SDWebImage源码探究（四）

UIImageView+normal 下载图片

　　这个应该是SDWebImage的最基本的使用了，就是利用给定的URL下载图片，采用的是UIImageView的分类webcache.

代码：

#import "JJWebImageVC.h"
#import "Masonry.h"
#import "UIImageView+WebCache.h"

@interface JJWebImageVC ()

@property (nonatomic, strong) UIImageView *imageView;


@end

@implementation JJWebImageVC

#pragma mark - Override Base Function

- (void)viewDidLoad
{
    [super viewDidLoad];
    
    self.view.backgroundColor = [UIColor lightGrayColor];
    
    [self setupUI];
    
    [self loadData];
}

#pragma mark - Object Private Function

- (void)setupUI
{
    UIImageView *imageView = [[UIImageView alloc] init];
    [self.view addSubview:imageView];
    self.imageView = imageView;
    
    [imageView mas_makeConstraints:^(MASConstraintMaker *make) {
        make.center.equalTo(self.view);
        make.height.width.equalTo(@200);
    }];
}

- (void)loadData
{
    [self.imageView sd_setImageWithURL:[NSURL URLWithString:@"http://picture.51rebo.cn/149991503036592"] placeholderImage:[UIImage imageNamed:@"placeholder"]];
}

@end

图片下载除了上面这个方法还有下面几个方法

- (void)sd_setImageWithURL:(nullable NSURL *)url
          placeholderImage:(nullable UIImage *)placeholder
                   options:(SDWebImageOptions)options;

- (void)sd_setImageWithURL:(nullable NSURL *)url
          placeholderImage:(nullable UIImage *)placeholder
                 completed:(nullable SDExternalCompletionBlock)completedBlock;

- (void)sd_setImageWithURL:(nullable NSURL *)url
          placeholderImage:(nullable UIImage *)placeholder
                   options:(SDWebImageOptions)options
                 completed:(nullable SDExternalCompletionBlock)completedBlock;

- (void)sd_setImageWithURL:(nullable NSURL *)url
          placeholderImage:(nullable UIImage *)placeholder
                   options:(SDWebImageOptions)options
                  progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                 completed:(nullable SDExternalCompletionBlock)completedBlock;

这里最后面这个方法的参数是最多的，达到了5个，多的不说了，主要说一下progress这个是下载进度的回调，completed下载完成的block，options这个参数是最重要的，是一个枚举值，主要对下载图片的情况进行配置，下面我们就看一下这个枚举值。

typedef NS_OPTIONS(NSUInteger, SDWebImageOptions) {
    /**
     * By default, when a URL fail to be downloaded, the URL is blacklisted so the library won't keep trying.
     * This flag disable this blacklisting.
     */
    //默认的情况下，当一个URL下载失败了，那么这个URL会被放在黑名单里面，不会继续尝试，这个标志位就是使黑名单效果失效，保证失败后可以重新尝试。
    SDWebImageRetryFailed = 1 << 0,

    /**
     * By default, image downloads are started during UI interactions, this flags disable this feature,
     * leading to delayed download on UIScrollView deceleration for instance.
     */
    //默认情况下，在UI交互的情况下就开始图像下载，该标志位使该特性失效，例如在UIScrollView减速时延迟下载。
    SDWebImageLowPriority = 1 << 1,

    /**
     * This flag disables on-disk caching
     */
    //该标志位使磁盘缓存失效，只是内存缓存。
    SDWebImageCacheMemoryOnly = 1 << 2,

    /**
     * This flag enables progressive download, the image is displayed progressively during download as a browser would do.
     * By default, the image is only displayed once completely downloaded.
     */
    //该标志位可以确保下载的回调，图像可以边下载边显示
    SDWebImageProgressiveDownload = 1 << 3,

    /**
     * Even if the image is cached, respect the HTTP response cache control, and refresh the image from remote location if needed.
     * The disk caching will be handled by NSURLCache instead of SDWebImage leading to slight performance degradation.
     * This option helps deal with images changing behind the same request URL, e.g. Facebook graph api profile pics.
     * If a cached image is refreshed, the completion block is called once with the cached image and again with the final image.
     *
     * Use this flag only if you can't make your URLs static with embedded cache busting parameter.
     */
    //刷新缓存
    SDWebImageRefreshCached = 1 << 4,

