blocks|key|177559|text|您可以这样使用let-function：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|177560|val+a+=+b?.let+{
++++//+If+b+is+not+null.
}+?:+run+{
++++//+If+b+is+null.
}|code-block|syntax|javascript|177561|注意，只有在需要代码块时才需要调用run函数。如果在猫王运算符(run-operator，?:)之后只有一个单线器，则可以删除?:。|177562|请注意，如果run为null，或者如果let-block计算为null，则将计算let块。|177563|因此，通常只需要一个if表达式。|177564|val+a+=+if+(b+==+null)+{
++++//+...
}+else+{
++++//+...
}|177565|在这种情况下，只有当else-block不是null时才会计算b。|177566|entityMap^0|7|3|0|0|H|3|W|3|19|2|1R|2|0|6|3|J|3|V|4|14|3|0|A|2|0|0|A|4|V|1|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|Z|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|10|8|@$9|11|A|12|B|C]|$9|13|A|14|B|C]|$9|15|A|16|B|C]|$9|17|A|18|B|C]]|D|@]|E|$]]|$1|M|3|N|5|6|7|19|8|@$9|1A|A|1B|B|C]|$9|1C|A|1D|B|C]|$9|1E|A|1F|B|C]|$9|1G|A|1H|B|C]]|D|@]|E|$]]|$1|O|3|P|5|6|7|1I|8|@$9|1J|A|1K|B|C]]|D|@]|E|$]]|$1|Q|3|R|5|H|7|1L|8|@]|D|@]|E|$I|J]]|$1|S|3|T|5|6|7|1M|8|@$9|1N|A|1O|B|C]|$9|1P|A|1Q|B|C]]|D|@]|E|$]]|$1|U|3|-4|5|6|7|1R|8|@]|D|@]|E|$]]]|V|$]]

You can use the <code>let</code>-function like this:

<pre><code>val a = b?.let {
 // If b is not null.
} ?: run {
 // If b is null.
}
</code></pre>

Note that you need to call the <code>run</code> function only if you need a block of code. You can remove the <code>run</code>-block if you only have a oneliner after the elvis-operator (<code>?:</code>).

Be aware that the <code>run</code> block will be evaluated either if <code>b</code> is null, or if the <code>let</code>-block evaluates to <code>null</code>.

Because of this, you usually want just an <code>if</code> expression.

<pre><code>val a = if (b == null) {
 // ...
} else {
 // ...
}
</code></pre>

In this case, the <code>else</code>-block will only be evaluated if <code>b</code> is not null.

blocks|key|1979679|text|让我们首先确保理解提供的Swift成语的语义：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1979680|if+let+a+=+<expr>+{
+++++//+then-block
}
else+{
+++++//+else-block
}|code-block|syntax|javascript|1979681|这意味着：“如果<expr>导致非零可选，则输入符号a绑定到未包装值的then-block，否则输入else块。|offset|length|style|CODE|1979682|特别要注意的是，a仅在then_-块中绑定。在Kotlin，您可以很容易地通过调用|1979683|<expr>?.also+{+a+->
++++//+then-block
}|1979684|您可以添加这样的else-block：|1979685|<expr>?.also+{+a+->
++++//+then-block
}+?:+run+{
++++//+else-block
}|1979686|这就产生了与Swift成语相同的语义。|1979687|entityMap^0|0|0|8|6|Q|1|Z|4|1E|4|0|8|1|B|4|0|0|8|4|0|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|Z|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|10|8|@$I|11|J|12|K|L]|$I|13|J|14|K|L]|$I|15|J|16|K|L]|$I|17|J|18|K|L]]|9|@]|A|$]]|$1|M|3|N|5|6|7|19|8|@$I|1A|J|1B|K|L]|$I|1C|J|1D|K|L]]|9|@]|A|$]]|$1|O|3|P|5|D|7|1E|8|@]|9|@]|A|$E|F]]|$1|Q|3|R|5|6|7|1F|8|@$I|1G|J|1H|K|L]]|9|@]|A|$]]|$1|S|3|T|5|D|7|1I|8|@]|9|@]|A|$E|F]]|$1|U|3|V|5|6|7|1J|8|@]|9|@]|A|$]]|$1|W|3|-4|5|6|7|1K|8|@]|9|@]|A|$]]]|X|$]]

Let's first ensure we understand the semantics of the provided Swift idiom:

<pre><code>if let a = &lt;expr&gt; {
 // then-block
}
else {
 // else-block
}
</code></pre>

It means this: "if the <code>&lt;expr&gt;</code> results in a non-nil optional, enter the <code>then</code>-block with the symbol <code>a</code> bound to the unwrapped value. Otherwise enter the <code>else</code> block.

