Golang : Take screen shot of browser with JQuery example

In this tutorial, we will learn how to do a screen capture of a client(browser) and send the capture screen shot back to the web server to be saved as a PNG file and download into the client's computer afterward(need user permission).

This tutorial uses JQuery & JavaScript HTML renderer - html2canvas to capture the browser content either by targeting certain <div> or entire page and pass the base64 data back to Golang web server.

This feature can be useful in situation where you want download a page screen shot at certain interval or use in tech support(with permission from client).

Here you go!

 package main

 import (
 "bytes"
 "encoding/base64"
 "fmt"
 "image"
 "image/png"
 "io/ioutil"
 "net/http"
 "os"
 )

 func SaveFile(w http.ResponseWriter, r *http.Request) {

 if r.Method == "POST" {

 imgBase64 := r.FormValue("data")

 // remove "data:image/png;base64,"
 imgBase64cleaned := imgBase64[len("data:image/png;base64,"):len(imgBase64)]

 // decode base64 to buffer bytes
 imgBytes, _ := base64.StdEncoding.DecodeString(imgBase64cleaned)

 // convert []byte to image for saving to file
 img, _, _ := image.Decode(bytes.NewReader(imgBytes))

 imgFile, err := os.Create("./screen-capture.png")
 if err != nil {
 panic(err)
 }

 // save to file on your webserver
 png.Encode(imgFile, img)
 fmt.Println("screen-capture.png file saved")

 }

 // NOTE : For some odd reason this part has to be outside the r.Method == "POST"
 // in order for the streaming/download straight to browser to work

 streamBytes, _ := ioutil.ReadFile("./screen-capture.png")

 //output to browser
 w.Header().Set("Content-Disposition", "attachment; filename='screen-capture.png'")
 w.Header().Set("Content-Type", "image/png")
 w.Header().Set("Content-Transfer-Encoding", "binary")
 w.Header().Set("Content-Description", "File Transfer")

 //w.Header().Set("Content-Length", string(len(streamBytes)))
 // will produce warning message : http: invalid Content-Length of "뚴"

 // ok, pump it out to the browser!
 b := bytes.NewBuffer(streamBytes)
 if _, err := b.WriteTo(w); err != nil {
 fmt.Fprintf(w, "%s", err)
 }

 }

 func ScreenCapture(w http.ResponseWriter, r *http.Request) {
 html := `<html><body>
 <script type="text/javascript" src="https://code.jquery.com/jquery-1.11.3.js"></script>
 <script type="text/javascript" src="https://socketloop.com/public/tutorial/html2canvas.js"></script>

 <style type="text/css">
 #buttons-area
 {
 margin:30px auto;
 padding:5px;
 border:solid #cdcdcd 1px;
 width:700px;
 text-color:white;
 background:#f4f4f4;
 }

 #target-area
 {
 margin:50px auto;
 padding:15px;
 border:solid #cdcdcd 1px;
 width:700px;
 background:blue;
 }
 button
 {
 text-align:center;
 width:48%;
 height:80px;
 border:1px solid #c3d825;
 font-family:Verdana, Geneva, sans-serif;
 border-radius:3px;
 background:#c3d825;
 color:#355105;
 text-shadow: 0 1pm #ecffba;
 font-weight:bolder;
 font-size:18px;
 border-radius:10px;
 }
 </style>

 <div id="target-area"></div>
 <div id="buttons-area">
 <h1>Take a screenshot with JQuery and Golang example.</h1>
 <button type="button" onclick="captureDiv()">capture div and replace</button>
 <button type="button" onclick="capturePage()">capture entire page & save</button>
 </div>
 </body></html>
 <script type="text/javascript">
 function captureDiv() {
 html2canvas([document.getElementById('buttons-area')], {
 onrendered: function(canvas)
 {
 var imgBase64 = canvas.toDataURL() // already base64
 var img2html = '<img src=' + imgBase64 + '>'
 document.getElementById('target-area').innerHTML = img2html;
 }
 });
 }
 function capturePage() {
 html2canvas(document.body, {
 onrendered: function(canvas)
 {
 var imgBase64 = canvas.toDataURL() // already base64
 var img2html = '<img src=' + imgBase64 + '>'

 // too big to fit into target-area div
 // so, we save to file
 //document.getElementById('target-area').innerHTML = img2html;

 //post screen capture base64 image data to save function
 $.post("/save", {data: imgBase64}, function () {
 window.location.href = "save"});
 }
 });
 }
 </script>`

 w.Write([]byte(html))
 }

 func main() {
 mux := http.NewServeMux()
 mux.HandleFunc("/", ScreenCapture)
 mux.HandleFunc("/save", SaveFile)

 http.ListenAndServe(":8080", mux)
 }

Hope this helps and happy coding!

References:

https://github.com/niklasvh/html2canvas

https://www.socketloop.com/tutorials/golang-how-to-stream-file-to-client-browser-or-write-to-http-responsewriter