动态将彩色图片动画过渡到黑白图片的BlackAndWhiteView

效果如下：

BlackAndWhiteView.h 与 BlackAndWhiteView.m

//
//  BlackAndWhiteView.h
//  BlackAndWhiteView
//
//  Created by YouXianMing on 14-10-4.
//  Copyright (c) 2014年 YouXianMing. All rights reserved.
//

#import <UIKit/UIKit.h>

@interface BlackAndWhiteView : UIView

@property (nonatomic)         float    blackAlpha;
@property (nonatomic, strong) UIImage *image;

- (void)startImageProcessing;

@end

//
//  BlackAndWhiteView.m
//  BlackAndWhiteView
//
//  Created by YouXianMing on 14-10-4.
//  Copyright (c) 2014年 YouXianMing. All rights reserved.
//

#import "BlackAndWhiteView.h"

@interface BlackAndWhiteView ()

@property (nonatomic, strong) UIImageView *normalView;
@property (nonatomic, strong) UIImageView *blackView;

@property (nonatomic, strong) UIImage     *blackImage;

@end

@implementation BlackAndWhiteView

- (instancetype)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if (self) {
        _normalView = [[UIImageView alloc] initWithFrame:self.bounds];
        _blackView  = [[UIImageView alloc] initWithFrame:self.bounds];
        _blackView.alpha = 0.f;

        [self addSubview:_normalView];
        [self addSubview:_blackView];
    }
    return self;
}

- (void)startImageProcessing
{
    if (_image) {
        _normalView.image = _image;

        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

            if (_blackImage == nil) {
                _blackImage = [self grayScale:_image];
            }

            dispatch_async(dispatch_get_main_queue(), ^{
                _blackView.image = _blackImage;
            });
        });
    }
}

#pragma mark - 私有方法
- (UIImage *)grayScale:(UIImage *)inputImage
{
    int width = inputImage.size.width;
    int height = inputImage.size.height;

    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceGray();

    CGContextRef context = CGBitmapContextCreate(nil,
                                                 width,
                                                 height,
                                                 8, // bits per component
                                                 0,
                                                 colorSpace,
                                                 kCGBitmapByteOrderDefault);

    CGColorSpaceRelease(colorSpace);

    if (context == NULL) {
        return nil;
    }

    CGContextDrawImage(context,
                       CGRectMake(0, 0, width, height), inputImage.CGImage);
    CGImageRef image = CGBitmapContextCreateImage(context);
    UIImage *grayImage = [UIImage imageWithCGImage:image];
    CFRelease(image);
    CGContextRelease(context);

    return grayImage;
}

#pragma mark - 重写setter方法
@synthesize blackAlpha = _blackAlpha;
- (void)setBlackAlpha:(float)blackAlpha
{
    _blackAlpha      = blackAlpha;
    _blackView.alpha = blackAlpha;
}

- (float)blackAlpha
{
    return _blackAlpha;
}

@end

使用的源码：

//
//  ViewController.m
//  BlackAndWhiteView
//
//  Created by YouXianMing on 14-10-4.
//  Copyright (c) 2014年 YouXianMing. All rights reserved.
//

#import "ViewController.h"
#import "BlackAndWhiteView.h"

@interface ViewController ()

@property (nonatomic, strong) BlackAndWhiteView *blackView;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    UIImage *image   = [UIImage imageNamed:@"red.jpg"];
    _blackView = [[BlackAndWhiteView alloc] initWithFrame:CGRectMake(0, 0,
                                                                     image.size.width,
                                                                     image.size.height)];
    _blackView.image  = image;
    _blackView.center = self.view.center;
    [_blackView startImageProcessing];
    [self.view addSubview:_blackView];

    [self performSelector:@selector(run) withObject:nil afterDelay:8];
}

- (void)run
{
    [UIView animateWithDuration:2 animations:^{
        _blackView.blackAlpha = 1.f;
    }];
}

@end