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

Table 14.1 Datafeed Messages
Message	Syntax	Explanation
Open Datafeed Datafeed	feed = Open Datafeed( commands )	Creates a data feed object. Any of the following can be used as commands inside Open Datafeed or sent as messages to an existing data feed object. Datafeed is a synonym.
Set Script	feed << Set Script( script )	Assigns the script that is run each time a line of data is received.
Get Line	feed << Get Line	Returns and removes one line from the data feed queue.
Get Lines	feed << Get Lines	Returns as a list and removes all lines from the data feed queue.
Queue Line	feed << Queue Line( string )	Sends one line to the end of the data feed queue.
Stop	feed << Stop	Stops processing queued lines.
Restart	feed << Restart	Restarts processing queued lines.
Close	feed << Close	Closes the data feed object and its window.
Connect	feed << Connect( Port( "com1:" \| "lpt1:" \| ... ), Baud( 9600 \| 4800 \| ... ), Data Bits( 8 \| 7 ), Parity( None \| Odd \| Even ), Stop Bits( 1 \| 0 \| 2 ), DTR_DSR( 0 \| 1 ), RTS_CTS( 0 \| 1 ), XON_XOFF( 1 \| 0 ) )	(Windows only) Sets up port settings for the connection to the device. The symbol \| between arguments represents “or”; choose one argument for each setting.
Disconnect	feed << Disconnect	(Windows only) Disconnects the device from the data feed queue but leaves the data feed object active.