【完美解决方案】控制div拖拽范围,让层在范围内拖动,不拖拽出边界值

var XNew = e.pageX;
var YNew = e.pageY;
var pageXOld = this.options.event.pageX;
var pageYOld = this.options.event.pageY;
var leftOld = this.options.oleft;
var topOld = this.options.otop;
var leftNew = leftOld + XNew - pageXOld;
var topNew = topOld + YNew - pageYOld;
var selfWidth = this.$element[0].offsetWidth;
var selfHeight = this.$element[0].offsetHeight;
var windowWidth = $(window).width();
var windowHeight = $(window).height();
if(leftNew > (windowWidth - selfWidth)){
this.$element.css('left', windowWidth - selfWidth);
return;
}
if(topNew > (windowHeight - selfHeight)){
this.$element.css('top', windowHeight - selfHeight);
return;
}
if(leftNew < 0){
this.$element.css('left', 0);
return;
}
if(topNew < 0){
this.$element.css('top', 0);
return;
}

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