Question

我的应用文档库中有很多图片。我想显示文档目录图像的缩略图。

我正在使用以下代码。

UIImage *thumbImage=[UIImage imageWithContentsOfFile:strImagePath];
if (thumbImage) {
    [thubImageView setImage:thumbImage];
}

它适用于我，但问题是应用程序因内存警告而冻结和崩溃。

我已经尝试了[1]：https://github.com/SlaunchaMan/GCDExample“GCDExample”，但我得到的结果相同。

ALAsset库用于获取缩略图图像。但我不知道如何将它与文档目录图像一起使用。

还有其他方法可以显示没有内存警告的缩略图。

提前致谢。

Answer 1

类似的问题已经回答here

你可以使用方法

- (UIImage *)thumbnailImage:(NSInteger)thumbnailSize
      transparentBorder:(NSUInteger)borderSize
           cornerRadius:(NSUInteger)cornerRadius
   interpolationQuality:(CGInterpolationQuality)quality;

用于创建缩略图图像。可用的UIImage类别here

希望这有帮助。

Answer 2

在没有看到更多代码的情况下，更准确地回答这个问题非常困难，但是这个问题的一般解决方案是在显示表格单元格时懒洋洋地加载所需的图像，并确保图像足够小留在记忆中。

提供的GCDExample使用全局GCD队列执行此操作。我不推荐这种方法，使用与I / O耦合的并发队列可能会导致意外的线程爆炸。请参阅这篇优秀的Mike Ash文章，进行非常详细的分析。在这里使用串行GCD队列是一种更明智的方法。

Answer 3

使所有图像的拇指图像为任何大小的ex：80x80在临时目录中使用它们在您需要的程序区域然后记得在完成任务的情况下删除这些临时目录。

此类别实施代码可以帮助您：

#import "UIImage+Resize.h"
@implementation UIImage (ResizeCategory)

-(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)!

    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

    UIGraphicsBeginImageContext(dstSize);

    CGContextRef context = UIGraphicsGetCurrentContext();

    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;}

-(UIImage*)resizedImageToFitInSize:(CGSize)boundingSize scaleIfSmaller:(BOOL)scale
{
    // get the image size (independant of imageOrientation)
    CGImageRef imgRef = self.CGImage;
    CGSize srcSize = CGSizeMake(CGImageGetWidth(imgRef), CGImageGetHeight(imgRef)); // not equivalent to self.size (which depends on the imageOrientation)!

    // adjust boundingSize to make it independant on imageOrientation too for farther computations
    UIImageOrientation orient = self.imageOrientation;  
    switch (orient) {
        case UIImageOrientationLeft:
        case UIImageOrientationRight:
        case UIImageOrientationLeftMirrored:
        case UIImageOrientationRightMirrored:
            boundingSize = CGSizeMake(boundingSize.height, boundingSize.width);
            break;
        default:
            // NOP
            break;
    }

    // Compute the target CGRect in order to keep aspect-ratio
    CGSize dstSize;

    if ( !scale && (srcSize.width < boundingSize.width) && (srcSize.height < boundingSize.height) ) {
        ////NSLog(@"Image is smaller, and we asked not to scale it in this case (scaleIfSmaller:NO)");
        dstSize = srcSize; // no resize (we could directly return 'self' here, but we draw the image anyway to take image orientation into account)
    } else {        
        CGFloat wRatio = boundingSize.width / srcSize.width;
        CGFloat hRatio = boundingSize.height / srcSize.height;

        if (wRatio < hRatio) {
            ////NSLog(@"Width imposed, Height scaled ; ratio = %f",wRatio);
            dstSize = CGSizeMake(boundingSize.width, srcSize.height * wRatio);
        } else {
            ////NSLog(@"Height imposed, Width scaled ; ratio = %f",hRatio);
            dstSize = CGSizeMake(srcSize.width * hRatio, boundingSize.height);
        }
    }

    return [self resizedImageToSize:dstSize];
}

@end

内存问题将自动解决。

ALAsset Library + Document Directory图片？

3 个答案: