我有一个允许用户下载动态生成文件的页面。生成它需要很长时间，所以我想显示一个“等待”指示器。问题是，我不知道如何检测浏览器何时接收到文件，以便我可以隐藏指示器。
我请求一个隐藏的表单，它post到服务器，并以一个隐藏的iframe作为其结果的目标。这样我就不会用结果替换整个浏览器窗口。我监听iframe上的“load”事件，希望它在下载完成时触发。
我将文件返回一个“Content-Disposition: attachment”头，这将导致浏览器显示“Save”对话框。但是浏览器不会在iframe中触发“load”事件。
我尝试过的一种方法是采用多部分回答。所以它会发送一个空的HTML文件，以及附加的可下载文件。
例如:
Content-type: multipart/x-mixed-replace;boundary="abcde"
--abcde
Content-type: text/html
--abcde
Content-type: application/vnd.fdf
Content-Disposition: attachment; filename=foo.fdf
file-content
--abcde
这在Firefox中是有效的;它接收空HTML文件，触发“load”事件，然后显示可下载文件的“Save”对话框。但它在ie和Safari浏览器上无法运行;Internet Explorer触发“load”事件，但它不下载文件，而Safari下载文件(带有错误的名称和内容类型)，但不触发“load”事件。
另一种方法可能是调用启动文件创建，轮询服务器直到准备就绪，然后下载已经创建的文件。但我宁愿避免在服务器上创建临时文件。
我该怎么办?
这个Java/Spring示例检测下载的结束，在这一点上它隐藏了“Loading…”指示符。
方法:在JavaScript方面，设置一个最大过期时间为2分钟的cookie，并每秒钟轮询一次cookie过期时间。然后服务器端用更早的过期时间覆盖这个cookie——服务器进程的完成。一旦在JavaScript轮询中检测到cookie过期，“Loading…”就会被隐藏。
JavaScript的一面
function buttonClick() { // Suppose this is the handler for the button that starts
$("#loadingProgressOverlay").show(); // Show loading animation
startDownloadChecker("loadingProgressOverlay", 120);
// Here you launch the download URL...
window.location.href = "myapp.com/myapp/download";
}
// This JavaScript function detects the end of a download.
// It does timed polling for a non-expired Cookie, initially set on the
// client-side with a default max age of 2 min.,
// but then overridden on the server-side with an *earlier* expiration age
// (the completion of the server operation) and sent in the response.
// Either the JavaScript timer detects the expired cookie earlier than 2 min.
// (coming from the server), or the initial JavaScript-created cookie expires after 2 min.
function startDownloadChecker(imageId, timeout) {
var cookieName = "ServerProcessCompleteChecker"; // Name of the cookie which is set and later overridden on the server
var downloadTimer = 0; // Reference to the timer object
// The cookie is initially set on the client-side with a specified default timeout age (2 min. in our application)
// It will be overridden on the server side with a new (earlier) expiration age (the completion of the server operation),
// or auto-expire after 2 min.
setCookie(cookieName, 0, timeout);
// Set a timer to check for the cookie every second
downloadTimer = window.setInterval(function () {
var cookie = getCookie(cookieName);
// If cookie expired (NOTE: this is equivalent to cookie "doesn't exist"), then clear "Loading..." and stop polling
if ((typeof cookie === 'undefined')) {
$("#" + imageId).hide();
window.clearInterval(downloadTimer);
}
}, 1000); // Every second
}
// These are helper JavaScript functions for setting and retrieving a Cookie
function setCookie(name, value, expiresInSeconds) {
var exdate = new Date();
exdate.setTime(exdate.getTime() + expiresInSeconds * 1000);
var c_value = escape(value) + ((expiresInSeconds == null) ? "" : "; expires=" + exdate.toUTCString());
document.cookie = name + "=" + c_value + '; path=/';
}
function getCookie(name) {
var parts = document.cookie.split(name + "=");
if (parts.length == 2 ) {
return parts.pop().split(";").shift();
}
}
Java/Spring服务器端
@RequestMapping("/download")
public String download(HttpServletRequest request, HttpServletResponse response) throws Exception {
//... Some logic for downloading, returning a result ...
// Create a Cookie that will override the JavaScript-created
// Max-Age-2min Cookie with an earlier expiration (same name)
Cookie myCookie = new Cookie("ServerProcessCompleteChecker", "-1");
myCookie.setMaxAge(0); // This is immediate expiration, but can also
// add +3 seconds for any flushing concerns
myCookie.setPath("/");
response.addCookie(myCookie);
//... -- presumably the download is writing to the Output Stream...
return null;
}
