golang模板内的变量

Question

html/text模板中变量的命名空间是什么？我认为变量$x可以改变模板内的值,但是这个例子告诉我我不能.

当我尝试按年度组合锦标赛时我失败了 - 就像这样(http://play.golang.org/p/EX1Aut_ULD):

package main

import (
    "fmt"
    "os"
    "text/template"
    "time"
)

func main() {
    tournaments := []struct {
        Place string
        Date  time.Time
    }{
        // for clarity - date is sorted, we don't need sort it again
        {"Town1", time.Date(2015, time.November, 10, 23, 0, 0, 0, time.Local)},
        {"Town2", time.Date(2015, time.October, 10, 23, 0, 0, 0, time.Local)},
        {"Town3", time.Date(2014, time.November, 10, 23, 0, 0, 0, time.Local)},
    }
    t, err := template.New("").Parse(`
{{$prev_year:=0}}
{{range .}}
    {{with .Date}}
        {{$year:=.Year}}
                    {{if ne $year $prev_year}}
                        Actions in year {{$year}}:
                {{$prev_year:=$year}}
            {{end}}
    {{end}}

        {{.Place}}, {{.Date}}
    {{end}}

    `)
    if err != nil {
        panic(err)
    }
    err = t.Execute(os.Stdout, tournaments)
    if err != nil {
        fmt.Println("executing template:", err)
    }
}

Answer 1

https://golang.org/pkg/text/template/#hdr-变量:

变量的范围扩展到声明它的控制结构("if","with"或"range")的"end"动作,或者如果没有这样的控制结构则延伸到模板的末尾.

所以$prev_year你定义{{$prev_year:=$year}}只有生命直到..下一行({{end}}).

似乎没有办法解决这个问题.

执行此操作的"正确"方法是从模板中取出该逻辑,并在Go代码中进行分组.

这是一个工作示例:https://play.golang.org/p/DZoSXo9WQR

package main

import (
    "fmt"
    "os"
    "text/template"
    "time"
)

type Tournament struct {
    Place string
    Date  time.Time
}

type TournamentGroup struct {
    Year        int
    Tournaments []Tournament
}

func groupTournamentsByYear(tournaments []Tournament) []TournamentGroup {
    if len(tournaments) == 0 {
        return nil
    }

    result := []TournamentGroup{
        {
            Year:        tournaments[0].Date.Year(),
            Tournaments: make([]Tournament, 0, 1),
        },
    }

    i := 0
    for _, tournament := range tournaments {
        year := tournament.Date.Year()
        if result[i].Year == year {
            // Add to existing group
            result[i].Tournaments = append(result[i].Tournaments, tournament)
        } else {
            // New group
            result = append(result, TournamentGroup{
                Year: year,
                Tournaments: []Tournament{
                    tournament,
                },
            })
            i++
        }
    }

    return result
}

func main() {
    tournaments := []Tournament{
        // for clarity - date is sorted, we don't need sort it again
        {"Town1", time.Date(2015, time.November, 10, 23, 0, 0, 0, time.Local)},
        {"Town2", time.Date(2015, time.October, 10, 23, 0, 0, 0, time.Local)},
        {"Town3", time.Date(2014, time.November, 10, 23, 0, 0, 0, time.Local)},
    }

    t, err := template.New("").Parse(`
{{$prev_year:=0}}
{{range .}}
    Actions in year {{.Year}}:
    {{range .Tournaments}}

            {{.Place}}, {{.Date}}
    {{end}}
    {{end}}

    `)
    if err != nil {
        panic(err)
    }
    err = t.Execute(os.Stdout, groupTournamentsByYear(tournaments))
    if err != nil {
        fmt.Println("executing template:", err)
    }
}

Answer 2

正如this answer所述，该变量“重新分配”的范围以{{end}}块结束。因此，仅使用标准变量无法解决问题，应该在执行模板的 Go 程序中解决。

然而，在某些框架中，这并不容易（例如 protoc-gen-gotemplate）。

该小枝库增加了额外的功能，以标准的模板语言。其中之一是可变映射，可以通过以下方式使用：

// init the dictionary (you can init it without initial key/values as well)
{{$myVar := dict "key" "value"}}

// getting the "key" from the dictionary (returns array) and then fetching the first element from that array
{{pluck "key" $myVar | first}}

// conditional update block
{{if eq "some" "some"}}
     // the $_ seems necessary because Go template functions need to return something
     {{$_ := set $myVar "key" "newValue"}}
{{end}}

// print out the updated value
{{pluck "key" $myVar | first}}

这个小例子打印出来：

value
newValue

一种实用的方法是对所有可变变量使用单个字典，并将它们作为键存储在相应的变量名下。

参考：

• http://masterminds.github.io/sprig/dicts.html
• https://github.com/Masterminds/sprig

Answer 3

在go1.11中，text / template和html / template可以设置现有变量的值，这意味着可以对原始代码进行很小的修改。

更改

{{$prev_year:=$year}}

至

{{$prev_year = $year}}

操场