Especially note that <code>a</code> is bound only within the <code>then</code>-block. In Kotlin you can easily get this by calling

<pre><code>&lt;expr&gt;?.also { a -&gt;
 // then-block
}
</code></pre>

and you can add an <code>else</code>-block like this:

<pre><code>&lt;expr&gt;?.also { a -&gt;
 // then-block
} ?: run {
 // else-block
}
</code></pre>

This results in the same semantics as the Swift idiom.

blocks|key|344247|text|我的回答完全是其他人的抄袭。但是，我很难理解它们的表达方式。所以我想最好能给出一个更容易理解的答案。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|344248|迅速：|344249|if+let+a+=+b.val+{
++//use+"a"+as+unwrapped
}
else+{

}|code-block|syntax|javascript|344250|在科特林：|344251|b.val?.let{a+->+
++//use+"a"+as+unwrapped
}+?:+run{
++//else+case
}|344252|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|P|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|Q|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|R|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|S|8|@]|9|@]|A|$G|H]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

My answer is totally a copy cat from the others. However, I cannot understand their expression easily. So I guess it would be nice to provide an more understandable answer.

In swift: 

<pre><code>if let a = b.val {
 //use "a" as unwrapped
}
else {

}
</code></pre>

In Kotlin:

<pre><code>b.val?.let{a -&gt; 
 //use "a" as unwrapped
} ?: run{
 //else case
}
</code></pre>

blocks|key|1979993|text|if+let语句|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1979994|Swift+if+let可选绑定(称为if-let语句)用于提取非可选值(如果存在)，或者如果值为nil，则不执行任何操作。|1979995|斯威夫特的if-let声明：|1979996|let+b:+Int?+=+50

if+let+a:+Int+=+b+{
++++print("Good+news!")
}+else+{
++++print("Equal+to+'nil'+or+not+set")
}

/*++RESULT:+Good+news!++*/|code-block|syntax|javascript|1979997|在Kotlin中，就像在Swift中一样，为了避免在不需要访问空值时尝试访问空值而造成的崩溃，为正确展开nullable+types提供了一个特定的语法(如第二个示例中的b.let+{+}+)|BOLD|1979998|Kotlin的equivalent1+of+Swift的if-let声明：|1979999|val+b:+Int?+=+null
val+a+=+b

if+(a+!=+null)+{+
++++println("Good+news!")
}+else+{+
++++println("Equal+to+'null'+or+not+set")
}

/*++RESULT:+Equal+to+'null'+or+not+set++*/|1980000|Kotlin的let方法与安全调用操作符?:结合使用时，提供了一种处理可空表达式的简洁方法。|1980001|斯威夫特inline+let+function的Kotlin算子和Elvis算子|1980002|val+b:+Int?+=+null

val+a+=+b.let+{+nonNullable+->+nonNullable+}+?:+"Equal+to+'null'+or+not+set"
println(a)

/*++RESULT:+Equal+to+'null'+or+not+set++*/|1980003|guard+let语句|1980004|Swift中的guard-let语句简单而强大。它检查某些条件，如果它的计算结果为false，则执行else语句，该语句通常将退出方法。|1980005|让我们研究一下Swift的guard-let语句：|1980006|let+b:+Int?+=+nil

func+method()+{
++++guard+let+a:+Int+=+b+else+{
++++++++print("Equal+to+'nil'+or+not+set")
++++++++return
++++}
++++print("Good+news!")
}
method()

/*++RESULT:+Equal+to+'nil'+or+not+set++*/|1980007|Kotlin类似斯威夫特的guard-let声明的效果：|1980008|与Swift不同的是，在Kotlin中，根本没有卫士语句。但是，您可以使用?:+-+Elvis+Operator来获得类似的效果。|1980009|val+b:+Int?+=+50

fun+method()+{
++++val+a+=+b+?:+return+println("Equal+to+'null'+or+not+set")
++++return+println("Good+news!")
}
method()

