objective-c - 从 uiimage 生成缩略图的有效方法

Question

我有一个 UICollectionView，每个项目都显示一个方形图像。每张图片都是大文件(> 3MB)，每次应用程序都会尝试跳转到此 View 。它会延迟2-3秒。我尝试从大文件创建缩略图，然后应用于每个收藏项。但似乎并没有节省时间。

有什么有效的方法吗？

下面是我用来创建缩略图的方法

-(UIImage*)resizedImageToSize:(CGSize)dstSize{
CGImageRef imgRef = self.CGImage;
// the below values are regardless of orientation : for UIImages from Camera, width>height (landscape)
CGSize  srcSize = CGSizeMake(CGImageGetWidth(imgRef), CGImageGetHeight(imgRef)); // not equivalent to self.size (which is dependant on the imageOrientation)!

/* Don't resize if we already meet the required destination size. */
if (CGSizeEqualToSize(srcSize, dstSize)) {
    return self;
}

CGFloat scaleRatio = dstSize.width / srcSize.width;
UIImageOrientation orient = self.imageOrientation;
CGAffineTransform transform = CGAffineTransformIdentity;
switch(orient) {

    case UIImageOrientationUp: //EXIF = 1
        transform = CGAffineTransformIdentity;
        break;

    case UIImageOrientationUpMirrored: //EXIF = 2
        transform = CGAffineTransformMakeTranslation(srcSize.width, 0.0);
        transform = CGAffineTransformScale(transform, -1.0, 1.0);
        break;

    case UIImageOrientationDown: //EXIF = 3
        transform = CGAffineTransformMakeTranslation(srcSize.width, srcSize.height);
        transform = CGAffineTransformRotate(transform, M_PI);
        break;

    case UIImageOrientationDownMirrored: //EXIF = 4
        transform = CGAffineTransformMakeTranslation(0.0, srcSize.height);
        transform = CGAffineTransformScale(transform, 1.0, -1.0);
        break;

    case UIImageOrientationLeftMirrored: //EXIF = 5
        dstSize = CGSizeMake(dstSize.height, dstSize.width);
        transform = CGAffineTransformMakeTranslation(srcSize.height, srcSize.width);
        transform = CGAffineTransformScale(transform, -1.0, 1.0);
        transform = CGAffineTransformRotate(transform, 3.0 * M_PI_2);
        break;

    case UIImageOrientationLeft: //EXIF = 6
        dstSize = CGSizeMake(dstSize.height, dstSize.width);
        transform = CGAffineTransformMakeTranslation(0.0, srcSize.width);
        transform = CGAffineTransformRotate(transform, 3.0 * M_PI_2);
        break;

    case UIImageOrientationRightMirrored: //EXIF = 7
        dstSize = CGSizeMake(dstSize.height, dstSize.width);
        transform = CGAffineTransformMakeScale(-1.0, 1.0);
        transform = CGAffineTransformRotate(transform, M_PI_2);
        break;

    case UIImageOrientationRight: //EXIF = 8
        dstSize = CGSizeMake(dstSize.height, dstSize.width);
        transform = CGAffineTransformMakeTranslation(srcSize.height, 0.0);
        transform = CGAffineTransformRotate(transform, M_PI_2);
        break;

    default:
        [NSException raise:NSInternalInconsistencyException format:@"Invalid image orientation"];

}

/////////////////////////////////////////////////////////////////////////////
// The actual resize: draw the image on a new context, applying a transform matrix
UIGraphicsBeginImageContextWithOptions(dstSize, NO, self.scale);

CGContextRef context = UIGraphicsGetCurrentContext();

if (!context) {
    return nil;
}

if (orient == UIImageOrientationRight || orient == UIImageOrientationLeft) {
    CGContextScaleCTM(context, -scaleRatio, scaleRatio);
    CGContextTranslateCTM(context, -srcSize.height, 0);
} else {
    CGContextScaleCTM(context, scaleRatio, -scaleRatio);
    CGContextTranslateCTM(context, 0, -srcSize.height);
}

CGContextConcatCTM(context, transform);

// we use srcSize (and not dstSize) as the size to specify is in user space (and we use the CTM to apply a scaleRatio)
CGContextDrawImage(UIGraphicsGetCurrentContext(), CGRectMake(0, 0, srcSize.width, srcSize.height), imgRef);
UIImage* resizedImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

return resizedImage;

Answer 1

有一种内置方法可以从压缩图像文件创建缩略图:直接使用 ImageIO。与将图像渲染到较小的位图上下文相比，它快如闪电。

这是完成这项工作的方法:

+ (UIImage *)thumbnailWithContentsOfURL:(NSURL *)URL maxPixelSize:(CGFloat)maxPixelSize
{
    CGImageSourceRef imageSource = CGImageSourceCreateWithURL((__bridge CFURLRef)URL, NULL);
    NSAssert(imageSource != NULL, @"cannot create image source");

    NSDictionary *imageOptions = @{
        (NSString const *)kCGImageSourceCreateThumbnailFromImageIfAbsent : (NSNumber const *)kCFBooleanTrue,
        (NSString const *)kCGImageSourceThumbnailMaxPixelSize            : @(maxPixelSize),
        (NSString const *)kCGImageSourceCreateThumbnailWithTransform     : (NSNumber const *)kCFBooleanTrue
    };
    CGImageRef thumbnail = CGImageSourceCreateThumbnailAtIndex(imageSource, 0, (__bridge CFDictionaryRef)imageOptions);
    CFRelease(imageSource);

    UIImage *result = [[UIImage alloc] initWithCGImage:thumbnail];
    CGImageRelease(thumbnail);

    return result;
}