爲什麼當我滾動我的UITableView時，CALayers堆疊在一起？

Question

我有一個UITableViewCell和我正在繪製的UIImage。當我滾動時，我得到了大量的子層，這使得性能變得非常不穩定。我如何確保我的CALayer只添加一次？

- (void) drawContentView:(CGRect)rect { 
    CGContextRef context = UIGraphicsGetCurrentContext(); 
    [[UIColor whiteColor] set]; 
    CGContextFillRect(context, rect); 

    NSString* caption = [NSString stringWithFormat:@"%@",[info objectForKey:@"caption"]]; 
    NSString* text = [info stringForKey:@"text"]; 

    CGFloat widthr = self.frame.size.width - 70; 

    [[UIColor grayColor] set]; 
    [text drawInRect:CGRectMake(63.0, 25.0, widthr, 20.0) withFont:system14 lineBreakMode:UILineBreakModeTailTruncation]; 

    if (self.image) { 
     UIImage *imageToDisplay; 
     imageToDisplay = self.image; 
     imageToDisplay = [self imageWithImage:imageToDisplay scaledToSize:CGSizeMake(imageToDisplay.size.width/1.5, imageToDisplay.size.height/1.5)]; 
     CGFloat width; 
     CGFloat height; 
     CGRect r; 
     if (imageToDisplay.size.width < 310 && imageToDisplay.size.height > 290) { 
      imageToDisplay = [self imageByCropping:imageToDisplay toRect:CGRectMake(0, 20, imageToDisplay.size.width, 270)]; 

     } 
     else if (imageToDisplay.size.width > 310 && imageToDisplay.size.height < 20) { 
      imageToDisplay = [self imageByCropping:imageToDisplay toRect:CGRectMake(30, 0, 290, imageToDisplay.size.height)]; 
     } 
     else { 
      if (![caption isEqualToString:@""]) { 
       imageToDisplay = [self imageByCropping:imageToDisplay toRect:CGRectMake(30, 0, 290, 230)]; 
      } 
      else { 
      imageToDisplay = [self imageByCropping:imageToDisplay toRect:CGRectMake(30, 0, 290, 270)]; 
      } 
     } 

     width = imageToDisplay.size.width; 
     height = imageToDisplay.size.height; 
     r = CGRectMake(5.0, 5.0, width, height); 

     //[imageToDisplay drawInRect:r]; 

     CALayer *sublayer = [CALayer layer]; 
     sublayer.contents = (id)imageToDisplay.CGImage; 
     sublayer.shadowOffset = CGSizeMake(0, 3); 
     sublayer.shadowRadius = 5.0; 
     sublayer.shadowColor = [UIColor blackColor].CGColor; 
     sublayer.shadowOpacity = 0.8; 
     sublayer.frame = CGRectMake(5.0, 5.0, imageToDisplay.size.width, imageToDisplay.size.height); 
     [self.layer addSublayer:sublayer]; 

     //Experimental shadow stuff with images 
     /*CALayer *layer = [CALayer layer]; 
     layer = [CALayer layer]; 
     layer.bounds = CGRectMake(5.0, 5.0, imageToDisplay.size.width, imageToDisplay.size.height); 
     layer.position = CGPointMake(150, 140); 
     layer.contents = (id)imageToDisplay.CGImage;  

     layer.shadowOffset = CGSizeMake(0, 2); 
     layer.shadowOpacity = 0.70; 

     [self.layer addSublayer:layer]; 

     [self bezierPathWithCurvedShadowForRect:layer.bounds];*/ 

     [[UIColor blackColor] set]; 
     [caption drawInRect:CGRectMake(10.0, height + 20 , widthr, 20.0) withFont:system14 lineBreakMode:UILineBreakModeWordWrap alignment:UITextAlignmentCenter]; 
    } 
} 

Answer 1

drawContentView:不是從正確的方法調用addSublayer:。在使用image構建對象時，或者在現有對象上設置image時，應該添加該圖層。或者，您可以自己在drawContentView:中繪製圖像，但它可能不會那麼快。