blocks|key|2948481|text|你应该看看Computed+Properties。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|2948482|在您的代码示例中，perimeter是一个未由类变量支持的属性。相反，它的值是使用get方法计算的，并通过set方法存储--通常称为getter和setter。|style|CODE|2948483|当您像这样使用该属性时：|2948484|var+cp+=+myClass.perimeter|code-block|syntax|javascript|2948485|您正在调用包含在get代码块中的代码，当您像这样使用它时：|2948486|myClass.perimeter+=+5.0|2948487|您正在调用set代码块中包含的代码，其中newValue将自动填充赋值操作符右侧提供的值。|2948488|如果同时指定了getter和setter，则计算属性可以是读/写的；如果仅指定了getter，则计算属性可以是只读的。|2948489|entityMap|0|LINK|mutability|MUTABLE|url|https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Properties.html#//apple_ref/doc/uid/TP40014097-CH14-XID_322^0|5|J|0|0|9|9|15|3|1H|3|0|0|0|8|3|0|0|5|3|K|8|0|0^^$0|@$1|2|3|4|5|6|7|14|8|@]|9|@$A|15|B|16|1|17]]|C|$]]|$1|D|3|E|5|6|7|18|8|@$A|19|B|1A|F|G]|$A|1B|B|1C|F|G]|$A|1D|B|1E|F|G]]|9|@]|C|$]]|$1|H|3|I|5|6|7|1F|8|@]|9|@]|C|$]]|$1|J|3|K|5|L|7|1G|8|@]|9|@]|C|$M|N]]|$1|O|3|P|5|6|7|1H|8|@$A|1I|B|1J|F|G]]|9|@]|C|$]]|$1|Q|3|R|5|L|7|1K|8|@]|9|@]|C|$M|N]]|$1|S|3|T|5|6|7|1L|8|@$A|1M|B|1N|F|G]|$A|1O|B|1P|F|G]]|9|@]|C|$]]|$1|U|3|V|5|6|7|1Q|8|@]|9|@]|C|$]]|$1|W|3|-4|5|6|7|1R|8|@]|9|@]|C|$]]]|X|$Y|$5|Z|10|11|C|$12|13]]]]

You should look at <a href="https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Properties.html#//apple_ref/doc/uid/TP40014097-CH14-XID_322" rel="nofollow noreferrer">Computed Properties</a>.
In your code sample, <code>perimeter</code> is a property not backed up by a class variable. Instead its value is computed using the <code>get</code> method and stored via the <code>set</code> method - usually referred to as getter and setter.
When you use that property like this:
<pre><code>var cp = myClass.perimeter
</code></pre>
you are invoking the code contained in the <code>get</code> code block, and when you use it like this:
<pre><code>myClass.perimeter = 5.0
</code></pre>
You are invoking the code contained in the <code>set</code> code block, where <code>newValue</code> is automatically filled with the value provided at the right of the assignment operator.
Computed properties can be read/write if both a getter and a setter are specified, or read-only if the getter only is specified.

blocks|key|154408|text|类中变量的获取和设置指的是检索(“获取”)或修改(“设置”)它们的内容。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|154409|考虑family类的变量members。当然，这个变量需要是一个整数，因为一个家庭永远不能由两个点组成。|offset|length|style|CODE|154410|因此，您可能会像这样定义members变量：|154411|class+family+{
++var+members:Int
}|code-block|syntax|javascript|154412|然而，这将使使用这个类的人有可能将家庭成员的数量设置为类似于0或1的东西。由于不存在1或0的家庭，这是相当不幸的。|154413|这就是getter和setter的用武之地。这样，您就可以自己决定如何更改变量以及它们可以接收什么值，以及决定它们返回的内容。|154414|返回到我们的家庭类，让我们确保没有人可以将members值设置为小于2的任何值：|154415|class+family+{
++var+_members:Int+=+2
++var+members:Int+{
++++get+{
++++++return+_members
++++}
++++set+(newVal)+{
++++++if+newVal+>=+2+{
++++++++_members+=+newVal
++++++}+else+{
++++++++println('error:+cannot+have+family+with+less+than+2+members')
++++++}
++++}
++}
}|154416|现在，我们可以像以前一样通过输入instanceOfFamily.members来访问members变量，并且通过setter函数，我们还可以像以前一样设置它的值，例如通过输入：instanceOfFamily.members+=+3。然而，改变的是，我们不能再将此变量设置为小于2的值。|154417|注意引入了_members变量，它是存储我们通过members设置器函数设置的值的实际变量。最初的members现在已经成为一个计算属性，这意味着它只作为一个接口来处理我们的实际变量。|154418|entityMap^0|0|2|6|C|7|0|C|7|0|0|0|0|L|7|0|0|G|O|17|7|2H|S|0|5|8|O|7|1D|7|0^^$0|@$1|2|3|4|5|6|7|12|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|13|8|@$D|14|E|15|F|G]|$D|16|E|17|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|18|8|@$D|19|E|1A|F|G]]|9|@]|A|$]]|$1|J|3|K|5|L|7|1B|8|@]|9|@]|A|$M|N]]|$1|O|3|P|5|6|7|1C|8|@]|9|@]|A|$]]|$1|Q|3|R|5|6|7|1D|8|@]|9|@]|A|$]]|$1|S|3|T|5|6|7|1E|8|@$D|1F|E|1G|F|G]]|9|@]|A|$]]|$1|U|3|V|5|L|7|1H|8|@]|9|@]|A|$M|N]]|$1|W|3|X|5|6|7|1I|8|@$D|1J|E|1K|F|G]|$D|1L|E|1M|F|G]|$D|1N|E|1O|F|G]]|9|@]|A|$]]|$1|Y|3|Z|5|6|7|1P|8|@$D|1Q|E|1R|F|G]|$D|1S|E|1T|F|G]|$D|1U|E|1V|F|G]]|9|@]|A|$]]|$1|10|3|-4|5|6|7|1W|8|@]|9|@]|A|$]]]|11|$]]

