# Event Handling and Callback Functions

## Introduction to Callback Functions

DifferentialEquations.jl allows for using callback functions to inject user code into the solver algorithms. It allows for safely and accurately applying events and discontinuities. Multiple callbacks can be chained together, and these callback types can be used to build libraries of extension behavior.

## The Callback Types

The callback types are defined as follows. There are two callback types: the `ContinuousCallback`

and the `DiscreteCallback`

. The `ContinuousCallback`

is applied when a continuous condition function hits zero. This type of callback implements what is known in other problem solving environments as an Event. A `DiscreteCallback`

is applied when its `condition`

function is `true`

.

### ContinuousCallbacks

```
ContinuousCallback(condition,affect!,affect_neg!=affect!;
rootfind = true,
initialize = (c,u,t,integrator) -> nothing,
save_positions = (true,true),
interp_points=10,
abstol=1e-9,reltol=0
idxs=nothing)
```

The arguments are defined as follows:

`condition`

: This is a function`condition(u,t,integrator)`

for declaring when the callback should be used. A callback is initiated if the condition hits`0`

within the time interval. See the Integrator Interface documentation for information about`integrator`

.`affect!`

: This is the function`affect!(integrator)`

where one is allowed to modify the current state of the integrator. If you do not pass an`affect_neg!`

function, it is called when`condition`

is found to be`0`

(at a root) and the cross is either an upcrossing (from negative to positive) or a downcrossing (from positive to negative). You need to explicitly pass`nothing`

as the`affect_neg!`

argument if it should only be called at upcrossings, e.g.`ContinuousCallback(condition, affect!, nothing)`

. For more information on what can be done, see the Integrator Interface manual page. Modifications to`u`

are safe in this function.`affect_neg!`

: This is the function`affect_neg!(integrator)`

where one is allowed to modify the current state of the integrator. This is called when`condition`

is found to be`0`

(at a root) and the cross is an downcrossing (from positive to negative). For more information on what can be done, see the Integrator Interface manual page. Modifications to`u`

are safe in this function.`rootfind`

: This is a boolean for whether to rootfind the event location. If this is set to`true`

, the solution will be backtracked to the point where`condition==0`

. Otherwise the systems and the`affect!`

will occur at`t+dt`

.`interp_points`

: The number of interpolated points to check the condition. The condition is found by checking whether any interpolation point / endpoint has a different sign. If`interp_points=0`

, then conditions will only be noticed if the sign of`condition`

is different at`t`

than at`t+dt`

. This behavior is not robust when the solution is oscillatory, and thus it's recommended that one use some interpolation points (they're cheap to compute!).`0`

within the time interval.`save_positions`

: Boolean tuple for whether to save before and after the`affect!`

. This saving will occur just before and after the event, only at event times, and does not depend on options like`saveat`

,`save_everystep`

, etc. (i.e. if`saveat=[1.0,2.0,3.0]`

, this can still add a save point at`2.1`

if true). For discontinuous changes like a modification to`u`

to be handled correctly (without error), one should set`save_positions=(true,true)`

.`idxs`

: The components which will be interpolated into the condition. Defaults to`nothing`

which means`u`

will be all components.`initialize`

: This is a function (c,u,t,integrator) which can be used to initialize the state of the callback`c`

. It should modify the argument`c`

and the return is ignored.

Additionally, keyword arguments for `abstol`

and `reltol`

can be used to specify a tolerance from zero for the rootfinder: if the starting condition is less than the tolerance from zero, then no root will be detected. This is to stop repeat events happening just after a previously rootfound event. The default has `abstol=1e-14`

and `reltol=0`

.

### DiscreteCallback

```
DiscreteCallback(condition,affect!;
save_positions=(true,true),
initialize = (c,u,t,integrator) -> nothing)
```

`condition`

: This is a function`condition(u,t,integrator)`

for declaring when the callback should be used. A callback is initiated if the condition evaluates to`true`

. See the Integrator Interface documentation for information about`integrator`

.`affect!`

: This is the function`affect!(integrator)`

where one is allowed to modify the current state of the integrator. For more information on what can be done, see the Integrator Interface manual page.`save_positions`

: Boolean tuple for whether to save before and after the`affect!`

. This saving will occur just before and after the event, only at event times, and does not depend on options like`saveat`

,`save_everystep`

, etc. (i.e. if`saveat=[1.0,2.0,3.0]`

, this can still add a save point at`2.1`

if true). For discontinuous changes like a modification to`u`

to be handled correctly (without error), one should set`save_positions=(true,true)`

.`initialize`

: This is a function (c,u,t,integrator) which can be used to initialize the state of the callback`c`

. It should modify the argument`c`

and the return is ignored.

