question: int addLayer ( ChildDrawable layer ) { final LayerState st = mLayerState ; final int N = st . mChildren != null ? st . mChildren . length : 0 ; final int i = st . mNum ; if ( i >= N ) { final ChildDrawable [ ] nu = new ChildDrawable [ N + 10 ] ; if ( i > 0 ) { System . arraycopy ( st . mChildren , 0 , nu , 0 , i ) ; } st . mChildren = nu ; } st . mChildren [ i ] = layer ; st . mNum ++ ; st . invalidateCache ( ) ; return i ; }
target:  Adds a new layer at the end of list of layers and returns its index.