将数据复制到可变数组时遇到问题

Question

我一直收到错误“终止应用程序，由于未捕获的异常'NSInvalidArgumentException'，原因：'+ [MainViewController minimalFormInContext：]：无法识别的选择器从这行代码发送到类” ： NSLog（@“访问特定的矿山实体”）;

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，我已经缩小误差在发生：

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

我该如何解决这个问题？

Answer 1

这可能是一个保留/发布错误。在XCode中执行“构建和分析”，并改进代码以删除所有警告。

这里有事情，我注意到：

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

这两条线都非常糟糕。你的mArray是什么？ m代表member或mutable？如果它是一个成员变量，你不应该只是分配一个新的数组，作为在

// 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，如果您还指定mutableCopy像你一样，

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

注意，在Objective- C，你在源代码中看到的变量只是一个指针，而不是对象本身。如果你给变量赋值，它不会使对象执行赋值操作，但它只是改变指针指向不同的东西。

事实上，你有这些线表明你在其他地方有类似的事情;它们中的任何一个最终都会导致你遇到的错误。所以你需要一一处理它们。祝你好运！

还有一点：当你准备变量predicate的if条款链叶predicate如果region匹配没有你列出的选择的不确定。这是非常危险的，因为在Objective-C，该行

NSPredicate* predicate; 

不不初始化predicate是nil。所以它可能是

[request setPredicate:predicate]; 

将设置一个垃圾的requrest的predicate。您应该将其更改为

NSPredicate* predicate=nil;