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

Question

我已经尝试过这段代码，但它在文档目录中创建了 BLANK database.sqlite 文件。

- (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]);

    }
}

如何确保在复制数据库文件时不会创建空数据库？

任何帮助将不胜感激。

Answer 1

首先，您的代码没有尝试创建数据库文件。

所以...您只想将数据库文件“prediction.sqlite”从源包复制到 UserDomain 目录？ 如果这是你的目的，你应该注意代码：

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

正确的应该是：

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

NSSearchPathForDirectoriesInDomains返回的目录不保证存在；如有必要，您需要自己创建它们（请参阅documentation）。 NSFileManager 的createDirectoryAtPath:withIntermediateDirectories:attributes:error: 可以提供帮助。@Anomie

尝试这些代码：（在复制“prediction.sqlite”文件之前，您需要将该文件添加到您的项目包中。）：

- (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]);

    }
}