objective-c Swift 只读外部，读写内部属性
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Swift readonly external, readwrite internal property
提问by Jasper Blues
In Swift, what is the conventional way to define the common pattern where a property is to be externally readonly, but modifiable internally by the class (and subclasses) that own it.
在 Swift 中，定义公共模式的常规方法是什么，其中属性在外部是只读的，但可以由拥有它的类（和子类）在内部修改。
In Objective-C, there are the following options:
在 Objective-C 中，有以下选项：
- Declare the property as readonly in the interface and use a class extension to access the property internally. This is message-based access, hence it works nicely with KVO, atomicity, etc.
- Declare the property as readonly in the interface, but access the backing ivar internally. As the default access for an ivar is protected, this works nicely in a class hierarchy, where subclasses will also be able to modify the value, but the field is otherwise readonly.
- 在接口中将该属性声明为只读，并使用类扩展在内部访问该属性。这是基于消息的访问，因此它可以很好地与 KVO、原子性等配合使用。
- 在接口中将属性声明为只读，但在内部访问支持的 ivar。由于 ivar 的默认访问权限受到保护，因此这在类层次结构中非常有效，其中子类也可以修改该值，但该字段是只读的。
In Java the convention is:
在 Java 中，约定是：
- Declare a protected field, and implement a public, read-only getter (method).
- 声明一个受保护的字段，并实现一个公共的、只读的 getter（方法）。
What is the idiom for Swift?
Swift 的成语是什么？
回答by Antonio
Given a class property, you can specify a different access level by prefixing the property declaration with the access modifier followed by getor setbetween parenthesis. For example, a class property with a public getter and a private setter will be declared as:
给定一个类属性，您可以通过在属性声明前加上访问修饰符后跟括号get或set括号之间来指定不同的访问级别。例如，具有公共 getter 和私有 setter 的类属性将声明为：
private(set) public var readonlyProperty: Int
Suggested reading: Getters and Setters
推荐阅读：Getter 和 Setter
Martin's considerations about accessibility level are still valid - i.e. there's no protectedmodifier, internalrestricts access to the module only, privateto the current file only, and publicwith no restrictions.
Martin 关于可访问性级别的考虑仍然有效 - 即没有protected修饰符，internal仅限制对模块的访问，仅对private当前文件的访问，并且public没有任何限制。
Swift 3 notes
斯威夫特 3 笔记
2 new access modifiers, fileprivateand openhave been added to the language, while privateand publichave been slightly modified:
2 个新的访问修饰符，fileprivate并open已添加到语言中，而private和public已稍作修改：
openapplies to class and class members only: it's used to allow a class to be subclassed or a member to be overridden outside of the module where they are defined.publicinstead makes the class or the member publicly accessible, but not inheritable or overridableprivatenow makes a member visible and accessible from the enclosing declaration only, whereasfileprivateto the entire file where it is contained
open仅适用于类和类成员：它用于允许类被子类化或成员在定义它们的模块之外被覆盖。public而是使类或成员可公开访问，但不可继承或可覆盖private现在使成员仅从封闭声明中可见和可访问，而fileprivate对包含它的整个文件
More details here.
更多细节在这里。
回答by Santosh
As per @Antonio, we can use a single property to access as the readOnlyproperty value publicly and readWriteprivately. Below is my illustration:
根据@Antonio，我们可以使用单个属性作为readOnly属性值公开和readWrite私有访问。下面是我的插图：
class MyClass {
private(set) public var publicReadOnly: Int = 10
//as below, we can modify the value within same class which is private access
func increment() {
publicReadOnly += 1
}
func decrement() {
publicReadOnly -= 1
}
}
let object = MyClass()
print("Initial valule: \(object.publicReadOnly)")
//For below line we get the compile error saying : "Left side of mutating operator isn't mutable: 'publicReadOnly' setter is inaccessible"
//object.publicReadOnly += 1
object.increment()
print("After increment method call: \(object.publicReadOnly)")
object.decrement()
print("After decrement method call: \(object.publicReadOnly)")
And here is the output:
这是输出：
Initial valule: 10
After increment method call: 11
After decrement method call: 10
