ios swift willSet didSet 并获取属性中的设置方法
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swift willSet didSet and get set methods in a property
提问by just ME
What is the difference between willSet- didSet, and get- set, when working with this inside a property?
在属性中使用 this 时willSet-didSet和get-之间有什么区别set？
From my point of view, both of them can set a value for a property. When, and why, should I use willSet- didSet, and when get- set?
在我看来，他们都可以为一个属性设置一个值。什么时候，为什么，我应该使用willSet- didSet，什么时候get- set？
I know that for willSetand didSet, the structure looks like this:
我知道对于willSetand didSet，结构如下所示：
var variable1 : Int = 0 {
didSet {
println (variable1)
}
willSet(newValue) {
..
}
}
var variable2: Int {
get {
return variable2
}
set (newValue){
}
}
回答by Maxim Shoustin
When and why should I use willSet/didSet
我何时以及为什么应该使用 willSet/didSet
willSetis called just beforethe value is stored.didSetis called immediately afterthe new value is stored.
willSet在存储值之前调用。didSet在存储新值后立即调用。
Consider your example with outputs:
考虑您的输出示例：
var variable1 : Int = 0 {
didSet{
print("didSet called")
}
willSet(newValue){
print("willSet called")
}
}
print("we are going to add 3")
variable1 = 3
print("we added 3")
Output:
输出：
we are going to add 3
willSet called
didSet called
we added 3
it works like pre/post -condition
它像前置/后置条件一样工作
On the other hand, you can use getif you want to add, for example, a read-only property:
另一方面，get如果要添加例如只读属性，则可以使用：
var value : Int {
get {
return 34
}
}
print(value)
value = 2 // error: cannot assign to a get-only property 'value'
回答by Antonio
@Maxim's answer is for the 1st part of your question.
@Maxim 的回答是针对您问题的第一部分。
As for when to use getand set: when you want a computed property. This:
至于何时使用get和set：何时需要计算属性。这个：
var x: Int
creates a stored property, which is automatically backed up by a variable (not directly accessible though). Setting a value to that property is translated in setting the value in the property, and similarly for getting.
创建一个存储属性，它由一个变量自动备份（虽然不能直接访问）。为该属性设置值被转换为在属性中设置值，并且与获取类似。
Instead:
反而：
var y = {
get { return x + 5 }
set { x = newValue - 5}
}
will create a computed property, which is not backed up by a variable - instead you have to provide the implementation of the getter and/or setter, usually reading and writing values from another property and more generally as a result of a computation (hence the computed propertyname)
将创建一个不由变量支持的计算属性 - 相反，您必须提供 getter 和/或 setter 的实现，通常从另一个属性读取和写入值，更一般地作为计算的结果（因此计算属性名称）
Suggested reading: Properties
推荐阅读：属性
Note: your code:
注意：您的代码：
var variable2: Int {
get{
return variable2
}
set (newValue){
}
}
is wrongbecause in the getyou are trying to return itself, which means calling getrecursively. And in fact the compiler will warn you with a message like Attempting to access 'variable2' within its own getter.
是错误的，因为在get您试图返回自身时，这意味着get递归调用。事实上，编译器会用类似Attempting to access 'variable2' within its own getter.
回答by NiravS
var variable1 : Int = 0 { //It's a store property
didSet {
print (variable1)
}
willSet(newValue) {
..
}
}
var variable2: Int { //It's a Computed Proprties
get {
return variable2
}
set (newValue){
}
}
For detail info of Store property and computed property
So when you are trying to assign the value into variable at that assigning time come the concept of 'didSet' & 'willSet'. As @Maxim says
有关Store 属性和计算属性的详细信息
因此，当您在分配时尝试将值分配给变量时，会出现“didSet”和“willSet”的概念。正如@Maxim 所说
- willSetis called just beforethe value is stored.
- didSetis called immediately afterthe new value is stored.
- willSet在值被存储之前被调用。
- didSet在存储新值后立即调用。
Example of 'willSet' & 'didSet' :
'willSet' 和 'didSet' 示例：
class Number {
var variable1 : Int = 0 {
didSet{
print("didSet called")
}
willSet(newValue){
print("willSet called")
}
}
}
print("we are going to add 3")
Number().variable1 = 3
print("we added 3")
//o/p:
we are going to add 3
willSet called
didSet called
we added 3
//o/p:
我们要添加 3 个
willSet 调用
didSet 调用
我们添加了 3
And generally when two property are dependent at that time 'get' & 'set' used. (It's also used in protocolthis is different concept.)
通常当两个属性依赖时，会使用 'get' 和 'set'。（它也用于协议中，这是不同的概念。）
Example of 'get' & 'set':
“获取”和“设置”的示例：
class EquilateralTriangle{
var sideLength: Double = 0.0
init(sideLength: Double){
self.sideLength = sideLength
}
var perimeter: Double {
get {
return 3.0 * sideLength
}
set {
sideLength = newValue / 3.0
}
}
}
var triangle = EquilateralTriangle(sideLength: 3.0)
print(triangle.perimeter) //o/p: 9.0
triangle.perimeter = 12.0
print(triangle.sideLength) //o/p: 4.0
回答by Hyman
get set:
设置：
getsetare Computed PropertiesWhich do not actually store a value. Instead, they provide a getter and an optional setter to retrieve and set other properties and values indirectly
getset是计算属性这实际上并不存储值。相反，它们提供了一个 getter 和一个可选的 setter 来间接检索和设置其他属性和值
Additionally you can define Read-Only Computed Properties. A read-only computed property always returns a value, and can be accessed through dot syntax, but cannot be set to a different value
此外，您可以定义只读计算属性。只读计算属性总是返回一个值，可以通过点语法访问，但不能设置为不同的值
Example get only property-
示例仅获取属性-
var number: Double {
return .pi*2
}
willSet didSet:
willSet didSet：
willSetdidSetare Property ObserversProperty observers observe and respond to changes in a property's value. Property observers are called every time a property's value is set, even if the new value is the same as the property's current value.
willSetdidSet是物业观察员属性观察者观察并响应属性值的变化。每次设置属性值时都会调用属性观察器，即使新值与属性的当前值相同。
- willSet is called just before the value is stored.
- didSet is called immediately after the new value is stored.
- willSet 在值被存储之前被调用。
- didSet 在存储新值后立即调用。
Example -
例子 -
var score: Int = 0 {
willSet(newScore) {
print("willSet score to \(newScore)")
}
didSet {
print("didSet score to \(oldValue) new score is: \(score)")
}
}
score = 10
//Output
//willSet score to 10
//didSet score to 0 new score is: 10
