如何保存网页的缩略图？

Question

我想保存网页的缩略图。所以我创建了一个webview。代码就是那个

- (void)webView:(WebView *)sender didFinishLoadForFrame:(WebFrame *)frame
{
if (frame != [sender mainFrame]) {
    return;
}
    [sender lockFocus];
    NSData* data = [sender dataWithPDFInsideRect:[sender bounds]];
    [sender unlockFocus];
    NSImage *img = [[NSImage alloc] initWithData:data];
    NSData* imageData = [img TIFFRepresentation];
    NSArray* deskTopArrayPaths = NSSearchPathForDirectoriesInDomains(NSDesktopDirectory, NSUserDomainMask, YES);
    NSString* deskTopPath = [deskTopArrayPaths objectAtIndex:0];
    NSString* pngPath = [NSString stringWithFormat:@"%@/SaveWebThumbnail.png",deskTopPath];
    [imageData writeToFile:pngPath atomically:NO];
}

但是图像中有一些空白位置被保存。我不知道为什么？我想当调用webView：didFinishLoadForFrame：时，webview还没有真正完成，不是吗？任何人都知道这个问题，请告诉我谢谢！