    /**
     * In iOS 4+, continue the download of the image if the app goes to background. This is achieved by asking the system for
     * extra time in background to let the request finish. If the background task expires the operation will be cancelled.
     */
    //后台下载
    SDWebImageContinueInBackground = 1 << 5,

    /**
     * Handles cookies stored in NSHTTPCookieStore by setting
     * NSMutableURLRequest.HTTPShouldHandleCookies = YES;
     */
    //处理NSHTTPCookieStore中存储的cookie
    SDWebImageHandleCookies = 1 << 6,

    /**
     * Enable to allow untrusted SSL certificates.
     * Useful for testing purposes. Use with caution in production.
     */
      //允许使用无效的SSL整数，但是仅用于测试，生产环境要小心
    SDWebImageAllowInvalidSSLCertificates = 1 << 7,

    /**
     * By default, images are loaded in the order in which they were queued. This flag moves them to
     * the front of the queue.
     */
    //默认的情况，图像下载都是按照队列内部的顺序进行的，这个标志位就是提高优先级，让其移动到队列的首位。
    SDWebImageHighPriority = 1 << 8,
    
    /**
     * By default, placeholder images are loaded while the image is loading. This flag will delay the loading
     * of the placeholder image until after the image has finished loading.
     */
    //默认的情况下是占位图在下载图像的时候加载，这个标志位延迟占位图的加载，直到图像下载完成。
    SDWebImageDelayPlaceholder = 1 << 9,

    /**
     * We usually don't call transformDownloadedImage delegate method on animated images,
     * as most transformation code would mangle it.
     * Use this flag to transform them anyway.
     */
    //我们一般不调用transformDownloadedImage代理方法处理动画图片，因为大多数的转换代码会损坏它，使用这个标志位可以转换它们。
    SDWebImageTransformAnimatedImage = 1 << 10,
    
    /**
     * By default, image is added to the imageView after download. But in some cases, we want to
     * have the hand before setting the image (apply a filter or add it with cross-fade animation for instance)
     * Use this flag if you want to manually set the image in the completion when success
     */
    //默认情况下，图像在下载完成后都是加载到UIImageview上，但是在一些情况下，我们想要在设置图像之前处理它，比如说加一个滤镜或者褪色的动画效果等等，使用这个标志位，如果你想要手动的在成功完成回调后设置这个图片。
    SDWebImageAvoidAutoSetImage = 1 << 11,
    
    /**
     * By default, images are decoded respecting their original size. On iOS, this flag will scale down the
     * images to a size compatible with the constrained memory of devices.
     * If `SDWebImageProgressiveDownload` flag is set the scale down is deactivated.
     */
    //默认情况下，图像都是根据他们原始尺寸进行解码，在ios中，这个标志位会缩小图片，以便能够兼容器件内存的限制，如果SDWebImageProgressiveDownload被设置了，那么这个位就失效了。
    SDWebImageScaleDownLargeImages = 1 << 12
};

UIImageView+highlightedImage图片下载

这个方法也是UIImageView的分类，专门用来下载highlightedImage。调用的是下面这个方法。

- (void)sd_setHighlightedImageWithURL:(nullable NSURL *)url
                              options:(SDWebImageOptions)options
                             progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                            completed:(nullable SDExternalCompletionBlock)completedBlock;

- (void)sd_internalSetImageWithURL:(nullable NSURL *)url
                  placeholderImage:(nullable UIImage *)placeholder
                           options:(SDWebImageOptions)options
                      operationKey:(nullable NSString *)operationKey
                     setImageBlock:(nullable SDSetImageBlock)setImageBlock
                          progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                         completed:(nullable SDExternalCompletionBlock)completedBlock;

判断是否是gif图

这个要根据UIImage的一个分类UIImage+GIF进行判断。UIImage对象有一个属性如下：

@property(nullable, nonatomic,readonly) NSArray<UIImage *> *images   NS_AVAILABLE_IOS(5_0); // default is nil for non-animated images

根据NSData创建动画的图像

这个方法也在分类UIImage+GIF里面，如下所示。

/**
 *  Compatibility method - creates an animated UIImage from an NSData, it will only contain the 1st frame image
 */
+ (UIImage *)sd_animatedGIFWithData:(NSData *)data;

UIButton设置图像

该框架也提供了UIButton设置图像的方法，以及设置背景图像的方法，还有就是取消的方法，下面我们接着看。

1.设置图像的方法

#pragma mark - Image

/**
 * Get the image URL for a control state.
 * 
 * @param state Which state you want to know the URL for. The values are described in UIControlState.
 */
- (nullable NSURL *)sd_imageURLForState:(UIControlState)state;

