主线程阻塞问题

问题

主线程阻塞会导致新添加在父视图的view不能及时显示。

如下代码:

-(void)saveButtonAction:(UIButton *)button
{
[SVProgressHUD showWithStatus:@"保存中。。。" maskType:SVProgressHUDMaskTypeClear];
int m = 0 ;
while (m < 100)
{
m++ ;
NSLog(@"test");
}
}
上述代码中，SVProgressHUD 将不会显示，直到 saveButtonAction 方法执行结束

解决办法
使用GCD异步执行，把需要执行的耗时操作，通过串行队列的方式添加到主线程队列，让主线程有时间的时候在执行
如下代码：

-(void)saveButtonAction:(UIButton *)button
{
[SVProgressHUD showWithStatus:@"保存中。。。" maskType:SVProgressHUDMaskTypeClear];
dispatch_async(dispatch_get_main_queue(), ^{
UIImage *image = [self getCapture];
[self saveImageToPhotos:image];
});
}

• (void)saveImageToPhotos:(UIImage*)savedImage
  {
  UIImageWriteToSavedPhotosAlbum(savedImage, self, @selector(image:didFinishSavingWithError:contextInfo:), NULL);
  }

// 指定回调方法

• (void)image:(UIImage *)image didFinishSavingWithError:(NSError *)error contextInfo:(void *)contextInfo
  {
  NSString *msg = nil ;
  if(error != NULL){
  msg = @"保存图片失败" ;
  [SVProgressHUD showErrorWithStatus:msg];
  }else{
  msg = @"保存图片成功" ;
  [SVProgressHUD showSuccessWithStatus:msg];
  if ([self.delegate respondsToSelector:@selector(saveButtonActionComplete:)])
  {
  [self.delegate saveButtonActionComplete:self];
  }
  }
  }

• (UIImage)getCapture
  {
  UIImage viewImage = nil;
  UICollectionView *scrollView = _collectionView;
  UIGraphicsBeginImageContextWithOptions(scrollView.contentSize, scrollView.opaque, 0.0);
  {
  CGPoint savedContentOffset = scrollView.contentOffset;
  CGRect savedFrame = scrollView.frame;

    scrollView.contentOffset = CGPointZero;
    scrollView.frame = CGRectMake(0, 0, scrollView.contentSize.width, scrollView.contentSize.height);
    [scrollView.layer renderInContext: UIGraphicsGetCurrentContext()];
    viewImage = UIGraphicsGetImageFromCurrentImageContext();
    
    scrollView.contentOffset = savedContentOffset;
    scrollView.frame = savedFrame;
  

  }
  UIGraphicsEndImageContext();
  return viewImage;
  }