### CallbackSet

Multiple callbacks can be chained together to form a `CallbackSet`

. A `CallbackSet`

is constructed by passing the constructor `ContinuousCallback`

, `DiscreteCallback`

, `VectorContinuousCallback`

or other `CallbackSet`

instances:

`CallbackSet(cb1,cb2,cb3)`

You can pass as many callbacks as you like. When the solvers encounter multiple callbacks, the following rules apply:

`ContinuousCallback`

s and`VectorContinuousCallback`

s are applied before`DiscreteCallback`

s. (This is because they often implement event-finding that will backtrack the timestep to smaller than`dt`

).- For
`ContinuousCallback`

s and`VectorContinuousCallback`

s, the event times are found by rootfinding and only the first`ContinuousCallback`

or`VectorContinuousCallback`

affect is applied. - The
`DiscreteCallback`

s are then applied in order. Note that the ordering only matters for the conditions: if a previous callback modifies`u`

in such a way that the next callback no longer evaluates condition to`true`

, its`affect`

will not be applied.

### VectorContinuousCallback

```
VectorContinuousCallback(condition,affect!,len;
initialize = INITIALIZE_DEFAULT,
idxs = nothing,
rootfind=true,
save_positions=(true,true),
affect_neg! = affect!,
interp_points=10,
abstol=10eps(),reltol=0)
```

`VectorContinuousCallback`

is also a subtype of `AbstractContinuousCallback`

. `CallbackSet`

is not feasible when you have a large number of callbacks, as it doesn't scale well. For this reason, we have `VectorContinuousCallback`

- it allows you to have a single callback for multiple events.

`condition`

- This is a function`condition(out, u, t, integrator)`

which should save the condition value in the array`out`

at the right index. Maximum index of`out`

should be specified in the`len`

property of callback. So this way you can have a chain of`len`

events, which would cause the`i`

th event to trigger when`out[i] = 0`

.`affect!`

- This is a function`affect!(integrator, event_index)`

which lets you modify`integrator`

and it tells you about which event occured using`event_idx`

i.e. gives you index`i`

for which`out[i]`

came out to be zero.`len`

- Number of callbacks chained. This is compulsory to be specified.

Rest of the fields have the same meaning as `ContinuousCallback`

.

## Using Callbacks

The callback type is then sent to the solver (or the integrator) via the `callback`

keyword argument:

`sol = solve(prob,alg,callback=cb)`

You can supply `nothing`

, a single `DiscreteCallback`

or `ContinuousCallback`

, or a `CallbackSet`

.

### Note About Saving

When a callback is supplied, the default saving behavior is turned off. This is because otherwise events would "double save" one of the values. To re-enable the standard saving behavior, one must have the first `save_positions`

value be true for at least one callback.

### Modifying the Stepping Within A Callback

A common issue with callbacks is that they cause a large discontinuous change, and so it may be wise to pull down `dt`

after such a change. To control the timestepping from a callback, please see the timestepping controls in the integrator interface. Specifically, `set_proposed_dt!`

is used to set the next stepsize, and `terminate!`

can be used to cause the simulation to stop.

## DiscreteCallback Examples

### Example 1: AutoAbstol

MATLAB's Simulink has the option for an automatic absolute tolerance. In this example we will implement a callback which will add this behavior to any JuliaDiffEq solver which implments the `integrator`

and callback interface.

The algorithm is as follows. The default value is set to start at `1e-6`

, though we will give the user an option for this choice. Then as the simulation progresses, at each step the absolute tolerance is set to the maximum value that has been reached so far times the relative tolerance. This is the behavior that we will implement in `affect!`

.

Since the effect is supposed to occur every timestep, we use the trivial condition:

```
condition = function (u,t,integrator)
true
end
```

which always returns true. For our effect we will overload the call on a type. This type will have a value for the current maximum. By doing it this way, we can store the current state for the running maximum. The code is as follows:

```
mutable struct AutoAbstolAffect{T}
curmax::T
end
# Now make `affect!` for this:
function (p::AutoAbstolAffect)(integrator)
p.curmax = max(p.curmax,integrator.u)
integrator.opts.abstol = p.curmax * integrator.opts.reltol
u_modified!(integrator,false)
end
```

This makes `affect!(integrator)`

use an internal mutating value `curmax`

to update the absolute tolerance of the integrator as the algorithm states.

