This pack gives trial mobs bossbars, one per spawner. It can hold a maximum of 30 trial bossbars at a time. Any naturally generating trial spawners function perfectly, and so do any trial spawners that were customised using spawn eggs, they even have custom colours like other trial mobs.*

The pack should work immediately upon applying. If trial mobs were already present before enabling the datapack, it is possible the bossbar doesn't appear yet, which just means they were too far away from the spawner to locate it. However, the next time that spawner spawns a mob, it should be marked and bossbars should then start functioning correctly.

As far as permanent changes to your world go, this datapack should only affect the following entities (no blocks or items):
 - All loaded persistent mobs will be checked to see if there are spawners near them, and once they have been checked they will be given the tag "checked.for.spawner" to prevent unnecessary double checking. These tags have no effect on the entity outside of telling the pack not to check them anymore.
 - To be able to read the data of the trial spawners, invisible markers are placed in their locations. These are used to store the positions of the spawners, and will be removed automatically if their spawner gets destroyed. However, they will remain present after removing the datapack. If need be you can run the following command to delete them:
/kill @e[type=marker,tag=trial.spawner]

This is my first datapack so if you have any ideas for efficiency, bugs to report, or other suggestions, any feedback is greatly appreciated!

Ideas I'm considering:
 - Adding some more obscure fights to the locator bar and/or giving them a bossbar, like skeleton traps or zombie sieges.
 - The ability to toggle whether bossbars show for trial spawners.
Let me know if you agree with these!

*It is worth noting that when using a datapack that customises trial spawners, the bossbars will likely not display accurate values, as I have not yet found a way to read the configuration files. If you have any ideas for how to do this, please let me know.