我的应用文档库中有很多图像。我想显示文档目录图像的缩略图图像。
我正在使用以下代码。
UIImage *thumbImage=[UIImage imageWithContentsOfFile:strImagePath];
if (thumbImage) {
[thubImageView setImage:thumbImage];
}
它对我有用，但问题是由于内存警告，应用程序冻结和崩溃。
我已经尝试过 [1]：https ://github.com/SlaunchaMan/GCDExample “GCDExample”，但我得到了相同的结果。
ALAsset 库用于获取缩略图。但我不知道如何将它与文档目录图像一起使用。
还有没有其他方法可以在没有内存警告的情况下显示缩略图。
提前致谢。
制作任何大小的所有图像缩略图例如：临时目录中的 80x80 在您所需的程序区域中使用它们，然后记住在完成任务的情况下删除这些临时目录。
此类别实现代码将帮助您：
#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
内存问题会自动解决。
如果没有看到更多代码，很难更准确地回答这个问题，但这个问题的一般解决方案是在显示表格单元格时延迟加载所需的图像，并确保图像足够小以容纳记忆。
提供的GCDExample使用全局 GCD 队列执行此操作。我不推荐这种方法，将并发队列与 I/O 结合使用会导致意外的线程爆炸。有关非常详细的分析，请参阅这篇出色的Mike Ash文章。在这里使用串行 GCD 队列是一种更明智的方法。