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$SynchronousEvaluation

$SynchronousEvaluation

is a symbol whose value is True if it is evaluated as part of a synchronous evaluation.

Details

A synchronous evaluation is generally triggered by controls in the front end. Controls that may generate synchronous evaluations include Button, EventHandler, and any control that takes a Dynamic value.
Manipulate may trigger synchronous evaluations, depending upon the value of the SynchronousUpdating and SynchronousInitialization options.
A synchronous evaluation is the same as a preemptive evaluation. Some controls, such as Button and EventHandler, support an option setting of Method->"Preemptive", which causes the control's evaluations to be synchronous.
Synchronous evaluations halt the system while being evaluated. A typical use of $SynchronousEvaluation is to write code that should behave differently to prevent halting the system excessively.

Examples

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Basic Examples (2)

Typical shift-return evaluations are asynchronous:

Evaluations within Dynamic are synchronous by default:

Scope (2)

Evaluations within DynamicModule Initialization are synchronous by default:

Manipulate has a different default for SynchronousUpdating:

Properties & Relations (3)

By default, Button and similar constructs do synchronous evaluation:

With the option Method->"Queued", the evaluation is done asynchronously:

Synchronous evaluation in Dynamic is controlled by the SynchronousUpdating option:

The initialization of DynamicModule is controlled by the SynchronousInitialization option:

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