title: why i change the alpha value of UIImageView but no effect?
body: <p>I am working on a reader app. when you read one magazine, it will display first five pages and  download the rest pages one by one. there is a scrollview to view the thumbnail image of pages. At the beginning, if the page needs downloading, the corresponding thumbnail view's alpha value is set to 0.5 (the thumbnail images are in the file,no need to download). when the page is downloaded, i will update the thumbnail view's value to 1.0. I use one operation to download the page, and when one is downloaded i use delegate to set thumbnail view's alpha.
    But when i update thumbnail view's alpha value, it still the same as the beginning. it seems the alpha has no effect. I wonder is there anything wrong with my code? some snippets are as follows:</p>
  
  <p>In the PageViewController.m</p>
  
  <pre><code>- (void)loadView
  {
     [super loadView];
     //...
     [self createSlideUpViewIfNecessary];
     [self downloadPages];
  }
  - (void)createSlideUpViewIfNecessary {
      if (!slideUpView) {
          [self createThumbScrollViewIfNecessary];
  
          // create container view that will hold scroll view and label
          CGRect frame = CGRectMake(CGRectGetMinX(self.view.bounds), CGRectGetMaxY(self.view.bounds), CGRectGetWidth(self.view.bounds), CGRectGetHeight(thumbScrollView.frame));
          slideUpView = [[UIView alloc] initWithFrame:frame];
          [slideUpView setBackgroundColor:[UIColor blackColor]];
          [slideUpView setOpaque:NO];
          [slideUpView setAlpha:0.75];
          [[self view] addSubview:slideUpView];
  
          // add subviews to container view
          [slideUpView addSubview:thumbScrollView];
      }
  }
  
  - (void)createThumbScrollViewIfNecessary {
      if (!thumbScrollView) {
          float scrollViewHeight = THUMB_HEIGHT + THUMB_V_PADDING;
          float scrollViewWidth  = CGRectGetWidth(self.view.bounds);
          thumbScrollView = [[UIScrollView alloc] initWithFrame:CGRectMake(0, 0, scrollViewWidth, scrollViewHeight)];
          [thumbScrollView setCanCancelContentTouches:NO];
          [thumbScrollView setClipsToBounds:NO];
  
          // now place all the thumb views as subviews of the scroll view
          // and in the course of doing so calculate the content width
          float xPosition = THUMB_H_PADDING;
  
          for (int i = 0; i &lt; magazine.pageNum ; i++) {
              Page *page = [magazine.pages objectAtIndex:i];
              NSString *name = page.pageName;
  
              NSString *mfjName =[name stringByReplacingOccurrencesOfString:@".mfj" withString:@"Small.mfj"];
              UIImage *thumbImage = nil;
              if([mfjName hasSuffix:@".mfj"])
                  thumbImage = [Reader loadMfjFromSprOrCache:magazine MFJ:mfjName];
  
              ThumbImageView *thumbView;
              if (thumbImage) {// sometimes mfjname is 0 which means white page in normal and black thumbnail in thumbnail scrollview.
                  if (!mThumbnailSizeUpdated) {
                      mThumbnailWidth = thumbImage.size.width;
                      mThumbnailHeight = thumbImage.size.height;
                      mThumbnailSizeUpdated = YES;
                  }
                  thumbView = [[ThumbImageView alloc] initWithImage:thumbImage];
              } else {
                  CGRect thumbFrame;
                  if (mThumbnailSizeUpdated) {
                      thumbFrame = CGRectMake(0, 0, mThumbnailWidth, mThumbnailHeight);
                  } else {
                      mThumbnailWidth = 80;
                      mThumbnailHeight = 100;
                      thumbFrame = CGRectMake(0, 0, mThumbnailWidth, mThumbnailHeight);
                  }
                  thumbView = [[ThumbImageView alloc] initWithFrame:thumbFrame];
              }
              NSString *mfjPath= [[magazine getDownloadPath] stringByAppendingPathComponent:name];
              if (![magazine getFileInfo:name]&amp;&amp;![[NSFileManager defaultManager] fileExistsAtPath:mfjPath]) {
                  thumbView.alpha = 0.5;
              }
              [thumbView setBackgroundColor:[UIColor blackColor]];
              [thumbView setTag:THUMBVIEW_OFFSET+i];
              [thumbView setDelegate:self];
              [thumbView setImageName:name];
              CGRect frame = [thumbView frame];
              frame.origin.y = THUMB_V_PADDING;
              frame.origin.x = xPosition;
              frame.size.width = frame.size.width+30;
              frame.size.height = frame.size.height+40;
              [thumbView setFrame:frame];
              [thumbScrollView addSubview:thumbView];
              UILabel *pageIndexLabel = [[UILabel alloc] initWithFrame:CGRectMake(xPosition, frame.origin.y+frame.size.height-THUMB_LABEL_HEIGHT, frame.size.width, THUMB_LABEL_HEIGHT)];
              [pageIndexLabel setBackgroundColor:[UIColor clearColor]];
              [pageIndexLabel setText:[NSString stringWithFormat:@"%d",(i+1)]];
              [pageIndexLabel setTextColor:[UIColor whiteColor]];
              [pageIndexLabel setTextAlignment:UITextAlignmentCenter];
              [thumbScrollView addSubview:pageIndexLabel];
  
              xPosition += (frame.size.width + THUMB_H_PADDING);
          }
          thumbScrollView.showsHorizontalScrollIndicator = NO;
          [thumbScrollView setContentSize:CGSizeMake(xPosition, scrollViewHeight)];
      }
  }
  - (void)downloadPages
  {
     DownloadOperation *op = [[DownloadOperation alloc] initWithMagazine:magazine];
     op.delegate = self;
     [[(AppDelegate *)[[UIApplication sharedApplication] delegate] sharedOperationQueue] addOperation:op];
  }
  
  - (void)downloadOperation:(DownloadOperation *)operation finishedAtIndex:(NSUInteger)index
  {
     if (thumbScrollView){
        [thumbScrollView viewWithTag:THUMBVIEW_OFFSET+index].alpha = 1.0;
     }
  }
  </code></pre>
  
  <p>In DownloadOperation.m</p>
  
  <pre><code>- (void)main
  {
     // ...
     NSUInteger index = 0;
     for (Page *page in mMagazine.pages)
     {
         if (/*page doesn't exist*/){
             // download the page;
             if ([delegate respondsToSelector:@selector(downloadOperation:finishedAtIndex:)]) {
                          [delegate downloadOperation:self finishedAtIndex:index];
              }
         }
         index++;
     }
  }
  </code></pre>