jQuery div向下滚动，向上滚动

Question

This lets me slide the #contact div in, but when I scroll up it doesn't slide back out.

$(window).scroll(function(){
    if ($(this).scrollTop() > 100) {
        $('#scrolltop').fadeIn();
        $('#contact').animate({
            right: "0px"
        }, 500 );
    } else {
        $('#scrolltop').fadeOut();
        $('#contact').animate({
           right: "-115px"
        }, 500 );
    }
});

Answer 1

The scroll event is fired many times when a user scrolls, and with your code, the animate function is called many times in quick succession which seems to be causing problems. I would recommend adding a flag to determine if you have already called animate. This code worked for me:

var animated = false;
$(window).scroll(function(){
if ($(this).scrollTop() > 100) {
    if(!animated){
        $('#scrolltop').fadeIn();
        $('#contact').animate({
            left: 0
        }, 500 );
        animated = true;
    }
} else if(animated){
    $('#scrolltop').fadeOut();
    $('#contact').animate({
       left: -115
    }, 500 );
    animated = false;
}

EDIT:

To solve the problem that multiple animate calls are made repeatedly when the user scrolls up and down quickly, I would additionally keep track of whether the element is currently animating, like this:

    var animated = false;
    var animating = false;
    $(window).scroll(scroll);

    function scroll(){
        if(!animating) {
            if ($(document).scrollTop() > 100) {
                    if(!animated){
                    animating = true;
                        $('#scrolltop').fadeIn();
                        $('#contact').animate({
                                left: 0
                        }, {"duration":500,"complete":complete});
                        animated = true;
                    }
            } else if(animated){
                animating = true;
                    $('#scrolltop').fadeOut();
                    $('#contact').animate({
                        left: -115
                    }, {"duration":500,"complete":complete} );
                    animated = false;
            }
        }
    }

    function complete(){
        animating = false;
        scroll();
    }

In this code, animated shows whether the element has slid in or out of the screen, whereas animating shows whether an animation (either in or out) is currently being used. I would recommend doing this rather than try and use timeouts.