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tag

button.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];