SDWebImage的底层介绍以及——需要了解和掌握的要点（包括面试题，图片格式，以及GIF的播放）

た入场券 2022-08-09 12:59 126阅读 0赞

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具体如下所示：

官方网站

$ git clone --recursive https://github.com/rs/SDWebImage.git

1> 面试题

**1\]如何防止一个url对应的图片重复下载**

**\* "cell下载图片思路 –有沙盒缓存"**

**2\] SDWebImage的默认缓存时长是多少？**

**\* 1个星期**

**3\] SDWebImage底层是怎么实现的？**

**\* 上课PPT的"cell下载图片思路 –有沙盒缓存"**

2>常用方法

\- (void)sd\_setImageWithURL:(NSURL \*)url placeholderImage:(UIImage \*)placeholder;

\- (void)sd\_setImageWithURL:(NSURL \*)url placeholderImage:(UIImage \*)placeholder options:(SDWebImageOptions)options;

\- (void)sd\_setImageWithURL:(NSURL \*)url placeholderImage:(UIImage \*)placeholder completed:(SDWebImageCompletionBlock)completedBlock;

\- (void)sd\_setImageWithURL:(NSURL \*)url placeholderImage:(UIImage \*)placeholder options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletionBlock)completedBlock;

**3>内存处理：当app接收到内存警告时，SDWebImage做了什么？ 在SDWebImageCache.m中可以找到**

**- SDWebImage会监听系统的UIApplicationDidReceiveMemoryWarningNotification通知，一旦收到通知，就会清理内存**

**- 应用程序将要终止的通知,UIApplicationWillTerminateNotification,清理磁盘**

**- 应用程序进入后台的通知，UIApplicationDidEnterBackgroundNotification，也会清理磁盘**

4> SDWebImageOptions

**\* SDWebImageRetryFailed :下载失败后，会自动重新下载**

**\* SDWebImageLowPriority :当正在进行UI交互时，自动暂停内部的一些下载操作**

**\* SDWebImageRetryFailed | SDWebImageLowPriority :拥有上面2个功能**

// 图片的格式

PNG: 无损压缩！压缩比较低。 PNG图片一般会JPG大。

\- GPU解压缩的消耗非常小。解压缩的速度比较快，比较清晰，苹果推荐使用

JPG: 有损压缩！压缩比非常高！照相机使用。

\- GPU解压缩的消耗比较大

GIF: 可动画的图片

BMP: (位图)，没有任何压缩。几乎不用

关于SDwebImage中枚举参数options有以下几种，用到时可以查阅

"留待查阅"
    typedefNS_OPTIONS(NSUInteger, SDWebImageOptions) {
        /**
         * By default, when a URL fail to be downloaded, the URL is blacklisted so the library won't keep trying.
         * 默认情况下，当一个 URL下载失败，会将该 URL放在黑名单中，不再尝试下载
         *
         * This flag disable this blacklisting.
         * 此标记取消黑名单
         */
        SDWebImageRetryFailed =1 <<0,
        
        /**
         * By default, image downloads are started during UI interactions, this flags disable this feature,
         * 默认情况下，在 UI交互时也会启动图像下载，此标记取消这一特性
         *
         * leading to delayed download on UIScrollView deceleration for instance.
         * 会推迟到滚动视图停止滚动之后再继续下载(这个注释是假的！)
         */
        SDWebImageLowPriority =1 <<1,
        
        /**
         * This flag disables on-disk caching
         * 此标记取消磁盘缓存，只有内存缓存
         */
        SDWebImageCacheMemoryOnly =1 <<2,
        
        /**
         * This flag enables progressive download, the image is displayed progressively during download as a browser would do.
         * 此标记允许渐进式下载，就像浏览器中那样，下载过程中，图像会逐步显示出来
         *
         * By default, the image is only displayed once completely downloaded.
         * 默认情况下，图像会在下载完成后一次性显示
         */
        SDWebImageProgressiveDownload =1 <<3,
        
