question on Trigger and updating of Manipulate expression

Experts,

I can't figure out why the Manipulate expression is being evaluated 2 times each time I click on the 'StepRightButton' which is part of a Trigger control.

When I click on the '+' button, I see the counter going up once as expected, but the expression is being evaluated 2 times, not one time as expected. Here is a small example

Manipulate[

Show[
Graphics@Text@Row[{currentTime,"   ",Date[]}],
Plot[Sin[x],{x,-Pi,Pi}]
],

Control[{{currentTime,0,Style["run",10]},0,100,0.1,
      ControlType->Trigger,DisplayAllSteps->True,ImageSize->Tiny,
      AnimationRate->Automatic,AnimationRepetitions->0,
      AppearanceElements->{"PlayButton","PauseButton","StepRightButton","ResetButton"}}
],

ContinuousAction->False,
SynchronousUpdating->False

]

again, clicking on '+' shows the 'currentTime' variabl开发者_运维问答e is being increased by the correct amount (0.1 in this example). But my question is, why is the expression being evaluated twice by the evidence that the Date[] printed can be seen to increment 2 times per each click on '+' button.

This causes me problem, as I was assuming the expression will evaluate once per 'tick'.

Is this a way to make sure the expression is evaluated once each time I click on '+' once?

thanks,

---

This can be solved by adding the option PerformanceGoal -> "Quality" to the Plot function.

During the display of manipulated items a lower fidelity sample of the object can be shown depending on the PerformanceGoal setting. A higher fidelity version is drawn at the end of the manipulation, if necessary. It is this later redrawing which seems to be causing the problem here.