The getting and setting of variables within classes refers to either retrieving (&quot;getting&quot;) or altering (&quot;setting&quot;) their contents.
Consider a variable <code>members</code> of a class <code>family</code>. Naturally, this variable would need to be an integer, since a family can never consist of two point something people.
So you would probably go ahead by defining the <code>members</code> variable like this:
<pre><code>class family {
 var members:Int
}
</code></pre>
This, however, will give people using this class the possibility to set the number of family members to something like 0 or 1. And since there is no such thing as a family of 1 or 0, this is quite unfortunate.
This is where the getters and setters come in. This way you can decide for yourself how variables can be altered and what values they can receive, as well as deciding what content they return.
Returning to our family class, let's make sure nobody can set the <code>members</code> value to anything less than 2:
<pre><code>class family {
 var _members:Int = 2
 var members:Int {
 get {
 return _members
 }
 set (newVal) {
 if newVal &gt;= 2 {
 _members = newVal
 } else {
 println('error: cannot have family with less than 2 members')
 }
 }
 }
}
</code></pre>
Now we can access the <code>members</code> variable as before, by typing <code>instanceOfFamily.members</code>, and thanks to the setter function, we can also set it's value as before, by typing, for example: <code>instanceOfFamily.members = 3</code>. What has changed, however, is the fact that we cannot set this variable to anything smaller than 2 anymore.
Note the introduction of the <code>_members</code> variable, which is the actual variable to store the value that we set through the <code>members</code> setter function. The original <code>members</code> has now become a computed property, meaning that it only acts as an interface to deal with our actual variable.

blocks|key|2735743|text|一个简单的问题后面应该有一个简短、简单和清晰的答案。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2735744|2735745|当我们获取属性的值时，它会触发它的get{}部件。|unordered-list-item|offset|length|style|CODE|2735746|2735747|2735748|当我们为属性设置一个值时，它会触发它的set{}部件。|2735749|2735750|PS。当为属性设置值时，Swift会自动创建一个常量"newValue“=我们正在设置的值。在属性的set{}部分中可以访问常量"newValue“之后。|2735751|示例：|2735752|var+A:Int+=+0
var+B:Int+=+0

var+C:Int+{
get+{return+1}
set+{print("Recived+new+value",+newValue,+"+and+stored+into+'B'+")
+++++B+=+newValue
+++++}
}

//+When+we+are+getting+a+value+of+C+it+fires+get{}+part+of+C+property
A+=+C
A++++++++++++//+Now+A+=+1

//+When+we+are+setting+a+value+to+C+it+fires+set{}+part+of+C+property
C+=+2
B++++++++++++//+Now+B+=+2|code-block|syntax|javascript|2735753|entityMap^0|0|0|H|5|0|0|0|J|5|0|0|1E|5|0|0|0^^$0|@$1|2|3|4|5|6|7|Z|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|10|8|@]|9|@]|A|$]]|$1|C|3|D|5|E|7|11|8|@$F|12|G|13|H|I]]|9|@]|A|$]]|$1|J|3|-4|5|6|7|14|8|@]|9|@]|A|$]]|$1|K|3|-4|5|6|7|15|8|@]|9|@]|A|$]]|$1|L|3|M|5|E|7|16|8|@$F|17|G|18|H|I]]|9|@]|A|$]]|$1|N|3|-4|5|6|7|19|8|@]|9|@]|A|$]]|$1|O|3|P|5|6|7|1A|8|@$F|1B|G|1C|H|I]]|9|@]|A|$]]|$1|Q|3|R|5|6|7|1D|8|@]|9|@]|A|$]]|$1|S|3|T|5|U|7|1E|8|@]|9|@]|A|$V|W]]|$1|X|3|-4|5|6|7|1F|8|@]|9|@]|A|$]]]|Y|$]]

