IOS只改变父视图的的透明度，不会改变子View的透明度

//正在滚动

-(void)scrollViewDidScroll:(UIScrollView *)scrollView{

float offset_Y = scrollView.contentOffset.y;

NSLog(@"%f",scrollView.contentOffset.y);

UIView *barView = [self.navigationController.view viewWithTag:1000];

if (offset_Y == -20) {

//colorWithAlphaComponent 只改变父视图的的透明度，不会改变子View的透明度

barView.backgroundColor = [[UIColor yellowColor] colorWithAlphaComponent:0.0];

}else if(offset_Y >= 160){

barView.alpha = 1.0;

}else{

barView.backgroundColor = [[UIColor yellowColor] colorWithAlphaComponent:(offset_Y) / 160];

}

}