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**YIELD**

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**YIELD** temporarily gives control to Windows to allow other concurrently executing Windows applications to process events they need to handle (except those events that would post messages back to the program containing the YIELD statement, or events that would change focus to the other application).

YIELD is used to ensure that long batch processing in a Clarion application does not completely "lock out" other applications from completing their tasks. This is known as "cooperative multi-tasking" and ensures that your Windows programs peacefully co-exist with any other Windows applications.

Within your Clarion application, YIELD only allows control to pass to EVENT:Timer events in other execution threads. This allows you to code a "background" procedure in its own execution thread using the TIMER attribute to perform some long batch processing without requiring the user to wait until the task is complete before continuing with other work in the application. This is an industry-standard Windows method of doing background processing within an application.

The example code on the next page demonstrates both approaches to performing batch processing: making the user wait for the process to complete, and processing in the background. Only the WaitForProcess procedure requires the YIELD statement, because it takes full control of the program. Background processing using EVENT:Timer does not need a YIELD statement, since the ACCEPT loop automatically performs cooperative multi-tasking with other Windows applications.

**Example:**

**StartProcess PROCEDURE**

**Win  WINDOW('Choose a Batch Process'),MDI**

**     BUTTON('Full Control'),USE(?FullControl)**

**     BUTTON('Background'),USE(?Background)**

**     BUTTON('Close'),USE(?Close)**

**    END**

** CODE**

** OPEN(Win)**

** ACCEPT**

**  CASE ACCEPTED()**

**  OF ?FullControl**

**   DISABLE(FIRSTFIELD(),LASTFIELD())   !Disable all buttons**

**   WaitForProcess                      !and call the batch process procedure**

**   ENABLE(FIRSTFIELD(),LASTFIELD())    !Enable buttons when batch is complete**

**  OF ?Background**

**   START(BackgroundProcess)            !Start new execution thread for the process**

**  OF ?Close**

**   BREAK**

**  END**

** END**

**WaitForProcess PROCEDURE               !Full control Batch process**

** CODE**

** SETCURSOR(CURSOR:Wait)                !Alert user to batch in progress**

** SET(File)                             !Set up a batch process**

** LOOP**

**  NEXT(File)**

**  IF ERRORCODE() THEN BREAK END**

**  !Perform some batch processing code**

** ****YIELD****                                !Yield to other applications and EVENT:Timer**

** END**

** SETCURSOR                             !Restore mouse cursor**

**BackgroundProcess  PROCEDURE           !Background processing batch process**

**Win  WINDOW('Batch Processing...'),TIMER(1),MDI**

**      BUTTON('Cancel'),STD(STD:Close)**

**     END**

** CODE**

** OPEN(Win)**

** SET(File)                             !Set up a batch process**

** ACCEPT**

**  CASE EVENT()**

**  OF EVENT:CloseWindow**

**   BREAK**

**  OF EVENT:Timer                       !Process records whenever the timer allows it**

**   LOOP 3 TIMES**

**    NEXT(File)**

**    IF ERRORCODE() **

**     POST(EVENT:CloseWindow)**

**     BREAK**

**    END**

**    !Perform some batch processing code**

**   END**

**  END**

** END**

**See Also:**

[[accept  the event processor .htm|ACCEPT]]

[[timer  set periodic event .htm|TIMER]]