ios - 缓存 URL 图片 iphone UITableview

Question

我正在寻找有关如何将从 url 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

但是代码不完整。我是一个客观的新手，所以我发现很难填补缺失的部分。

Answer 1

这是一个使用 NSCache 的简单 ImageCache 实现。ImageCache 是一个单例。

图像缓存.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

图像缓存.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

例子

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

在这里找到了一个很好的工作示例

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

Answer 3

您也可以尝试使用由 enormego 的敏锐研究员编写的令人敬畏的 EgoImage 库来完成此操作。它使用起来非常简单，在后台有效利用缓存，非常适合满足您的要求。

这是包含演示应用程序的库的github 路径。

Answer 4

我为我一直在开发的应用程序编写了这个（概念和一些代码取自 Lane Roathe 的优秀 UIImageView+Cache 类别）。它也使用ASIHTTPRequest类，这很棒。这绝对可以改进..例如，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.. 但它对我的目的来说效果很好。

@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 的任何所有者知道他们应该何时重新显示 - 在这种情况下，它在 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

您可能还想检查HJCache。它带有一个与 UIImageView 兼容的视图类，可以透明地执行所有缓存，适合在滚动性能很重要的 UITableViewCells 中使用。