所以我试图使用JavaScript滚动调用一个函数。但是我想知道我是否可以不使用jQuery检测滚动的方向。如果没有，有什么变通办法吗?
我想只放一个“顶部”按钮，但如果可以的话，我想避免这个按钮。
我现在刚刚尝试使用这段代码，但它不起作用:
if document.body.scrollTop <= 0 {
alert ("scrolling down")
} else {
alert ("scrolling up")
}
可以通过存储之前的scrollTop值并将当前scrollTop值与其进行比较来检测它。
JavaScript:
var lastScrollTop = 0;
// element should be replaced with the actual target element on which you have applied scroll, use window in case of no target element.
element.addEventListener("scroll", function(){ // or window.addEventListener("scroll"....
var st = window.pageYOffset || document.documentElement.scrollTop; // Credits: "https://github.com/qeremy/so/blob/master/so.dom.js#L426"
if (st > lastScrollTop) {
// downscroll code
} else if (st < lastScrollTop) {
// upscroll code
} // else was horizontal scroll
lastScrollTop = st <= 0 ? 0 : st; // For Mobile or negative scrolling
}, false);
使用这个来找到滚动方向。这只是为了找到垂直滚动的方向。支持所有跨浏览器。
var scrollableElement = document.body; //document.getElementById('scrollableElement');
scrollableElement.addEventListener('wheel', checkScrollDirection);
function checkScrollDirection(event) {
if (checkScrollDirectionIsUp(event)) {
console.log('UP');
} else {
console.log('Down');
}
}
function checkScrollDirectionIsUp(event) {
if (event.wheelDelta) {
return event.wheelDelta > 0;
}
return event.deltaY < 0;
}
例子
这是对prateek的回答的补充。在IE的代码中似乎有一个小故障，所以我决定修改它一点也不花哨(只是另一个条件)
$('document').ready(function() {
var lastScrollTop = 0;
$(window).scroll(function(event){
var st = $(this).scrollTop();
if (st > lastScrollTop){
console.log("down")
}
else if(st == lastScrollTop)
{
//do nothing
//In IE this is an important condition because there seems to be some instances where the last scrollTop is equal to the new one
}
else {
console.log("up")
}
lastScrollTop = st;
});});
捕捉所有滚动事件(触摸和滚轮)的简单方法
window.onscroll = function(e) {
// print "false" if direction is down and "true" if up
console.log(this.oldScroll > this.scrollY);
this.oldScroll = this.scrollY;
}
你可以试试这样做。
function scrollDetect(){ var lastScroll = 0; window.onscroll = function() { let currentScroll = document.documentElement.scrollTop || document.body.scrollTop; // Get Current Scroll Value if (currentScroll > 0 && lastScroll <= currentScroll){ lastScroll = currentScroll; document.getElementById("scrollLoc").innerHTML = "Scrolling DOWN"; }else{ lastScroll = currentScroll; document.getElementById("scrollLoc").innerHTML = "Scrolling UP"; } }; } scrollDetect(); html,body{ height:100%; width:100%; margin:0; padding:0; } .cont{ height:100%; width:100%; } .item{ margin:0; padding:0; height:100%; width:100%; background: #ffad33; } .red{ background: red; } p{ position:fixed; font-size:25px; top:5%; left:5%; } <div class="cont"> <div class="item"></div> <div class="item red"></div> <p id="scrollLoc">0</p> </div>
初始化一个oldValue 通过监听事件获取newValue 两者相减 从结果得出结论 用newValue更新oldValue
/ /初始化
let oldValue = 0;
//监听事件
window.addEventListener('scroll', function(e){
// Get the new Value
newValue = window.pageYOffset;
//Subtract the two and conclude
if(oldValue - newValue < 0){
console.log("Up");
} else if(oldValue - newValue > 0){
console.log("Down");
}
// Update the old value
oldValue = newValue;
});
我个人使用这段代码来检测javascript中的滚动方向… 你只需要定义一个变量来存储lastscrollvalue，然后使用这个if&else
let lastscrollvalue;
function headeronscroll() {
// document on which scroll event will occur
