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add a final section with a complete example

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Rob Pike 2009-11-04 17:29:20 -08:00
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@ -2360,6 +2360,137 @@ for try := 0; try < 2; try++ {
}
</pre>
<h2 id="web_server">A web server</h2>
<p>
Let's finish with a complete Go program, a web server.
This one is actually a kind of web re-server.
Google provides a service at
<a href="http://chart.apis.google.com">http://chart.apis.google.com</a>
that does automatic formatting of data into charts and graphs.
It's hard to use interactively, though,
because you need to put the data into the URL as a query.
The program here provides a nicer interface to one form of data: given a short piece of text,
it calls on the chart server to produce a QR code, a matrix of boxes that encode the
text.
That image can be grabbed with your cell phone's camera and interpreted as,
for instance, a URL, saving you typing the URL into the phone's tiny keyboard.
</p>
<p>
Here's the complete program.
An explanation follows.
</p>
<pre>
package main
import (
"flag";
"http";
"io";
"log";
"strings";
"template";
)
var addr = flag.String("addr", ":1718", "http service address") // Q = 17, R = 18
var fmap = template.FormatterMap{
"html": template.HtmlFormatter,
"url+html": UrlHtmlFormatter,
}
var templ = template.MustParse(templateStr, fmap)
func main() {
flag.Parse();
http.Handle("/", http.HandlerFunc(QR));
err := http.ListenAndServe(*addr, nil);
if err != nil {
log.Exit("ListenAndServe:", err);
}
}
func QR(c *http.Conn, req *http.Request) {
templ.Execute(req.FormValue("s"), c);
}
func UrlHtmlFormatter(w io.Writer, v interface{}, fmt string) {
template.HtmlEscape(w, strings.Bytes(http.URLEscape(v.(string))));
}
const templateStr = `
&lt;html&gt;
&lt;head&gt;
&lt;title&gt;QR Link Generator&lt;/title&gt;
&lt;/head&gt;
&lt;body&gt;
{.section @}
&lt;img src="http://chart.apis.google.com/chart?chs=300x300&amp;cht=qr&amp;choe=UTF-8&amp;chl={@|url+html}"
/&gt;
&lt;br&gt;
{@|html}
&lt;br&gt;
&lt;br&gt;
{.end}
&lt;form action="/" name=f method="GET"&gt;&lt;input maxLength=1024 size=70
name=s value="" title="Text to QR Encode"&gt;&lt;input type=submit
value="Show QR" name=qr&gt;
&lt;/form&gt;
&lt;/body&gt;
&lt;/html&gt;
`
</pre>
<p>
The pieces up to <code>main</code> should be easy to follow.
The one flag sets a default HTTP port for our server. The template
variable <code>templ</code> is where the fun happens. It builds an HTML template
that will be executed by the server to display the page; more about
that in a moment.
</p>
<p>
The <code>main</code> function parses the flags and, using the mechanism
we talked about above, binds the function <code>QR</code> to the root path
for the server. Then <code>http.ListenAndServe</code> is called to start the
server; it blocks while the server runs.
</p>
<p>
<code>QR</code> just receives the request, which contains form data, and
executes the template on the data in the field named <code>s</code>.
</p>
<p>
The template package, inspired by <a
href="http://code.google.com/p/json-template">json-template</a>, is
powerful;
this program just touches on its capabilities.
In essence, it rewrites a piece of text on the fly by substituting elements derived
from data items passed to <code>templ.Execute</code>, in this case the
string in the form data.
Within the template text (<code>templateStr</code>),
brace-delimited pieces denote template actions.
The piece from the <code>{.section @}</code>
to <code>{.end}</code> executes with the value of the data item <code>@</code>,
which is a shorthand for &ldquo;the current item&rdquo;, in this case the form data.
(When the string is empty, this piece of the template is suppressed.)
</p>
<p>
The snippet <code>{@|url+html}</code> says to run the data through the formatter
installed in the formatter map (<code>fmap</code>)
under the name <code>"url+html"</code>.
That is the function <code>UrlHtmlFormatter</code>, which sanitizes the string
for safe display on the web page.
</p>
<p>
The rest of the template string is just the HTML to show when the page loads.
If this is too quick an explanation, see the <a href="/pkg/template/">documentation</a>
for the template package for a more thorough discussion.
</p>
<p>
And there you have it: a useful webserver in a few lines of code plus some
data-driven HTML text.
Go is powerful enough to make a lot happen in a few lines.
</p>
<!--
TODO
<pre>