iOS 图片压缩

发帖的时候需要用到从相册中选取图片然后上传，由于每次上传图片的最大数量为9张，所以需要对图片进行压缩。开始时用了以前经常用的压缩的方法：

//压缩图片质量
+(UIImage *)reduceImage:(UIImage *)image percent:(float)percent
{
    NSData *imageData = UIImageJPEGRepresentation(image, percent);
    UIImage *newImage = [UIImage imageWithData:imageData];
    return newImage;
}
//压缩图片尺寸
+ (UIImage*)imageWithImageSimple:(UIImage*)image scaledToSize:(CGSize)newSize
{
    // Create a graphics image context
    UIGraphicsBeginImageContext(newSize);
    // new size
    [image drawInRect:CGRectMake(0,0,newSize.width,newSize.height)];
    // Get the new image from the context
    UIImage* newImage = UIGraphicsGetImageFromCurrentImageContext();

    // End the context
    UIGraphicsEndImageContext();
    // Return the new image.
    return newImage;
}

//压缩图片质量
+(UIImage *)reduceImage:(UIImage *)image percent:(float)percent
{
    NSData *imageData = UIImageJPEGRepresentation(image, percent);
    UIImage *newImage = [UIImage imageWithData:imageData];
    return newImage;
}
//压缩图片尺寸
+ (UIImage*)imageWithImageSimple:(UIImage*)image scaledToSize:(CGSize)newSize
{
    // Create a graphics image context
    UIGraphicsBeginImageContext(newSize);
    // new size
    [image drawInRect:CGRectMake(0,0,newSize.width,newSize.height)];
    // Get the new image from the context
    UIImage* newImage = UIGraphicsGetImageFromCurrentImageContext();
    
    // End the context
    UIGraphicsEndImageContext();
    // Return the new image.
    return newImage;
}

上面的方法比较常见，可是需要加载到内存中来处理图片，当图片数量多了的时候就会收到内存警告，程序崩溃。研究半天终于在一篇博客中找到了解决方法：

static size_t getAssetBytesCallback(voidvoid *info, voidvoid *buffer, off_t position, size_t count) {
    ALAssetRepresentation *rep = (__bridge id)info;

    NSError *error = nil;
    size_t countRead = [rep getBytes:(uint8_t *)buffer fromOffset:position length:count error:&error];

    if (countRead == 0 && error) {
        // We have no way of passing this info back to the caller, so we log it, at least.
        NDDebug(@"thumbnailForAsset:maxPixelSize: got an error reading an asset: %@", error);
    }

    return countRead;
}

static void releaseAssetCallback(voidvoid *info) {
    // The info here is an ALAssetRepresentation which we CFRetain in thumbnailForAsset:maxPixelSize:.
    // This release balances that retain.
    CFRelease(info);
}

// Returns a UIImage for the given asset, with size length at most the passed size.
// The resulting UIImage will be already rotated to UIImageOrientationUp, so its CGImageRef
// can be used directly without additional rotation handling.
// This is done synchronously, so you should call this method on a background queue/thread.
- (UIImage *)thumbnailForAsset:(ALAsset *)asset maxPixelSize:(NSUInteger)size {
    NSParameterAssert(asset != nil);
    NSParameterAssert(size > 0);

    ALAssetRepresentation *rep = [asset defaultRepresentation];

    CGDataProviderDirectCallbacks callbacks = {
        .version = 0,
        .getBytePointer = NULL,
        .releaseBytePointer = NULL,
        .getBytesAtPosition = getAssetBytesCallback,
        .releaseInfo = releaseAssetCallback,
    };

    CGDataProviderRef provider = CGDataProviderCreateDirect((voidvoid *)CFBridgingRetain(rep), [rep size], &callbacks);
    CGImageSourceRef source = CGImageSourceCreateWithDataProvider(provider, NULL);

    CGImageRef imageRef = CGImageSourceCreateThumbnailAtIndex(source, 0, (__bridge CFDictionaryRef) @{
                                                                                                      (NSString *)kCGImageSourceCreateThumbnailFromImageAlways : @YES,
                                                                                                      (NSString *)kCGImageSourceThumbnailMaxPixelSize : [NSNumber numberWithInt:size],
                                                                                                      (NSString *)kCGImageSourceCreateThumbnailWithTransform : @YES,
                                                                                                      });
    CFRelease(source);
    CFRelease(provider);

    if (!imageRef) {
        return nil;
    }

    UIImage *toReturn = [UIImage imageWithCGImage:imageRef];

    CFRelease(imageRef);

    return toReturn;
}

static size_t getAssetBytesCallback(void *info, void *buffer, off_t position, size_t count) {
    ALAssetRepresentation *rep = (__bridge id)info;
    
    NSError *error = nil;
    size_t countRead = [rep getBytes:(uint8_t *)buffer fromOffset:position length:count error:&error];
    
    if (countRead == 0 && error) {
        // We have no way of passing this info back to the caller, so we log it, at least.
        NDDebug(@"thumbnailForAsset:maxPixelSize: got an error reading an asset: %@", error);
    }
    
    return countRead;
}

static void releaseAssetCallback(void *info) {
    // The info here is an ALAssetRepresentation which we CFRetain in thumbnailForAsset:maxPixelSize:.
    // This release balances that retain.
    CFRelease(info);
}

// Returns a UIImage for the given asset, with size length at most the passed size.
// The resulting UIImage will be already rotated to UIImageOrientationUp, so its CGImageRef
// can be used directly without additional rotation handling.
// This is done synchronously, so you should call this method on a background queue/thread.
- (UIImage *)thumbnailForAsset:(ALAsset *)asset maxPixelSize:(NSUInteger)size {
    NSParameterAssert(asset != nil);
    NSParameterAssert(size > 0);
    
    ALAssetRepresentation *rep = [asset defaultRepresentation];
    
    CGDataProviderDirectCallbacks callbacks = {
        .version = 0,
        .getBytePointer = NULL,
        .releaseBytePointer = NULL,
        .getBytesAtPosition = getAssetBytesCallback,
        .releaseInfo = releaseAssetCallback,
    };
    
    CGDataProviderRef provider = CGDataProviderCreateDirect((void *)CFBridgingRetain(rep), [rep size], &callbacks);
    CGImageSourceRef source = CGImageSourceCreateWithDataProvider(provider, NULL);
    
    CGImageRef imageRef = CGImageSourceCreateThumbnailAtIndex(source, 0, (__bridge CFDictionaryRef) @{
                                                                                                      (NSString *)kCGImageSourceCreateThumbnailFromImageAlways : @YES,
                                                                                                      (NSString *)kCGImageSourceThumbnailMaxPixelSize : [NSNumber numberWithInt:size],
                                                                                                      (NSString *)kCGImageSourceCreateThumbnailWithTransform : @YES,
                                                                                                      });
    CFRelease(source);
    CFRelease(provider);
    
    if (!imageRef) {
        return nil;
    }
    
    UIImage *toReturn = [UIImage imageWithCGImage:imageRef];
    
    CFRelease(imageRef);
    
    return toReturn;
}

采用上面的方法之后内存占用率很低！