Lastly, we can wrap it in a nice little constructor:

```
function AutoAbstol(save=true;init_curmax=1e-6)
affect! = AutoAbstolAffect(init_curmax)
condtion = (u,t,integrator) -> true
save_positions = (save,false)
DiscreteCallback(condtion,affect!,save_positions=save_positions)
end
```

This creates the `DiscreteCallback`

from the `affect!`

and `condition`

functions that we implemented. Now

`cb = AutoAbstol(save=true;init_curmax=1e-6)`

returns the callback that we created. We can then solve an equation using this by simply passing it with the `callback`

keyword argument. Using the integrator interface rather than the solve interface, we can step through one by one to watch the absolute tolerance increase:

```
integrator = init(prob,BS3(),callback=cb)
at1 = integrator.opts.abstol
step!(integrator)
at2 = integrator.opts.abstol
@test at1 < at2
step!(integrator)
at3 = integrator.opts.abstol
@test at2 < at3
```

Note that this example is contained in DiffEqCallbacks.jl, a library of useful callbacks for JuliaDiffEq solvers.

### Example 2: A Control Problem

Another example of a `DiscreteCallback`

is the control problem demonstrated on the DiffEq-specific arrays page.

## ContinuousCallback Examples

### Example 1: Bouncing Ball

Let's look at the bouncing ball. Let the first variable `y`

is the height which changes by `v`

the velocity, where the velocity is always changing at `-g`

which is the gravitational constant. This is the equation:

```
function f(du,u,p,t)
du[1] = u[2]
du[2] = -p
end
```

All we have to do in order to specify the event is to have a function which should always be positive with an event occurring at 0. For now at least that's how it's specified. If a generalization is needed we can talk about this (but it needs to be "root-findable"). For here it's clear that we just want to check if the ball's height ever hits zero:

```
function condition(u,t,integrator) # Event when event_f(u,t) == 0
u[1]
end
```

Notice that here we used the values `u`

instead of the value from the `integrator`

. This is because the values `u,t`

will be appropriately modified at the interpolation points, allowing for the rootfinding behavior to occur.

Now we have to say what to do when the event occurs. In this case we just flip the velocity (the second variable)

```
function affect!(integrator)
integrator.u[2] = -integrator.u[2]
end
```

The callback is thus specified by:

`cb = ContinuousCallback(condition,affect!)`

Then you can solve and plot:

```
u0 = [50.0,0.0]
tspan = (0.0,15.0)
p = 9.8
prob = ODEProblem(f,u0,tspan,p)
sol = solve(prob,Tsit5(),callback=cb)
plot(sol)
```

As you can see from the resulting image, DifferentialEquations.jl is smart enough to use the interpolation to hone in on the time of the event and apply the event back at the correct time. Thus one does not have to worry about the adaptive timestepping "overshooting" the event as this is handled for you. Notice that the event macro will save the value(s) at the discontinuity.

The callback is robust to having multiple discontinuities occur. For example, we can integrate for long time periods and get the desired behavior:

```
u0 = [50.0,0.0]
tspan = (0.0,100.0)
prob = ODEProblem(f,u0,tspan,p)
sol = solve(prob,Tsit5(),callback=cb)
plot(sol,plotdensity=10000)
```

### Example 2: Terminating an Integration

In many cases you might want to terminate an integration when some condition is satisfied. To terminate an integration, use `terminate!(integrator)`

as the `affect!`

in a callback.

In this example we will solve the differential equation:

```
u0 = [1.,0.]
function fun2(du,u,p,t)
du[2] = -u[1]
du[1] = u[2]
end
tspan = (0.0,10.0)
prob = ODEProblem(fun2,u0,tspan)
```

which has cosine and -sine as the solutions respectively. We wish to solve until the sine part, `u[2]`

becomes positive. There are two things we may be looking for.

A `DiscreteCallback`

will cause this to halt at the first step such that the condition is satisfied. For example, we could use:

```
condition(u,t,integrator) = u[2]>0
affect!(integrator) = terminate!(integrator)
cb = DiscreteCallback(condition,affect!)
sol = solve(prob,Tsit5(),callback=cb)
```

However, in many cases we wish to halt exactly at the point of time that the condition is satisfied. To do that, we use a continuous callback. The condition must thus be a function which is zero at the point we want to halt. Thus we use the following:

```
condition(u,t,integrator) = u[2]
affect!(integrator) = terminate!(integrator)
cb = ContinuousCallback(condition,affect!)
sol = solve(prob,Tsit5(),callback=cb)
```

Note that this uses rootfinding to approximate the "exact" moment of the crossing. Analytically we know the value is `pi`

, and here the integration terminates at

`sol.t[end] # 3.1415902502224307`

Using a more accurate integration increases the accuracy of this prediction:

```
sol = solve(prob,Vern8(),callback=cb,reltol=1e-12,abstol=1e-12)
sol.t[end] # 3.1415926535896035
#π = 3.141592653589703...
```

Now say we wish to find the when the first period is over, i.e. we want to ignore the upcrossing and only stop on the downcrossing. We do this by ignoring the `affect!`

and only passing an `affect!`

for the second:

```
condition(u,t,integrator) = u[2]
affect!(integrator) = terminate!(integrator)
cb = ContinuousCallback(condition,nothing,affect!)
sol = solve(prob,Tsit5(),callback=cb)
```

Notice that passing only one `affect!`

is the same as `ContinuousCallback(condition,affect!,affect!)`

, i.e. both upcrossings and downcrossings will activate the event. Using `ContinuousCallback(condition,affect!,nothing)`

will thus be the same as above because the first event is an upcrossing.

