layer.shadowSpread < number>

Makes shadows larger in all directions. The shadow is expanded by the given value. Negative values cause the shadow to contract. If shadowX, shadowY and shadowBlur are all set to 0, this will appear as a border. A visible shadow will only appear if the shadowColor property is also defined.
译：使得阴影在所有方向上较大。阴影是由给定的值扩大。负值会使阴影收缩。的阴影，阴影和shadowBlur都设置为0，这将显示为边框。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 = 1
layerA.shadowBlur = 4
layerA.shadowSpread = 2