/*++RESULT:+Good+news!++*/|1980010|entityMap^0|0|6|0|6|6|J|6|1D|3|0|5|6|0|0|1G|E|1G|E|2D|9|0|S|6|0|0|7|3|K|2|0|4|J|0|0|0|9|0|7|9|1E|4|0|D|9|0|0|D|9|0|O|2|11|2|11|2|16|E|0|0^^$0|@$1|2|3|4|5|6|7|1H|8|@$9|1I|A|1J|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|1K|8|@$9|1L|A|1M|B|C]|$9|1N|A|1O|B|C]|$9|1P|A|1Q|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|1R|8|@$9|1S|A|1T|B|C]]|D|@]|E|$]]|$1|J|3|K|5|L|7|1U|8|@]|D|@]|E|$M|N]]|$1|O|3|P|5|6|7|1V|8|@$9|1W|A|1X|B|Q]|$9|1Y|A|1Z|B|C]|$9|20|A|21|B|C]]|D|@]|E|$]]|$1|R|3|S|5|6|7|22|8|@$9|23|A|24|B|C]]|D|@]|E|$]]|$1|T|3|U|5|L|7|25|8|@]|D|@]|E|$M|N]]|$1|V|3|W|5|6|7|26|8|@$9|27|A|28|B|C]|$9|29|A|2A|B|C]]|D|@]|E|$]]|$1|X|3|Y|5|6|7|2B|8|@$9|2C|A|2D|B|C]]|D|@]|E|$]]|$1|Z|3|10|5|L|7|2E|8|@]|D|@]|E|$M|N]]|$1|11|3|12|5|6|7|2F|8|@$9|2G|A|2H|B|C]]|D|@]|E|$]]|$1|13|3|14|5|6|7|2I|8|@$9|2J|A|2K|B|C]|$9|2L|A|2M|B|C]]|D|@]|E|$]]|$1|15|3|16|5|6|7|2N|8|@$9|2O|A|2P|B|C]]|D|@]|E|$]]|$1|17|3|18|5|L|7|2Q|8|@]|D|@]|E|$M|N]]|$1|19|3|1A|5|6|7|2R|8|@$9|2S|A|2T|B|C]]|D|@]|E|$]]|$1|1B|3|1C|5|6|7|2U|8|@$9|2V|A|2W|B|Q]|$9|2X|A|2Y|B|Q]|$9|2Z|A|30|B|C]|$9|31|A|32|B|C]]|D|@]|E|$]]|$1|1D|3|1E|5|L|7|33|8|@]|D|@]|E|$M|N]]|$1|1F|3|-4|5|6|7|34|8|@]|D|@]|E|$]]]|1G|$]]

<h2><code>if let</code> statement</h2>
Swift <code>if let</code> Optional Binding (so called <code>if-let</code> statement) is used to extract a non-optional value if one exists, or to do nothing if a value is <code>nil</code>.
Swift's <code>if-let</code> statement:
<pre class="lang-swift prettyprint-override"><code>let b: Int? = 50

if let a: Int = b {
 print(&quot;Good news!&quot;)
} else {
 print(&quot;Equal to 'nil' or not set&quot;)
}

/* RESULT: Good news! */
</code></pre>
In Kotlin, like in Swift, to avoid crashes caused by trying to access a null value when it’s not expected, a specific syntax (like <code>b.let { }</code> in second example) is provided for properly unwrapping <code>nullable types</code>:
Kotlin's equivalent1 of Swift's <code>if-let</code> statement:
<pre class="lang-kotlin prettyprint-override"><code>val b: Int? = null
val a = b

if (a != null) { 
 println(&quot;Good news!&quot;)
} else { 
 println(&quot;Equal to 'null' or not set&quot;)
}

/* RESULT: Equal to 'null' or not set */
</code></pre>
Kotlin’s <code>let</code> method, when used in combination with the safe-call operator <code>?:</code>, provides a concise way to handle nullable expressions.
Kotlin's <code>inline let function</code> and Elvis Operator of Swift's <code>nil coalescing operator</code>:
<pre class="lang-kotlin prettyprint-override"><code>val b: Int? = null

val a = b.let { nonNullable -&gt; nonNullable } ?: &quot;Equal to 'null' or not set&quot;
println(a)

/* RESULT: Equal to 'null' or not set */
</code></pre>
 
<h2><code>guard let</code> statement</h2>
<code>guard-let</code> statement in Swift is simple and powerful. It checks for some condition and if it evaluates to be false, then the <code>else</code> statement executes which normally will exit a method.
Let's explore a Swift's <code>guard-let</code> statement:
<pre class="lang-swift prettyprint-override"><code>let b: Int? = nil

func method() {
 guard let a: Int = b else {
 print(&quot;Equal to 'nil' or not set&quot;)
 return
 }
 print(&quot;Good news!&quot;)
}
method()

/* RESULT: Equal to 'nil' or not set */
</code></pre>
Kotlin's similar effect of Swift's <code>guard-let</code> statement:
Unlike Swift, in Kotlin, there is no guard statement at all. However, you can use the <code>Elvis Operator</code> – <code>?:</code> for getting a similar effect.
<pre class="lang-kotlin prettyprint-override"><code>val b: Int? = 50

fun method() {
 val a = b ?: return println(&quot;Equal to 'null' or not set&quot;)
 return println(&quot;Good news!&quot;)
}
method()

