# Events¤

Events allow for interrupting a differential equation solve, and changing its internal state, or terminating the solve before `t1`

is reached.

At the moment a single kind of event is supported: discrete events which are checked at the end of every step, and which halt the integration once they become true.

`diffrax.AbstractDiscreteTerminatingEvent`

####
```
diffrax.AbstractDiscreteTerminatingEvent
```

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Evaluated at the end of each integration step. If true then the solve is stopped at that time.

#####
`__call__(self, state, **kwargs) -> bool`

`abstractmethod`

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**Arguments:**

`state`

: a dataclass of the evolving state of the system, including in particular the solution`state.y`

at time`state.tprev`

.`**kwargs`

: the integration options held constant throughout the solve are passed as keyword arguments:`terms`

,`solver`

,`args`

. etc.

**Returns**

A boolean. If true then the solve is terminated.

#### ¤

##### ¤

####
```
diffrax.DiscreteTerminatingEvent (AbstractDiscreteTerminatingEvent)
```

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Terminates the solve if its condition is ever active.

#####
`__init__(self, cond_fn: Callable[..., bool])`

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**Arguments:**

`cond_fn`

: A function`(state, **kwargs) -> bool`

that is evaluated on every step of the differential equation solve. If it returns`True`

then the solve is finished at that timestep.`state`

is a dataclass of the evolving state of the system, including in particular the solution`state.y`

at time`state.tprev`

. Passed as keyword arguments are the`terms`

,`solver`

,`args`

etc. that are constant throughout the solve.

####
```
diffrax.SteadyStateEvent (AbstractDiscreteTerminatingEvent)
```

¤

Terminates the solve once it reaches a steady state.

#####
`__init__(self, rtol: Optional[float] = None, atol: Optional[float] = None, norm: Callable[[PyTree[Array]], Union[float, int]] = <function rms_norm>)`

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**Arguments:**

`rtol`

: The relative tolerance for determining convergence. Defaults to the same`rtol`

as passed to an adaptive step controller if one is used.`atol`

: The absolute tolerance for determining convergence. Defaults to the same`atol`

as passed to an adaptive step controller if one is used.`norm`

: A function`PyTree -> Scalar`

, which is called to determine whether the vector field is close to zero.