Add SDWebImagePrefetcher, an utility class to preventively warm-up SDWebImage cache (fix #32)

Contributed by https://github.com/mystcolor

SDWebImagePrefetcher.h

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+/*
+ * This file is part of the SDWebImage package.
+ * (c) Olivier Poitrey <rs@dailymotion.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+#import <Foundation/Foundation.h>
+#import "SDWebImageManagerDelegate.h"
+
+@interface SDWebImagePrefetcher : NSObject <SDWebImageManagerDelegate>
+{
+    NSArray *_prefetchURLs;
+    NSUInteger _skippedCount;
+    NSUInteger _finishedCount;
+    NSUInteger _requestedCount;
+    NSTimeInterval _startedTime;
+}
+
+/**
+ * Maximum number of URLs to prefetch at the same time. Defaults to 3.
+ */
+@property (nonatomic, assign) NSUInteger maxConcurrentDownloads;
+
++ (SDWebImagePrefetcher *)sharedImagePrefetcher;
+
+/**
+ * Assign list of URLs to let SDWebImagePrefetcher to queue the prefetching,
+ * currently one image is downloaded at a time,
+ * and skips images for failed downloads and proceed to the next image in the list
+ *
+ * @param NSArray list of URLs to prefetch
+ */
+- (void)prefetchURLs:(NSArray *)urls;
+
+
+/**
+ * Remove and cancel queued list
+ */
+- (void)cancelPrefetching;
+
+
+@end

SDWebImagePrefetcher.m

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+/*
+ * This file is part of the SDWebImage package.
+ * (c) Olivier Poitrey <rs@dailymotion.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+#import "SDWebImagePrefetcher.h"
+#import "SDWebImageManager.h"
+
+@interface SDWebImagePrefetcher ()
+@property (nonatomic, retain) NSArray *prefetchURLs;
+@end
+
+@implementation SDWebImagePrefetcher
+
+static SDWebImagePrefetcher *instance;
+
+@synthesize prefetchURLs;
+@synthesize maxConcurrentDownloads;
+
++ (SDWebImagePrefetcher *)sharedImagePrefetcher
+{
+    if (instance == nil)
+    {
+        instance = [[SDWebImagePrefetcher alloc] init];
+        instance.maxConcurrentDownloads = 3;
+    }
+
+    return instance;
+}
+
+- (void)startPrefetchingAtIndex:(NSUInteger)index withManager:(SDWebImageManager *)imageManager
+{
+    if (index >= [self.prefetchURLs count]) return;
+    _requestedCount++;
+    [imageManager downloadWithURL:[self.prefetchURLs objectAtIndex:index] delegate:self options:SDWebImageLowPriority];
+}
+
+- (void)reportStatus
+{
+    NSUInteger total = [self.prefetchURLs count];
+    NSLog(@"Finished prefetching (%d successful, %d skipped, timeElasped %.2f)", total - _skippedCount, _skippedCount, CFAbsoluteTimeGetCurrent() - _startedTime);
+}
+
+- (void)prefetchURLs:(NSArray *)urls
+{
+    [self cancelPrefetching]; // Prevent duplicate prefetch request
+    _startedTime = CFAbsoluteTimeGetCurrent();
+    self.prefetchURLs = urls;
+
+    // Starts prefetching from the very first image on the list with the max allowed concurrency
+    int listCount = [self.prefetchURLs count];
+    SDWebImageManager *manager = [SDWebImageManager sharedManager];
+    for (int i = 0; i < self.maxConcurrentDownloads && _requestedCount < listCount; i++)
+    {
+        [self startPrefetchingAtIndex:i withManager:manager];
+    }
+}
+
+- (void)cancelPrefetching
+{
+    self.prefetchURLs = nil;
+    _skippedCount = 0;
+    _requestedCount = 0;
+    _finishedCount = 0;
+    [[SDWebImageManager sharedManager] cancelForDelegate:self];
+}
+
+#pragma mark SDWebImagePrefetcher (SDWebImageManagerDelegate)
+
+- (void)webImageManager:(SDWebImageManager *)imageManager didFinishWithImage:(UIImage *)image
+{
+    _finishedCount++;
+    NSLog(@"Prefetched %d out of %d", _finishedCount, [self.prefetchURLs count]);
+
+    if ([self.prefetchURLs count] > _requestedCount)
+    {
+        [self startPrefetchingAtIndex:_requestedCount withManager:imageManager];
+    }
+    else if (_finishedCount == _requestedCount)
+    {
+        [self reportStatus];
+    }
+}
+
+- (void)webImageManager:(SDWebImageManager *)imageManager didFailWithError:(NSError *)error
+{
+    _finishedCount++;
+    NSLog(@"Prefetched %d out of %d (Failed)", _finishedCount, [self.prefetchURLs count]);
+
+    // Add last failed
+    _skippedCount++;
+
+    if ([self.prefetchURLs count] > _requestedCount)
+    {
+        [self startPrefetchingAtIndex:_requestedCount withManager:imageManager];
+    }
+    else if (_finishedCount == _requestedCount)
+    {
+        [self reportStatus];
+    }
+}
+
+- (void)dealloc
+{
+    self.prefetchURLs = nil;
+    [super dealloc];
+}
+
+@end