Question

我正在寻找一个关于如何将从网址加载的图像缓存到uitableview的单元格中的教程。

我在这里找到了一个例子

http://www.ericd.net/2009/05/iphone-caching-images-in-memory.html#top

但代码不完整。我是一个客观的新手，所以我发现填补缺失的部分非常困难。

Answer 1

这是一个使用NSCache的简单ImageCache实现。 ImageCache是一个singelton。

<强> ImageCache.h

    #import <Foundation/Foundation.h>

    @interface ImageCache : NSObject

    @property (nonatomic, retain) NSCache *imgCache;


    #pragma mark - Methods

    + (ImageCache*)sharedImageCache;
    //- (void) AddImage:(NSString *)imageURL: (UIImage *)image;
   - (void) AddImage:(NSString *)imageURL withImage:(UIImage *)image;
    - (UIImage*) GetImage:(NSString *)imageURL;
    - (BOOL) DoesExist:(NSString *)imageURL;

    @end

<强> ImageCache.m

  #import "ImageCache.h"

    @implementation ImageCache

    @synthesize imgCache;

    #pragma mark - Methods

    static ImageCache* sharedImageCache = nil;

    +(ImageCache*)sharedImageCache
    {
        @synchronized([ImageCache class])
        {
            if (!sharedImageCache)
                sharedImageCache= [[self alloc] init];

            return sharedImageCache;
        }

        return nil;
    }

    +(id)alloc
    {
        @synchronized([ImageCache class])
        {
            NSAssert(sharedImageCache == nil, @"Attempted to allocate a second instance of a singleton.");
            sharedImageCache = [super alloc];

            return sharedImageCache;
        }

        return nil;
    }

    -(id)init 
    {
        self = [super init];
        if (self != nil) 
        {
            imgCache = [[NSCache alloc] init];
        }

        return self;
    }

   // - (void) AddImage:(NSString *)imageURL: (UIImage *)image
- (void) AddImage:(NSString *)imageURL withImage:(UIImage *)image
    {
        [imgCache setObject:image forKey:imageURL];
    }

    - (NSString*) GetImage:(NSString *)imageURL
    {
        return [imgCache objectForKey:imageURL];
    }

    - (BOOL) DoesExist:(NSString *)imageURL
    {
        if ([imgCache objectForKey:imageURL] == nil)
        {
            return false;
        }

        return true;
    }


    @end

示例

UIImage *image; // 1. Check the image cache to see if the image already exists. If so, then use it. If not, then download it. if ([[ImageCache sharedImageCache] DoesExist:imgUrl] == true) { image = [[ImageCache sharedImageCache] GetImage:imgUrl]; } else { NSData *imageData = [[NSData alloc] initWithContentsOfURL: [NSURL URLWithString: imgUrl]]; image = [[UIImage alloc] initWithData:imageData]; // Add the image to the cache //[[ImageCache sharedImageCache] AddImage:imgUrl :image]; [[ImageCache sharedImageCache] AddImage:imgUrl withImage:image]; }

Answer 2

这里找到了一个很好的工作实例

http://ezekiel.vancouver.wsu.edu/~wayne/yellowjacket/YellowJacket.zip

Answer 3

您还可以尝试使用由enormego的尖锐研究员编写的精彩EgoImage库来实现此目的。它使用起来非常简单，可以在幕后有效地使用缓存，非常适合满足您的要求。

这是包含演示应用程序的库的github path。

Answer 4

我为我正在研究的应用程序编写了这个（带有概念和一些代码来自Lane Roathe优秀的UIImageView + Cache类别）。它也使用ASIHTTPRequest类，这很好。这肯定可以改进..例如，通过允许在不再需要时取消请求，或者通过使用通知userInfo来允许更精确的UI更新......但它对我的目的起作用。

@implementation ImageFetcher
#define MAX_CACHED_IMAGES 20
static NSMutableDictionary* cache = nil;

+ (void)asyncImageFetch:(UIImage**)anImagePtr withURL:(NSURL*)aUrl {

    if(!cache) {
        cache = [[NSMutableDictionary dictionaryWithCapacity:MAX_CACHED_IMAGES] retain];
    }

    UIImage* newImage = [cache objectForKey:aUrl.description];
    if(!newImage) { // cache miss - doh!
        ASIHTTPRequest *imageRequest = [ASIHTTPRequest requestWithURL:aUrl];    
        imageRequest.userInfo = [NSDictionary dictionaryWithObject:[NSValue valueWithPointer:anImagePtr] forKey:@"imagePtr"];
        imageRequest.delegate = self;
        [imageRequest setDidFinishSelector:@selector(didReceiveImage:)];
        [imageRequest setDidFailSelector:@selector(didNotReceiveImage:)];
        [imageRequest startAsynchronous];
    }
    else { // cache hit - good!
        *anImagePtr = [newImage retain];    
    }
}

+ (void)didReceiveImage:(ASIHTTPRequest *)request {
    NSLog(@"Image data received.");
    UIImage **anImagePtr = [(NSValue*)[request.userInfo objectForKey:@"imagePtr"] pointerValue];

    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
    UIImage *newImage = [[UIImage imageWithData:[request responseData]] retain];

    if(!newImage) {
        NSLog(@"UIImageView: LoadImage Failed");
    }
    else {
        *anImagePtr = newImage;
        // check to see if we should flush existing cached items before adding this new item
        if( [cache count] >= MAX_CACHED_IMAGES)
            [cache removeAllObjects];

        [cache setValue:newImage forKey:[request url].description];

        NSNotificationCenter *nc = [NSNotificationCenter defaultCenter];
        [nc postNotificationName: @"ImageDidLoad" object: self userInfo:request.userInfo];
    }

    [pool drain];
}

您可以按如下方式调用此代码：

[ImageFetcher asyncImageFetch:&icon withURL:url];

我也在使用通知，无论好坏，让相应UIImage的任何所有者知道何时应该重新显示 - 在这种情况下，它位于tableView上下文中：

- (void)viewDidLoad {
    [super viewDidLoad];
    NSNotificationCenter *nc = [NSNotificationCenter defaultCenter];
    [nc addObserver:self selector:@selector(imageDidLoad:) name:@"ImageDidLoad" object:nil];
}

- (void)imageDidLoad:(NSNotification*)notif {
    NSLog(@"Received icon load notification.");
    // reload table view so that new image appears.. would be better if I could
    // only reload the particular UIImageView that holds this image, oh well...
    [self.tableView reloadData];
}

- (void)dealloc {
    NSNotificationCenter *nc = [NSNotificationCenter defaultCenter];
    [nc removeObserver:self];
        // ...
}

Answer 5

您也可以查看HJCache。它带有一个UIImageView兼容的视图类，可以透明地执行所有缓存，适合在UITableViewCells中使用，其中滚动性能非常重要。

缓存URL图片iphone UITableview

5 个答案: