SDWebImage指定缓存图片大小

这几天在做iOS上的网络图片的异步加载和缓存。网上查了下，决定用SDWebImage实现。按照网络的教程，走了一遍，没什么问题。但是程序运行起来发现加载起来的图片都变得很模糊，本人有一点平面设计的经验。推测是因为图片太大的造成的细节都是，从而使图片变的模糊。进到程序的缓存文件目录，可以看到下载的图片果真和直接从网页上下载的一样大，即图片没有经过压缩就被缓存起来了。但是我的要求应该是图片应该在保存时被压缩的啊，就像Android开源框架Afinal中的FinalBitmap做的一样。

那么作为一个iOS新手真的不想去改SDWebImage得源码，于是刚开始就一直在google上找，找了不少时间，没找到。无奈之下，只能自己尝试着改了。还好要改动的东西比较简单。只需要在保存图片时将图片压缩一下在保存就好了。工程文件可以在本文最后下载。

先上运行结果:

测试用的两张图都是1920x1080的原图，从左边可以看出位于下方的那张图片已经变得很模糊了。从右边可看出，下面那张原图的大小是2.1M，而上面的那张图片只有34k，说明，缓存文件已经被处理过了。

其实SDWebImage已经做了这方面的考虑，就是那个SDWebImageManager中的SDWebImageManagerDelegate中，

/*** Allows to transform the image immediately after it has been downloaded and just before to cache it on disk and memory.* NOTE: This method is called from a global queue in order to not to block the main thread.** @param imageManager The current `SDWebImageManager`* @param image The image to transform* @param imageURL The url of the image to transform** @return The transformed image object.*/
- (UIImage *)imageManager:(SDWebImageManager *)imageManager transformDownloadedImage:(UIImage *)image withURL:(NSURL *)imageURL;

只可惜前面我每仔细看框架，没发现有这个东西，自己在那边建Category，然后各种试，还好最终还是发现了这个方法。那么现在问题就比较简单了。经过代码追踪(其实就是按照关键字，进行代码的全文查找，哈哈)，发现在

- (id<SDWebImageOperation>)downloadWithURL:(NSURL *)url options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletedWithFinishedBlock)completedBlock;

方法中调用了transformDownloadedImage方法，从transformDownloadedImage的注释可以看到这个回调方法会将下载得到UIImage对象作为参数，然后将处理过后的UIImage对象返回。而downloadWithURL...方法则会先判断URL指向的图片是否已经缓存过了，如果没有缓存过话，就将下载得到的UIImage对象交给

