问如何在jQuery中检测页面滚动到某一点？

EN

var target = $(".myPara").offset().top;
var interval = setInterval(function() {
    if ($(window).scrollTop() >= target) {
        alert("made it!");
        clearInterval(interval);
    }
}, 250);

var target = $(".mypara").offset().top,
    timeout = null;

$(window).scroll(function () {
    if (!timeout) {
        timeout = setTimeout(function () {
            console.log('scroll');            
            clearTimeout(timeout);
            timeout = null;
            if ($(window).scrollTop() >= target) {
                alert('made it');
            }
        }, 250);
    }
});

$(window).scroll(function(){
    console.log($('#myPara').offset().top < $(this).height() + $(this).scrollTop());
});

var showMessageInterval = window.setInterval(showMessageIfNeeded, 500);
// you could adjust the interval to the animation duration of the 
// message showing. In this way, the delay will be more "natural"

function showMessageIfNeeded() {
    var scrollTop = $(window).scrollTop();
    var targetTop = $(".myPara").offset().top;
    if (scrollTop > targetTop) {
        alert('Show message');
        window.clearInterval(showMessageInterval);
    }
}