與UIImageView性能不佳（本地圖像）

Question

我正在開發一個帶有圖庫的應用程序。 所有圖像都在設備光盤（文檔目錄）上，但無法順利顯示。

由於某些圖片很大（2000px * 2000px），我在後臺線程中加載它們，縮小它們並將它們顯示在主線程中。

這裏是我的UIImageView擴展：

@implementation UIImageView (BackgroundResize) 

- (void)localImageFromPath: (NSString *) path scaledTo: (CGSize) size { 
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0); 
    dispatch_async(queue, ^{ 
     //Load image 
     NSData * imgData = [[NSData alloc] initWithContentsOfFile:path]; 
     UIImage * original = [[[UIImage alloc] initWithData:imgData] autorelease]; 
     [imgData release]; 
     CGSize imgSize = original.size; 
     float widthScale = size.width/imgSize.width; 
     float heightScale = size.height/imgSize.height; 
     float scaleFactor = widthScale; 
     if (heightScale < scaleFactor) { 
      scaleFactor = heightScale; 
     } 
     UIImage * result = nil; 
     //Scale if necessary 
     if (scaleFactor < 1.0) { 
      result = [[UIImage alloc] initWithCGImage:original.CGImage scale:1/scaleFactor orientation:original.imageOrientation]; 
     } else { 
      result = [original retain]; 
     } 
     NSLog(@"Image prepeared on backgroud thread, update ui image on main.."); 
     dispatch_async(dispatch_get_main_queue(), ^{ 
      NSLog(@"Begin updating image on main thread"); 
      self.image = result; 
      [result release]; 
      [[NSNotificationCenter defaultCenter] postNotificationName:NTF_IMG_LOADED object:self]; 
      NSLog(@"Updated"); 
     }); 

    }); 
    dispatch_release(queue); 

} 
@end 

在控制檯輸出似乎一切都完美，所有的調試字符串顯示，在不到1秒（6個非常大的圖像），但是UI被阻止並顯示圖像延遲10秒。

在iOS上顯示大型本地圖像的正確方法是什麼？

應用是建立的iOS5 + ..

感謝，

Answer 1

你實際上並沒有調整圖片大小。 scale參數-initWithCGImage:scale:設置圖像像素與屏幕座標點的比率;它不會對底層圖像做任何事情。您需要真正將圖像繪製到CGContext以縮小圖像：

//Load image 
    UIImage * original = [[[UIImage alloc] initWithContentsOfFile:path] autorelease]; 

    CGSize imgSize = original.size; 
    float widthScale = size.width/imgSize.width; 
    float heightScale = size.height/imgSize.height; 
    float scaleFactor = widthScale; 
    if (heightScale < scaleFactor) { 
     scaleFactor = heightScale; 
    } 

    UIImage * result = nil; 
    //Scale if necessary 
    if (scaleFactor < 1.0) { 
     CGSize newSize = CGSizeMake(floorf(imgSize.width * scaleFactor), 
            floorf(imgSize.height * scaleFactor)); 
     UIGraphicsBeginImageContextWithOptions(newSize, NO, 0); 
     CGContextRef context = UIGraphicsGetCurrentContext(); 
     CGContextSetInterpolationQuality(context, kCGInterpolationHigh); 
     [original drawInRect:CGRectMake(0, 0, newSize.width, newSize.height)]; 
     result = UIGraphicsGetImageFromCurrentImageContext(); 
     [result retain]; 
     UIGraphicsEndImageContext(); 
    } else { 
     result = [original retain]; 
    }