objective-c - 我可以访问用于 Core Data 中外部二进制存储的文件吗？

Question

我正在开发媒体数据库应用程序。我有一个带有数据存储的自定义模型，并考虑将其重写为 Core Data。我特别感兴趣的一个用例是电影存储。我将电影文件存储在数据库中，但媒体框架只能从文件(而不是数据)中读取电影。

Core Data 提供了一种称为“外部二进制存储”的便捷功能，其中实体数据不存储在数据库中，而是存储在外部文件中。这对 Core Data API 用户是透明的。我的问题是，我能否获得外部文件的路径，以便我可以使用 Core Data 存储电影，然后轻松地从其 Core Data 外部文件加载它？

Answer 1

是的，您可以访问存储在外部存储中的文件。这需要一些技巧，而且可能与 Apple 的 App Store 不完全一致，但您可以相当轻松地做到这一点。

假设我们有一个 NSManagedObject 子类“媒体”，其“数据”属性已在核心数据编辑器中设置为“允许外部存储”:

//  Media.h
//  Examples
//
//  Created by Garrett Shearer on 11/21/12.
//  Copyright (c) 2012 Garrett Shearer. All rights reserved.
//

#import <Foundation/Foundation.h>
#import <CoreData/CoreData.h>


@interface CRMMedia : NSManagedObject

@property (nonatomic, retain) NSString * name;
@property (nonatomic, retain) NSData * data;

@end

还有一个方便的 NSString 类别:

//  NSString+Parse.m
//  Examples
//
//  Created by Garrett Shearer on 11/21/12.
//  Copyright (c) 2012 Garrett Shearer. All rights reserved.
//

#import "NSString+Parse.h"

@implementation NSString (Parse)

- (NSString*)returnBetweenString:(NSString *)inString1
                       andString:(NSString *)inString2
{
    NSRange substringRange = [self rangeBetweenString:inString1
                                            andString:inString2];
    logger(@"substringRange: (%d, %d)",substringRange.location,substringRange.length);
    logger(@"string (self): %@",self);
    return [self substringWithRange:substringRange];
}


/*
 Return the range of a substring, searching between a starting and ending delimeters
 Original Source: <http://cocoa.karelia.com/Foundation_Categories/NSString/Return_the_range_of.m>
 (See copyright notice at <http://cocoa.karelia.com>)
 */

/*" Find a string between the two given strings with the default options; the delimeter strings are not included in the result.
 "*/

- (NSRange) rangeBetweenString:(NSString *)inString1 andString:(NSString *)inString2
{
    return [self rangeBetweenString:inString1 andString:inString2 options:0];
}

/*" Find a string between the two given strings with the given options inMask; the delimeter strings are not included in the result.  The inMask parameter is the same as is passed to [NSString rangeOfString:options:range:].
 "*/

- (NSRange) rangeBetweenString:(NSString *)inString1 andString:(NSString *)inString2
                       options:(unsigned)inMask
{
    return [self rangeBetweenString:inString1 andString:inString2
                            options:inMask
                              range:NSMakeRange(0,[self length])];
}

/*" Find a string between the two given strings with the given options inMask and the given substring range inSearchRange; the delimeter strings are not included in the result.  The inMask parameter is the same as is passed to [NSString rangeOfString:options:range:].
 "*/

- (NSRange) rangeBetweenString:(NSString *)inString1 andString:(NSString *)inString2
                       options:(unsigned)inMask range:(NSRange)inSearchRange
{
    NSRange result;
    unsigned int foundLocation = inSearchRange.location;    // if no start string, start here
    NSRange stringEnd = NSMakeRange(NSMaxRange(inSearchRange),0); // if no end string, end here
    NSRange endSearchRange;
    if (nil != inString1)
    {
        // Find the range of the list start
        NSRange stringStart = [self rangeOfString:inString1 options:inMask range:inSearchRange];
        if (NSNotFound == stringStart.location)
        {
            return stringStart; // not found
        }
        foundLocation = NSMaxRange(stringStart);
    }
    endSearchRange = NSMakeRange( foundLocation, NSMaxRange(inSearchRange) - foundLocation );
    if (nil != inString2)
    {
        stringEnd = [self rangeOfString:inString2 options:inMask range:endSearchRange];
        if (NSNotFound == stringEnd.location)
        {
            return stringEnd;   // not found
        }
    }
    result = NSMakeRange( foundLocation, stringEnd.location - foundLocation );
    return result;
}


