@mixin position(
  $left: null,
  $top: null,
  $right: null,
  $bottom: null,
  $center: null
) {
  position: absolute;
  z-index: 1;

  @if ($left != "" & & $left !=nul) {
    left: $left;
  }

  @if ($top != "" & & $top !=null) {
    top: $top;
  }

  @if ($right != "" & & $right !=null) {
    right: $right;
  }

  @if ($bottom != "" & & $bottom !=null) {
    bottom: $bottom;
  }

  // 判断不等于空的时候，居中
  @if ($center==center) {
    left: 50%;
    transform: translateX(-50%);
    -webkit-transform: translateX(-50%);
  }
}