充分利用异步回调(JavaScript 应用程序中实现数据源协调)

function getResource(uri, data_callback, error_callback, timeout) {
var tryAgain = function () {
getResource(uri, data_callback, error_callback, timeout);
}
var r = new XMLHttpRequest();
var timer = setTimeout(
function() {
r.abort();
r.onreadystatechange = null;
setTimeout(tryAgain, timeout);
},
timeout);
r.open("GET", uri, true);
r.onreadystatechange = function() {
if (r.readyState != 4) {
// Ignore non-loaded readyStates
// ...will timeout if do not get to "Loaded"
return;
}
clearTimeout(timer);  // readyState==4, no more timer
if (r.status==200) {  // "OK status"
data_callback(r.responseText);
}
else if (r.status==304) {
// "Not Modified": No change to display
}
else if (r.status >= 400 && r.status < 500) {
// Client error, probably bad URI
error_callback(r)
}
else if (r.status >= 500 && r.status < 600) {
// Server error, try again after delay
setTimeout(tryAgain, timeout);
}
else {
error_callback(r);
}
}
r.send(null);
return r;
}

var other_data = null;
function processOtherData(responseText) {
other_data = responseText;
}
function processData(this_data) {
var delay = 1000;     // Keep trying at 1-second intervals
if (other_data == null) {
setTimeout(function() { processData(this_data); }, delay);
return;
}
// We have both this_data and other_data
displayThisAndThat(this_data, other_data);
// Reset other_data now that we have consumed it
other_data = null;
}