/* RESULT: Good news! */
</code></pre>

blocks|key|177783|text|上面有两个答案，两种答案都得到了广泛的接受：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|177784|让{}+?：运行{}|ordered-list-item|offset|length|style|BOLD|177785|也是{}+?：运行{}|177786|这两种方法在大多数用法中似乎都有效，但在下面的测试中，#1可能失败：|177787|​|177788|📷|atomic|177789|177790|第二条似乎更好。|177791|entityMap|0|IMAGE|mutability|IMMUTABLE|imageUrl|https://developer.qcloudimg.com/http-save/yehe-900000/5f7011b20affe2129303c084fc0a1275.png|imageAlt^0|0|0|1|0|0|2|0|0|0|0|1|0|0|0|0^^$0|@$1|2|3|4|5|6|7|13|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|14|8|@$E|15|F|16|G|H]]|9|@]|A|$]]|$1|I|3|J|5|D|7|17|8|@$E|18|F|19|G|H]]|9|@]|A|$]]|$1|K|3|L|5|6|7|1A|8|@]|9|@]|A|$]]|$1|M|3|N|5|6|7|1B|8|@]|9|@]|A|$]]|$1|O|3|P|5|Q|7|1C|8|@]|9|@$E|1D|F|1E|1|1F]]|A|$]]|$1|R|3|N|5|6|7|1G|8|@]|9|@]|A|$]]|$1|S|3|T|5|6|7|1H|8|@]|9|@]|A|$]]|$1|U|3|-4|5|6|7|1I|8|@]|9|@]|A|$]]]|V|$W|$5|X|Y|Z|A|$10|11|12|-4]]]]

there are two answers above, both got a lot acceptances:
<ol>
<li>str?.let{ } ?: run { }</li>
<li>str?.also{ } ?: run { }</li>
</ol>
Both seem to work in most of the usages, but #1 would fail in the following test:
<a href="https://i.stack.imgur.com/eOXhp.png" rel="noreferrer"><img src="https://i.stack.imgur.com/eOXhp.png" alt="enter image description here" /></a>
#2 seems better.

blocks|key|1979903|text|与Swift不同，在Kotlin中使用可选选项之前，不必打开它。我们只需检查该值是否为null，编译器将跟踪有关您执行的检查的信息，并允许将其用作未包装的检查。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1979904|在Swift：|1979905|if+let+a+=+b.val+{
++//use+"a"+as+unwrapped
}+else+{

}|code-block|syntax|javascript|1979906|在科特林：|1979907|if+b.val+!=+null+{
++//use+"b.val"+as+unwrapped
}+else+{

}|1979908|有关更多这样的用例，请参考文档：(零-安全)|offset|length|1979909|entityMap|0|LINK|mutability|MUTABLE|url|https://kotlinlang.org/docs/reference/null-safety.html^0|0|0|0|0|0|D|9|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|Z|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|10|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|11|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|12|8|@]|9|@]|A|$G|H]]|$1|M|3|N|5|6|7|13|8|@]|9|@$O|14|P|15|1|16]]|A|$]]|$1|Q|3|-4|5|6|7|17|8|@]|9|@]|A|$]]]|R|$S|$5|T|U|V|A|$W|X]]]]

Unlike Swift, Its not necessary to unwrap the optional before using it in Kotlin. We could just check if the value is non null and the compiler tracks the information about the check you performed and allows to use it as unwrapped.

In Swift:

<pre><code>if let a = b.val {
 //use "a" as unwrapped
} else {

}
</code></pre>

In Kotlin:

<pre><code>if b.val != null {
 //use "b.val" as unwrapped
} else {

}
</code></pre>

Refer <a href="https://kotlinlang.org/docs/reference/null-safety.html" rel="noreferrer">Documentation: (null-safety)</a> for more such use cases

blocks|key|1979723|text|下面是如何仅在name不为null时执行代码：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1979724|var+name:+String?+=+null
name?.let+{+nameUnwrapp+->
++++println(nameUnwrapp)++//+not+printed+because+name+was+null
}
name+=+"Alex"
name?.let+{+nameUnwrapp+->
++++println(nameUnwrapp)++//+printed+"Alex"
}|code-block|syntax|javascript|1979725|entityMap^0|7|4|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|P|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|Q|8|@]|D|@]|E|$]]]|L|$]]

Here's how to only execute code when <code>name</code> is not null:

<pre><code>var name: String? = null
name?.let { nameUnwrapp -&gt;
 println(nameUnwrapp) // not printed because name was null
}
name = "Alex"
name?.let { nameUnwrapp -&gt;
 println(nameUnwrapp) // printed "Alex"
}
</code></pre>

blocks|key|344381|text|这是我的变体，仅限于非常常见的“如果不是空”的情况。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|344382|首先，在某个地方定义如下：|344383|inline+fun+<T>+ifNotNull(obj:+T?,+block:+(T)+->+Unit)+{
++++if+(obj+!=+null)+{
++++++++block(obj)
++++}
}|code-block|syntax|javascript|344384|它可能应该是internal，以避免冲突。|offset|length|style|CODE|344385|现在，转换以下Swift代码：|344386|if+let+item+=+obj.item+{
++++doSomething(item)
}|344387|到这个Kotlin代码：|344388|ifNotNull(obj.item)+{+item+->+
++++doSomething(item)
}|344389|注意，和Kotlin中的块一样，您可以删除参数并使用it|344390|ifNotNull(obj.item)+{
++++doSomething(it)
}|344391|但是，如果块超过1-2行，那么最好是显式的。|344392|这和我能找到的斯威夫特很相似。|344393|entityMap^0|0|0|0|6|8|0|0|0|0|0|Q|2|0|0|0|0^^$0|@$1|2|3|4|5|6|7|16|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|17|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|18|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|19|8|@$K|1A|L|1B|M|N]]|9|@]|A|$]]|$1|O|3|P|5|6|7|1C|8|@]|9|@]|A|$]]|$1|Q|3|R|5|F|7|1D|8|@]|9|@]|A|$G|H]]|$1|S|3|T|5|6|7|1E|8|@]|9|@]|A|$]]|$1|U|3|V|5|F|7|1F|8|@]|9|@]|A|$G|H]]|$1|W|3|X|5|6|7|1G|8|@$K|1H|L|1I|M|N]]|9|@]|A|$]]|$1|Y|3|Z|5|F|7|1J|8|@]|9|@]|A|$G|H]]|$1|10|3|11|5|6|7|1K|8|@]|9|@]|A|$]]|$1|12|3|13|5|6|7|1L|8|@]|9|@]|A|$]]|$1|14|3|-4|5|6|7|1M|8|@]|9|@]|A|$]]]|15|$]]

Here's my variant, limited to the very common "if not null" case.

First of all, define this somewhere:

<pre><code>inline fun &lt;T&gt; ifNotNull(obj: T?, block: (T) -&gt; Unit) {
 if (obj != null) {
 block(obj)
 }
}
</code></pre>

It should probably be <code>internal</code>, to avoid conflicts.

Now, convert this Swift code:

<pre class="lang-swift prettyprint-override"><code>if let item = obj.item {
 doSomething(item)
}
</code></pre>

To this Kotlin code:

<pre><code>ifNotNull(obj.item) { item -&gt; 
 doSomething(item)
}
</code></pre>

Note that as always with blocks in Kotlin, you can drop the argument and use <code>it</code>:

<pre><code>ifNotNull(obj.item) {
 doSomething(it)
}
</code></pre>

But if the block is more than 1-2 lines, it's probably best to be explicit.

This is as similar to Swift as I could find.

blocks|key|176895|text|kotlin也有类似的方法来实现斯威夫特的风格|type|unstyled|depth|inlineStyleRanges|entityRanges|data|176896|if+(val+a+=+b)+{
++++a.doFirst()
++++a.doSecond()
}|code-block|syntax|javascript|176897|您还可以分配多个可空值。|176898|if+(val+name+=+nullableName,+val+age+=+nullableAge)+{
++++doSomething(name,+age)
}|176899|如果使用了1次以上的可空值，这种方法将更适合。在我看来，它从性能方面有所帮助，因为可空值将只检查一次。|176900|来源：Kotlin讨论|offset|length|176901|entityMap|0|LINK|mutability|MUTABLE|url|https://discuss.kotlinlang.org/t/kotlin-null-check-for-multiple-nullable-vars/1946/22^0|0|0|0|0|0|3|8|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|Z|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|10|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|11|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|12|8|@]|9|@]|A|$]]|$1|M|3|N|5|6|7|13|8|@]|9|@$O|14|P|15|1|16]]|A|$]]|$1|Q|3|-4|5|6|7|17|8|@]|9|@]|A|$]]]|R|$S|$5|T|U|V|A|$W|X]]]]