/**
 * Set the imageView `image` with an `url`.
 *
 * The download is asynchronous and cached.
 *
 * @param url   The url for the image.
 * @param state The state that uses the specified title. The values are described in UIControlState.
 */
- (void)sd_setImageWithURL:(nullable NSURL *)url
                  forState:(UIControlState)state;

/**
 * Set the imageView `image` with an `url` and a placeholder.
 *
 * The download is asynchronous and cached.
 *
 * @param url         The url for the image.
 * @param state       The state that uses the specified title. The values are described in UIControlState.
 * @param placeholder The image to be set initially, until the image request finishes.
 * @see sd_setImageWithURL:placeholderImage:options:
 */
- (void)sd_setImageWithURL:(nullable NSURL *)url
                  forState:(UIControlState)state
          placeholderImage:(nullable UIImage *)placeholder;

/**
 * Set the imageView `image` with an `url`, placeholder and custom options.
 *
 * The download is asynchronous and cached.
 *
 * @param url         The url for the image.
 * @param state       The state that uses the specified title. The values are described in UIControlState.
 * @param placeholder The image to be set initially, until the image request finishes.
 * @param options     The options to use when downloading the image. @see SDWebImageOptions for the possible values.
 */
- (void)sd_setImageWithURL:(nullable NSURL *)url
                  forState:(UIControlState)state
          placeholderImage:(nullable UIImage *)placeholder
                   options:(SDWebImageOptions)options;

/**
 * Set the imageView `image` with an `url`.
 *
 * The download is asynchronous and cached.
 *
 * @param url            The url for the image.
 * @param state          The state that uses the specified title. The values are described in UIControlState.
 * @param completedBlock A block called when operation has been completed. This block has no return value
 *                       and takes the requested UIImage as first parameter. In case of error the image parameter
 *                       is nil and the second parameter may contain an NSError. The third parameter is a Boolean
 *                       indicating if the image was retrieved from the local cache or from the network.
 *                       The fourth parameter is the original image url.
 */
- (void)sd_setImageWithURL:(nullable NSURL *)url
                  forState:(UIControlState)state
                 completed:(nullable SDExternalCompletionBlock)completedBlock;

/**
 * Set the imageView `image` with an `url`, placeholder.
 *
 * The download is asynchronous and cached.
 *
 * @param url            The url for the image.
 * @param state          The state that uses the specified title. The values are described in UIControlState.
 * @param placeholder    The image to be set initially, until the image request finishes.
 * @param completedBlock A block called when operation has been completed. This block has no return value
 *                       and takes the requested UIImage as first parameter. In case of error the image parameter
 *                       is nil and the second parameter may contain an NSError. The third parameter is a Boolean
 *                       indicating if the image was retrieved from the local cache or from the network.
 *                       The fourth parameter is the original image url.
 */
- (void)sd_setImageWithURL:(nullable NSURL *)url
                  forState:(UIControlState)state
          placeholderImage:(nullable UIImage *)placeholder
                 completed:(nullable SDExternalCompletionBlock)completedBlock;

/**
 * Set the imageView `image` with an `url`, placeholder and custom options.
 *
 * The download is asynchronous and cached.
 *
 * @param url            The url for the image.
 * @param state          The state that uses the specified title. The values are described in UIControlState.
 * @param placeholder    The image to be set initially, until the image request finishes.
 * @param options        The options to use when downloading the image. @see SDWebImageOptions for the possible values.
 * @param completedBlock A block called when operation has been completed. This block has no return value
 *                       and takes the requested UIImage as first parameter. In case of error the image parameter
 *                       is nil and the second parameter may contain an NSError. The third parameter is a Boolean
 *                       indicating if the image was retrieved from the local cache or from the network.
 *                       The fourth parameter is the original image url.
 */
- (void)sd_setImageWithURL:(nullable NSURL *)url
                  forState:(UIControlState)state
          placeholderImage:(nullable UIImage *)placeholder
                   options:(SDWebImageOptions)options
                 completed:(nullable SDExternalCompletionBlock)completedBlock;

2.获取按钮图像的URL

/**
 * Get the current image URL.
 */
- (nullable NSURL *)sd_currentImageURL;

