Callback dosen't work when is set to "save_positions = (false, true)"

[yunan-l]

Hi, I tried to set the "save_positions = (false, true)" to only save the state values after the event, but failed. It still saved the values before and after the event (“double save” ). Please see the below:

using DifferentialEquations, Plots
function f(du, u, p, t)
    du[1] = -u[1]
end
u0 = [10.0]
prob = ODEProblem(f, u0, (0.0, 10.0))

condition(u, t, integrator) = t == 4
affect!(integrator) = integrator.u[1] += 10
cb = DiscreteCallback(condition, affect!; save_positions = (false, true))

sol = solve(prob, Tsit5(), callback = cb, tstops = [4])

sol.t

22-element Vector{Float64}:
  0.0
  0.10000199992000479
  0.3420229369016172
  0.6552769527588096
  1.0310596557404956
  1.470610835625779
  1.9654372701948286
  2.510852255671655
  3.0991540633746
  3.7244761095148275
  4.0
  4.0
  4.535515171750724
  4.995616597492678
  5.575642288964066
  6.1561472937094255
  6.795295351347132
  7.451558338536082
  8.139599569463904
  8.84425844508113
  9.567790810446223
 10.0

Is this a bug?

[ChrisRackauckas]

One is from the callback while the other is because you have the integrator set to save each step.