文本/模板如何确定地图的“默认文本表示”？
[英]How does text/template determine the “default textual representation” of a map?
问题描述
Per the documentation of the text/template package in the Go standard library, ( html/template would be the same here as far as I know) simply using the pipeline operator will spit out a "default textual representation" of whatever that is: 
根据Go标准库中的text/template包的文档 ，（据我所知， html/template在这里是相同的）简单地使用管道操作符将吐出任何不同的“默认文本表示”：
{{pipeline}}
The default textual representation of the value of the pipeline is copied to the output.
In the case of a map, you get a nice printed-out format with key names and everything...incidentally, this is valid JavaScript, so it makes it easy to pass whole structs into your JS code if you want. 在地图的情况下，你会得到一个很好的打印格式，其中包含键名和所有内容......顺便说一句，这是有效的JavaScript，因此如果你愿意，它可以很容易地将整个结构传递给你的JS代码。
My question is, how is this textual representation determined, and more specifically can I hook into it? 我的问题是，这个文本表示是如何确定的，更具体地说，我可以加入它吗？ I thought perhaps it would check to see if the pipeline was a fmt.Stringer and I could give my map subtype a String() string method, but that doesn't seem to be the case. 我想也许它会检查管道是否是fmt.Stringer ，我可以给我的map子类型一个String() string方法，但似乎并非如此。 I'm hunting through the text/template code but I seem to be missing how it does this. 我正在寻找text/template代码，但我似乎不知道它是如何做到的。
How does text/template determine "default textual representation"? text/template如何确定“默认文本表示”？
1 个解决方案
解决方案1
3 已采纳 2016-07-22 06:57:29
The default textual representation is determined by how the fmt package prints the value. 默认文本表示由fmt包打印值的方式决定。 So you were barking up the right tree. 所以你正在吠叫正确的树。
See this example: 看这个例子：
t := template.Must(template.New("").Parse("{{.}}"))
m := map[string]interface{}{"a": "abc", "b": 2}
t.Execute(os.Stdout, m)
It outputs: 它输出：
map[a:abc b:2]
Now if we use a custom map type with a String() method: 现在，如果我们使用带有String()方法的自定义地图类型：
type MyMap map[string]interface{}
func (m MyMap) String() string { return "custom" }
mm := MyMap{"a": "abc", "b": 2}
t.Execute(os.Stdout, mm)
Output is: 输出是：
custom
Try these (and below examples) on the Go Playground . 在Go Playground上试试这些（及以下示例）。
What to look out for? 需要注意什么？
Note that MyMap.String() has a value receiver (not a pointer). 请注意， MyMap.String()有一个值接收器（不是指针）。 And I pass a value of MyMap , so it works. 我传递了MyMap的值，所以它有效。 If you change the receiver type to pointer to MyMap , it won't work. 如果将接收器类型更改为指向MyMap指针，则它将不起作用。 And it is because then only a value of type *MyMap will have a String() method, but not a value of MyMap . 这是因为只有*MyMap类型的值才会有String()方法，而不是MyMap的值。
If the String() method has a pointer receiver, you have to pass &mm (a value of type *MyMap ) if you want your custom representation to work. 如果String()方法有一个指针接收器，如果希望自定义表示工作，则必须传递&mm （类型为*MyMap的值）。
Also note that in case of html/template , the template engine does contextual escaping, so the result of the fmt package may further be escaped. 另请注意，在html/template情况下，模板引擎执行上下文转义，因此fmt包的结果可能会被进一步转义。
For example if your custom String() method would return something "unsafe": 例如，如果您的自定义String()方法将返回“不安全”的内容：
func (m MyMap2) String() string { return "<html>" }
Trying to insert it: 试图插入它：
mm2 := MyMap2{"a": "abc", "b": 2}
t.Execute(os.Stdout, mm2)
Gets escaped: 获取转义：
<html>
Implementation 履行
This is where it is implemented in the text/template package: text/template/exec.go , unexported function state.PrintValue() , currently line #848: 这是在text/template包中实现的地方： text/template/exec.go ，unexported function state.PrintValue() ，当前行＃848：
_, err := fmt.Fprint(s.wr, iface)
If you're using the html/template package, it's implemented in html/template/content.go , unexported function stringify() , currently line #135: 如果你正在使用html/template包，它是用html/template/content.go ，未实现的函数stringify() ，当前第135行：
return fmt.Sprint(args...), contentTypePlain
Further options 更多选择
Also note that if the value implements error , the Error() method will be called and it takes precedence over String() : 另请注意，如果值实现error ，将调用Error()方法，它优先于String() ：
type MyMap map[string]interface{}
func (m MyMap) Error() string { return "custom-error" }
func (m MyMap) String() string { return "custom" }
t := template.Must(template.New("").Parse("{{.}}"))
mm := MyMap{"a": "abc", "b": 2}
t.Execute(os.Stdout, mm)
Will output: 将输出：
custom-error
Instead of custom . 而不是custom 。 Try it on the Go Playground . 在Go Playground尝试一下。
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