实现方式主要使用了CAGradientLayer。先在ImageView的底端加一个UIView，并在这个UIView上插入一个从透明到黑色的CAGradientLayer，然后再UIView上插入一个Label就行了。
实现代码如下：
import "ViewController.h"
@interface ViewController ()
@end
@implementation ViewController
{
CAGradientLayer *_gradientLayer;
UIView *_layerView;
UIImageView *_imageView;
}
-
(void)viewDidLoad
{
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.
self.view.backgroundColor = [UIColor blueColor];UIButton *button = [[UIButton alloc] initWithFrame:CGRectMake(10, 350, 150, 100)];
[button setTitle:@"显示/隐藏标题" forState:UIControlStateNormal];
[button addTarget:self action:@selector(buttonPressed) forControlEvents:UIControlEventTouchUpInside];
[self.view addSubview:button];_imageView = [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"test"]];
_imageView.frame = CGRectMake(0, 0, 320, 320);UILabel *label = [[UILabel alloc] initWithFrame:CGRectMake(10, 25, 320, 100)];
label.text = @"我是标题标题标题！";
label.textColor = [UIColor whiteColor];_layerView = [[UIView alloc] initWithFrame:CGRectMake(0, 320-100, 320, 100)];
_gradientLayer = [CAGradientLayer layer]; // 设置渐变效果
_gradientLayer.bounds = _layerView.bounds;
_gradientLayer.borderWidth = 0;_gradientLayer.frame = _layerView.bounds;
_gradientLayer.colors = [NSArray arrayWithObjects:
(id)[[UIColor clearColor] CGColor],
(id)[[UIColor blackColor] CGColor], nil nil];
_gradientLayer.startPoint = CGPointMake(0.5, 0.5);
_gradientLayer.endPoint = CGPointMake(0.5, 1.0);[_layerView.layer insertSublayer:_gradientLayer atIndex:0];
[_imageView addSubview:_layerView];
[_layerView addSubview:label];
[self.view addSubview:_imageView];
} (void)buttonPressed
{
static BOOL yesOrNo = YES;
if (yesOrNo) {
[_layerView removeFromSuperview];
} else {
[_imageView addSubview:_layerView];
}
yesOrNo = !yesOrNo;
}
@end
