Manage a Data Feed with Messages

A data feed object responds to several messages, including Connect and Set Script. These are detailed above as arguments for Open Datafeed(). They can also be sent as messages to a data feed object that already exists:

feed << Connect( port( "com1:" ), baud( 4800 ), databits( 7 ), parity( odd ), stopbits( 2 ) );

feed << Set Script( myScript );

The following messages could also be used as arguments to On Datafeed(). However, it would be more common to send them as messages to a data feed object that is already present.

You can send lines to a data feed from a script. This is a quick way to test a data feed. If you do not have a device available, Queue Line does not send data to a device; it simulates getting data from a device. Include a text argument or a global that stores text:

feed << Queue Line( "14" );

feed << Queue Line( myValue );

Here is a test script to queue five lines of data:

feed << Queue Line( "11" );

feed << Queue Line( "22" );

feed << Queue Line( "33" );

feed << Queue Line( "44" );

feed << Queue Line( "55" );

Figure 14.2 Datafeed: 5 Lines Queued 

To get the first line currently waiting in the queue, use a Get Line (singular) message. When you get a line, it is removed from the queue. Five lines were queued with the test script above, and Get Line returns the first line and removes it from the queue:

feed << Get Line

"11"

Figure 14.3 Datafeed: 4 Lines Queued 

To empty all lines from the queue into a list, use Get Lines (plural). This returns the next four lines from the test script in list { } format.

myList = feed << GetLines;

{ "22", "33", "44", "55" }

Figure 14.4 Datafeed: 0 Lines Queued 

To stop and later restart the processing of queued lines, either click the Stop and Restart buttons in the Datafeed window, or send the equivalent messages:

feed << Stop;

feed << Restart;

To close the data feed and its window:

feed << Close;

To disconnect from the live data source:

feed << Disconnect

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