A simple question should be followed by a short, simple and clear answer.
<ul>
<li>When we are getting a value of the property it fires its <code>get{}</code> part.
</li>
<li>When we are setting a value to the property it fires its <code>set{}</code> part.
</li>
</ul>
PS. When setting a value to the property, Swift automatically creates a constant named &quot;newValue&quot; = a value we are setting. After a constant &quot;newValue&quot; becomes accessible in the property's <code>set{}</code> part.
Example:
<pre><code>var A:Int = 0
var B:Int = 0

var C:Int {
get {return 1}
set {print(&quot;Recived new value&quot;, newValue, &quot; and stored into 'B' &quot;)
 B = newValue
 }
}

// When we are getting a value of C it fires get{} part of C property
A = C
A // Now A = 1

// When we are setting a value to C it fires set{} part of C property
C = 2
B // Now B = 2
</code></pre>

blocks|key|151677|text|变量在类中的声明和调用如下所示：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|151678|class+X+{
++++var+x:+Int+=+3

}

var+y+=+X()
print("value+of+x+is:+",+y.x)

//value+of+x+is:++3|code-block|syntax|javascript|151679|现在，您希望程序使x的默认值大于或等于3。现在，假设x小于3:您的程序将失败。|151680|所以，你希望人们放入3或大于3。Swift让你很容易，理解这一点很重要-这是一种高级的确定变量值的方法，因为他们将在iOS开发中广泛使用它。现在让我们看看如何在这里使用get和set。|151681|class+X+{
++++var+_x:+Int+=+3
++++var+x:+Int+{
++++++++get+{
++++++++++++return+_x
++++++++}
++++++++set(newVal)+{++//+Set+always+take+one+argument
++++++++++++if+newVal+>=+3+{
++++++++++++++++_x+=+newVal+//+Updating+_x+with+the+input+value+by+the+user
++++++++++++++++print("new+value+is:+",+_x)
++++++++++++}
++++++++++++else+{
++++++++++++++++print("error+must+be+greater+than+3")
++++++++++++}
++++++++}
++++}
}

let+y+=+X()
y.x+=+1
print(y.x)+//+The+error+must+be+greater+than+3
y.x+=+8+//+//+The+new+value+is:+8|151682|如果您仍然有疑问，只需记住get和set的用法就是按照我们希望的方式更新任何变量。get和set可以让你更好地控制你的逻辑。它是一个强大的工具，因此不容易理解。|151683|entityMap^0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|S|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|T|8|@]|9|@]|A|$]]|$1|K|3|L|5|D|7|U|8|@]|9|@]|A|$E|F]]|$1|M|3|N|5|6|7|V|8|@]|9|@]|A|$]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

A variable declares and is called like this in a class:
<pre><code>class X {
 var x: Int = 3

}

var y = X()
print(&quot;value of x is: &quot;, y.x)

//value of x is: 3

</code></pre>
Now you want the program to make the default value of x more than or equal to 3. Now take the hypothetical case if x is less than 3: your program will fail.
So, you want people to either put in 3 or more than 3. Swift got it easy for you and it is important to understand this bit - it is an advanced way of dating the variable value, because they will extensively use it in iOS development. Now let's see how get and set will be used here.
<pre><code>class X {
 var _x: Int = 3
 var x: Int {
 get {
 return _x
 }
 set(newVal) { // Set always take one argument
 if newVal &gt;= 3 {
 _x = newVal // Updating _x with the input value by the user
 print(&quot;new value is: &quot;, _x)
 }
 else {
 print(&quot;error must be greater than 3&quot;)
 }
 }
 }
}

let y = X()
y.x = 1
print(y.x) // The error must be greater than 3
y.x = 8 // // The new value is: 8

</code></pre>
If you still have doubts, just remember the use of get and set is to update any variable the way we want it to be updated. get and set will give you better control to rule your logic. It is a powerful tool, hence not easily understandable.

I'm learning Swift and I'm reading The Swift Programming Language from Apple. I don't have any Objective-C background (only <a href="https://en.wikipedia.org/wiki/PHP" rel="noreferrer">PHP</a>, JavaScript, and others, but not Objective-C).
On page 24-25 I see this code:
<pre><code>//...Class definition stuff...

var perimeter: Double {
 get {
 return 3.0 * sideLength
 }
 set {
 sideLength = newValue / 3.0
 }
}

//...Class continues...
</code></pre>
This part is not specified in the book, and I can't get what those are for.
What are get and set?

What are 'get' and 'set' in Swift?

Objective-C

Swift

我正在学习Swift，我正在阅读Apple的Swift编程语言。我没有任何Objective-C背景(只有、JavaScript和其他，但没有Objective-C)。在第24-25页上，我看到了以下代码：//...Class definition stuff...var perimeter: Double {    ...

问在Swift中'get‘和'set’是什么？
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