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<hr>
<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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