var a = document.querySelector('.refcontainer');
if (lastscrollvalue == undefined) {
lastscrollvalue = a.scrollTop;
// sets lastscrollvalue
} else if (a.scrollTop > lastscrollvalue) {
// downscroll rules will be here
lastscrollvalue = a.scrollTop;
} else if (a.scrollTop < lastscrollvalue) {
// upscroll rules will be here
lastscrollvalue = a.scrollTop;
}
}
这段简单的代码可以工作:检查控制台的结果。
let scroll_position = 0;
let scroll_direction;
window.addEventListener('scroll', function(e){
scroll_direction = (document.body.getBoundingClientRect()).top > scroll_position ? 'up' : 'down';
scroll_position = (document.body.getBoundingClientRect()).top;
console.log(scroll_direction);
});
虽然公认的答案是可行的，但值得注意的是，这将以很高的速率发射。对于计算成本高的操作，这可能会导致性能问题。
MDN的建议是限制这些事件。下面是他们的样例的修改，增强检测滚动方向。
修改自:https://developer.mozilla.org/en-US/docs/Web/API/Document/scroll_event
// ## function declaration
function scrollEventThrottle(fn) {
let last_known_scroll_position = 0;
let ticking = false;
window.addEventListener("scroll", function () {
let previous_known_scroll_position = last_known_scroll_position;
last_known_scroll_position = window.scrollY;
if (!ticking) {
window.requestAnimationFrame(function () {
fn(last_known_scroll_position, previous_known_scroll_position);
ticking = false;
});
ticking = true;
}
});
}
// ## function invocation
scrollEventThrottle((scrollPos, previousScrollPos) => {
if (previousScrollPos > scrollPos) {
console.log("going up");
} else {
console.log("going down");
}
});
如果有人想用React钩子实现它
const [scrollStatus, setScrollStatus] = useState({
scrollDirection: null,
scrollPos: 0,
});
useEffect(() => {
window.addEventListener("scroll", handleScrollDocument);
return () => window.removeEventListener("scroll", handleScrollDocument);
}, []);
function handleScrollDocument() {
setScrollStatus((prev) => { // to get 'previous' value of state
return {
scrollDirection:
document.body.getBoundingClientRect().top > prev.scrollPos
? "up"
: "down",
scrollPos: document.body.getBoundingClientRect().top,
};
});
}
console.log(scrollStatus.scrollDirection)
修改Prateek的答案，如果lastScrollTop没有变化，那么它将是一个水平滚动(在x方向溢出，可以使用鼠标使用水平滚动条或使用滚动轮+ shift来使用。
const containerElm = document.getElementById("container");
let lastScrollTop = containerElm.scrollTop;
containerElm.addEventListener("scroll", (evt) => {
const st = containerElm.scrollTop;
if (st > lastScrollTop) {
console.log("down scroll");
} else if (st < lastScrollTop) {
console.log("up scroll");
} else {
console.log("horizontal scroll");
}
lastScrollTop = Math.max(st, 0); // For mobile or negative scrolling
});
这似乎工作得很好。
document.addEventListener('DOMContentLoaded', () => {
var scrollDirectionDown;
scrollDirectionDown = true;
window.addEventListener('scroll', () => {
if (this.oldScroll > this.scrollY) {
scrollDirectionDown = false;
} else {
scrollDirectionDown = true;
}
this.oldScroll = this.scrollY;
// test
if (scrollDirectionDown) {
console.log('scrolling down');
} else {
console.log('scrolling up');
}
});
});
有时滚动行为不一致，不能正确更新元素的scrollTop属性。在决定滚动方向之前设置一些阈值会更安全。
let lastScroll = 0
let threshold = 10 // must scroll by 10 units to know the direction of scrolling
element.addEventListener("scroll", () => {
let newScroll = element.scrollTop
if (newScroll - lastScroll > threshold) {
// "up" code here
} else if (newScroll - lastScroll < -threshold) {
// "down" code here
}
lastScroll = newScroll
})
