Notes about the cache database:

• It is a cache, but the user-created links between tags and resources exist only in the database. We cannot drop the database and recreate it based on what is in the resources folder, unless we back-up the tag links in an external file.

Logic for creating/recreating the database:

• There is no database yet: create the database
• There is a database: open and verify the version. If the version is too low, migration is needed
• There is a database, but not all tables are present: migration is needed
• There is a database, but not entries in the lookup tables are present: migration is needed.

What do we do when we remove an entry from a lookup table -- remove a resource type?

Notes about the lifecycle of the resource system

1. Startup

• On application startup, the KisResourceLoader instances are registered with the KisResourceLoaderRegistry singleton.
• The cache database is opened. KisResourceCacheDb is accessible from this point
• The KisResourceLocator singleton is created
• KisResourceLocator will make a list of all available KisResourceStorages in the configured directory.
• KisResourceLocator will ask the database to synchronize itself.

2. Synchronization

1. we discover there are no resources in any of the listed storages. -> this means that we need to reinstall the default resources that come with Krita

2. we discover new resources -> we will add them to the database

3. we discover a resource has changed (date has changed, md5 sum has changed) -> we will add a new version to the database

4. we discover a resource has disappeared -> we will remove the resource from the database (but keep the version history and mark the brush as deleted? Do we need to make a difference between deleted from disk, deleted from the ui, deleted by updating krita)

5. we discover new tags.

3. Runtime

• From now on resources can be retrieved.
• KisResourceModel and KisTagModel provide a view of all resources as cached in the database.
• If the user selects an item from the KisResourceModel, the resource will be loaded using KisResourceLoader and returned. KisResourceLocator will cache X resources of each type.

• If the user changes the place where storages are located, KisResourceLocator will ask the database to re-synchronize.

• Q: will we use QFileSystemWatcher to trigger a re-synch if a resource is copied or removed into or from the storage location? A: No idea yet.

• If the user removes a resource using the gui, the database will be updated. The flag deleted will be set on the last versioned resource.

• If the user creates a resource or adds it through the resource manager, the database will be updated. A new resource will be inserted.

• If the user modifies a resource, a new version will be created on disk and in the database

• If the user tags a resource, the database will be updated
• If the user untags a resource, the database will be updated

• If the user removes a tag using the gui, the database will be updated and the flag active will be set to false
• If the user removes a tag from disk...

4. Shutdown

• ...

Note about first installation:

I first thought that checking whether the resource location was empty would be a good idea to figure out whether a first-time install of krita's default resources would be needed, but this would break on updating an existing installation to the first version with the new resource system.