transformDownloadedImage处理一下，然后调用保存数据方法storeImage:imageData:key:toDisk方法进行数据保存。

那么可以拷贝一下SDWebImageManager中的downloadWithURL...方法，然后添加两个参数height和width

/*** 重写下载图片方法，可以指定要缓存的图片大小(长、宽)，同时作用于内存缓存和磁盘缓存*/
- (id<SDWebImageOperation>)downloadWithURL:(NSURL *)url options:(SDWebImageOptions)optionswidth:(NSInteger)w height:(NSInteger)hprogress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletedWithFinishedBlock)completedBlock
{// Invoking this method without a completedBlock is pointlessNSParameterAssert(completedBlock);// Very common mistake is to send the URL using NSString object instead of NSURL. For some strange reason, XCode won't// throw any warning for this type mismatch. Here we failsafe this error by allowing URLs to be passed as NSString.if ([url isKindOfClass:NSString.class]){url = [NSURL URLWithString:(NSString *)url];}// Prevents app crashing on argument type error like sending NSNull instead of NSURLif (![url isKindOfClass:NSURL.class]){url = nil;}__block SDWebImageCombinedOperation *operation = SDWebImageCombinedOperation.new;__weak SDWebImageCombinedOperation *weakOperation = operation;BOOL isFailedUrl = NO;@synchronized(self.failedURLs){isFailedUrl = [self.failedURLs containsObject:url];}if (!url || (!(options & SDWebImageRetryFailed) && isFailedUrl)){dispatch_main_sync_safe(^{NSError *error = [NSError errorWithDomain:NSURLErrorDomain code:NSURLErrorFileDoesNotExist userInfo:nil];completedBlock(nil, error, SDImageCacheTypeNone, YES);});return operation;}@synchronized(self.runningOperations){[self.runningOperations addObject:operation];}NSString *key = [self cacheKeyForURL:url];operation.cacheOperation = [self.imageCache queryDiskCacheForKey:key done:^(UIImage *image, SDImageCacheType cacheType){if (operation.isCancelled){@synchronized(self.runningOperations){[self.runningOperations removeObject:operation];}return;}if ((!image || options & SDWebImageRefreshCached) && (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url])){if (image && options & SDWebImageRefreshCached){dispatch_main_sync_safe(^{// If image was found in the cache bug SDWebImageRefreshCached is provided, notify about the cached image// AND try to re-download it in order to let a chance to NSURLCache to refresh it from server.completedBlock(image, nil, cacheType, YES);});}// download if no image or requested to refresh anyway, and download allowed by delegateSDWebImageDownloaderOptions downloaderOptions = 0;if (options & SDWebImageLowPriority) downloaderOptions |= SDWebImageDownloaderLowPriority;if (options & SDWebImageProgressiveDownload) downloaderOptions |= SDWebImageDownloaderProgressiveDownload;if (options & SDWebImageRefreshCached) downloaderOptions |= SDWebImageDownloaderUseNSURLCache;if (image && options & SDWebImageRefreshCached){// force progressive off if image already cached but forced refreshingdownloaderOptions &= ~SDWebImageDownloaderProgressiveDownload;// ignore image read from NSURLCache if image if cached but force refreshingdownloaderOptions |= SDWebImageDownloaderIgnoreCachedResponse;}id<SDWebImageOperation> subOperation = [self.imageDownloader downloadImageWithURL:url options:downloaderOptions progress:progressBlock completed:^(UIImage *downloadedImage, NSData *data, NSError *error, BOOL finished){if (weakOperation.isCancelled){dispatch_main_sync_safe(^{completedBlock(nil, nil, SDImageCacheTypeNone, finished);});}else if (error){dispatch_main_sync_safe(^{completedBlock(nil, error, SDImageCacheTypeNone, finished);});if (error.code != NSURLErrorNotConnectedToInternet){@synchronized(self.failedURLs){[self.failedURLs addObject:url];}}}else{BOOL cacheOnDisk = !(options & SDWebImageCacheMemoryOnly);if (options & SDWebImageRefreshCached && image && !downloadedImage){// Image refresh hit the NSURLCache cache, do not call the completion block}// NOTE: We don't call transformDownloadedImage delegate method on animated images as most transformation code would mangle itelse if (downloadedImage && !downloadedImage.images && [self.delegate respondsToSelector:@selector(imageManager:transformDownloadedImage:withURL:width:height:)]) //定义新的回调方法{dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{   //将设定的h和w传递给回调方法...tranformDownloadedImage...，这个回调方法需要在具体的Delegate中实现UIImage *transformedImage = [self.delegate imageManager:self transformDownloadedImage:downloadedImage withURL:urlwidth:w height:h];dispatch_main_sync_safe(^{completedBlock(transformedImage, nil, SDImageCacheTypeNone, finished);});if (transformedImage && finished){NSData *dataToStore = [transformedImage isEqual:downloadedImage] ? data : nil;[self.imageCache storeImage:transformedImage imageData:dataToStore forKey:key toDisk:cacheOnDisk];}});}else{dispatch_main_sync_safe(^{completedBlock(downloadedImage, nil, SDImageCacheTypeNone, finished);});if (downloadedImage && finished){[self.imageCache storeImage:downloadedImage imageData:data forKey:key toDisk:cacheOnDisk];}}}if (finished){@synchronized(self.runningOperations){[self.runningOperations removeObject:operation];}}}];operation.cancelBlock = ^{[subOperation cancel];};}else if (image){dispatch_main_sync_safe(^{completedBlock(image, nil, cacheType, YES);});@synchronized(self.runningOperations){[self.runningOperations removeObject:operation];}}else{// Image not in cache and download disallowed by delegatedispatch_main_sync_safe(^{completedBlock(nil, nil, SDImageCacheTypeNone, YES);});@synchronized(self.runningOperations){[self.runningOperations removeObject:operation];}}}];return operation;
}

