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<li><em>KusonStack一站式可编程配置技术栈: <a href="https://github.com/KusionStack/kusion">https://github.com/KusionStack/kusion</a></em></li>
<li><em>凹语言™: <a href="https://github.com/wa-lang/wa">https://github.com/wa-lang/wa</a></em></li>
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<h2 id="64-方法值和方法表达式"><a class="header" href="#64-方法值和方法表达式">6.4. 方法值和方法表达式</a></h2>
<p>我们经常选择一个方法并且在同一个表达式里执行比如常见的p.Distance()形式实际上将其分成两步来执行也是可能的。p.Distance叫作“选择器”选择器会返回一个方法“值”-&gt;一个将方法Point.Distance绑定到特定接收器变量的函数。这个函数可以不通过指定其接收器即可被调用即调用时不需要指定接收器译注因为已经在前文中指定过了只要传入函数的参数即可</p>
<pre><code class="language-go">p := Point{1, 2}
q := Point{4, 6}
distanceFromP := p.Distance // method value
fmt.Println(distanceFromP(q)) // &quot;5&quot;
var origin Point // {0, 0}
fmt.Println(distanceFromP(origin)) // &quot;2.23606797749979&quot;, sqrt(5)
scaleP := p.ScaleBy // method value
scaleP(2) // p becomes (2, 4)
scaleP(3) // then (6, 12)
scaleP(10) // then (60, 120)
</code></pre>
<p>在一个包的API需要一个函数值、且调用方希望操作的是某一个绑定了对象的方法的话方法“值”会非常实用``=_=`真是绕。举例来说下面例子中的time.AfterFunc这个函数的功能是在指定的延迟时间之后来执行一个译注另外的函数。且这个函数操作的是一个Rocket对象r</p>
<pre><code class="language-go">type Rocket struct { /* ... */ }
func (r *Rocket) Launch() { /* ... */ }
r := new(Rocket)
time.AfterFunc(10 * time.Second, func() { r.Launch() })
</code></pre>
<p>直接用方法“值”传入AfterFunc的话可以更为简短</p>
<pre><code class="language-go">time.AfterFunc(10 * time.Second, r.Launch)
</code></pre>
<p>译注:省掉了上面那个例子里的匿名函数。</p>
<p>和方法“值”相关的还有方法表达式。当调用一个方法时与调用一个普通的函数相比我们必须要用选择器p.Distance语法来指定方法的接收器。</p>
<p>当T是一个类型时方法表达式可能会写作<code>T.f</code>或者<code>(*T).f</code>,会返回一个函数“值”,这种函数会将其第一个参数用作接收器,所以可以用通常(译注:不写选择器)的方式来对其进行调用:</p>
<pre><code class="language-go">p := Point{1, 2}
q := Point{4, 6}
distance := Point.Distance // method expression
fmt.Println(distance(p, q)) // &quot;5&quot;
fmt.Printf(&quot;%T\n&quot;, distance) // &quot;func(Point, Point) float64&quot;
scale := (*Point).ScaleBy
scale(&amp;p, 2)
fmt.Println(p) // &quot;{2 4}&quot;
fmt.Printf(&quot;%T\n&quot;, scale) // &quot;func(*Point, float64)&quot;
// 译注这个Distance实际上是指定了Point对象为接收器的一个方法func (p Point) Distance()
// 但通过Point.Distance得到的函数需要比实际的Distance方法多一个参数
// 即其需要用第一个额外参数指定接收器后面排列Distance方法的参数。
// 看起来本书中函数和方法的区别是指有没有接收器,而不像其他语言那样是指有没有返回值。
</code></pre>
<p>当你根据一个变量来决定调用同一个类型的哪个函数时方法表达式就显得很有用了。你可以根据选择来调用接收器各不相同的方法。下面的例子变量op代表Point类型的addition或者subtraction方法Path.TranslateBy方法会为其Path数组中的每一个Point来调用对应的方法</p>
<pre><code class="language-go">type Point struct{ X, Y float64 }
func (p Point) Add(q Point) Point { return Point{p.X + q.X, p.Y + q.Y} }
func (p Point) Sub(q Point) Point { return Point{p.X - q.X, p.Y - q.Y} }
type Path []Point
func (path Path) TranslateBy(offset Point, add bool) {
var op func(p, q Point) Point
if add {
op = Point.Add
} else {
op = Point.Sub
}
for i := range path {
// Call either path[i].Add(offset) or path[i].Sub(offset).
path[i] = op(path[i], offset)
}
}
</code></pre>
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