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# TSSetEventHandler
Sets a function used for detecting events 
## Synopsis
```
#include "petscts.h" 
PetscErrorCode TSSetEventHandler(TS ts, PetscInt nevents, PetscInt direction[], PetscBool terminate[], PetscErrorCode (*eventhandler)(TS, PetscReal, Vec, PetscScalar[], void *), PetscErrorCode (*postevent)(TS, PetscInt, PetscInt[], PetscReal, Vec, PetscBool, void *), void *ctx)
```
Logically Collective


## Input Parameters

- ***ts -*** the `TS` context obtained from `TSCreate()`
- ***nevents -*** number of local events
- ***direction -*** direction of zero crossing to be detected. -1 => Zero crossing in negative direction,
+1 => Zero crossing in positive direction, 0 => both ways (one for each event)
- ***terminate -*** flag to indicate whether time stepping should be terminated after
event is detected (one for each event)
- ***eventhandler -*** a change in sign of this function (see `direction`) is used to determine an even has occurred
- ***postevent -*** [optional] post-event function, this function can change properties of the solution, ODE etc at the time of the event
- ***ctx       -*** [optional] user-defined context for private data for the
event detector and post event routine (use `NULL` if no
context is desired)



## Calling sequence of `eventhandler`
```none
PetscErrorCode eventhandler(TS ts, PetscReal t, Vec U, PetscScalar fvalue[], void* ctx)
```

- ***ts  -*** the `TS` context
- ***t   -*** current time
- ***U   -*** current iterate
- ***ctx -*** [optional] context passed with eventhandler
- ***fvalue    -*** function value of events at time t



## Calling sequence of `postevent`
```none
PetscErrorCode postevent(TS ts, PetscInt nevents_zero, PetscInt events_zero[], PetscReal t, Vec U, PetscBool forwardsolve, void *ctx)
```

- ***ts -*** the `TS` context
- ***nevents_zero -*** number of local events whose event function is zero
- ***events_zero  -*** indices of local events which have reached zero
- ***t            -*** current time
- ***U            -*** current solution
- ***forwardsolve -*** Flag to indicate whether `TS` is doing a forward solve (1) or adjoint solve (0)
- ***ctx          -*** the context passed with eventhandler





## Note
The `eventhandler` is actually the event detector function and the `postevent` function actually handles the desired changes that
should take place at the time of the event


## See Also
 [](ch_ts), `TSEvent`, `TSCreate()`, `TSSetTimeStep()`, `TSSetConvergedReason()`

## Level
intermediate

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ts/event/tsevent.c.html#TSSetEventHandler">src/ts/event/tsevent.c</A>

## Examples
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ts/tutorials/ex40.c.html">src/ts/tutorials/ex40.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ts/tutorials/ex41.c.html">src/ts/tutorials/ex41.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ts/tutorials/ex44.c.html">src/ts/tutorials/ex44.c</A><BR>


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[Edit on GitLab](https://gitlab.com/petsc/petsc/-/edit/release/src/ts/event/tsevent.c)


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