原文链接：https://www.jianshu.com/p/eb859cb99974

加载超清大图是会引起内存爆表的问题，最近一直困扰着我。

SDWebImage在加载大图时做的不是很好，加载大图内存爆表。YYWebImage会好一点，但还是不行。

当不要求图片质量的情况下，最好是在上传图片的时候就压缩图片，如果显示的时候再压缩的话也会导致内存暴涨，压缩也是很占内存的。

而对于UIImageJPEGRepresentation压缩方法，不能降低显示内存。因为显示图片所占的内存大小只与图片的分辨率有关，与图片的大小无关。UIImageJPEGRepresentation压缩方法只能降低图片大小，分辨率不变。

压缩图片的两张方式看我写的这篇http://www.jianshu.com/p/8150a8e7c0e4

后来找到了苹果的一个官方建议加载大图的demo

https://developer.apple.com/library/ios/samplecode/LargeImageDownsizing/

我改装之后的代码，可以直接用：

#import"LargeImageDispose.h"#define IPAD1_IPHONE3GS#ifdef IPAD1_IPHONE3GS#  define kDestImageSizeMB 60.0f // The resulting image will be (x)MB of uncompressed image data.#  define kSourceImageTileSizeMB 20.0f // The tile size will be (x)MB of uncompressed image data.#endif/* These constants are suggested initial values for iPad2, and iPhone 4 *///#define IPAD2_IPHONE4#ifdef IPAD2_IPHONE4#  define kDestImageSizeMB 120.0f // The resulting image will be (x)MB of uncompressed image data.#  define kSourceImageTileSizeMB 40.0f // The tile size will be (x)MB of uncompressed image data.#endif/* These constants are suggested initial values for iPhone3G, iPod2 and earlier devices *///#define IPHONE3G_IPOD2_AND_EARLIER#ifdef IPHONE3G_IPOD2_AND_EARLIER#  define kDestImageSizeMB 30.0f // The resulting image will be (x)MB of uncompressed image data.#  define kSourceImageTileSizeMB 10.0f // The tile size will be (x)MB of uncompressed image data.#endif#define bytesPerMB 1048576.0f#define bytesPerPixel 4.0f#define pixelsPerMB ( bytesPerMB / bytesPerPixel )#define destTotalPixels kDestImageSizeMB * pixelsPerMB#define tileTotalPixels kSourceImageTileSizeMB * pixelsPerMB#define destSeemOverlap 2.0f@interfaceLargeImageDispose(){CGContextRefdestContext;}@property(strong,nonatomic)UIImage*destImage;@end@implementationLargeImageDispose-(UIImage*)downsizeLargeImage:(UIImage*)image {// create an image from the image filename constant. Note this// doesn't actually read any pixel information from disk, as that// is actually done at draw time.UIImage*sourceImage = image;if( sourceImage ==nil)NSLog(@"input image not found!");// get the width and height of the input image using// core graphics image helper functions.CGSizesourceResolution;    sourceResolution.width =CGImageGetWidth(sourceImage.CGImage);    sourceResolution.height =CGImageGetHeight(sourceImage.CGImage);// use the width and height to calculate the total number of pixels// in the input image.floatsourceTotalPixels = sourceResolution.width * sourceResolution.height;// calculate the number of MB that would be required to store// this image uncompressed in memory.floatsourceTotalMB = sourceTotalPixels / pixelsPerMB;NSLog(@"%.2f",sourceTotalMB);// determine the scale ratio to apply to the input image// that results in an output image of the defined size.// see kDestImageSizeMB, and how it relates to destTotalPixels.floatimageScale = destTotalPixels / sourceTotalPixels;NSLog(@"%.2f",destTotalPixels);// use the image scale to calcualte the output image width, heightCGSizedestResolution;    destResolution.width = (int)( sourceResolution.width * imageScale );    destResolution.height = (int)( sourceResolution.height * imageScale );// create an offscreen bitmap context that will hold the output image// pixel data, as it becomes available by the downscaling routine.// use the RGB colorspace as this is the colorspace iOS GPU is optimized for.CGColorSpaceRefcolorSpace =CGColorSpaceCreateDeviceRGB();intbytesPerRow = bytesPerPixel * destResolution.width;// allocate enough pixel data to hold the output image.void* destBitmapData = malloc( bytesPerRow * destResolution.height );if( destBitmapData ==NULL)NSLog(@"failed to allocate space for the output image!");// create the output bitmap contextdestContext =CGBitmapContextCreate( destBitmapData, destResolution.width, destResolution.height,8, bytesPerRow, colorSpace, kCGImageAlphaPremultipliedLast );//    self.destContext = destContext;// remember CFTypes assign/check for NULL. NSObjects assign/check for nil.if( destContext ==NULL) {        free( destBitmapData );NSLog(@"failed to create the output bitmap context!");    }// release the color space object as its job is doneCGColorSpaceRelease( colorSpace );// flip the output graphics context so that it aligns with the// cocoa style orientation of the input document. this is needed// because we used cocoa's UIImage -imageNamed to open the input file.CGContextTranslateCTM( destContext,0.0f, destResolution.height );CGContextScaleCTM( destContext,1.0f,-1.0f );// now define the size of the rectangle to be used for the// incremental blits from the input image to the output image.// we use a source tile width equal to the width of the source// image due to the way that iOS retrieves image data from disk.// iOS must decode an image from disk in full width 'bands', even// if current graphics context is clipped to a subrect within that// band. Therefore we fully utilize all of the pixel data that results// from a decoding opertion by achnoring our tile size to the full// width of the input image.CGRectsourceTile;    sourceTile.size.width = sourceResolution.width;// the source tile height is dynamic. Since we specified the size// of the source tile in MB, see how many rows of pixels high it// can be given the input image width.sourceTile.size.height = (int)( tileTotalPixels / sourceTile.size.width );NSLog(@"source tile size: %f x %f",sourceTile.size.width, sourceTile.size.height);    sourceTile.origin.x =0.0f;// the output tile is the same proportions as the input tile, but// scaled to image scale.CGRectdestTile;    destTile.size.width = destResolution.width;    destTile.size.height = sourceTile.size.height * imageScale;    destTile.origin.x =0.0f;NSLog(@"dest tile size: %f x %f",destTile.size.width, destTile.size.height);// the source seem overlap is proportionate to the destination seem overlap.// this is the amount of pixels to overlap each tile as we assemble the ouput image.floatsourceSeemOverlap = (int)( ( destSeemOverlap / destResolution.height ) * sourceResolution.height );NSLog(@"dest seem overlap: %f, source seem overlap: %f",destSeemOverlap, sourceSeemOverlap);CGImageRefsourceTileImageRef;// calculate the number of read/write opertions required to assemble the// output image.intiterations = (int)( sourceResolution.height / sourceTile.size.height );// if tile height doesn't divide the image height evenly, add another iteration// to account for the remaining pixels.intremainder = (int)sourceResolution.height % (int)sourceTile.size.height;if( remainder ) iterations++;// add seem overlaps to the tiles, but save the original tile height for y coordinate calculations.floatsourceTileHeightMinusOverlap = sourceTile.size.height;    sourceTile.size.height += sourceSeemOverlap;    destTile.size.height += destSeemOverlap;NSLog(@"beginning downsize. iterations: %d, tile height: %f, remainder height: %d", iterations, sourceTile.size.height,remainder );for(inty =0; y < iterations; ++y ) {NSLog(@"iteration %d of %d",y+1,iterations);        sourceTile.origin.y = y * sourceTileHeightMinusOverlap + sourceSeemOverlap;        destTile.origin.y = ( destResolution.height ) - ( ( y +1) * sourceTileHeightMinusOverlap * imageScale + destSeemOverlap );// create a reference to the source image with its context clipped to the argument rect.sourceTileImageRef =CGImageCreateWithImageInRect( sourceImage.CGImage, sourceTile );// if this is the last tile, it's size may be smaller than the source tile height.// adjust the dest tile size to account for that difference.if( y == iterations -1&& remainder ) {floatdify = destTile.size.height;            destTile.size.height =CGImageGetHeight( sourceTileImageRef ) * imageScale;            dify -= destTile.size.height;            destTile.origin.y += dify;        }// read and write a tile sized portion of pixels from the input image to the output image.CGContextDrawImage( destContext, destTile, sourceTileImageRef );/* release the source tile portion pixel data. note,

        releasing the sourceTileImageRef doesn't actually release the tile portion pixel

        data that we just drew, but the call afterward does. */CGImageRelease( sourceTileImageRef );/* while CGImageCreateWithImageInRect lazily loads just the image data defined by the argument rect,

        that data is finally decoded from disk to mem when CGContextDrawImage is called. sourceTileImageRef

        maintains internally a reference to the original image, and that original image both, houses and

        caches that portion of decoded mem. Thus the following call to release the source image. */// we reallocate the source image after the pool is drained since UIImage -imageNamed// returns us an autoreleased object.if( y < iterations -1) {            sourceImage = image;            [selfperformSelectorOnMainThread:@selector(createImageFromContext) withObject:nilwaitUntilDone:YES];        }    }NSLog(@"downsize complete.");//    [self performSelectorOnMainThread:@selector(initializeScrollView:) withObject:nil waitUntilDone:YES];// free the context since its job is done. destImageRef retains the pixel data now.CGContextRelease( destContext );returnself.destImage;}-(void)createImageFromContext {// create a CGImage from the offscreen image contextCGImageRefdestImageRef =CGBitmapContextCreateImage( destContext );if( destImageRef ==NULL)NSLog(@"destImageRef is null.");// wrap a UIImage around the CGImageself.destImage = [UIImageimageWithCGImage:destImageRef scale:1.0f orientation:UIImageOrientationDownMirrored];// release ownership of the CGImage, since destImage retains ownership of the object now.CGImageRelease( destImageRef );if(self.destImage ==nil)NSLog(@"destImage is nil.");}@end

这段代码只是改变了图片的渲染方式，利用GPU 进行渲染，有效降低内存，不改变图片的质量，亲测加载24M图片so easy，只占20多内存

不能用于加载太小的图片，只能用于加载大图（大概1M以上）

最近看了这篇文章http://www.jianshu.com/p/1c9de8dea3ea

导致加载大图内存暴涨的原因是对大图的解压缩加载。

在SDWebImage中大图禁用解压缩，可以防止内存暴涨：

[[SDImageCache sharedImageCache]setShouldDecompressImages:NO];[[SDWebImageDownloader sharedDownloader]setShouldDecompressImages:NO];

测试只有iPhone7，7p,SE,有用，其他机型不起作用，和系统版本没有关系