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