明getter & setter 在幹嘛 ＸＤ
3. property的關鍵字表達了額外的行為資訊：如copy,assign,weak等等
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;
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As a result, all of the following work:
if (color.blue) { }
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if (color.isBlue) { }
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if ([color isBlue]) { }
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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
請注意下列幾項不是property
1. init 方法
2. copy, mutableCopy 方法
3. 類別工廠方法
4. 發起行動後並回傳BOOL結果的方法
5. 明確低改變內部狀態（副作用）的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.
3. getter應idempotent(當getter被呼叫兩次，兩次的結果應該是要相同的)。然而，getter被呼叫的時候才去計算他的結果也是可以被接受的