        /**
         * Even if the image is cached, respect the HTTP response cache control, and refresh the image from remote location if needed.
         * 即使图像被缓存，遵守 HTPP响应的缓存控制，如果需要，从远程刷新图像
         *
         * The disk caching will be handled by NSURLCache instead of SDWebImage leading to slight performance degradation.
         * 磁盘缓存将由 NSURLCache处理，而不是 SDWebImage，这会对性能有轻微的影响
         *
         * This option helps deal with images changing behind the same request URL, e.g. Facebook graph api profile pics.
         * 此选项有助于处理同一个请求 URL的图像发生变化
         *
         * If a cached image is refreshed, the completion block is called once with the cached image and again with the final image.
         * 如果缓存的图像被刷新，会调用一次 completion block，并传递最终的图像
         *
         * Use this flag only if you can't make your URLs static with embeded cache busting parameter.
         * 仅在无法使用嵌入式缓存清理参数确定图像 URL时，使用此标记
         */
        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
         * 在 iOS 4+，当 App进入后台后仍然会继续下载图像。这是向系统请求额外的后台时间以保证下载请求完成的
         *
         * extra time in background to let the request finish. If the background task expires the operation will be cancelled.
         * 如果后台任务过期，请求将会被取消
         */
        SDWebImageContinueInBackground =1 <<5,
        
        /**
         * Handles cookies stored in NSHTTPCookieStore by setting
         * 通过设置
         * NSMutableURLRequest.HTTPShouldHandleCookies = YES;
         * 处理保存在 NSHTTPCookieStore中的 cookies
         */
        SDWebImageHandleCookies =1 <<6,
         /**
         * Enable to allow untrusted SSL ceriticates.
         * 允许不信任的 SSL证书
         *
         * Useful for testing purposes. Use with caution in production.
         * 可以出于测试目的使用，在正式产品中慎用
         */
        SDWebImageAllowInvalidSSLCertificates =1 << 7,
        
        /**
         * By default, image are loaded in the order they were queued. This flag move them to
         * 默认情况下，图像会按照在队列中的顺序被加载，此标记会将它们移动到队列前部立即被加载
         *
         * the front of the queue and is loaded immediately instead of waiting for the current queue to be loaded (which
         * 而不是等待当前队列被加载，等待队列加载会需要一段时间
         * could take a while).
         */
        SDWebImageHighPriority =1 <<8,
        
        /**
         * By default, placeholder images are loaded while the image is loading. This flag will delay the loading
         * 默认情况下，在加载图像时，占位图像已经会被加载。而此标记会延迟加载占位图像，直到图像已经完成加载
         *
         * of the placeholder image until after the image has finished loading.
         */
        SDWebImageDelayPlaceholder =1 <<9,
        
        /**
         * We usually don't call transformDownloadedImage delegate method on animated images,
         * 通常不会在可动画的图像上调用 transformDownloadedImage代理方法，因为大多数转换代码会破坏动画文件
         *
         * as most transformation code would mangle it.
         * Use this flag to transform them anyway.
         * 使用此标记尝试转换
         */
        SDWebImageTransformAnimatedImage =1 <<10,
    };

**SDWebImage播放GIF**

//
    //  ViewController.m
    //  SDWebImage播放GIF
    //
    //  Created by apple on 15/10/25.
    //  Copyright (c) 2015年 LiuXun. All rights reserved.
    //
    
    #import "ViewController.h"
    #import "UIImageView+WebCache.h"
    #import "UIImage+GIF.h"
    @interface ViewController ()
    @property(nonatomic, strong) UIImageView *imageVIew;
    @end
    
    @implementation ViewController
    
    -(void)viewDidLoad
    {
        self.imageVIew = [[UIImageView alloc] initWithFrame:self.view.bounds ];
        [self.view addSubview:self.imageVIew];
    }
    
    -(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
    {
        [self.imageVIew sd_setImageWithURL:[[NSBundle mainBundle] URLForResource:@"bd698b0fjw1e0ph8unyg6g.gif" withExtension:nil]];
    }
    
    /**
     PNG: 无损压缩！ 压缩程度没有JPG高。PNG图片一般比JPG大，比较清晰，解压速度比较快, GPU解压消耗比较小，苹果推荐使用。
     JPG：有损压缩！压缩比非常高！照相机使用 GPU压缩的消耗比较大
     GIF: 可动画的图片
     BMP: 位图 没有任何压缩，无论如何放大都不会失真。开发中一般不使用。
     */
    @end

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