# Objective
I'm build a UI system for bevy. In this UI system there is a concept of a system per UI entity. I had an issue where change detection wasn't working how I would expect and it's because when a function system is ran the `last_change_tick` is updated with the latest tick(from world). In my particular case I want to "wait" to update the `last_change_tick` until after my system runs for each entity.
## Solution
Initially I thought bypassing the change detection all together would be a good fix, but on talking to some users in discord a simpler fix is to just expose `last_change_tick` to the end users. This is achieved by adding the following to the `System` trait:
```rust
/// Allows users to get the system's last change tick.
fn get_last_change_tick(&self) -> u32;
/// Allows users to set the system's last change tick.
fn set_last_change_tick(&mut self, last_change_tick: u32);
```
This causes a bit of weirdness with two implementors of `System`. `FixedTimestep` and `ChainSystem` both implement system and thus it's required that some sort of implementation be given for the new functions. I solved this by outputting a warning and not doing anything for these systems.
I think it's important to understand why I can't add the new functions only to the function system and not to the `System` trait. In my code I store the systems generically as `Box<dyn System<...>>`. I do this because I have differing parameters that are being passed in depending on the UI widget's system. As far as I can tell there isn't a way to take a system trait and cast it into a specific type without knowing what those parameters are.
In my own code this ends up looking something like:
```rust
// Runs per entity.
let old_tick = widget_system.get_last_change_tick();
should_update_children = widget_system.run((widget_tree.clone(), entity.0), world);
widget_system.set_last_change_tick(old_tick);
// later on after all the entities have been processed:
for system in context.systems.values_mut() {
system.set_last_change_tick(world.read_change_tick());
}
```
## Changelog
- Added `get_last_change_tick` and `set_last_change_tick` to `System`'s.
9e34c748c6