同时拷贝SDWebImageManagerDelegate中的回调方法...transformDownloadedImage...方法，修改并增加参数height和width,在SDWebImageManager.h中的SDWebImageManagerDelegate中有：

//自定义方法,指定要转换的图片的长、宽
- (UIImage *)imageManager:(SDWebImageManager *)imageManager transformDownloadedImage:(UIImage *)image withURL:(NSURL *)imageURL width:(NSInteger)w height:(NSInteger)h;

最后调用，使ViewController.h实现SDWebImageManagerDelegate，下面是 ViewController.m:

//
//  ViewController.m
//  MySDWEBIMAGE_0928
//
//  Created by wly on 13-9-28.
//  Copyright (c) 2013年 wly. All rights reserved.
//#import "ViewController.h"
#import "SDImageCache.h"
#import "UIImageView+WebCache_HW.h"
@interface ViewController ()@end@implementation ViewController- (void)viewDidLoad
{[super viewDidLoad];SDWebImageManager *manager = [SDWebImageManager sharedManager];manager.delegate = self;//添加图片一,指定缓存大小是原图的1/10，即192x108UIImageView *imageView = [[UIImageView alloc]initWithFrame:CGRectMake(40, 100, 100, 100)];imageView.contentMode = UIViewContentModeScaleAspectFill;[manager downloadWithURL:[NSURL URLWithString:@"http://c.hiphotos.baidu.com/album/w%3D1920%3Bcrop%3D0%2C0%2C1920%2C1080/sign=4e447d5d7acb0a4685228f305953cd47/c995d143ad4bd113314728955bafa40f4afb058a.jpg"]options:0 width:192 height:108progress:nilcompleted:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished){if (image){// do something with imageimageView.image = image;}}];//添加图片二，指定缓存大小和原图大小一样，为1920x1080UIImageView *imageView2 = [[UIImageView alloc]initWithFrame:CGRectMake(40, 300, 100, 100)];imageView2.contentMode = UIViewContentModeScaleAspectFill;[manager downloadWithURL:[NSURL URLWithString:@"http://c.hiphotos.baidu.com/album/w%3D1920%3Bcrop%3D0%2C0%2C1920%2C1080/sign=52823b0355e736d158138801a96074a1/10dfa9ec8a136327885cb7ec908fa0ec09fac7b8.jpg"]options:0 width:1920 height:1080progress:nilcompleted:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished){if (image){// do something with imageimageView2.image = image;}}];[self.view addSubview:imageView];[self.view addSubview:imageView2];
}- (void)didReceiveMemoryWarning
{[super didReceiveMemoryWarning];// Dispose of any resources that can be recreated.
}-(UIImage*)imageManager:(SDWebImageManager *)imageManager transformDownloadedImage:(UIImage *)image withURL:(NSURL *)imageURL width:(NSInteger)w height:(NSInteger)h {//缩放图片// Create a graphics image contextUIGraphicsBeginImageContext(CGSizeMake(w, h));// Tell the old image to draw in this new context, with the desired// new size[image drawInRect:CGRectMake(0,0,w, h)];// Get the new image from the contextUIImage* newImage = UIGraphicsGetImageFromCurrentImageContext();// End the contextUIGraphicsEndImageContext();return newImage;
}
@end

再是ViewController.h

//
//  ViewController.h
//  MySDWEBIMAGE_0928
//
//  Created by wly on 13-9-28.
//  Copyright (c) 2013年 wly. All rights reserved.
//#import <UIKit/UIKit.h>
#import "SDWebImageManager.h"
@interface ViewController : UIViewController <SDWebImageManagerDelegate>@end

最后，在调试过程中遇到一个问题，就是传递CGFloat参数时，得到了一个很小的负数,即出现了奇怪的问题。有兴趣的朋友可看一下stackoverflow上的一篇文章:http://stackoverflow.com/questions/3018978/cgfloat-argument-type-incorrectly-traces-out-with-zero-value-why

转载请保留出处:http://blog.csdn.net/u011638883/article/details/12160963

工程文件:http://download.csdn.net/detail/u011638883/6338379

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