How can I use AJAX to fetch data and store it in javascript variables?

问题

This is a sample dynamically updated chart: http://www.highcharts.com/demo/dynamic-update

The chart is updated every second with the the date as the x value and a random number as the y value.

load: function() {            
    // set up the updating of the chart each second
    var series = this.series[0];
    setInterval(function() {
        var x = (new Date()).getTime(), // current time
            y = Math.random();
        series.addPoint([x, y], true, true);
    }, 1000);
}

How would I rewrite load to fetch x and y from another webpage using AJAX, rather than generating the values within the function?

回答1:

I take it what you want is the sample load method but with the lines that set x and y replaced with an AJAX call. You'll need to perform a fairly basic continuation-passing code transformation, turning the code after the point you want the asynchronous call into a function that's passed to the asynchronous call. "Continuation" simply means "the rest of the calculation, from a given point forward". In the code sample, that's the call to series.addPoint. The pattern for this is you transform:

function f(...) {
    (1)
    result = sync(...);
    (2)
}

into:

function f(...) {
    (1)
    async(..., function (result) {
          (2)
      });
}

If (2) returned a value in the original function, then the call to f must also be rewritten using continuation passing style, adding an extra parameter to hold the continuation.

The other thing you should do is make sure the PHP script outputs the numeric pair as valid JSON, either as an array of two numbers (which can be passed directly to the series.addPoint call after parsing), or as an object with properties "x" and "y".

Note that, due to the nature of network communication, the data may not arrive in a timely manner, resulting in a graph with irregular intervals.

Here's the gist, wrapping up the nuts-and-bolts of the asynchronous call into a function named ajaj. The sample assumes the browser supports the necessary standards for XMLHttpRequest and the JSON parser.

/* Asynchronous Javascript And Json */
function ajaj(url, succeed, fail) {
    if (! fail) {
        fail = function() {};
    }
    var xhr = new XMLHttpRequest;
    xhr.open('GET', url);
    xhr.onreadystatechange = function() {
        if (xhr.readyState==4) {
            if (200 <= xhr.status && xhr.status < 300) {
                succeed(JSON.parse(xhr.responseText));
            } else {
                // error
                fail(xhr.status, xhr.statusText, xhr.responseText);
            }
        }
    };
    xhr.send();
    return xhr;
}

...

    url: '...',

    load: function() {
        // ensure only one data load interval is running for this object
        if (this.updateInterval) {
            return;
        }
        // set up the updating of the chart each second
        var series = this.series[0];

        this.updateInterval = setInterval(function() {
            ajaj(this.url, 
                 function(point) { // success
                     series.addPoint(point, true, true);
                 },
                 function(status, statusText, response) { // failure
                     ...
                 }
            );
        }, 1000);
    }

JS libraries provide their own version of ajaj, often with more features. If you're doing anything of any complexity for a production site, look into adopting one. If you're using jQuery (as the tag suggests), you can, for example, use jQuery.get:

    load: function() {
        if (this.updateInterval) {
            return;
        }
        // set up the updating of the chart each second
        var series = this.series[0];

        this.updateInterval = setInterval(function() {
            jQuery.get(this.url, 
                 function(point, textStatus, jqXHR) { // success
                     series.addPoint(point, true, true);
                 }, 'json'
            );
        }, 1000);
    }

The server side of things is dead simple. time() returns a Unix timestamp, rand() returns a (not very) pseudorandom number (though good enough for a demo), and json_encode(), well, you can figure that out.

<?php
header('Content-type: application/json');

echo json_encode(
        array(
            time(),
            rand() / getrandmax(),
        ));

回答2:

I think you want a recursive call to setTimeout:

function update(series){
    var x = (new Date()).getTime(), // current time
        y = Math.random();
    series.addPoint([x, y], true, true);
    setTimeout(function() { update(series); }, 1000);
}

And then:

load: function() {            
    var series = this.series[0];
    update(series);
}

Or better yet, try something like this:

function update(){
    var series = yourChart.series[0];
    var x = (new Date()).getTime(), // current time
        y = Math.random();
    series.addPoint([x, y], true, true);
    setTimeout(update, 1000);
}

And then: