iOS 中collectionView flowLayout 私有公共屬性沿腰。
左中右對齊
來源：http://www.reibang.com/p/de08c2679241
SEL sel = NSSelectorFromString(@"_setRowAlignmentsOptions:");
if ([self.colView.collectionViewLayout respondsToSelector:sel]) {
((void(*)(id,SEL,NSDictionary*))objc_msgSend)(self.colView.collectionViewLayout,sel,
@{@"UIFlowLayoutCommonRowHorizontalAlignmentKey":@(NSTextAlignmentLeft),
@"UIFlowLayoutLastRowHorizontalAlignmentKey" : @(NSTextAlignmentLeft),
@"UIFlowLayoutRowVerticalAlignmentKey" : @(NSTextAlignmentCenter)});
}