There is a similar way in kotlin to achieve Swift's style if-let

<pre><code>if (val a = b) {
 a.doFirst()
 a.doSecond()
}
</code></pre>

You can also assigned multiple nullable values

<pre><code>if (val name = nullableName, val age = nullableAge) {
 doSomething(name, age)
}
</code></pre>

This kind of approach will be more suitable if the nullable values is used for more than 1 times. In my opinion, it helps from the performance aspect because the nullable value will be checked only once.

source: <a href="https://discuss.kotlinlang.org/t/kotlin-null-check-for-multiple-nullable-vars/1946/22" rel="nofollow noreferrer">Kotlin Discussion</a>

blocks|key|176988|text|我加入这个答案是为了澄清被接受的答案，因为它太大了，不能发表评论。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|176989|这里的一般模式是，您可以使用Kotlin中可用的任何范围功能组合，用埃尔维斯算子隔开，如下所示：|offset|length|176990|<nullable>?.<scope+function>+{
++++//+code+if+not+null
}+:?+<scope+function>+{
++++//+code+if+null
}|code-block|syntax|javascript|176991|例如：|176992|val+gradedStudent+=+student?.apply+{
++++grade+=+newGrade
}+:?+with(newGrade)+{
++++Student().apply+{+grade+=+newGrade+}
}|176993|entityMap|0|LINK|mutability|MUTABLE|url|https://kotlinlang.org/docs/reference/scope-functions.html|1|https://kotlinlang.org/docs/reference/null-safety.html#elvis-operator^0|0|Q|4|0|Y|6|1|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|Z|8|@]|9|@$D|10|E|11|1|12]|$D|13|E|14|1|15]]|A|$]]|$1|F|3|G|5|H|7|16|8|@]|9|@]|A|$I|J]]|$1|K|3|L|5|6|7|17|8|@]|9|@]|A|$]]|$1|M|3|N|5|H|7|18|8|@]|9|@]|A|$I|J]]|$1|O|3|-4|5|6|7|19|8|@]|9|@]|A|$]]]|P|$Q|$5|R|S|T|A|$U|V]]|W|$5|R|S|T|A|$U|X]]]]

I'm adding this answer to clarify the accepted answer because it's too big for a comment.

The general pattern here is that you can use any combination of the <a href="https://kotlinlang.org/docs/reference/scope-functions.html" rel="nofollow noreferrer">Scope Functions</a> available in Kotlin separated by the <a href="https://kotlinlang.org/docs/reference/null-safety.html#elvis-operator" rel="nofollow noreferrer">Elvis Operator</a> like this:

<pre><code>&lt;nullable&gt;?.&lt;scope function&gt; {
 // code if not null
} :? &lt;scope function&gt; {
 // code if null
}
</code></pre>

For example:

<pre><code>val gradedStudent = student?.apply {
 grade = newGrade
} :? with(newGrade) {
 Student().apply { grade = newGrade }
}
</code></pre>

blocks|key|344472|text|在我看来，最干净的选择是|type|unstyled|depth|inlineStyleRanges|entityRanges|data|344473|斯威夫特|344474|if+let+a+=+b.val+{

}+else+{

}|code-block|syntax|javascript|344475|Kotlin|344476|b.val.also+{+a+->

}+?:+run+{

}|344477|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|P|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|Q|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|R|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|S|8|@]|9|@]|A|$G|H]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

The cleanest option in my opinion is this

Swift:

<pre><code>if let a = b.val {

} else {

}
</code></pre>

Kotlin

<pre><code>b.val.also { a -&gt;

} ?: run {

}
</code></pre>

blocks|key|1980166|text|伊夫特·伊特声明在科特林|type|blockquote|depth|inlineStyleRanges|entityRanges|data|1980167|简单的回答是使用简单的IF-作为评论的时候，在Kotlin+LET中没有对应的，|unstyled|offset|length|style|BOLD|1980168|++++if(A.isNull()){
//+A+is+null
++++}else{
//+A+is+not+null
++++}|code-block|syntax|javascript|1980169|entityMap^0|0|B|3|0|0^^$0|@$1|2|3|4|5|6|7|P|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|Q|8|@$E|R|F|S|G|H]]|9|@]|A|$]]|$1|I|3|J|5|K|7|T|8|@]|9|@]|A|$L|M]]|$1|N|3|-4|5|D|7|U|8|@]|9|@]|A|$]]]|O|$]]

<blockquote>
 Swift if let statement in Kotlin
</blockquote>

The short answer is use simple IF-ELSE as by the time of this comment there is no equivalent in Kotlin LET, 

