如何在Cocoa的Application Support目录中复制数据库文件而不丢失数据？

Question

I have tried this code but it creates BLANK database.sqlite file in document directory.

- (void)createEditableCopyOfDatabaseIfNeeded
{
    // First, test for existence.
    BOOL success;
    NSFileManager *fileManager = [NSFileManager defaultManager];
    NSError *error;
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSApplicationSupportDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex:0];

    NSString *writableDBPath = [documentsDirectory stringByAppendingPathComponent:@"prediction.sqlite"];



    success = [fileManager fileExistsAtPath:writableDBPath];

    if (success) return;
    // The writable database does not exist, so copy the default to the appropriate location.
    NSString *defaultDBPath = [[[NSBundle mainBundle] resourcePath] stringByAppendingPathComponent:@"prediction.sqlite"];

    success = [fileManager copyItemAtPath:defaultDBPath toPath:writableDBPath error:&error];


    if (!success) {
        NSAssert1(0, @"Failed to create writable database file with message '%@'.", [error localizedDescription]);

    }
}

How do I ensure that when I copy database file it does not create an empty database?

Any help would be greatly appreciated.

Answer 1

First your code were not attempts to create a database file.

So... You just want to copy the database file “prediction.sqlite” from source bundle to UserDomain dir? If this is your purpose，You should pay attention to the code:

  if (success) return;
// The writable database does not exist, so copy the default to the appropriate location.

The correct should be：

      if (!success) return;
// The writable database does not exist, so copy the default to the appropriate location.

The directories returned by NSSearchPathForDirectoriesInDomains are not guaranteed to exist; you need to create them yourself, if necessary (see the documentation ). NSFileManager's createDirectoryAtPath:withIntermediateDirectories:attributes:error: can help with that.@Anomie

try those code:(before you copy the "prediction.sqlite" file, you need add the file in to your project bundle.):

- (void)createEditableCopyOfDatabaseIfNeeded
{
    // First, test for existence.
    BOOL success;
    NSFileManager *fileManager = [NSFileManager defaultManager];
    NSError *error;
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSApplicationSupportDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex:0];

    NSString *writableDBPath = [documentsDirectory stringByAppendingPathComponent:@"prediction.sqlite"];

    success = [fileManager fileExistsAtPath:writableDBPath];

    if (!success){
        // create the directories, because the directories returned by NSSearchPathForDirectoriesInDomains are not guaranteed to exist
        if (![fileManager createFileAtPath:writableDBPath contents:nil attributes:nil]) {
            return;
        }
    }

    // The writable database does not exist, so copy the default to the appropriate location.
    NSString *defaultDBPath = [[[NSBundle mainBundle] resourcePath] stringByAppendingPathComponent:@"prediction.sqlite"];

    // remove the same file, before you copy the file
    [fileManager removeItemAtPath:writableDBPath error:nil];

    success = [fileManager copyItemAtPath:defaultDBPath toPath:writableDBPath error:&error];

    if (!success) {
        NSAssert1(0, @"Failed to create writable database file with message '%@'.", [error localizedDescription]);

    }
}