SDWebImage开源库详细使用指南

@SDWebImage提供一个UIImageView的类别以支持加载来自网络的远程图片。具有缓存管理、异步下载、同一个URL下载次数控制和优化等特征.

它提供了:

• 添加网络图像和缓存管理到Cocoa Touch framework的UIImageView类别
• 异步图像下载
• An asynchronous memory + disk image caching with automatic cache expiration handling
• 支持GIF动画
• 支持WebP格式
• 后台图像解压
• 保证相同的url不会下载多次
• 保证伪造的URL不会尝试一遍又一遍的下载
• 保证主线程永远不会被阻塞
• Performances!
• 使用GCD和ARC

@目前来说,主要用到了"UIImageView+WebCache.h",给出一个代码示例:

        它是UIImageView的一个类目,在要使用它的类中加入#import "UIImageView+WebCache.h",调用                setImageWithURL:placeholderImage:方法。从异步下载到缓存管理，一切都会为你处理。

@得到1个图片的url用SDWebImage缓存后的文件名

NSLog(@"%s__%d__|%@",__FUNCTION__,__LINE__,[[SDImageCache sharedImageCache] cachedFileNameForKey:@"http://c.hiphotos.baidu.com/image/w%3D2048/sign=396e9d640b23dd542173a068e531b2de/cc11728b4710b9123a8117fec1fdfc039245226a.jpg"]); 

对于不可见的ImageView取消图片的异步加载

不可见的ImageView加载图片的必要性大大降低，取消其加载任务可以加速其他图片的加载，也有助于减少资源占用。特别是在可以reuse的view中，一定要记得在prepareForReuse中取消掉当前imageView的图片加载。否则之前的加载任务，还会影响到当前reuse以前视图的imageView.

- (void)prepareForReuse
{
    [super prepareForReuse];
    [_imageView cancelCurrentImageLoad];
}

非必要情况下，避免使用SDWebImageManager

SDWebImageManager是UIImageView+WebCache的底层实现，通过SDWebImageManager可以实现异步的下载UIImage，官方也给出了示例代码。

SDWebImageManager *manager = [SDWebImageManager sharedManager];
[manager downloadWithURL:imageURL
 options:0
 progress:^(NSInteger receivedSize, NSInteger expectedSize)
 {
 // progression tracking code
 }
 completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished)
 {
 if (image)
 {
 // do something with image
 }
 }];

但是SDWebImageManager并没有提供取消特定任务的方法，想要取消下载任务，会很麻烦。

如果只是希望使用SDWebImageManager实现pregress的效果显示加载进度。UIImageView+WebCache也提供了类似的方法。

[_imageView setImageWithURL:[NSURL URLWithString:url]
 placeholderImage:nil
 options:0
 progress:^(NSInteger receivedSize, NSInteger expectedSize) {
 [weakSelf.downloadProgressLabel setText:[NSString stringWithFormat:@"%d%%",(int)(receivedSize*100/expectedSize)]];
 }
 completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType) {
 _downloadProgressLabel.hidden = YES;
 }];

正如之前说的，使用[_imageView cancleCurrentImageLoad]可以取消这个ImageView的图片加载。

如何使用

API 文档地址 http://hackemist.com/SDWebImage/doc/。

Using UIImageView+WebCache category with UITableView

#import 

...

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *MyIdentifier = @"MyIdentifier";

    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:MyIdentifier];

    if (cell == nil)
    {
        cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault
                                       reuseIdentifier:MyIdentifier] autorelease];
    }

    // Here we use the new provided setImageWithURL: method to load the web image
    [cell.imageView setImageWithURL:[NSURL URLWithString:@"http://www.domain.com/path/to/image.jpg"]
                   placeholderImage:[UIImage imageNamed:@"placeholder.png"]];

    cell.textLabel.text = @"My Text";
    return cell;
}

// Here we use the new provided setImageWithURL: method to load the web image
[cell.imageView setImageWithURL:[NSURL URLWithString:@"http://www.domain.com/path/to/image.jpg"]
               placeholderImage:[UIImage imageNamed:@"placeholder.png"]
                      completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType) {... completion code here ...}];

Using SDWebImageManager

      下面是如何使用SDWebImageManager的代码：

SDWebImageManager *manager = [SDWebImageManager sharedManager];
[manager downloadWithURL:imageURL
                 options:0
                 progress:^(NSUInteger receivedSize, long long expectedSize)
                 {
                     // progression tracking code
                 }
                 completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType)
                 {
                     if (image)
                     {
                         // do something with image
                     }
                 }];

[SDWebImageDownloader.sharedDownloader downloadImageWithURL:imageURL
                                                    options:0
                                                   progress:^(NSUInteger receivedSize, long long expectedSize)
                                                   {
                                                       // progression tracking code
                                                   }
                                                   completed:^(UIImage *image, NSData *data, NSError *error, BOOL finished)
                                                   {
                                                       if (image && finished)
                                                       {
                                                           // do something with image
                                                       }
                                                   }];