<pre><code> if(A.isNull()){
// A is null
 }else{
// A is not null
 }
</code></pre>

blocks|key|1980257|text|我们可以使用if+let获得与Swift+inline+fun相同的解包装语法|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1980258|inline+fun+<T:Any?>+T?.unwrap(callback:+(T)->+Unit)+:+Boolean+{
++++return+if+(this+!=+null)+{
++++++++this?.let(callback)
++++++++true
++++}else+{
++++++++false
++++}
}|code-block|syntax|javascript|1980259|用途：：|BOLD|1980260|++++++++val++name+:+String?+=+null
++++++++val++rollNo+:+String?+=+""
++++++++var+namesList:+ArrayList<String>?++=+null

++++++++if+(name.unwrap+{+name+->

++++++++++++++++Log.i("Dhiru",+"Name+have+value+on+it++$name")

++++++++++++})else+if+(+rollNo.unwrap+{
++++++++++++++++Log.i("Dhiru","Roll+have+value+on+it")

++++++++++++})+else+if+(namesList.unwrap+{++namesList++->
++++++++++++++++Log.i("Dhiru","This+is+Called+when+names+list+have+value+")
++++++++++++})++{
+++++++++++++Log.i("Dhiru","No+Field+have+value+on+it+")
++++++++}|1980261|entityMap^0|6|6|L|A|0|0|0|3|0|0^^$0|@$1|2|3|4|5|6|7|R|8|@$9|S|A|T|B|C]|$9|U|A|V|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|W|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|X|8|@$9|Y|A|Z|B|M]]|D|@]|E|$]]|$1|N|3|O|5|H|7|10|8|@]|D|@]|E|$I|J]]|$1|P|3|-4|5|6|7|11|8|@]|D|@]|E|$]]]|Q|$]]

we can get the same Unwraping syntax like Swift <code>if let</code> using <code>inline fun</code>
<pre><code>inline fun &lt;T:Any?&gt; T?.unwrap(callback: (T)-&gt; Unit) : Boolean {
 return if (this != null) {
 this?.let(callback)
 true
 }else {
 false
 }
}
</code></pre>
Uses: :
<pre><code> val name : String? = null
 val rollNo : String? = &quot;&quot;
 var namesList: ArrayList&lt;String&gt;? = null

 if (name.unwrap { name -&gt;

 Log.i(&quot;Dhiru&quot;, &quot;Name have value on it $name&quot;)

 })else if ( rollNo.unwrap {
 Log.i(&quot;Dhiru&quot;,&quot;Roll have value on it&quot;)

 }) else if (namesList.unwrap { namesList -&gt;
 Log.i(&quot;Dhiru&quot;,&quot;This is Called when names list have value &quot;)
 }) {
 Log.i(&quot;Dhiru&quot;,&quot;No Field have value on it &quot;)
 }
</code></pre>

blocks|key|1980368|text|Any?.let+{}+?:+run+{}结构的问题是：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1980369|它只允许每个语句进行一次非空检查。|unordered-list-item|1980370|如果let块返回null，则无论如何对run块进行计算。|1980371|不可能在开关/何时样式中执行多个检查|1980372|解决大多数这些问题的解决方案是定义如下函数：|1980373|private+inline+fun+<A>+ifNotNull(p1:+A?,+block:+(A)+->+Unit):+Unit?+{
++++if+(p1+!=+null)+{
++++++++return+block.invoke(p1)
++++}
++++return+null
}

private+inline+fun+<A,+B>+ifNotNull(p1:+A?,+p2:+B?,+block:+(A,+B)+->+Unit):+Unit?+{
++++if+(p1+!=+null+&&+p2+!=+null)+{
++++++++return+block.invoke(p1,+p2)
++++}
++++return+null
}

