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HTML how to tell which elements are visible?

I have seen several solutions which determine when an element is visible in the viewport whilst scrolling the page, but I haven't seen any which do this for elements that are contained in a scrolling container div as in the example here.

How would I detect the items as they scroll into view via the scrolling div? And by contrast how would I detect them if they fell out of view. In all cases the overflow elements are not hidden at the outset.

HTML

    <div id="mainContainer" class="main">
        <div id="scrollContainer"class="scroller">
            <div id="picturesContainer"class="holder">
                <div id="pictureContainer1" class="picture position1">
                pictureContainer1</div>

                <div id="pictureContainer2" class="picture position2">
                pictureContainer2</div>
                <div id="pictureContainer3" class="picture position3">
                pictureContainer3</div>
                <div id="pictureContainer4" class="picture position4">
                pictureContainer4</div>
                <div id="pictureContainer5" class="picture position5">
                pictureContainer5</div>
                <div id="pictureContainer6" class="picture position6">
                pictureContainer6</div>
                <div id="pictureContainer7" class="picture position7">
                pictureContainer7</div>
                <div id="pictureContainer8" class="picture position8">
                pictureContainer8</div>
                <div id="pictureContainer9" class="picture position9">
                pictureContainer9</div>
            </div>

        </div>
    </div>

CSS

    .main{
        position:absolute;
        top:0px;
        left:0px;
        height: 200px;
        width:200px;
        background-color: grey;
        border: 1px solid black;
    }

    .scroller{
        position:absolute;
        top:0px;
        left:0px;
        height: 250px;
        width:250px;
        background-color: lightblue;
        border: 1px solid black;
        overflow: scroll;
    }

    .picture{
        position:absolute;
        height: 200px;
        width: 200px;
        background-color: lightyellow;
        border: 1px solid black;
    }

    .position1{
        top:0px;
        left:0px;
    }

    .position2{
        top:0px;
        left:200px;
    }

    .position3{
        top:0px;
        left:400px;
    }

    .position4{
        top:200px;
        left:0px;
    }

    .position5{
        top:200px;
        left:200px;
    }

    .position6{
        top:200px;
        left:400px;
    }

    .position7{
        top:400px;
        left:0px;
    }

    .position8{
        top:400px;
        left:200px;
    }

    .position9{
        top:400px;
        left:400px;
    }

    .holder{
        position:absolute;
        top:0px;
        left:0px;
        width:600px;
        height:600px;
        background-color:lightgreen;
    }
like image 959
T9b Avatar asked May 31 '11 18:05

T9b


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How do I know if my element is visible on screen?

Summary. Use the getBoundingClientRect() method to get the size of the element and its relative position to the viewport. Compare the position of the element with the viewport height and width to check if the element is visible in the viewport or not.

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1 Answers

include jQuery library on page first.

function getObjDims(obj){
    if (!obj) return false;
    var off = $(obj).offset();
    var t = {
        top:off.top,
        left:off.left,
        width:$(obj).width(),
        height:$(obj).height()
    };
    return {x1:t.left, y1:t.top, x2:t.left+t.width,y2:t.top+t.height}
};
function testInside(pic,box){
    var d=getObjDims(pic);
    return (box.x1<=d.x1 && box.y1<=d.y1 && box.x2>=d.x2 && box.y2>=d.y2)?1:-1;
};
$(document).ready(function(){
    var inside={};
    var port=$('#scrollContainer');
    var box=getObjDims(port);
    $(window).resize(function(){
        box=getObjDims(port);
    });
    $(port).scroll(function(){      
        var str=[];
        $('.picture').each(function(i){
            var oldState = inside[this.id]!=undefined?inside[this.id]:0;            
            var newState = testInside(this,box);
            inside[this.id]=newState;
            if (oldState!=newState)
                switch (newState){
                    case 1:str.push(this.id);break;// go IN
                    case -1: break;// go OUT
                }
        });
        $('#picStatus').text(str.join(', '));
    });
});

Add in HTML for results output:

<div style='margin-top:280px;'>Pictures in window (numbers):</div>
<div id='picStatus'></div>

It is code based on object's coords, witch are recalculated on scroll event. There are some things to know. IE and, seems to be, Opera takes into consideration width and height of scrollbars themselves, that demands curtain code tuning. I just have suggested solution direction and did not spent much time for debugging this.

And yet, maybe will be useful following (from jquery api about offset):

Note: jQuery does not support getting the offset coordinates of hidden elements or accounting for borders, margins, or padding set on the body element.

like image 52
Igor Avatar answered Sep 27 '22 16:09

Igor