Activating OnBeforeUnload ONLY when field values have changed

问题

What I'm trying to achieve is to Warn the user of unsaved changes if he/she tries to close a page or navigate away from it without saving first.

I've managed to get the OnBeforeUnload() dialog to pop-up... but I don't want it to be displayed at all if the user hasn't modified any field values. For this, I'm using this hidden input field called is_modified that starts with a default value of false and flips to true when any field is edited.

I tried to bind the change event to this is_modified field to try and detect for value change... and only then activate OnBeforeUnload.

$( '#is_modified' ).change( function() {
    if( $( '#is_modified' ).val() == 'true' )
        window.onbeforeunload = function() { return "You have unsaved changes."; }
});

But from what I figure is that the change() event works only after these 3 steps - a field receives focus, a value is changed and the field looses focus. In case of the hidden input field, I'm not sure how this receiving and loosing focus part works! Hence, the onbeforeunload function is never being activated.

Can anyone suggest a way to maintain a trigger over is_modified?

Thanks.

回答1:

I had a similar requirement so came up with following jQuery script:

$(document).ready(function() {
    needToConfirm = false; 
    window.onbeforeunload = askConfirm;
});

function askConfirm() {
    if (needToConfirm) {
        // Put your custom message here 
        return "Your unsaved data will be lost."; 
    }
}

$("select,input,textarea").change(function() {
    needToConfirm = true;
});

The above code checks the needToConfirm variable, if its true then it will display warning message. Whenever input, select or textarea elements value is changed, needToConfirm variable is set to true.

PS: Firefox > 4 don't allow custom message for onbeforeunload.
Reference: https://bugzilla.mozilla.org/show_bug.cgi?id=588292

UPDATE: If you are a performance freak, you will love @KyleMit's suggestion. He wrote a jQuery extension only() which will be executed only once for any element.

$.fn.only = function (events, callback) {
    //The handler is executed at most once for all elements for all event types.
    var $this = $(this).on(events, myCallback);
    function myCallback(e) {
        $this.off(events, myCallback);
        callback.call(this, e);
    }
    return this
};    

$(":input").only('change', function() {
    needToConfirm = true;
});

回答2:

We just use Window.onbeforeunload as our "changed" flag. Here's what we're doing, (using lowpro):

Event.addBehavior({
  "input[type=radio]:change,input[type=text]:change,input[type=checkbox]:change,select:change": function(ev) {
       window.onbeforeunload = confirmLeave;
  }
  ".button.submit-button:click": function(ev) {
       window.onbeforeunload = null;
  },
});


function confirmLeave(){
    return "Changes to this form have not been saved. If you leave, your changes will be lost."  
}

回答3:

The following works well in jQuery:

var needToConfirm = false;

$("input,textarea").on("input", function() {
  needToConfirm = true;
});

$("select").change(function() {
  needToConfirm = true;
});

window.onbeforeunload = function(){        
  if(needToConfirm) {
    return "If you exit this page, your unsaved changes will be lost.";        
  }
}

And if the user is submitting a form to save the changes, you might want to add this (change #mainForm to the ID of the form they're submitting):

$("#mainForm").submit(function() {
  needToConfirm = false;
});

回答4:

$(window).bind('beforeunload',function() {
 return "'Are you sure you want to leave the page. All data will be lost!";
});

$('#a_exit').live('click',function() {
    $(window).unbind('beforeunload');
});

Above works For me.

回答5:

Try your logic in a different manner. Meaning, put the logic for checking the value of the input field in your onbeforeunload method.

window.onbeforeunload = function () { 
    if ($("#is_modified").val() == 'true') { 
        return "You have unsaved changes."; 
    } else { 
        return true; // I think true is the proper value here 
    } 
};