将CAGradientLayer用作maskView的遮罩图层

//
//  CAGradientView.h
//  AlphaView
//
//  Created by YouXianMing on 15/5/4.
//  Copyright (c) 2015年 YouXianMing. All rights reserved.
//

#import <UIKit/UIKit.h>

@interface CAGradientView : UIView

@property (nonatomic, strong) NSArray *colors;
@property (nonatomic, strong) NSArray *locations;
@property (nonatomic)         CGPoint  startPoint;
@property (nonatomic)         CGPoint  endPoint;

- (void)alphaType;

@end

//
//  CAGradientView.m
//  AlphaView
//
//  Created by YouXianMing on 15/5/4.
//  Copyright (c) 2015年 YouXianMing. All rights reserved.
//

#import "CAGradientView.h"

@interface CAGradientView () {
    CAGradientLayer   *_gradientLayer;
}

@end

@implementation CAGradientView

/**
 *  修改当前view的backupLayer为CAGradientLayer
 *
 *  @return CAGradientLayer类名字
 */
+ (Class)layerClass {
    return [CAGradientLayer class];
}

- (instancetype)initWithFrame:(CGRect)frame {
    self = [super initWithFrame:frame];
    if (self) {
        _gradientLayer = (CAGradientLayer *)self.layer;
    }
    return self;
}

- (void)alphaType {
    self.colors     = @[[UIColor clearColor], [UIColor blackColor], [UIColor clearColor]];
    self.locations  = @[@(0.25), @(0.5), @(0.75)];
    self.startPoint = CGPointMake(0, 0);
    self.endPoint   = CGPointMake(1, 0);
}

/**
 *  重写setter,getter方法
 */
@synthesize colors = _colors;
- (void)setColors:(NSArray *)colors {
    _colors = colors;
    
    // 将color转换成CGColor
    NSMutableArray *cgColors = [NSMutableArray array];
    for (UIColor *tmp in colors) {
        id cgColor = (__bridge id)tmp.CGColor;
        [cgColors addObject:cgColor];
    }
    
    // 设置Colors
    _gradientLayer.colors = cgColors;
}
- (NSArray *)colors {
    return _colors;
}

@synthesize locations = _locations;
- (void)setLocations:(NSArray *)locations {
    _locations               = locations;
    _gradientLayer.locations = _locations;
}
- (NSArray *)locations {
    return _locations;
}

@synthesize startPoint = _startPoint;
- (void)setStartPoint:(CGPoint)startPoint {
    _startPoint               = startPoint;
    _gradientLayer.startPoint = startPoint;
}
- (CGPoint)startPoint {
    return _startPoint;
}

@synthesize endPoint = _endPoint;
- (void)setEndPoint:(CGPoint)endPoint {
    _endPoint               = endPoint;
    _gradientLayer.endPoint = endPoint;
}
- (CGPoint)endPoint {
    return _endPoint;
}
@end

//
//  ViewController.m
//  AlphaView
//
//  Created by YouXianMing on 15/5/4.
//  Copyright (c) 2015年 YouXianMing. All rights reserved.
//

#import "ViewController.h"
#import "CAGradientView.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
 
    // 加了mask的view
    UIImageView *imageView = [[UIImageView alloc] initWithFrame:CGRectMake(40, 40, 200, 200)];
    imageView.image        = [UIImage imageNamed:@"base"];
    [self.view addSubview:imageView];
    
    CAGradientView *gradientView = [[CAGradientView alloc] initWithFrame:imageView.bounds];
    [gradientView alphaType];
    imageView.maskView = gradientView;
    
    
    // 对照组
    UIImageView *baseImageView = [[UIImageView alloc] initWithFrame:CGRectMake(40, 40 + 200 + 40, 200, 200)];
    baseImageView.image        = [UIImage imageNamed:@"base"];
    [self.view addSubview:baseImageView];
}

@end