Why does my JavaScript code fire prematurely?

Question

I have an event listener added in onDeviceReady to listen for back button event, and respond with stopWatch when it is pressed. However it responds when the app is started.

    <!DOCTYPE html>
<html>
  <head>
    <title>UNH BSApp</title>

    <script type="text/javascript" charset="utf-8" src="phonegap.js"></script>
    <script type="text/javascript" charset="utf-8">

    // The watch id references the current `watchAcceleration`
    var watchID = null;

    // Wait for PhoneGap to load
    //
    document.addEventListener("deviceready", onDeviceReady, false);

    // PhoneGap is ready
    //
    function onDeviceReady() {
        document.addEventListener("backbutton", stopWatch(), false);
        startWatch();
    }

    // Start watching the acceleration
    //
    function startWatch() {
        document.addEventListener("menubutton", stopWatch(), false);
        // Update acceleration every 0.1 seconds
        var options = { frequency: 10 };

        watchID = navigator.accelerometer.watchAcceleration(onSuccess, onError, options);
    }

    // Stop watching the acceleration
    //
    function stopWatch() {
        alert("Hello!");
        if (watchID) {
            navigator.accelerometer.clearWatch(watchID);
            watchID = null;
        }
    }

    // onSuccess: Get a snapshot of the current acceleration
    //
    function onSuccess(acceleration) {
        var element = document.getElementById('accelerometer');
        element.innerHTML = 'Acceleration X: ' + acceleration.x + '<br />' +
                            'Acceleration Y: ' + acceleration.y + '<br />' +
                            'Acceleration Z: ' + acceleration.z + '<br />' +
                            'Timestamp: '      + acceleration.timestamp + '<br />';
    }

    // onError: Failed to get the acceleration
    //
    function onError() {
        alert('onError!');
    }

    </script>
    <style>
        #start {
            display:block;
            border:solid;
        }
    </style>
  </head>
  <body>
    <div id="accelerometer">Waiting for accelerometer...</div>
    <div id="start">Start</div>
  </body>
</html>

Answer 1

 document.addEventListener("menubutton", stopWatch(), false);

This calls the stopWatch function immediately. You want something like:

 document.addEventListener("menubutton", stopWatch, false);
                                                 ^^ no parens!

Answer 2

从侦听器的设置中删除() -这是在调用函数而不将其作为参数传递：

document.addEventListener("menubutton", stopWatch, false);

Why does my JavaScript code fire prematurely?

Question

2 answers

solution1
4 ACCPTED 2012-02-22 19:07:55

solution2
0 2012-02-22 19:07:33

Why does my JavaScript code fire prematurely?

Question

2 answers

solution1 4 ACCPTED 2012-02-22 19:07:55

solution2 0 2012-02-22 19:07:33

solution1
4 ACCPTED 2012-02-22 19:07:55

solution2
0 2012-02-22 19:07:33