拉伸按钮背景图片:stretchableImageWithLeftCapWidth:

 // 1. 拉伸按钮背景图片

    // 1) 登录按钮

    UIImage *loginImage = [UIImage imageNamed:@"LoginGreenBigBtn"];

    loginImage = [loginImage stretchableImageWithLeftCapWidth:loginImage.size.width * 0.5 topCapHeight:loginImage.size.height * 0.5];

    [_loginButton setBackgroundImage:loginImage forState:UIControlStateNormal];