SDImageCache *imageCache = [SDImageCache.alloc initWithNamespace:@"myNamespace"];
[imageCache queryDiskCacheForKey:myCacheKey done:^(UIImage *image)
{
    // image is not nil if image was found
}];

[[SDImageCache sharedImageCache] storeImage:myImage forKey:myCacheKey];

[[SDImageCache sharedImageCache]clearDisk];
[[SDImageCache sharedImageCache]clearMemory];

// Subscribe to app events
[[NSNotificationCenter defaultCenter] addObserver:self
 selector:@selector(clearMemory)
 name:UIApplicationDidReceiveMemoryWarningNotification
 object:nil];

[[NSNotificationCenter defaultCenter] addObserver:self
 selector:@selector(cleanDisk)
 name:UIApplicationWillTerminateNotification
 object:nil];

[[NSNotificationCenter defaultCenter] addObserver:self
 selector:@selector(backgroundCleanDisk)
 name:UIApplicationDidEnterBackgroundNotification
 object:nil];

[SDImageCache sharedImageCache].maxCacheAge = 1 * 24 * 60 * 60; // 1 day

static const NSInteger kDefaultCacheMaxCacheAge = 60 * 60 * 24 * 7; // 1 week

typedef NS_ENUM(NSInteger, SDImageCacheType) {
 /**
 * The image wasn't available the SDWebImage caches, but was downloaded from the web.
 */
 SDImageCacheTypeNone,
 /**
 * The image was obtained from the disk cache.
 */
 SDImageCacheTypeDisk,
 /**
 * The image was obtained from the memory cache.
 */
 SDImageCacheTypeMemory
};

//other code
completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType) {
 _imageView.alpha = 0;
 if (cacheType == SDImageCacheTypeNone){
 [UIView animateWithDuration:0.3 animations:^{
 _imageView.alpha = 1;
 }];
 }
}];

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    SDWebImageManager.sharedManager.cacheKeyFilter:^(NSURL *url)
    {
        url = [[[NSURL alloc] initWithScheme:url.scheme host:url.host path:url.path] autorelease];
        return [url absoluteString];
    };

    // Your app init code...
    return YES;
}

[imageView setImageWithURL:[NSURL URLWithString:@"https://graph.facebook.com/olivier.poitrey/picture"]
          placeholderImage:[UIImage imageNamed:@"avatar-placeholder.png"]
                   options:SDWebImageRefreshCached];

SDWebImageDecoder的作用

SDWebImageDecoder只包含一个静态方法：

+ (UIImage *)decodedImageWithImage:(UIImage *)image;

该方法用来对已经加载的UIImage进行解码。

由于UIImage的imageWithData函数是每次画图的时候才将Data解压成ARGB的图像， 
所以在每次画图的时候，会有一个解压操作，这样效率很低，但是只有瞬时的内存需求。 
为了提高效率通过SDWebImageDecoder将包装在Data下的资源解压，然后画在另外一张图片上，这样这张新图片就不再需要重复解压了。 
这种做法是典型的空间换时间的做法。

上面是这段话来自摇滚诗人 http://www.cnblogs.com/biosli/archive/2012/07/21.html

显示GIF图片

SDWebImage显示Gif很占内存，这和SDWebImage对GIF的解码方式有关，相关代码如下：

+ (UIImage *)sd_animatedGIFWithData:(NSData *)data {
 if (!data) {
 return nil;
 }
 CGImageSourceRef source = CGImageSourceCreateWithData((__bridge CFDataRef)data, NULL);
 size_t count = CGImageSourceGetCount(source); 
 UIImage *animatedImage;
 if (count <= 1) {
 animatedImage = [[UIImage alloc] initWithData:data];
 } else {
 NSMutableArray *images = [NSMutableArray array];
 NSTimeInterval duration = 0.0f;
 for (size_t i = 0; i < count; i++) {
 CGImageRef image = CGImageSourceCreateImageAtIndex(source, i, NULL);
 duration += [self frameDurationAtIndex:i source:source];
 [images addObject:[UIImage imageWithCGImage:image scale:[UIScreen mainScreen].scale orientation:UIImageOrientationUp]];
 CGImageRelease(image);
 }
 if (!duration) {
 duration = (1.0f / 10.0f) * count;
 }
 animatedImage = [UIImage animatedImageWithImages:images duration:duration];
 }
 CFRelease(source);
 return animatedImage;
}

这里的解码方式，单独对gif的每一帧进行解码，然后加入到animatedImage里面，这样，对于帧数很多的gif，则非常的占用内存。如果对GIF显示并不在乎GIF的显示的话，可以修改size_t count = CGImageSourceGetCount(source); 这段代码为sizt_t count = 1;这样GIF图片只会渲染第一帧，也就变成了一个静态图片展示，内存消耗也变得可以接受的范围。当然，如果要坚持使用GIF动画的话，也可以使用其他的开源项目，有些开源项目可以以占用内存非常少得方式显示GIF动画，比如说VVeboImage.

Add a progress indicator（添加进度指示）

Installation（安装）

参考：http://blog.csdn.net/shenjx1225/article/details/10444449

http://www.mzule.com/posts/SDWebImage_usage_experience.html