How to open/create UIManagedDocument synchronously?

问题

As mentioned in title, I would like to open UIManagedDocument synchronously, i.e, I would like my execution to wait till open completes. I'm opening document on mainThread only.

Current API to open uses block

[UIManagedDocument openWithCompletionHandler:(void (^)(BOOL success))];

Locks usage mentioned at link works well on threads other than main thread. If I use locks on mainThread, it freezes execution of app.

Any advice would be helpful. Thanks.

回答1:

First, let me say that I strongly discourage doing this. Your main thread just waits, and does nothing while waiting for the call to complete. Under certain circumstances, the system will kill your app if it does not respond on the main thread. This is highly unusual.

I guess you should be the one to decide when/how you should use various programming tools.

This one does exactly what you want... block the main thread until the completion handler runs. Again, I do not recommend doing this, but hey, it's a tool, and I'll take the NRA stance: guns don't kill people...

__block BOOL waitingOnCompletionHandler = YES;
[object doSomethingWithCompletionHandler:^{
    // Do your work in the completion handler block and when done...
    waitingOnCompletionHandler = NO;
}];
while (waitingOnCompletionHandler) {
    usleep(USEC_PER_SEC/10);
}

Another option is to execute the run loop. However, this isn't really synchronous, because the run loop will actually process other events. I've used this technique in some unit tests. It is similar to the above, but still allows other stuff to happen on the main thread (for example, the completion handler may invoke an operation on the main queue, which may not get executed in the previous method).

__block BOOL waitingOnCompletionHandler = YES;
[object doSomethingWithCompletionHandler:^{
    // Do your work in the completion handler block and when done...
    waitingOnCompletionHandler = NO;
}];
while (waitingOnCompletionHandler) {
    NSDate *futureTime = [NSDate dateWithTimeIntervalSinceNow:0.1];
    [[NSRunLoop currentRunLoop] runUntilDate:futureTime];
}

There are other methods as well, but these are simple, easy to understand, and stick out like a sore thumb so it's easy to know you are doing something unorthodox.

I should also note that I've never encountered a good reason to do this in anything other than tests. You can deadlock your code, and not returning from the main run loop is a slippery slope (even if you are manually executing it yourself - note that what called you is still waiting and running the loop again could re-enter that code, or cause some other issue).

Asynchronous APIs are GREAT. The condition variable approach or using barriers for concurrent queues are reasonable ways to synchronize when using other threads. Synchronizing the main thread is the opposite of what you should be doing.

Good luck... and make sure you register your guns, and always carry your concealed weapons permit. This is certainly the wild west. There's always a John Wesley Harden out there looking for a gun fight.

来源：https://stackoverflow.com/questions/10314190/how-to-open-create-uimanageddocument-synchronously

标签

ios

objective-c-blocks

uimanageddocument