在學習AFNetWorking的過程中,經常看到類似: __weak __typeof ( self )weakSelf = self ; 然後在block中,看到: __strong __typeof (weakSelf)strongSelf = weakSelf;
如下代碼:
- (void)setShouldExecuteAsBackgroundTaskWithExpirationHandler:(void (^)(void))handler {
[self.lock lock];
if (!self.backgroundTaskIdentifier) {
UIApplication *application = [UIApplication sharedApplication];
__weak __typeof(self)weakSelf = self;
self.backgroundTaskIdentifier = [application beginBackgroundTaskWithExpirationHandler:^{
__strong __typeof(weakSelf)strongSelf = weakSelf;
if (handler) {
handler();
}
if (strongSelf) {
[strongSelf cancel];
[application endBackgroundTask:strongSelf.backgroundTaskIdentifier];
strongSelf.backgroundTaskIdentifier = UIBackgroundTaskInvalid;
}
}];
}
[self.lock unlock];
}
這是因為:沒有添加__strong 引用的話,編譯器會有警告,為什麼會警告呢,因為弱引用的weakself會在某個時間被釋放,有可能是在執行之後的block之前就會被釋放,這樣在後續的操作操作就有可能出錯,是以最好是添加一個對weakSelf的__strong引用。
具體可以參考:
http://stackoverflow.com/questions/11899134/how-to-properly-address-weak-receiver-may-be-unpredictably-null-in-arc-mode
http://stackoverflow.com/questions/19018456/ios-blocks-and-strong-weak-references-to-self