将数据复制到可变数组时遇到问题

Question

我不断收到错误“由于未捕获的异常‘NSInvalidArgumentException’而终止应用程序，原因：‘+[MainViewControllerimalFormInContext:]：发送到类的无法识别的选择器” 从这行代码： NSLog(@"访问特定矿场实体");

NSEntityDescription *entity = [NSEntityDescription entityForName:@"Mine" inManagedObjectContext:managedObjectContext];   
NSFetchRequest *request = [[[NSFetchRequest alloc] init] autorelease];   
NSError *error = nil;
[request setEntity:entity];  
NSPredicate *predicate;
NSPredicate *metalFilter;

NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
NSString *region = [defaults stringForKey:@"mineArray"];

if([region isEqualToString:@"Butte & Plumas"])
{
    predicate = [NSPredicate predicateWithFormat:@"(county Contains %@)  OR (county Contains %@)",@"Butte",@"Plumas"];
}
else if([region isEqualToString:@"Sutter, Yuba, & Sierra"])
{
    predicate = [NSPredicate predicateWithFormat:@"(county Contains %@)  OR (county Contains %@) OR (county Contains %@)",@"Sutter",@"Yuba",@"Sierra"];
}
else if([region isEqualToString:@"Nevada & Placer"])
{
    predicate = [NSPredicate predicateWithFormat:@"(county Contains %@)  OR (county Contains %@)",@"Nevada",@"Placer"];
}
else if([region isEqualToString:@"Sacramento & El Dorado"])
{
    predicate = [NSPredicate predicateWithFormat:@"(county Contains %@)  OR (county Contains %@)",@"Sacramento",@"El Dorado"];
}
else if([region isEqualToString:@"San Joaquin, Amador, & Calaveras"])
{
    predicate = [NSPredicate predicateWithFormat:@"(county Contains %@)  OR (county Contains %@) OR (county Contains%@)",@"San Joaquin",@"Amador", @"Calaveras"];
}
else if([region isEqualToString:@"Tuolumne & Stanislaus"])
{
    predicate = [NSPredicate predicateWithFormat:@"(county Contains %@)  OR (county Contains %@)",@"Tuolumne",@"Stanislaus"];
}
else if([region isEqualToString:@"Merced, Mariposa, & Madera"])
{
    predicate = [NSPredicate predicateWithFormat:@"(county Contains %@) OR (county Contains %@) OR (county Contains %@)",@"Merced",@"Mariposa",@"Madera"];
}

[request setPredicate:predicate];
mArray = [[NSMutableArray alloc] init];
mArray = [[managedObjectContext executeFetchRequest:request error:&error] mutableCopy];

使用调试器，我已经缩小了发生在以下位置的错误范围：

mArray = [[managedObjectContext executeFetchRequest:request error:&error] mutableCopy];

如何修复此问题？

Answer 1

这很可能是一个保留/释放错误。在 XCode 中进行“构建和分析”，并改进代码以删除所有警告。

以下是我注意到的事情：

mArray = [[NSMutableArray alloc] init];
mArray = [[managedObjectContext executeFetchRequest:request error:&error] mutableCopy];

这两行非常糟糕。你的mArray是什么？ m 代表 member 还是 mutable ？如果它是一个成员变量，您不应该像在中那样只为其分配一个新数组

// mArray points to an array at this time, say X
mArray = [[NSMutableArray alloc] init];
// at this point, mArray points to an array Y created by alloc init. X is lost! 

此外，如果您像您那样进一步分配一个 mutableCopy ，

mArray = [[managedObjectContext executeFetchRequest:request error:&error] mutableCopy];
// at this point, mArray points to an array Z created by mutableCopy. Y is lost, too!

请注意，在 Objective-C 中，您在源代码只是一个指针，而不是对象本身。如果将某些内容分配给变量，它不会使对象执行分配操作，而只是更改指针以指向不同的内容。

事实上，你有这些线条表明你在其他地方也有类似的东西；其中任何一个最终都可能导致您遇到的错误。所以你需要一一处理。祝你好运！

另一点：当您准备变量 predicate 时，如果 region 不匹配，则 if 子句链会使 predicate 未定义您列出的选项。这是非常危险的，因为在 Objective-C 中，该行

 NSPredicate* predicate;

不会将谓词初始化为nil。因此，可能会

[request setPredicate:predicate];

为 requrest 的 predicate 设置垃圾。你应该将其更改为

 NSPredicate* predicate=nil;