### Example 3: Growing Cell Population

Another interesting issue is with models of changing sizes. The ability to handle such events is a unique feature of DifferentialEquations.jl! The problem we would like to tackle here is a cell population. We start with 1 cell with a protein `X`

which increases linearly with time with rate parameter `α`

. Since we are going to be changing the size of the population, we write the model in the general form:

```
const α = 0.3
function f(du,u,p,t)
for i in 1:length(u)
du[i] = α*u[i]
end
end
```

Our model is that, whenever the protein `X`

gets to a concentration of 1, it triggers a cell division. So we check to see if any concentrations hit 1:

```
function condition(u,t,integrator) # Event when event_f(u,t) == 0
1-maximum(u)
end
```

Again, recall that this function finds events as when `condition==0`

, so `1-maximum(u)`

is positive until a cell has a concentration of `X`

which is 1, which then triggers the event. At the event, we have that the cell splits into two cells, giving a random amount of protein to each one. We can do this by resizing the cache (adding 1 to the length of all of the caches) and setting the values of these two cells at the time of the event:

```
function affect!(integrator)
u = integrator.u
resize!(integrator,length(u)+1)
maxidx = findmax(u)[2]
Θ = rand()
u[maxidx] = Θ
u[end] = 1-Θ
nothing
end
```

As noted in the Integrator Interface, `resize!(integrator,length(integrator.u)+1)`

is used to change the length of all of the internal caches (which includes `u`

) to be their current length + 1, growing the ODE system. Then the following code sets the new protein concentrations. Now we can solve:

```
callback = ContinuousCallback(condition,affect!)
u0 = [0.2]
tspan = (0.0,10.0)
prob = ODEProblem(f,u0,tspan)
sol = solve(prob,callback=callback)
```

The plot recipes do not have a way of handling the changing size, but we can plot from the solution object directly. For example, let's make a plot of how many cells there are at each time. Since these are discrete values, we calculate and plot them directly:

```
plot(sol.t,map((x)->length(x),sol[:]),lw=3,
ylabel="Number of Cells",xlabel="Time")
```

Now let's check-in on a cell. We can still use the interpolation to get a nice plot of the concentration of cell 1 over time. This is done with the command:

```
ts = range(0, stop=10, length=100)
plot(ts,map((x)->x[1],sol.(ts)),lw=3,
ylabel="Amount of X in Cell 1",xlabel="Time")
```

Notice that every time it hits 1 the cell divides, giving cell 1 a random amount of `X`

which then grows until the next division.

Note that one macro which was not shown in this example is `deleteat!`

on the caches. For example, to delete the second cell, we could use:

`deleteat!(integrator,2)`

This allows you to build sophisticated models of populations with births and deaths.

## VectorContinuousCallback Example

### Example 1: Bouncing Ball with multiple walls

This is similar to the above Bouncing Ball example, but now we have two more vertical walls, at `x = 0`

and `x = 10.0`

. We have our ODEFunction as -

```
function f(du,u,p,t)
du[1] = u[2]
du[2] = -p
du[3] = u[4]
du[4] = 0.0
end
```

where `u[1]`

denotes `y`

-coordinate, `u[2]`

denotes velocity in `y`

-direction, `u[3]`

denotes `x`

-coordinate and `u[4]`

denotes velocity in `x`

-direction. We will make a `VectorContinuousCallback`

of length 2 - one for `x`

axis collision, one for walls parallel to `y`

axis.

```
function condition(out,u,t,integrator) # Event when event_f(u,t) == 0
out[1] = u[1]
out[2] = (u[3] - 10.0)u[3]
end
function affect!(integrator, idx)
if idx == 1
integrator.u[2] = -0.9integrator.u[2]
elseif idx == 2
integrator.u[4] = -0.9integrator.u[4]
end
end
cb = VectorContinuousCallback(condition,affect!,2)
```

It is evident that `out[2]`

will be zero when `u[3]`

(x-coordinate) is either `0.0`

or `10.0`

. And when that happens, we flip the velocity with some coefficient of restitution (`0.9`

).

Completeting rest of the code-

```
u0 = [50.0,0.0,0.0,2.0]
tspan = (0.0,15.0)
p = 9.8
prob = ODEProblem(f,u0,tspan,p)
sol = solve(prob,Tsit5(),callback=cb,dt=1e-3,adaptive=false)
plot(sol,vars=(1,3))
```

And you get the following output: