layer.placeBefore(layer)

Places this layer before another layer. This changes the layer.index property for at least one of the layers. This method only works on layers that have the same parent, or no parent at all.
译：这一层放在另一层之前。 这层的变化。至少一层的索引属性。此方法仅适用于相同的层上的父母，或者根本就没有父。

Arguments（参数）

layer — A layer object. 译：一层对象

layerA = new Layer
layerB = new Layer

# Draw layerB on top 
layerB.placeBefore(layerA)