Spent some more time working on dynamo-browse, this time adding a back-stack. This can be used to go back to the previously viewed table, query or filter by pressing the Backspace key, kind of like how a browser back button works.
This is the first feature that makes use of a workspace, which is a concept that I’ve been thinking about since the start of the project. A workspace is basically a file storing various bits of state that could be recalled in future launches of the tool. A workspace is always created, even if one is not explicitly specified by the user. The workspace filename be set by using the
-w switch on launch, or if one is not specified, then a new workspace filename within the temp directory is generated. The workspace file itself is a Bolt database, which is a very simple, embeddable key/value store that uses B-Trees and memory-mapped data access. I’m actually using StormDB to access this file, since it provides a nice interface for storing and querying Go structs without having to worry about marshalling or unmarshalling.
The back-stack consist of view-snapshots, which are essentially records of the currently view table, filter, and query. The view-snapshots are stored as a linked list, and the back-stack simply pushes new snapshots as the view changes (because this is stored on file, the backlinks are actually ID references and not physical pointers). I had a bit of trouble getting this right at first. The initial version actually pushed a snapshot of the previous view before it changed (i.e. a new table was selected or the filter was changed). But this resulted in a bit of messy implementation, with push calls copied and pasted across the various controllers with various special cases, etc.
So I went back and reimplemented it so that the top of the back-stack is actually a snapshot of the current view. Pressing the Backspace key will actually pop the current snapshot, then read the head of the stack and use that to update the view. This means that there will always be one item in the stack, which bothered me a little at first, but that did make the code easier to implement.
There is another benefit to this arrangement which just came to me while I was writing this. Since the current state of the view is always at the top of the stack, I could add the ability to restore the last view when launching dynamo-browse with an existing workspace, saving the user from selecting a table and running their last query. This will come a bit later. For the meantime, I need to get this back-stack working with user scripts, so that when a script changes the view, the user can still go back.
Finally, a shoutout to boltcli which is a useful CLI tool for browsing a Bolt database. This tool has come in handy for debugging this feature.