Question

I have this code:

jQuery:

var mouseDown=false;
var lastPositionX;
var lastPositionY;
var newX;
var newY;
var lastPositionLeft;
var lastPositionTop;
var newLeft;
var newTop;
    $("#insideMap").on("mousedown",function(){
        mouseDown=true;
        lastPositionX=e.pageX;
        lastPositionY=e.pageY;
    });
    $("#insideMap").on("mouseup",function(u){
        mouseDown=false;
    });
    $("#insideMap").on("mouseleave",function(){
        mouseDown=false;
    });
    $("#insideMap").on("mousemove",function(n){
        if (mouseDown){
            mouseDown = true; // mantain the boolean to prevent mouseleave trigger
            newX=n.pageX-lastPositionX;
            newY=n.pageY-lastPositionY;
            lastPositionLeft=$("#insideMap").position().left;
            lastPositionTop=$("#insideMap").position().top;
            newLeft=newX+lastPositionLeft;
            newTop=newY+lastPositionTop;
            $("#insideMap").css({"left":newLeft,"top":newTop});
            lastPositionX=n.pageX;
            lastPositionY=n.pageY;
        }
    });

Demo: http://jsfiddle.net/SkWeX/3/

but something is still bad 🙁 it doesnt work well, it is still lagging,
can you help me with it?

Answer 1

🙂 the browser wants sometimes to drag your image (bg), you need to prevent this!

jsFiddle demo

Just add this to your #insideMap element’s CSS:

-webkit-touch-callout: none;
-webkit-user-select: none;
-khtml-user-select: none;
-moz-user-select: none;
-ms-user-select: none;
user-select: none;

And this code is all you need:

var mouseDown=false,
    posX   = 0,
    posY   = 0,
    innerX = 0,
    innerY = 0;

$("#insideMap").on("mousedown",function(e){
    innerX = e.pageX - $(this).offset().left;
    innerY = e.pageY - $(this).offset().top;
    mouseDown= true;
}).on("mouseup",function(){
    mouseDown=false;
});

$(document).on("mousemove",function(e){    
    if (mouseDown){
        var m = {  x: e.pageX-innerX,  y: e.pageY-innerY };        
        $('#insideMap').css({ left: m.x, top: m.y });       
    }
});

Some explanation:

Your code is slow cause on any registered mousemovement (and there are many) you are recalculating the element position. Wrong.

• I grab the mouse position inside the element on click vars innerX and innerX
• On mousemove I just move the CSS element position to the actual mouse position (you guess right: ) but minus the position where the element was clicked (innerX, innerY) (to prevent the element jump to mouse position) .