private+inline+fun+<A,+B,+C>+ifNotNull(p1:+A?,+p2:+B?,+p3:+C?,+block:+(A,+B,+C)+->+Unit):+Unit?+{
++++if+(p1+!=+null+&&+p2+!=+null+&&+p3+!=+null)+{
++++++++return+block.invoke(p1,+p2,+p3)
++++}
++++return+null
}|code-block|syntax|javascript|1980374|这将允许这样的语句：|1980375|ifNotNull(a,+b)+{+a,+b+->
++++//+code+when+a,+b+are+not+null
}+?:
ifNotNull(c)+{+c+->
++++//+code+when+a,+b+are+null+and+c+not+null
}+?:+
ifNotNull(d,+e,+f)+{+d,+e,+f+->
++++//+code+when+a,+b,+c+are+null+and+d,+e,+f+not+null
}+?:+run+{
++++//+code+which+should+be+performed+if+a,+b,+c,+d,+e+and+f+are+null
}|1980376|唯一的警告是，如果在循环中执行continue和break语句，则不支持与Swift的if+let等效语句相比较。|1980377|entityMap^0|0|L|0|0|2|3|0|0|0|0|0|0|F|8|O|5|17|6|0^^$0|@$1|2|3|4|5|6|7|11|8|@$9|12|A|13|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|14|8|@]|D|@]|E|$]]|$1|I|3|J|5|H|7|15|8|@$9|16|A|17|B|C]]|D|@]|E|$]]|$1|K|3|L|5|H|7|18|8|@]|D|@]|E|$]]|$1|M|3|N|5|6|7|19|8|@]|D|@]|E|$]]|$1|O|3|P|5|Q|7|1A|8|@]|D|@]|E|$R|S]]|$1|T|3|U|5|6|7|1B|8|@]|D|@]|E|$]]|$1|V|3|W|5|Q|7|1C|8|@]|D|@]|E|$R|S]]|$1|X|3|Y|5|6|7|1D|8|@$9|1E|A|1F|B|C]|$9|1G|A|1H|B|C]|$9|1I|A|1J|B|C]]|D|@]|E|$]]|$1|Z|3|-4|5|6|7|1K|8|@]|D|@]|E|$]]]|10|$]]

The problem with the <code>Any?.let {} ?: run {}</code> constructions is that:
<ul>
<li>It only allows for one non-null check per statement</li>
<li>If the <code>let</code> block returns null the run block is evaluated anyway</li>
<li>It's not possible to perform multiple checks in a switch/when style</li>
</ul>
A solution which tackles most of these problems is to define functions like the following:
<pre><code>private inline fun &lt;A&gt; ifNotNull(p1: A?, block: (A) -&gt; Unit): Unit? {
 if (p1 != null) {
 return block.invoke(p1)
 }
 return null
}

private inline fun &lt;A, B&gt; ifNotNull(p1: A?, p2: B?, block: (A, B) -&gt; Unit): Unit? {
 if (p1 != null &amp;&amp; p2 != null) {
 return block.invoke(p1, p2)
 }
 return null
}

private inline fun &lt;A, B, C&gt; ifNotNull(p1: A?, p2: B?, p3: C?, block: (A, B, C) -&gt; Unit): Unit? {
 if (p1 != null &amp;&amp; p2 != null &amp;&amp; p3 != null) {
 return block.invoke(p1, p2, p3)
 }
 return null
}
</code></pre>
This would allow for a statement like:
<pre><code>ifNotNull(a, b) { a, b -&gt;
 // code when a, b are not null
} ?:
ifNotNull(c) { c -&gt;
 // code when a, b are null and c not null
} ?: 
ifNotNull(d, e, f) { d, e, f -&gt;
 // code when a, b, c are null and d, e, f not null
} ?: run {
 // code which should be performed if a, b, c, d, e and f are null
}
</code></pre>
The only caveat is that <code>continue</code> and <code>break</code> statements are not supported if executed within a loop compared to Swift's <code>if let</code> equivalent.

blocks|key|1980387|text|也许我很晚了，不过，打开包装的最简单的方法是|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1980388|yourOptionalString+?:+return|code-block|syntax|javascript|1980389|下面的所有行都有未包装的字符串|1980390|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

Probably I am very late however the easiest way to unwrap and option is
<pre><code>yourOptionalString ?: return
</code></pre>
this was all the following lines will have unwrapped string

blocks|key|344610|text|如果b是一个成员变量，那么这种方法在我看来是最易读的：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|344611|val+b+=+this.b
if+(b+==+null)+{
++++return
}
println("non+nullable+:+${b}")|code-block|syntax|javascript|344612|这也与其工作方式是一致的，其中一个新的局部变量遮挡了成员变量。|344613|entityMap^0|2|1|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|R|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|S|8|@]|D|@]|E|$]]|$1|M|3|-4|5|6|7|T|8|@]|D|@]|E|$]]]|N|$]]

If <code>b</code> is a member variable then this approach seems most readable to me:
<pre class="lang-kotlin prettyprint-override"><code>val b = this.b
if (b == null) {
 return
}
println(&quot;non nullable : ${b}&quot;)
</code></pre>
This is also consistent with how it works in swift, where a new local variable shadows the member variable.

In Kotlin is there an equivalent to the Swift code below?
<pre><code>if let a = b.val {

} else {

}
</code></pre>

Swift 'if let' statement equivalent in Kotlin

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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在Kotlin中，是否有相当于以下Swift代码的代码？if let a = b.val {} else {}

问相当于Kotlin的“如果让”语句
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