Fix the default download config & cache config to readonly. You can modify the property of config, but not the override the default config instance itself.

SDWebImage/SDImageCacheConfig.h

@@ -13,11 +13,10 @@
 @interface SDImageCacheConfig : NSObject <NSCopying>
 
 /**
- Gets/Sets the default cache config used for shared instance or initialization when it does not provide any cache config. Such as `SDImageCache.sharedImageCache`.
+ Gets the default cache config used for shared instance or initialization when it does not provide any cache config. Such as `SDImageCache.sharedImageCache`.
  @note You can modify the property on default cache config, which can be used for later created cache instance. The already created cache instance does not get affected.
- @note You should not pass nil to this value.
  */
-@property (nonatomic, class, nonnull) SDImageCacheConfig *defaultCacheConfig;
+@property (nonatomic, class, readonly, nonnull) SDImageCacheConfig *defaultCacheConfig;
 
 /**
  * Whether or not to disable iCloud backup

SDWebImage/SDImageCacheConfig.m

@@ -23,12 +23,6 @@ static const NSInteger kDefaultCacheMaxCacheAge = 60 * 60 * 24 * 7; // 1 week
     return _defaultCacheConfig;
 }
 
-+ (void)setDefaultCacheConfig:(SDImageCacheConfig *)defaultCacheConfig {
-    if (defaultCacheConfig) {
-        _defaultCacheConfig = defaultCacheConfig;
-    }
-}
-
 - (instancetype)init {
     if (self = [super init]) {
         _shouldDisableiCloud = YES;

SDWebImage/SDWebImageDownloaderConfig.h

@@ -24,11 +24,10 @@ typedef NS_ENUM(NSInteger, SDWebImageDownloaderExecutionOrder) {
 @interface SDWebImageDownloaderConfig : NSObject <NSCopying>
 
 /**
- Gets/Sets the default downloader config used for shared instance or initialization when it does not provide any downloader config. Such as `SDWebImageDownloader.sharedDownloader`.
+ Gets the default downloader config used for shared instance or initialization when it does not provide any downloader config. Such as `SDWebImageDownloader.sharedDownloader`.
  @note You can modify the property on default downloader config, which can be used for later created downloader instance. The already created downloader instance does not get affected.
- @note You should not pass nil to this value.
  */
-@property (nonatomic, class, nonnull) SDWebImageDownloaderConfig *defaultDownloaderConfig;
+@property (nonatomic, class, readonly, nonnull) SDWebImageDownloaderConfig *defaultDownloaderConfig;
 
 /**
  * The maximum number of concurrent downloads.

SDWebImage/SDWebImageDownloaderConfig.m

@@ -20,12 +20,6 @@ static SDWebImageDownloaderConfig * _defaultDownloaderConfig;
     return _defaultDownloaderConfig;
 }
 
-+ (void)setDefaultDownloaderConfig:(SDWebImageDownloaderConfig *)defaultDownloaderConfig {
-    if (defaultDownloaderConfig) {
-        _defaultDownloaderConfig = defaultDownloaderConfig;
-    }
-}
-
 - (instancetype)init {
     self = [super init];
     if (self) {