layer.shadowY < number>

Defines the shadow direction on the y-axis. A positive value will produce a shadow from the bottom edge of a layer, whereas a negative value will produce a shadow from the top edge. A visible shadow will only appear if the shadowColor property is also defined.
译：定义在y轴的阴影的方向。正值将产生从一个层的底部边缘上的阴影，而负值将产生一个阴影，从顶部边缘。如果则shadowColor属性还定义了一个明显的阴影才会出现。

layerA = new Layer
layerA.shadowY = 10