int main (int argc, const char * argv[])
{
NSLog(@" === Core Data Tutorial ===");
// Enable GC
//
objc_startCollectorThread();
NSError *error = nil;
NSManagedObjectModel *moModel = managedObjectModel();
NSLog(@"The managed object model is defined as follows:\n%@", moModel);
if (applicationLogDirectory() == nil) {
exit(1);
}
NSManagedObjectContext *moContext = managedObjectContext();
// Create an Instance of the Run Entity
//
NSEntityDescription *runEntity = [[moModel entitiesByName] objectForKey:@"Run"];
Run *run = [[Run alloc] initWithEntity:runEntity insertIntoManagedObjectContext:moContext];
NSProcessInfo *processInfo = [NSProcessInfo processInfo];
run.processID = [processInfo processIdentifier];
if (![moContext save: &error]) {
NSLog(@"Error while saving\n%@", ([error localizedDescription] != nil) [error localizedDescription] : @"Unknown Error");
exit(1);
}
// Fetching Run Objects
//
NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:runEntity];
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"date" ascending:YES];
[request setSortDescriptors:[NSArray arrayWithObject:sortDescriptor]];
error = nil;
NSArray *array = [moContext executeFetchRequest:request error:&error];
if ((error != nil) || (array == nil))
{
NSLog(@"Error while fetching\n%@", ([error localizedDescription] != nil) [error localizedDescription] : @"Unknown Error");
exit(1);
}
// Display the Results
//
NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
[formatter setDateStyle:NSDateFormatterMediumStyle];
[formatter setTimeStyle:NSDateFormatterMediumStyle];
NSLog(@"%@ run history:", [processInfo processName]);
for (run in array)
{
NSLog(@"On %@ as process ID %ld", [formatter stringForObjectValue:run.date], run.processID);
}
return 0;
}
在上面的代码中：
1）我们先获得全局的 NSManagedObjectModel 和 NSManagedObjectContext 对象：moModel 和 moContext；
2）并创建一个Run Entity，设置其 Property processID 为当前进程的 ID；
3）将该数据对象保存到持久化文件中：[moContext save: &error]。我们无需与 PersistentStoreCoordinator 打交道，只需要给 ManagedObjectContext 发送 save 消息即可，NSManagedObjectContext 会透明地在后面处理对持久化数据文件的读写；
4）然后我们创建一个 FetchRequest 来查询持久化数据文件中保存的数据记录，并将结果按照日期升序排列。查询操作也是由 ManagedObjectContext 来处理的：[moContextexecuteFetchRequest:request error:&error];
5）将查询结果打印输出；
最后，不要忘记导入相关头文件：
#import
#import
#include
#import "Run.h"
好！大功告成！编译运行，我们可以得到如下显示：
2011-09-03 21:42:47.556 CoreDataTutorial[992:903] CoreDataTutorial run history:
2011-09-03 21:42:47.557 CoreDataTutorial[992:903] On 2011-9-3 下午09:41:56 as process ID 940
2011-09-03 21:42:47.557 CoreDataTutorial[992:903] On 2011-9-3 下午09:42:16 as process ID 955
2011-09-03 21:42:47.558 CoreDataTutorial[992:903] On 2011-9-3 下午09:42:20 as process ID 965
2011-09-03 21:42:47.558 CoreDataTutorial[992:903] On 2011-9-3 下午09:42:24 as process ID 978
2011-09-03 21:42:47.559 CoreDataTutorial[992:903] On 2011-9-3 下午09:42:47 as process ID 992
通过这个例子，我们可以更好理解 Core Data 的运作机制。在 Core Data 中我们最常用的就是 ManagedObjectContext，它几乎参与对数据对象的所有操作，包括对 undo/redo 的支持；而 Entity 对应的运行时类为 ManagedObject，我们可以理解为抽象数据结构 Entity 在内存中由 ManagedObject 来体现，而 Perp