自定义button

改变button内部label和imageView的frame

- (CGRect)titleRectForContentRect:(CGRect)contentRect
- (CGRect)imageRectForContentRect:(CGRect)contentRect

这两个方法在初始化的时候就会调用，此时button自身可能还没有frame，所以可以使用

- (void)setTitle:(NSString *)title forState:(UIControlState)state
{
[super setTitle:title forState:state];

[self sizeToFit];
}

- (void)setImage:(UIImage *)image forState:(UIControlState)state
{
[super setImage:image forState:state];

[self sizeToFit];
}

在layoutSubviews中设置其内部控件的frame