在显示在UITableViewCells之前调整大小和裁剪图像

Question

我在我的应用程序中有一个UITableView，我必须加载一些固定宽度但高度不同的图像。我使用NSOperationQueue下载图像异步，并调整大小和裁剪我尝试使用Jane Sales在此帖子link text提供的解决方案。

我做了一个自定义的UITableViewCell类，并在其中有一个方法，当排队的操作完成下载图像时调用。该方法被正确调用并显示图像。当我尝试使用Jane提出的方法调整图像大小时，会出现问题。当它到达[sourceImage drawInRect：thumbnailRect];我收到一个exec访问错误，我找不出原因。我所说的方法是这样的：

- (void) setupImage:(UIImage *) anImage{ 
    UIImage *resized = [anImage imageByScalingAndCroppingForSize:CGSizeMake(64, 59)]; 
    if(resized == nil) 
     resized = [UIImage newImageFromResource:@"thumb2.png"]; 
    [thumbnailView setImage:resized]; 
} 

setupImage是调用的函数时NSOperationQueue完成anImage的下载操作。

有人能给我一个线索，为什么当我尝试调整大小和裁剪图像时，为什么我收到执行不良访问错误？我尝试在表格视图外使用相同的功能。 80％的情况下，但有些情况下，当我收到相同的执行不良访问错误。

谢谢你在前进， 索林

Answer 1

这是我使用的调整和croping的UIImages（代码是从Jane Sales solution）

@implementation UIImage (Extras) 
#pragma mark - 
#pragma mark Scale and crop image 

- (UIImage*)imageByScalingAndCroppingForSize:(CGSize)targetSize 
{ 
UIImage *sourceImage = self; 
UIImage *newImage = nil;   
CGSize imageSize = sourceImage.size; 
CGFloat width = imageSize.width; 
CGFloat height = imageSize.height; 
CGFloat targetWidth = targetSize.width; 
CGFloat targetHeight = targetSize.height; 
CGFloat scaleFactor = 0.0; 
CGFloat scaledWidth = targetWidth; 
CGFloat scaledHeight = targetHeight; 
CGPoint thumbnailPoint = CGPointMake(0.0,0.0); 

if (CGSizeEqualToSize(imageSize, targetSize) == NO) 
     { 
     CGFloat widthFactor = targetWidth/width; 
     CGFloat heightFactor = targetHeight/height; 

     if (widthFactor > heightFactor) 
       scaleFactor = widthFactor; // scale to fit height 
     else 
       scaleFactor = heightFactor; // scale to fit width 
     scaledWidth = width * scaleFactor; 
     scaledHeight = height * scaleFactor; 

     // center the image 
     if (widthFactor > heightFactor) 
       { 
       thumbnailPoint.y = (targetHeight - scaledHeight) * 0.5; 
       } 
     else 
       if (widthFactor < heightFactor) 
         { 
         thumbnailPoint.x = (targetWidth - scaledWidth) * 0.5; 
         } 
     }  

UIGraphicsBeginImageContext(targetSize); // this will crop 

CGRect thumbnailRect = CGRectZero; 
thumbnailRect.origin = thumbnailPoint; 
thumbnailRect.size.width = scaledWidth; 
thumbnailRect.size.height = scaledHeight; 

[sourceImage drawInRect:thumbnailRect]; 

newImage = UIGraphicsGetImageFromCurrentImageContext(); 
if(newImage == nil) 
     NSLog(@"could not scale image"); 

//pop the context to get back to the default 
UIGraphicsEndImageContext(); 
return newImage; 
} 

什么我调用上述功能等我以前说过：

- (void) setupImage:(UIImage *) anImage 
{ 
    UIImage *resized = [anImage imageByScalingAndCroppingForSize:CGSizeMake(64, 59)]; 
    [thumbnailView setImage:resized]; 
} 

的图像下载异步时被显示在表视图的细胞。 setupImage函数从NSOperation接收图像，并将其下载到异步。问题是，就像我上面所说的，当它到达[sourceImage drawInRect：thumbnailRect];

thumbnailView是一个UIImageView，它是我自定义的TableViewCell的子视图。

希望这可以清楚我在代码中使用的内容。 谢谢你的帮助。 Sorin

Answer 2

尝试下面的代码。 createImage：image：width：height方法需要一个源CGImageRef和将返回的最终UIImage的新宽度和高度。当使用作为源的UIImage的，就可以得到相应的CGImageRef如下：

UIImage *sourceImage; 
CGImageRef *sourceRef = [sourceImage CGImage]; 

     // Draw the image into a pixelsWide x pixelsHigh bitmap and use that bitmap to 
     // create a new UIImage 
     - (UIImage *) createImage: (CGImageRef) image width: (int) pixelWidth height: (int) pixelHeight 
     { 
      // Set the size of the output image 
      CGRect aRect = CGRectMake(0.0f, 0.0f, pixelWidth, pixelHeight); 
      // Create a bitmap context to store the new thumbnail 
      CGContextRef context = MyCreateBitmapContext(pixelWidth, pixelHeight); 
      // Clear the context and draw the image into the rectangle 
      CGContextClearRect(context, aRect); 
      CGContextDrawImage(context, aRect, image); 
      // Return a UIImage populated with the new resized image 
      CGImageRef myRef = CGBitmapContextCreateImage (context); 

      UIImage *img = [UIImage imageWithCGImage:myRef]; 

      free(CGBitmapContextGetData(context)); 
      CGContextRelease(context); 
      CGImageRelease(myRef); 

      return img; 
     } 



     // MyCreateBitmapContext: Source based on Apple Sample Code 
     CGContextRef MyCreateBitmapContext (int pixelsWide, 
              int pixelsHigh) 
     { 
      CGContextRef context = NULL; 
      CGColorSpaceRef colorSpace; 
      void *   bitmapData; 
      int    bitmapByteCount; 
      int    bitmapBytesPerRow; 

      bitmapBytesPerRow = (pixelsWide * 4); 
      bitmapByteCount  = (bitmapBytesPerRow * pixelsHigh); 

      colorSpace = CGColorSpaceCreateDeviceRGB(); 
      bitmapData = malloc(bitmapByteCount); 
      if (bitmapData == NULL) 
      { 
       fprintf (stderr, "Memory not allocated!"); 
       CGColorSpaceRelease(colorSpace); 
       return NULL; 
      } 
      context = CGBitmapContextCreate (bitmapData, 
              pixelsWide, 
              pixelsHigh, 
              8, 
              bitmapBytesPerRow, 
              colorSpace, 
              kCGImageAlphaPremultipliedLast); 
      if (context== NULL) 
      { 
       free (bitmapData); 
       CGColorSpaceRelease(colorSpace); 
       fprintf (stderr, "Context not created!"); 
       return NULL; 
      } 
      CGColorSpaceRelease(colorSpace); 

      return context; 
     }