3、获取按钮背景backgroundImage

#pragma mark - Background image

/**
 * Set the backgroundImageView `image` with an `url`.
 *
 * The download is asynchronous and cached.
 *
 * @param url   The url for the image.
 * @param state The state that uses the specified title. The values are described in UIControlState.
 */
- (void)sd_setBackgroundImageWithURL:(nullable NSURL *)url
                            forState:(UIControlState)state;

/**
 * Set the backgroundImageView `image` with an `url` and a placeholder.
 *
 * The download is asynchronous and cached.
 *
 * @param url         The url for the image.
 * @param state       The state that uses the specified title. The values are described in UIControlState.
 * @param placeholder The image to be set initially, until the image request finishes.
 * @see sd_setImageWithURL:placeholderImage:options:
 */
- (void)sd_setBackgroundImageWithURL:(nullable NSURL *)url
                            forState:(UIControlState)state
                    placeholderImage:(nullable UIImage *)placeholder;

/**
 * Set the backgroundImageView `image` with an `url`, placeholder and custom options.
 *
 * The download is asynchronous and cached.
 *
 * @param url         The url for the image.
 * @param state       The state that uses the specified title. The values are described in UIControlState.
 * @param placeholder The image to be set initially, until the image request finishes.
 * @param options     The options to use when downloading the image. @see SDWebImageOptions for the possible values.
 */
- (void)sd_setBackgroundImageWithURL:(nullable NSURL *)url
                            forState:(UIControlState)state
                    placeholderImage:(nullable UIImage *)placeholder
                             options:(SDWebImageOptions)options;

/**
 * Set the backgroundImageView `image` with an `url`.
 *
 * The download is asynchronous and cached.
 *
 * @param url            The url for the image.
 * @param state          The state that uses the specified title. The values are described in UIControlState.
 * @param completedBlock A block called when operation has been completed. This block has no return value
 *                       and takes the requested UIImage as first parameter. In case of error the image parameter
 *                       is nil and the second parameter may contain an NSError. The third parameter is a Boolean
 *                       indicating if the image was retrieved from the local cache or from the network.
 *                       The fourth parameter is the original image url.
 */
- (void)sd_setBackgroundImageWithURL:(nullable NSURL *)url
                            forState:(UIControlState)state
                           completed:(nullable SDExternalCompletionBlock)completedBlock;

/**
 * Set the backgroundImageView `image` with an `url`, placeholder.
 *
 * The download is asynchronous and cached.
 *
 * @param url            The url for the image.
 * @param state          The state that uses the specified title. The values are described in UIControlState.
 * @param placeholder    The image to be set initially, until the image request finishes.
 * @param completedBlock A block called when operation has been completed. This block has no return value
 *                       and takes the requested UIImage as first parameter. In case of error the image parameter
 *                       is nil and the second parameter may contain an NSError. The third parameter is a Boolean
 *                       indicating if the image was retrieved from the local cache or from the network.
 *                       The fourth parameter is the original image url.
 */
- (void)sd_setBackgroundImageWithURL:(nullable NSURL *)url
                            forState:(UIControlState)state
                    placeholderImage:(nullable UIImage *)placeholder
                           completed:(nullable SDExternalCompletionBlock)completedBlock;

/**
 * Set the backgroundImageView `image` with an `url`, placeholder and custom options.
 *
 * The download is asynchronous and cached.
 *
 * @param url            The url for the image.
 * @param placeholder    The image to be set initially, until the image request finishes.
 * @param options        The options to use when downloading the image. @see SDWebImageOptions for the possible values.
 * @param completedBlock A block called when operation has been completed. This block has no return value
 *                       and takes the requested UIImage as first parameter. In case of error the image parameter
 *                       is nil and the second parameter may contain an NSError. The third parameter is a Boolean
 *                       indicating if the image was retrieved from the local cache or from the network.
 *                       The fourth parameter is the original image url.
 */
- (void)sd_setBackgroundImageWithURL:(nullable NSURL *)url
                            forState:(UIControlState)state
                    placeholderImage:(nullable UIImage *)placeholder
                             options:(SDWebImageOptions)options
                           completed:(nullable SDExternalCompletionBlock)completedBlock;

4、按钮下载图片的取消方法

/**
 * Cancel the current image download
    //取消图片下载
 */
- (void)sd_cancelImageLoadForState:(UIControlState)state;

/**
 * Cancel the current backgroundImage download
    //取消背景图片的下载
 */
- (void)sd_cancelBackgroundImageLoadForState:(UIControlState)state;

判断获取图片的格式

我们根据NSdata获取图片，有的时候需要知道获取图片的格式，分类NSData+ImageContentType就很好的解决了这个问题，我们先看这个方法。

typedef NS_ENUM(NSInteger, SDImageFormat) {
    SDImageFormatUndefined = -1,
    SDImageFormatJPEG = 0,
    SDImageFormatPNG,
    SDImageFormatGIF,
    SDImageFormatTIFF,
    SDImageFormatWebP
};

@interface NSData (ImageContentType)

/**
 *  Return image format
 *
 *  @param data the input image data
 *
 *  @return the image format as `SDImageFormat` (enum)
 */
+ (SDImageFormat)sd_imageFormatForImageData:(nullable NSData *)data;

@end

它的返回值是一个枚举，会返回6中类型，其中一个是未定义类型，下面我们就研究下该方法是如何实现的。

+ (SDImageFormat)sd_imageFormatForImageData:(nullable NSData *)data {
    if (!data) {
        return SDImageFormatUndefined;
    }
    
    uint8_t c;
    [data getBytes:&c length:1];
    switch (c) {
        case 0xFF:
            return SDImageFormatJPEG;
        case 0x89:
            return SDImageFormatPNG;
        case 0x47:
            return SDImageFormatGIF;
        case 0x49:
        case 0x4D:
            return SDImageFormatTIFF;
        case 0x52:
            // R as RIFF for WEBP
            if (data.length < 12) {
                return SDImageFormatUndefined;
            }
            
            NSString *testString = [[NSString alloc] initWithData:[data subdataWithRange:NSMakeRange(0, 12)] encoding:NSASCIIStringEncoding];
            if ([testString hasPrefix:@"RIFF"] && [testString hasSuffix:@"WEBP"]) {
                return SDImageFormatWebP;
            }
    }
    return SDImageFormatUndefined;
}

缓存图像

缓存图像这里就包括缓存到disk和memory两种情况。涉及到的类主要就是SDImageCache。下面四中方法都是缓存图像到内存和硬盘，前三种方法都是异步存储，后面一种方法是同步存储。

#pragma mark - Store Ops

/**
 * Asynchronously store an image into memory and disk cache at the given key.
//异步存储指定key对应的图片到内存和disk
 *
 * @param image           The image to store
 * @param key             The unique image cache key, usually it's image absolute URL
 * @param completionBlock A block executed after the operation is finished
 */
- (void)storeImage:(nullable UIImage *)image
            forKey:(nullable NSString *)key
        completion:(nullable SDWebImageNoParamsBlock)completionBlock;

/**
 * Asynchronously store an image into memory and disk cache at the given key.
//异步存储指定key对应的图片到内存和disk
 *
 * @param image           The image to store
 * @param key             The unique image cache key, usually it's image absolute URL
 * @param toDisk          Store the image to disk cache if YES
 * @param completionBlock A block executed after the operation is finished
 */
- (void)storeImage:(nullable UIImage *)image
            forKey:(nullable NSString *)key
            toDisk:(BOOL)toDisk
        completion:(nullable SDWebImageNoParamsBlock)completionBlock;

/**
 * Asynchronously store an image into memory and disk cache at the given key.
//异步存储指定key对应的图片到内存和disk
 *
 * @param image           The image to store
 * @param imageData       The image data as returned by the server, this representation will be used for disk storage
 *                        instead of converting the given image object into a storable/compressed image format in order
 *                        to save quality and CPU
 * @param key             The unique image cache key, usually it's image absolute URL
 * @param toDisk          Store the image to disk cache if YES
 * @param completionBlock A block executed after the operation is finished
 */
- (void)storeImage:(nullable UIImage *)image
         imageData:(nullable NSData *)imageData
            forKey:(nullable NSString *)key
            toDisk:(BOOL)toDisk
        completion:(nullable SDWebImageNoParamsBlock)completionBlock;

/**
 * Synchronously store image NSData into disk cache at the given key.
 //同步存储指定key对应的NSData对象到disk
 *
 * @warning This method is synchronous, make sure to call it from the ioQueue
 *
 * @param imageData  The image data to store
 * @param key        The unique image cache key, usually it's image absolute URL
 */
- (void)storeImageDataToDisk:(nullable NSData *)imageData forKey:(nullable NSString *)key;