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Go - Using Templates for Go Web Applications

Problem: You want to use Go’s templating system to create a web application.


Solution: Use the html/template package to create a web application.

 

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package main

import (
    "html/template"
    "net/http"
)

func hello(w http.ResponseWriter, r *http.Request) {
    t, _ := template.ParseFiles("hello.html")
    t.Execute(w, "Hello World!")
}

func main() {
    http.HandleFunc("/", hello)
    http.ListenAndServe(":8000", nil)
}
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The only difference from the earlier web application is that you’re using the html/template package to parse the template file into a template , which is an instance of template.Template . You then call the Execute method on the template, passing it the data to be rendered. 

In the template file hello.html , you can use the {{.}} syntax to render the data:

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<!DOCTYPE html>
<html>
    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
        <title>Go Cookbook</title>
    </head>
    <body>
        <h1>{{ . }}</h1>
    </body>
</html>
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The dot (.) between the double braces is an action , and it’s a command for the Go template engine to replace it with a value when the template is executed. In this case, the value is the string “Hello World!”

Actions are a critical part of the template engine. They are used to control the flow of the template, and they can also be used to perform operations on the data in the template. 

Some of the more commonly used actions are:
• The most important commonly used action is the dot {{.}} action. It’s used to render the data passed to the template.
• The {{range}} action is used to iterate over a slice or a map.
• The {{if}} , {{else}} action is used to perform conditional checks.
The dot action represents the data passed to the template. It can be a string or an integer, any primitive type, or it can be a struct, slice, or map. With a struct, you can access the fields using the dot action. 

Take this struct:

type Person struct {
    Name      string
    Gender    string
    Homeworld string
}

Send this struct to the template from the handler:

func person(w http.ResponseWriter, r *http.Request) {
    t, _ := template.ParseFiles("person.html")
    t.Execute(w, Person{
        Name:      "Luke Skywalker",
        Gender:    "male",
        Homeworld: "Tatooine",
    })
}

In the template file, you can access the fields using the dot action:

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<!DOCTYPE html>
<html>
    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
        <title>Go Cookbook</title>
    </head>
    <body>
        <h1>{{ .Name }}</h1>
        <ul>
            <li><b>Gender: </b>{{ .Gender }}</li>
            <li><b>Home world: </b>{{ .Homeworld }}</li>
        </ul>
    </body>
</html>
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For actions that wrap around a section of the template, the opening action is followed by an {{end}} action. For example, the {{range}} action is followed by an {{end}} action. 

Within these wrapped-around sections, you can use the dot action to render the data. For example, in the {{range}} action, you can use the dot action to render the data in the slice or map. 

Here’s how this works:

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func people(w http.ResponseWriter, r *http.Request) {
    t, _ := template.ParseFiles("people.html")
    t.Execute(w, []string{"Luke", "Leia", "Han", "Chewbacca"})
}

func main() {
    http.HandleFunc("/people", people)
    http.ListenAndServe(":8000", nil)
}
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As you can see, you are passing to the people.html template a slice of names. In the template file, you can use the {{range}} action to iterate over the slice and render the data:

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<!DOCTYPE html>
<html>
    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
        <title>Go Cookbook</title>
    </head>
    <body>
        <div>This is a list of people in Start Wars:</div>
        <ul>
            {{ range . }}
            <li>{{ . }}</li>
            {{ end }}
        </ul>
    </body>
</html>
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You might have noticed that you are ranging through the dot (.), but within the {{range}} action, you are using the dot action again. The dot action within the range refers to the item in the slice. This is equivalent to:

    for _, item := range people { //  people  is  the  dot  for  the  range
        item //  item  is  the  dot  within  the  range
    }

The {{if}} , {{else}} action is used to perform conditional checks. For example, you can use it to check if the slice is empty.

Send an empty slice to the people.html template:

func people(w http.ResponseWriter, r *http.Request) {
    t, _ := template.ParseFiles("people.html")
    t.Execute(w, []string{})
}

Now you can use the {{if}} , {{else}} action to check if the slice is empty:

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<!DOCTYPE html>
<html>
    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
        <title>Go Cookbook</title>
    </head>
    <body>
    {{ if gt (len .) 0 }}
        <div>This is a list of people in Start Wars:</div>
        <ul>
            {{ range . }}
            <li>{{ . }}</li>
            {{ end }}
        </ul>
    {{ else }}
        <div>There are no people in this list.</div>
    {{ end }}
    </body>
</html>
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So far, you’ve been ignoring the error that’s returned along with the parsed template. The usual Go practice is to handle the error, but Go provides another mechanism to handle errors returned by parsing templates:

t := template.Must(template.ParseFiles("people.html"))

The Must function wraps around a function that returns a pointer to a template and an error and panics if the error is not a nil .

 

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