## Crafter.Debug

This app can be driven while it runs: call its commands, read and set its state, pause and step it, wait for conditions, read its log, take screenshots. It is only in debug builds, under this project's debug flag (the notes above name it, usually `--debug-api`).

### Driving it

- Build with the debug flag first, in the foreground: `crafter-build <debug flag>`. That build also compiles the client `./build/crafter-debug` runs. The first one takes a few minutes.
- Then start the app in the background, since it does not exit: `crafter-build <debug flag> -r`.
- From the project directory, use `./build/crafter-debug`:
  - `ls --wait 60s` until it is listed; `status`, `cmds`, `help <command>`, `get '*'`
  - `call <command> key=value ...`, `set <path> <value>`
  - `pause --when '<condition>'` then `wait --paused`; `step 3`, `resume`
  - `wait '<condition>'`, `wait --frames 30`, `wait --event <Name>`
  - `trace <path> --frames 120`, `log --level warn`, `log --follow` (under Monitor)
  - `screenshot`: open the PNG it prints with Read
  - `quit` when done
- `./build/crafter-debug guide` is the full reference. Errors name the next step.
- Exit codes: 2 usage, 3 no instance, 4 app and client from different versions, 5 timed out, 6 the app answered an error.
- Requests run when the app calls `Debug::Pump()`, once per frame; `status` shows when it last did.
- Stopping the background `-r` task can leave the app running. Stop it with `quit`, or `quit --force` if its loop is stuck.
- Run long `wait`s in the background, and chain short steps with `&&`.
- Other projects' debug apps may be running too. Commands pick this project's app and leave the others alone, and `ls` marks this project's. `-i <name|id>` picks another project's app, or one of two copies of this one.
- `debugger` prints how to attach lldb or gdb for breakpoints and memory.

### Coding with it

Missing a command or watch for the task at hand? Add it. It is debug-only code.

- **Where it goes:**
  - The hook points sit in `main`, under the project's debug define, or in a shim module: `Debug::Session debug({ .name = "..." });` after `ThreadPool::Start()`, and `Debug::Pump()` once per frame on the app's thread.
  - Registrations go in debug-only sources, which `project.cpp` adds only when the flag is on.
- **Pause and step** hold only what is gated on them. In a frame loop: `while (acc >= dt && Debug::Advance()) { Tick(); acc -= dt; }`. In a fixed-step loop: `Debug::Paused()` and `Debug::TakeSteps()`.
- **Registering** (`import Crafter.Debug;`, namespace `Crafter::Debug`):

```cpp
struct SpawnArgs { Debug::Required<std::string> kind; float x = 0; float y = 0; };   // fields become key=value
Debug::Command("World_Spawn", "Spawn an entity; returns its id.",
    [](const SpawnArgs& a) -> Debug::Result<std::int32_t> {
        if (!world) return Debug::Error{ "NoWorld", "no world loaded; call Level_Load first" };
        return world->Spawn(*a.kind, a.x, a.y);
    });
Debug::Watch("physics.gravity", gravity, { .min = -50, .max = 0, .help = "m/s^2" });   // get and set
Debug::Watch("scene", [] { return SceneName(); });                                       // read-only
Debug::Sample("solver.residual", residual);                    // hot code, any thread; `trace --history`
Debug::Check(mass > 0, "physics", "body {} has mass {}", id, mass);   // a soft assert; also Log, LogWarn, LogError, Emit
```

- **Rules:**
  - **Threading:** handlers, getters and setters run on the app's thread inside `Pump`, so they read state without locks, but must not block. For a result that takes frames (a GPU readback, a network round trip), return `Debug::Async<T>` with a poller.
  - **Reuse the app's own paths:** call the same code the UI or game uses, rather than re-implementing it in the command, because a re-implemented command drifts from the real behaviour.
  - **Errors** are `Debug::Error{ "Code", "message" }`, with the message saying what to do next.
  - **Names:** commands are `Area_Verb`; watches are dotted paths.
  - **Per-object state** goes through a `Debug::Group` member, `Debug::Group debug{ std::format("fort{}", slot) }`, which prefixes paths and unregisters with its owner. It can't be moved, so keep such objects in `std::unique_ptr` or `std::deque`.
  - **Types:** arguments and results are aggregates (public fields with default initializers), scoped enums, strings, numbers or std containers. For anything else, specialize `Debug::Codec<T>`.
  - **Screenshots:** `Debug::SetCapture` makes them work. Crafter.Debug's README has the Crafter.Graphics version.