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# Objective Add interactive system debugging capabilities to bevy, providing step/break/continue style capabilities to running system schedules. * Original implementation: #8063 - `ignore_stepping()` everywhere was too much complexity * Schedule-config & Resource discussion: #8168 - Decided on selective adding of Schedules & Resource-based control ## Solution Created `Stepping` Resource. This resource can be used to enable stepping on a per-schedule basis. Systems within schedules can be individually configured to: * AlwaysRun: Ignore any stepping state and run every frame * NeverRun: Never run while stepping is enabled - this allows for disabling of systems while debugging * Break: If we're running the full frame, stop before this system is run Stepping provides two modes of execution that reflect traditional debuggers: * Step-based: Only execute one system at a time * Continue/Break: Run all systems, but stop before running a system marked as Break ### Demo https://user-images.githubusercontent.com/857742/233630981-99f3bbda-9ca6-4cc4-a00f-171c4946dc47.mov Breakout has been modified to use Stepping. The game runs normally for a couple of seconds, then stepping is enabled and the game appears to pause. A list of Schedules & Systems appears with a cursor at the first System in the list. The demo then steps forward full frames using the spacebar until the ball is about to hit a brick. Then we step system by system as the ball impacts a brick, showing the cursor moving through the individual systems. Finally the demo switches back to frame stepping as the ball changes course. ### Limitations Due to architectural constraints in bevy, there are some cases systems stepping will not function as a user would expect. #### Event-driven systems Stepping does not support systems that are driven by `Event`s as events are flushed after 1-2 frames. Although game systems are not running while stepping, ignored systems are still running every frame, so events will be flushed. This presents to the user as stepping the event-driven system never executes the system. It does execute, but the events have already been flushed. This can be resolved by changing event handling to use a buffer for events, and only dropping an event once all readers have read it. The work-around to allow these systems to properly execute during stepping is to have them ignore stepping: `app.add_systems(event_driven_system.ignore_stepping())`. This was done in the breakout example to ensure sound played even while stepping. #### Conditional Systems When a system is stepped, it is given an opportunity to run. If the conditions of the system say it should not run, it will not. Similar to Event-driven systems, if a system is conditional, and that condition is only true for a very small time window, then stepping the system may not execute the system. This includes depending on any sort of external clock. This exhibits to the user as the system not always running when it is stepped. A solution to this limitation is to ensure any conditions are consistent while stepping is enabled. For example, all systems that modify any state the condition uses should also enable stepping. #### State-transition Systems Stepping is configured on the per-`Schedule` level, requiring the user to have a `ScheduleLabel`. To support state-transition systems, bevy generates needed schedules dynamically. Currently it’s very difficult (if not impossible, I haven’t verified) for the user to get the labels for these schedules. Without ready access to the dynamically generated schedules, and a resolution for the `Event` lifetime, **stepping of the state-transition systems is not supported** --- ## Changelog - `Schedule::run()` updated to consult `Stepping` Resource to determine which Systems to run each frame - Added `Schedule.label` as a `BoxedSystemLabel`, along with supporting `Schedule::set_label()` and `Schedule::label()` methods - `Stepping` needed to know which `Schedule` was running, and prior to this PR, `Schedule` didn't track its own label - Would have preferred to add `Schedule::with_label()` and remove `Schedule::new()`, but this PR touches enough already - Added calls to `Schedule.set_label()` to `App` and `World` as needed - Added `Stepping` resource - Added `Stepping::begin_frame()` system to `MainSchedulePlugin` - Run before `Main::run_main()` - Notifies any `Stepping` Resource a new render frame is starting ## Migration Guide - Add a call to `Schedule::set_label()` for any custom `Schedule` - This is only required if the `Schedule` will be stepped --------- Co-authored-by: Carter Anderson <mcanders1@gmail.com> |
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