@end

现在是施展魔法的时候了……我们将创建一个 Category 方法来从 [data description] 字符串中解析文件名。在 Media 子类的实例上操作时，'data' 实际上是一个'External Storage Reference'，而不是 NSData 对象。实际数据的文件名存储在描述字符串中。

//  Media+ExternalData.m
//  Examples
//
//  Created by Garrett Shearer on 11/21/12.
//  Copyright (c) 2012 Garrett Shearer. All rights reserved.
//

#import "Media+ExternalData.h"
#import "NSString+Parse.h"

@implementation Media (ExternalData)

- (NSString*)filePathString
{
    // Parse out the filename
    NSString *description = [self.data description];
    NSString *filename = [description returnBetweenString:@"path = " andString:@" ;"];
    // Determine the name of the store
    NSPersistentStoreCoordinator *psc = self.managedObjectContext.persistentStoreCoordinator;
    NSPersistentStore *ps = [psc.persistentStores objectAtIndex:0];
    NSURL *storeURL = [psc URLForPersistentStore:ps];
    NSString *storeNameWithExt = [storeURL lastPathComponent];
    NSString *storeName = [storeNameWithExt stringByDeletingPathExtension];
    // Generate path to the 'external data' directory
    NSString *documentsPath = [[[[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory
                                                                       inDomains:NSUserDomainMask] lastObject] path];
    NSString *pathComponentToExternalStorage = [NSString stringWithFormat:@".%@_SUPPORT/_EXTERNAL_DATA",storeName];
    NSString *pathToExternalStorage = [documentsPath stringByAppendingPathComponent:pathComponentToExternalStorage];
    // Generate path to the media file
    NSString *pathToMedia = [pathToExternalStorage stringByAppendingPathComponent:filename];
    logger(@"pathToMedia: %@",pathToMedia);
    return pathToMedia;
}

- (NSURL*)filePathUrl
{
    NSURL *urlToMedia = [NSURL fileURLWithPath:[self filePathString]];
    return urlToMedia;
}

@end

现在你有了文件的 NSString 路径和 NSURL 路径。 快乐!!!

需要注意的一点是，我在使用这种方法加载电影时遇到了问题...但我也想出了一个解决方法。看来MPMoviePlayer不会访问这个目录下的文件，所以解决方法是暂时把文件复制到documents目录下，然后播放。然后在我卸载 View 时删除临时副本:

- (void)viewDidLoad
{
    [super viewDidLoad];
    [self copyTmpFile];
}

- (void)viewDidUnload
{
    logger(@"viewDidUnload");
    [_moviePlayer stop];
    [_moviePlayer.view removeFromSuperview];
    [self cleanupTmpFile];
    [super viewDidUnload];
}

- (NSString*)tmpFilePath
{
    NSString *documentsPath = [[[[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory
                                                                       inDomains:NSUserDomainMask] lastObject] path];
    NSString *tmpFilePath = [documentsPath stringByAppendingPathComponent:@"temp_video.m4v"];
    return tmpFilePath;
}

- (void)copyTmpFile
{
    NSString *tmpFilePath = [self tmpFilePath];
    NSFileManager *mgr = [NSFileManager defaultManager];
    NSError *err = nil;
    if([mgr fileExistsAtPath:tmpFilePath])
    {
        [mgr removeItemAtPath:tmpFilePath error:nil];
    }

    [mgr copyItemAtPath:_media.filePathString toPath:tmpFilePath error:&err];
    if(err)
    {
        logger(@"error: %@",err.description);
    }
}

- (void)cleanupTmpFile
{
    NSString *tmpFilePath = [self tmpFilePath];
    NSFileManager *mgr = [NSFileManager defaultManager];
    if([mgr fileExistsAtPath:tmpFilePath])
    {
        [mgr removeItemAtPath:tmpFilePath error:nil];
    }
}

祝你好运!