CALL WORKER

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4D v20.6

CALL WORKER


CALL WORKER ( process ; formula {; param}{; param2 ; ... ; paramN} )

Parameter	Type	Description
process	Text, Longint	Name or number of worker process
formula	Object, Text	Formula object or Name of project method
param	Expression	Parameter(s) passed to formula

Description

The CALL WORKER command creates or calls the worker process whose name or ID you passed in process, and requests the execution of the code designated by formula in its context with the optional param parameter(s).

The CALL WORKER command encapsulates the param parameters into a message and posts it in the worker's message box. For more information on worker processes, please refer to the About workers section.

In the process parameter, you can specify the worker using its process name or its process number:

If you pass the number of a process that does not exist, or if the process specified was not created by CALL WORKER nor by 4D itself (such as the main application process), CALL WORKER does nothing.
If you pass the name of a process that does not exist, a new worker process is created.

Note: The main process, created by 4D for the user interface and the application mode when a database is opened, is a worker process and can be called by CALL WORKER. However, since its name can vary depending on the 4D language, it is preferable to designate this process using its number (always 1) when you use CALL WORKER.

The worker process appears in the list of processes of the Runtime Explorer and is returned by the PROCESS PROPERTIES command when applied to this process.

In formula, you designate the 4D code to execute in the context of the worker process. You can pass either:

a formula object (see Formula Objects). Formula objects can encapsulate any executable expressions, including functions and project methods;
a string containing the name of a project method. You can pass an empty string; in this case, the worker executes the method that was originally used to start its process, if any (i.e., the startup method of the worker).
Note: It is not possible to pass an empty string in formula when the command calls the main process (process number 1) since it was not started using a project method. As a result, CALL WORKER (1;"") does nothing.

You can also pass parameters to the formula using one or more optional param parameters. You can use sequential parameters or, if the formula expression is a function or a project method, named parameters. Upon starting execution in the context of the process, the process formula receives the parameter values either in the named parameters, or in $1, $2, and so on. Remember that arrays cannot be passed as parameters. Furthermore, in the context of the CALL WORKER command, the following additional considerations need to be taken into account:

Pointers to tables or fields are allowed.
Pointers to variables, particularly local and process variables, are not recommended since these variables may be undefined at the moment they are being accessed by the process method.
Standard object or collection type parameters are passed by copy, i.e. 4D will create a copy of the object or the collection in the destination process if the worker is in a process different from the one calling the CALL WORKER command. In this context, if you want to pass an object or collection parameter by reference, you must use a shared object or collection (see Shared objects and shared collections).

A worker process remains alive until the application is closed or the KILL WORKER command is explicitly called for it. To free up memory, do not forget to call this command once a worker process is no longer needed.

Example

In a form, a button starts a computation: for example, statistics for the selected year. The button creates or calls a worker process that computes the data while the user can continue to work in the form.

The method of the button is:

  //call the worker myWorker with the parameter

 var $vYear: Integer

 $vYear:=2015 // could have been selected by the user in the form

 CALL WORKER("myWorker";Formula(workerMethod);$vYear;Current form window)

The code of workerMethod is:

  // this is the method of the worker

  // it can be preemptive or cooperative

 #DECLARE($vYearInteger;$windowInteger) //year and window reference

 var $vStatResults : Object //to store statistical results

 ... //compute statistics

  //once finished, calls the form back with calculated values

  //$vStatResults can display results in the form

 CALL FORM($window;Formula(displayStats);$vStatResults)