Using Roomy

Undo a clean

Put files back from the Trash and see the Activity Log.

Changed your mind after a clean? Most things Roomy removes go to the Trash, so you can put them back in a few clicks, as long as you haven't emptied the Trash yet.

What you can undo

Moved to the Trash (you can restore these): downloads, installers, device backups, Xcode archives, duplicates, large files, uninstalled apps and their leftovers, and launch agents you removed in Optimize.
Deleted (these rebuild by themselves): caches, logs, temporary files, build data and package caches. They aren't in the Trash, but apps and tools recreate them automatically when needed, so there's nothing to restore.

Before every clean, the confirmation sheet tells you how many items are deleted and how many move to the Trash.

Put items back from the Trash

Click the Trash icon at the end of the Dock.
To see the newest items first, choose View → as List, then View → Show View Options, tick Date Added, and click the Date Added column heading.
Select the items you want back. Hold ⌘ to select several.
Right-click (or Control-click) the selection and choose Put Back. Each item returns to the folder it came from.

If Put Back isn't available, for example for an app that was installed for all users, drag the item to where it belongs instead. Apps go back into Applications.

Restore an app you uninstalled

Put the app back in Applications as above.
Also put back its leftovers if you want them, such as files with the app's name from ~/Library.
Open the app. If you left Also remove app data & settings off when uninstalling, its documents, settings and logins were never removed, so it picks up where you left off.

If you removed a background helper (launch agent) in Optimize and want it back, put its file back and then log out and back in so macOS loads it again. Reinstalling the app it belongs to also restores it.

See what was cleaned in the Activity Log

Click Activity Log in the sidebar under Tools. Every clean is recorded with:

the Date and the Action, such as "Clean Safe Items" or an uninstall;
how many Items were removed and how much Space was freed, marked (Trash) when items went to the Trash;
the Result: Done, or how many items were skipped.

The Activity Log stays on your Mac and is never sent anywhere. Clear Log empties it.

Tips

Prefer a safety net for everything? Turn on Settings → General → Move everything to the Trash instead of deleting. Caches and logs will then go to the Trash too. Remember to empty the Trash to actually get the space back.
Wait a day or two before emptying the Trash after a big clean.
Right-click anything you always want to keep and choose Never Clean This.
Keep regular backups with Time Machine. If you already emptied the Trash, a backup is the only way to get files back.

If that didn't work

The item isn't in the Trash: it was probably a cache or log, which is deleted rather than moved. It rebuilds on its own.
The Trash was emptied: files are permanently gone from the Trash. Restore them from a Time Machine backup if you have one.
Put Back says you don't have permission: drag the item to its folder instead, and enter your administrator password if macOS asks.