制作了一个通过multipart/x-mixed-replace Content-Type标题将PNG图像流式传输到浏览器的程序后，我注意到标签中仅显示最后一帧<img>，而不是最近发送的帧。
这种行为非常烦人，因为我只在图像更改以节省带宽时才发送更新，这意味着在我等待更新时屏幕上会出现错误的帧。
具体来说，我使用的是 Brave 浏览器（基于 chromium），但是当我尝试上下使用“盾牌”时，我认为这个问题至少也会出现在其他基于 chromium 的浏览器中。
搜索问题只产生一个相关结果（以及许多不相关的结果），即这个HowToForge 线程，没有回复。同样，我也认为问题与缓冲有关，但我确保刷新缓冲区无济于事，这与线程中的用户非常相似。用户确实报告说它可以在他们的一个服务器上运行，而不是在另一个服务器上运行，这让我相信它可能与特定的 HTTP 标头或类似的东西有关。我的第一个猜测是Content-Length因为浏览器可以判断图像何时完成，但它似乎没有任何效果。
所以本质上，我的问题是：有没有办法告诉浏览器显示最新的multipart/x-mixed-replace而不是以前的？而且，如果这不是标准行为，原因可能是什么？
当然，这是相关的源代码，尽管我认为这更像是一个一般的 HTTP 问题，而不是与代码有关的问题：
服务器
package routes
import (
"crypto/md5"
"fmt"
"image/color"
"net/http"
"time"
brain "path/to/image/generator/module"
)
func init() {
RouteHandler{
function: func(w http.ResponseWriter, r *http.Request) {
w.Header().Set("Content-Type", "multipart/x-mixed-replace; boundary=frame")
w.Header().Set("Cache-Control", "no-cache") // <- Just in case
w.WriteHeader(200)
// If the request contains a token and the token maps to a valid "brain", start consuming frames from
// the brain and returning them to the client
params := r.URL.Query()
if val, ok := params["token"]; ok && len(val) > 0 {
if b, ok := SharedMemory["brains"].(map[string]*brain.Brain)[val[0]]; ok && !b.CheckHasExit() {
// Keep a checksum of the previous frame to avoid sending frames which haven't changed. Frames cannot
// be compared directly (at least efficiently) as they are slices not arrays
previousFrameChecksum := [16]byte{}
for {
if !b.CheckHasExit() {
frame, err := b.GetNextFrame(SharedMemory["conf"].(map[string]interface{})["DISPLAY_COL"].(color.Color))
if err == nil && md5.Sum(frame) != previousFrameChecksum {
// Only write the frame if we succesfully read it and it's different to the previous
_, err = w.Write([]byte(fmt.Sprintf("--frame\r\nContent-Type: image/png\r\nContent-Size: %d\r\n\r\n%s\r\n", len(frame), frame)))
if err != nil {
// The client most likely disconnected, so we should end the stream. As the brain still exists, the
// user can re-connect at any time
return
}
// Update the checksum to this frame
previousFrameChecksum = md5.Sum(frame)
// If possible, flush the buffer to make sure the frame is sent ASAP
if flusher, ok := w.(http.Flusher); ok {
flusher.Flush()
}
}
// Limit the framerate to reduce CPU usage
<-time.After(time.Duration(SharedMemory["conf"].(map[string]interface{})["FPS_LIMITER_INTERVAL"].(int)) * time.Millisecond)
} else {
// The brain has exit so there is no more we can do - we are braindead :P
return
}
}
}
}
},
}.Register("/stream", "/stream.png")
}
客户端（start()在正文中运行onload）
function start() {
// Fetch the token from local storage. If it's empty, the server will automatically create a new one
var token = localStorage.getItem("token");
// Create a session with the server
http = new XMLHttpRequest();
http.open("GET", "/startsession?token="+(token)+"&w="+(parent.innerWidth)+"&h="+(parent.innerHeight));
http.send();
http.onreadystatechange = (e) => {
if (http.readyState === 4 && http.status === 200) {
// Save the returned token
token = http.responseText;
localStorage.setItem("token", token);
// Create screen
var img = document.createElement("img");
img.alt = "main display";
// Hide the loader when it loads
img.onload = function() {
var loader = document.getElementById("loader");
loader.remove();
}
// Start loading
img.src = "/stream.png?token="+token;
// Start capturing keystrokes
document.onkeydown = function(e) {
// Send the keypress to the server as a command (ignore the response)
cmdsend = new XMLHttpRequest();
cmdsend.open("POST", "/cmd?token="+(token));
cmdsend.send("keypress:"+e.code);
// Catch special cases
if (e.code === "Escape") {
// Clear local storage to remove leftover token
localStorage.clear();
// Remove keypress handler
document.onkeydown = function(e) {}
// Notify the user
alert("Session ended succesfully and the screen is inactive. You may now close this tab.");
}
// Cancel whatever it is the keypress normally does
return false;
}
// Add screen to body
document.getElementById("body").appendChild(img);
} else if (http.readyState === 4) {
alert("Error while starting the session: "+http.responseText);
}
}
}