在800px 后滚动显示 div

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You've got a few things going on there. One, why a class? Do you actually have multiple of these on the page? The CSS suggests you can't. If not you should use an ID - it's faster to select both in CSS and jQuery:

<div id=bottomMenu>You read it all.</div>

Second you've got a few crazy things going on in that CSS - in particular the z-index is supposed to just be a number, not measured in pixels. It specifies what layer this tag is on, where each higher number is closer to the user (or put another way, on top of/occluding tags with lower z-indexes).

The animation you're trying to do is basically .fadeIn(), so just set the div to display: none; initially and use .fadeIn() to animate it:

$('#bottomMenu').fadeIn(2000);

.fadeIn() works by first doing display: (whatever the proper display property is for the tag), opacity: 0, then gradually ratcheting up the opacity.

Full working example:

http://jsfiddle.net/b9chris/sMyfT/

CSS:

#bottomMenu {
display: none;
position: fixed;
left: 0; bottom: 0;
width: 100%; height: 60px;
border-top: 1px solid #000;
background: #fff;
z-index: 1;
}

JS:

var $win = $(window);


function checkScroll() {
if ($win.scrollTop() > 100) {
$win.off('scroll', checkScroll);
$('#bottomMenu').fadeIn(2000);
}
}


$win.scroll(checkScroll);

小开

最佳答案

If you want to show a div after scrolling a number of pixels:

Working Example

$(document).scroll(function() {
var y = $(this).scrollTop();
if (y > 800) {
$('.bottomMenu').fadeIn();
} else {
$('.bottomMenu').fadeOut();
}
});

$(document).scroll(function() {
var y = $(this).scrollTop();
if (y > 800) {
$('.bottomMenu').fadeIn();
} else {
$('.bottomMenu').fadeOut();
}
});

body {
height: 1600px;
}
.bottomMenu {
display: none;
position: fixed;
bottom: 0;
width: 100%;
height: 60px;
border-top: 1px solid #000;
background: red;
z-index: 1;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<p>Scroll down... </p>
<div class="bottomMenu"></div>

Its simple, but effective.

Documentation for .scroll()
Documentation for .scrollTop()

If you want to show a div after scrolling a number of pixels,

without jQuery:

Working Example

myID = document.getElementById("myID");


var myScrollFunc = function() {
var y = window.scrollY;
if (y >= 800) {
myID.className = "bottomMenu show"
} else {
myID.className = "bottomMenu hide"
}
};


window.addEventListener("scroll", myScrollFunc);

myID = document.getElementById("myID");


var myScrollFunc = function() {
var y = window.scrollY;
if (y >= 800) {
myID.className = "bottomMenu show"
} else {
myID.className = "bottomMenu hide"
}
};


window.addEventListener("scroll", myScrollFunc);

body {
height: 2000px;
}
.bottomMenu {
position: fixed;
bottom: 0;
width: 100%;
height: 60px;
border-top: 1px solid #000;
background: red;
z-index: 1;
transition: all 1s;
}
.hide {
opacity: 0;
left: -100%;
}
.show {
opacity: 1;
left: 0;
}

<div id="myID" class="bottomMenu hide"></div>

Documentation for .scrollY
Documentation for .className
Documentation for .addEventListener

If you want to show an element after scrolling to it:

Working Example

$('h1').each(function () {
var y = $(document).scrollTop();
var t = $(this).parent().offset().top;
if (y > t) {
$(this).fadeIn();
} else {
$(this).fadeOut();
}
});

$(document).scroll(function() {
//Show element after user scrolls 800px
var y = $(this).scrollTop();
if (y > 800) {
$('.bottomMenu').fadeIn();
} else {
$('.bottomMenu').fadeOut();
}


// Show element after user scrolls past
// the top edge of its parent
$('h1').each(function() {
var t = $(this).parent().offset().top;
if (y > t) {
$(this).fadeIn();
} else {
$(this).fadeOut();
}
});
});

body {
height: 1600px;
}
.bottomMenu {
display: none;
position: fixed;
bottom: 0;
width: 100%;
height: 60px;
border-top: 1px solid #000;
background: red;
z-index: 1;
}
.scrollPast {
width: 100%;
height: 150px;
background: blue;
position: relative;
top: 50px;
margin: 20px 0;
}
h1 {
display: none;
position: absolute;
bottom: 0;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<p>Scroll Down...</p>
<div class="scrollPast">
<h1>I fade in when you scroll to my parent</h1>


