UIButton中addSubview后,UIButton中的文字为什么显示在添加的View的上方

Question

UIButton *button = [[UIButton alloc] initWithFrame:CGRectMake(i*100,  0, 100,40)];NSString *buttonTitle = [types objectAtIndex:i];[button setTitle:buttonTitle forState:UIControlStateNormal];[button setTitleColor:[UIColor blackColor] forState:UIControlStateNormal];[button addTarget:self action:@selector(buttonAction:) forControlEvents:UIControlEventTouchUpInside];//区分不同的button  1、可以根据frame的位置 2 设置tagbutton.tag = 10000+i;button.titleLabel.font = [UIFont systemFontOfSize:15.0];[scrollView addSubview:button];if (i==0) {button.layer.borderColor = [UIColor cyanColor].CGColor;button.layer.borderWidth = 2.0;button.layer.cornerRadius = 5.0;//CGPoint 只定义promptview的位置  宽高由文字大小定义PromptView *promptView = [[PromptView alloc] initWithPoint:CGPointMake(button.frame.size.width-20, 0) message:5];[button addSubview:promptView];