__block ViewController *weakSelf = self;
 [self.imageView imageWithUrl:@"" completedBlock:^(UIImage *image, NSError *error) { 
 NSLog(@"%@",weakSelf.imageView.description);
 }];

- (void)setImageView:(UIImageView *)imageView
{
 if (![_imageView isEqual:imageView])
 {
 [_imageView release];
 _imageView = [imageView retain];
 }
}

@interface ViewController ()
@property (strong,nonatomic) NSMutableArray *array;
@end

for (int i = 0; i < 100; i++) {
 dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
  self.array = [[NSMutableArray alloc] init];
 });
 } 

if(handler == nil)
{
 hander = [[Handler alloc] init];
}
return handler;

Exception Type: SIGSEGV
Exception Codes: SEGV_ACCERR at 0x12345678
Triggered by Thread: 1

 @synchronized(self){
 if(handler == nil)
 {
  hander = [[Handler alloc] init];
 }
 }
return handler;

if (self.xxx) {
 [self.dict setObject:@"ah" forKey:self.xxx];
}

__strong id val = self.xxx;
if (val) {
 [self.dict setObject:@"ah" forKey:val];
}

dispatch_group_t serviceGroup = dispatch_group_create();
dispatch_group_notify(serviceGroup, dispatch_get_main_queue(), ^{
 NSLog(@"Finish downloading :%@", downloadUrls);
});
// t 是一个包含一堆字符串的数组 
[downloadUrls enumerateObjectsUsingBlock:^(id _Nonnull obj, NSUInteger idx, BOOL * _Nonnull stop) {
 dispatch_group_enter(serviceGroup);
 SDWebImageCompletionWithFinishedBlock completion =
 ^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
  dispatch_group_leave(serviceGroup);
  NSLog(@"idx:%zd",idx);
 };
 [[SDWebImageManager sharedManager] downloadImageWithURL:[NSURL URLWithString: downloadUrls[idx]]      options:SDWebImageLowPriority      progress:nil             completed:completion];
}];

dispatch_barrier_sync(self.barrierQueue, ^{
 SDWebImageDownloaderOperation *operation = self.URLOperations[url];
 if (!operation) {
  operation = createCallback();
  // ****注意这行****
  self.URLOperations[url] = operation;
  __weak SDWebImageDownloaderOperation *woperation = operation;
  operation.completionBlock = ^{
   SDWebImageDownloaderOperation *soperation = woperation;
   if (!soperation) return;
   if (self.URLOperations[url] == soperation) {
    [self.URLOperations removeObjectForKey:url];
   };
  };
 }
// ****注意这行****
id downloadOperationCancelToken = [operation addHandlersForProgress:progressBlock completed:completedBlock];
}

- (void)finishEditing
{
 [Manager removeObject:self];
 [self doSomething];
}

Thread 0 Crashed:
0 libobjc.A.dylib     0x00000001816ec160 _objc_release :16 (in libobjc.A.dylib)
1 libobjc.A.dylib     0x00000001816edae8 __ZN12_GLOBAL__N_119AutoreleasePoolPage3popEPv :508 (in libobjc.A.dylib)
2 CoreFoundation     0x0000000181f4c9fc __CFAutoreleasePoolPop :28 (in CoreFoundation)
3 CoreFoundation     0x0000000182022bc0 ___CFRunLoopRun :1636 (in CoreFoundation)
4 CoreFoundation     0x0000000181f4cc50 _CFRunLoopRunSpecific :384 (in CoreFoundation)
5 GraphicsServices    0x0000000183834088 _GSEventRunModal :180 (in GraphicsServices)
6 UIKit       0x0000000187236088 _UIApplicationMain :204 (in UIKit)
7 Tmall4iPhone     0x00000001000b7ae4 main main.m:50 (in Tmall4iPhone)
8 libdyld.dylib     0x0000000181aea8b8 _start :4 (in libdyld.dylib)

__weak typeof(self) weakSelf = self;
[NSObject cancelPreviousPerformRequestsWithTarget:self];
if (!weakSelf)
{
//NSLog(@"self被销毁");
 return;
}
[self doOther];