</div>
<div class="scrollPast">
<h1>I fade in when you scroll to my parent</h1>


</div>
<div class="scrollPast">
<h1>I fade in when you scroll to my parent</h1>


</div>
<div class="bottomMenu">I fade in when you scroll past 800px</div>

Note that you can't get the offset of elements set to display: none;, grab the offset of the element's parent instead.

Documentation for .each()
Documentation for .parent()
Documentation for .offset()

If you want to have a nav or div stick or dock to the top of the page once you scroll to it and unstick/undock when you scroll back up:

Working Example

$(document).scroll(function () {
//stick nav to top of page
var y = $(this).scrollTop();
var navWrap = $('#navWrap').offset().top;
if (y > navWrap) {
$('nav').addClass('sticky');
} else {
$('nav').removeClass('sticky');
}
});


#navWrap {
height:70px
}
nav {
height: 70px;
background:gray;
}
.sticky {
position: fixed;
top:0;
}

$(document).scroll(function () {
//stick nav to top of page
var y = $(this).scrollTop();
var navWrap = $('#navWrap').offset().top;
if (y > navWrap) {
$('nav').addClass('sticky');
} else {
$('nav').removeClass('sticky');
}
});

body {
height:1600px;
margin:0;
}
#navWrap {
height:70px
}
nav {
height: 70px;
background:gray;
}
.sticky {
position: fixed;
top:0;
}
h1 {
margin: 0;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<p>Zombie ipsum reversus ab viral inferno, nam rick grimes malum cerebro. De carne lumbering animata corpora quaeritis. Summus brains sit, morbo vel maleficia? De apocalypsi gorger omero undead survivor dictum mauris. Hi mindless mortuis soulless creaturas,
imo evil stalking monstra adventus resi dentevil vultus comedat cerebella viventium. Qui animated corpse, cricket bat max brucks terribilem incessu zomby. The voodoo sacerdos flesh eater, suscitat mortuos comedere carnem virus. Zonbi tattered for solum
oculi eorum defunctis go lum cerebro. Nescio brains an Undead zombies. Sicut malus putrid voodoo horror. Nigh tofth eliv ingdead.</p>
<div id="navWrap">
<nav>
<h1>I stick to the top when you scroll down and unstick when you scroll up to my original position</h1>


</nav>
</div>
<p>Zombie ipsum reversus ab viral inferno, nam rick grimes malum cerebro. De carne lumbering animata corpora quaeritis. Summus brains sit, morbo vel maleficia? De apocalypsi gorger omero undead survivor dictum mauris. Hi mindless mortuis soulless creaturas,
imo evil stalking monstra adventus resi dentevil vultus comedat cerebella viventium. Qui animated corpse, cricket bat max brucks terribilem incessu zomby. The voodoo sacerdos flesh eater, suscitat mortuos comedere carnem virus. Zonbi tattered for solum
oculi eorum defunctis go lum cerebro. Nescio brains an Undead zombies. Sicut malus putrid voodoo horror. Nigh tofth eliv ingdead.</p>

小开

SCROLLBARS & $(window).scrollTop()

What I have discovered is that calling such functionality as in the solution thankfully provided above, (there are many more examples of this throughout this forum - which all work well) is only successful when scrollbars are actually visible and operating.

If (as I have maybe foolishly tried), you wish to implement such functionality, and you also wish to, shall we say, implement a minimalist "clean screen" free of scrollbars, such as at this discussion, then $(window).scrollTop() will not work.

It may be a programming fundamental, but thought I'd offer the heads up to any fellow newbies, as I spent a long time figuring this out.

If anyone could offer some advice as to how to overcome this or a little more insight, feel free to reply, as I had to resort to show/hide onmouseover/mouseleave instead here

Live long and program, CollyG.

小开

You can also, do this.

$(window).on("scroll", function () {
if ($(this).scrollTop() > 800) {
#code here
} else {
#code here
}
});