NSManagedObjectContext executeFetchRequest返回erratic对象，导致EXC_BAD_ACCESS，SIGABRT等 [英] NSManagedObjectContext executeFetchRequest returns erratic objects, causes EXC_BAD_ACCESS, SIGABRT, etc

问题描述

我的猜测是我在这里缺少一些核心数据的基本理解，但这里：

My guess is I'm missing some Core Data fundamental understanding here, but here goes:

我有几个获取请求在我的应用程序处理检索不同的东西。在某些情况下，代码运行正常，返回请求的对象。

I have several fetch requests in my app to handle retrieval of different things. In some cases the code runs fine, returning the requested objects.

在某些情况下，它返回似乎是已经释放的对象后来试图引用返回的结果给出EXC_BAD_ACCESS）。由于我在代码中设置了各种断点和日志语句，因此它也会在代码中的其他位置得到偶尔的SIGABRT或EXC_BAD_ACCESS。

In some cases, it returns what appear to be already-released objects (e.g. just a few lines of code later trying to reference the returned result gives EXC_BAD_ACCESS). As I set various breakpoints and log statements in the code and step through it will also get the occasional SIGABRT or EXC_BAD_ACCESS in other locations in the code.

在每种情况下，它似乎是我参考获取请求的结果。

In every case, it appears to be when I go to reference the result of a fetch request.

以下是一个此类提取请求的示例：

Here's a sample of one such fetch request:

// Who am I?
NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
NSString *signedInPersonId = [defaults stringForKey:@"signedInPersonId"];

// Return (if any) the Request object with given UUID
RequestStrings *r = [[RequestStrings alloc] init];
NSEntityDescription *description = [NSEntityDescription entityForName:r.table_Request inManagedObjectContext:moc];

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"(UUID == %@) && (dataOwnerId == %@)", UUID, signedInPersonId];
NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:description];
[request setPredicate:predicate];
NSError *error = nil;

NSArray *requests = [moc executeFetchRequest:request error:&error];

Request *returnRequest = nil;
if (requests != nil) {        
    if ([requests count] > 0) {
        NSLog(@"getRequestWithId - requests array: %@, first: %@", requests, [requests objectAtIndex:0]);
        returnRequest = [requests objectAtIndex:0];
    }
    else {
        returnRequest = nil;
    }
}

[r release];
[request release];
return returnRequest;

这里有一些更多的信息
在某些情况下，同样的代码将返回所需的对象，或者抛出一个异常，说明 [NSCFNumber length] 是一个无法识别的选择器。不确定同一个实体说明+撷取要求如何在一种情况下传回数组，在另一种情况下传回数字。

P.S. Here's some more info In some instances the same code will return the desired objects, or throw an exception stating that [NSCFNumber length] is an unrecognized selector. Not sure how the same entity description + fetch request could return an array in one case and a Number in the other.

推荐答案

基本上这样做：

// (1) Create an array of stuff
NSArray *myArray = [NSarray arrayWithObjects:a, b, c, nil];

// (2) Take the first one off
id myObject = [myArray objectAtIndex:0];

// (3) Release everything
[myArray release];

// (4) Return myObject
return myObject;

您只是使用CoreData执行步骤（1）。

You're just using CoreData to do step (1).

步骤（1）返回一个对象数组。保留这些对象的唯一的东西是他们在*中的数组。如果您释放数组（步骤3），您将释放其中的所有对象。你为什么期望myObject仍然存在的步骤（4）？

Step (1) returns an array of objects. The only thing retaining these objects is the array that they're in*. If you release the array (step 3), you will release all the objects that are inside it. Why do you expect myObject to still exist by step (4)?

试试这个：

// Make sure that we keep a retain of returnRequest
returnRequest = [[[requests objectAtIndex:0] retain] autorelease];

---

做出了一个假设，使我的回答更简单。在现实世界中，你不知道什么是保留你的对象 - 这是由框架和你的代码。但是，保留任何您期望保留的内容是一个好习惯。

---

*For the pedantic out there : I've made an assumption that makes my answer simpler. In the real world you don't know what's retaining your objects - that's up to the framework as well as your code. However, it's good practice to retain anything you expect to keep around.

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