Limitations¶

Some things can't change once a session is built. This page lists them and says what to do instead. For the parts redsun leaves to you, see Why redsun exists.

Can I add a value after startup?¶

No. Every shared value reaches the session before it starts running, in one of three ways: from a component, from a method marked with provides, or from a plugin's provider. You can't hand the session a new value later.

If something fills up over time, share a single object that changes, and let the other components read it as it changes.

Can a shared value be optional?¶

No. The session decides what exists by reading types before it builds, and Roi | None is a type of its own: the session reports it as a value no component asks for, even when one asks for Roi | None. The value still arrives while it is a Roi, but when the method returns None, a component asking for Roi fails its setup:

@provides
def roi(self) -> Roi | None:  # does not do what it looks like
    return self._roi if self.camera.has_roi else None

Always share a value, and let the value say it's empty:

@provides
def roi(self) -> Roi:
    return self._roi if self.camera.has_roi else Roi.empty()

Whether the value exists at all then depends only on whether the component that shares it is in the session.

No. A type names one value, so two components sharing Readings is an error:

TypeError: 'plot_b' and 'plot_a' both share 'Readings'.
A shared type identifies one value; give them distinct types.

Give each value its own type.

Can I import parameter types under `if TYPE_CHECKING:`?¶

Not the types the session reads to decide what to pass: it reads them while the program runs, when a type imported only under if TYPE_CHECKING:, a block only type checkers read, doesn't exist. What happens depends on where the type is used:

A type imported only for type checkers

A component left out shows in the log like this:

Failed to build presenter 'motor_ctrl': cannot read the constructor of
MotorPresenter: 'Calibration' is not available at runtime. A type a component
is injected by must be imported outside 'if TYPE_CHECKING', because the graph
evaluates the annotation.

Either way, import those types with a normal import.

Can I run two frontends?¶

Not in one process. A QtSession owns the QApplication, and Qt allows only one per process. Two sessions in one process also need different names, because each registers an application under its own name.

What happens when a service crashes?¶

The session logs it and emits a signal, but nothing restarts the service, so it stays down:

A launched service that stops on its own

See Services.