CoreData的基本使用

概述：把对象存储进CoreData，大概是这个情况：


那么问题来了：
1.哪设置存储文件路径（full path）？
2.

1.关于UIManagedDocument
1.1Creating a UIManagedDocument
NSFileManager *fileManager = [NSFileManager defaultManager];
NSURL *documentsDirectory = [[fileManager URLsForDirectory:NSDocumentDirectory
inDomains:NSUserDomainMask] firstObject];
NSString *documentName = @“MyDocument”;
NSURL *url = [documentsDirectory URLByAppendingPathComponent:documentName];
UIManagedDocument *document = [[UIManagedDocument alloc] initWithFileURL:url];
注意：This creates the UIManagedDocument instance, but does not open nor create the underlying file（只创建了UIManagedDocument，并还没有打开或者创建了底层的文件）。

1.2How to open or create a UIManagedDocument
1）First, check to see if the UIManagedDocument’s underlying file exists on disk …
BOOL fileExists = [[NSFileManager defaultManager] fileExistsAtPath:[url path]] ;
… if it does, open the document using …
[document openWithCompletionHandler:^(BOOL success) { /* block to execute when open */ }];
… if it does not, create the document using …
[document saveToURL:url // could (should?) use document. fileURL property here
forSaveOperation:UIDocumentSaveForCreating
competionHandler:^(BOOL success) { /* block to execute when create is done */ }];
解释：用NSFileManager 去检测是否已创建了对应URL([URL path])的文件，是就打开该文件（openWithCompletionHandler:），否则创建该文件（saveToURL:forSaveOperation:completionHandler:）。具体看苹果官方文档：https://developer.apple.com/library/ios/documentation/UIKit/Reference/UIDocument_Class/index.html#//apple_ref/occ/cl/UIDocument
2） Example
self.document = [[UIManagedDocument alloc] initWithFileURL: (URL *)url];
if ([[NSFileManager defaultManager] fileExistsAtPath: [url path]]) {
[document openWithCompletionHandler: ^(BOOL success) {
if (success) [self documentIsReady];
if (!success) NSLog(@“couldn’t open document at %@”, url);
}];
} else {
[document saveToURL: url forSaveOperation: UIDocumentSaveForCreating
completionHandler: ^(BOOL success) {
if (success) [self documentIsReady];
if (!success) NSLog(@“couldn’t create document at %@”, url);
}];
}
// can’t do anything with the document yet (do it in documentIsReady)
检查文件状态
Once document is open/created, you can start using it
But you might want to check the documentState when you do …
- (void) documentIsReady
{
if (self.document. documentState == UIDocumentStateNormal) {
// start using document
}
}