Polling vs. Events - Revision history

Jdecoux: Redirected page to Using Events

2019-02-28T20:19:26Z

Redirected page to Using Events

← Older revision		Revision as of 20:19, 28 February 2019
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	~~[[Category:Programming]]~~		#REDIRECT [[Using Events]]

	The Phidget library operates asynchronously. The library fires attach and detach events when a device channel appears and disappears (plugged in or unplugged). After a user channel is attached, data and status updates from the device channel begin being delivered. These events cause the library to update properties of the user channel, and to call any event handlers set by the program.

	~~===Discussion===~~

	~~====Attach====~~
	Once a channel is opened, the Phidget library begins attempting to match the channel to a device channel. While the channel is not '''attached''', the {{Code\|Attached}} property of the channel will be '''false'''. As soon as the Phidget library matches the channel with a device channel the {{Code\|Attached}} property is updated to be '''true'''. A program could poll the {{Code\|Attached}} property to determine when the channel becomes '''attached''', or set an attach handler to be notified asynchronously.

	~~====Detach====~~
	While a channel is '''open''' and '''attached''', the device channel could disappear for some reason (could be unplugged). As a result, the Phidget library would '''detach''' the user channel, and update the {{Code\|Attached}} property to be '''false'''. A program could poll the {{Code\|Attached}} property to detect when the channel becomes '''dettached''', or set a detach handler to be notified asynchronously.

	~~====Data====~~
	While a channel is '''open''' and '''attached''', data and status updates received by the Phidget library are delivered to the channel. A program could poll data and status properties (for example, {{Code\|Acceleration}}, {{Code\|Temperature}}, and {{Code\|State}}), or set handlers that would be called when the data or state changes.

	~~===Recommendations===~~
	Generally, setting event handlers is preferred and the most flexible. The code required to poll for attach and detach states, and to detect data and state changes can be complicated and error prone. Event handlers are simple and work in a very predicable manor.

	A simple program that only wants to read a small amount of data from a device could simply open a channel and wait for attachment, read a data or state property, and then close the channel. In that case, the code to implement and set event handlers might not be worth the effort. Error checking however becomes very important, as accessing some channel properties while the channel is '''detached''' will result in an error or exception that could kill the program. There will always be a race between checking the {{Code\|Attached}} property and accessing a data or state property.

	All but the most simple program should set handlers that will be called when a channel attaches or detaches, and when the channel receives data or changes state. The channel will only receive change events while '''attached''', and no external calls are required to access the changed data, which eliminates a chance for error. In addition, change triggers and data interval settings will not get reset accidentally if they are always set from within an attach handler, eliminating another common mistake.

	~~When in doubt, use event handlers.~~

	~~=== Further Reading ===~~

	~~[[Phidget Programming Basics]] - Here you can find the basic concepts to help you get started with making your own programs that use Phidgets.~~

	~~[[Data Interval/Change Trigger]] - Learn about these two properties that control how much data comes in from your sensors.~~

	[[Using ~~Multiple Phidgets]] - It can be difficult to figure out how to use more than one Phidget in your program. This page will guide you through the steps.~~

	~~[[Logging, Exceptions, and Errors]] - Learn about all the tools you can use to debug your program.~~

	~~[[Phidget Network Server]] - Phidgets can be controlled and communicated with over your network- either wirelessly or over ethernet.~~

	~~[[Best Phidgets Practices~~]] ~~- Good programming habits that will save you from common problems when writing code for your Phidgets.~~

Chad at 16:38, 12 July 2017

2017-07-12T16:38:11Z

Show changes

Mparadis at 19:11, 5 May 2017

2017-05-05T19:11:45Z

← Older revision		Revision as of 19:11, 5 May 2017
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			[[Category:Programming]]
	In your program, user and device actions can be handled by either:		In your program, user and device actions can be handled by either:
	*Letting the program tell you when they happen and then doing something ('''event driven''' code)		*Letting the program tell you when they happen and then doing something ('''event driven''' code)

Mparadis: /* Further Reading */

2017-05-03T19:48:42Z

Mparadis at 19:40, 28 April 2017

2017-04-28T19:40:30Z

← Older revision		Revision as of 19:40, 28 April 2017
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	[[Logging, Exceptions, and Errors]] - Learn about all the tools you can use to debug your program.		[[Logging, Exceptions, and Errors]] - Learn about all the tools you can use to debug your program.

	[[Phidget Network ~~Service~~]] - Phidgets can be controlled and communicated with over your network- either wirelessly or over ethernet.		[[Phidget Network Server]] - Phidgets can be controlled and communicated with over your network- either wirelessly or over ethernet.

Mparadis: /* Further Reading */

2017-01-13T19:13:48Z

Mparadis: /* Logic Code */

2017-01-13T17:23:08Z

Logic Code

← Older revision		Revision as of 17:23, 13 January 2017
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	<br>		<br>

	[[Image:logic.png"link=]]		[[Image:logic.png\|link=]]

	<br>		<br>

Mparadis: /* Logic Code */

2017-01-13T17:22:46Z

Logic Code

← Older revision		Revision as of 17:22, 13 January 2017
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	<br>		<br>

	[[Image:logic.png]]		[[Image:logic.png"link=]]

	<br>		<br>

Mparadis: /* Event Driven Code */

2017-01-13T17:22:34Z

Event Driven Code

← Older revision		Revision as of 17:22, 13 January 2017
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	For example, creating and opening a Phidget in an event-driven program may look like this:		For example, creating and opening a Phidget in an event-driven program may look like this:

	[[Image:event.png]]		[[Image:event.png\|link=]]

	As you can see, the attach handler and event handler are separate functions that trigger when the Phidget is plugged in or when some data is available, respectively. The rest of the program is logic-driven, and simply initializes the Phidget and GUI, sets up the event handlers, and then waits for events to fire and make updates to the GUI. This continues until the user decides to close the program.		As you can see, the attach handler and event handler are separate functions that trigger when the Phidget is plugged in or when some data is available, respectively. The rest of the program is logic-driven, and simply initializes the Phidget and GUI, sets up the event handlers, and then waits for events to fire and make updates to the GUI. This continues until the user decides to close the program.
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	Once you have written this code flow, the actual order of events that occur within the program look something like this:		Once you have written this code flow, the actual order of events that occur within the program look something like this:

	[[Image:Eventhandler.jpg\|160px]]		[[Image:Eventhandler.jpg\|link=\|160px]]

	Note that the device itself (not the program), initiates the function call by 'firing' the event. This allows you to update only when events fire, and ensures that you can capture and respond to all data events.		Note that the device itself (not the program), initiates the function call by 'firing' the event. This allows you to update only when events fire, and ensures that you can capture and respond to all data events.

Mparadis: /* Further Reading */

2017-01-13T17:20:56Z

← Older revision		Revision as of 19:48, 3 May 2017
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	[[Phidget Network Server]] - Phidgets can be controlled and communicated with over your network- either wirelessly or over ethernet.		[[Phidget Network Server]] - Phidgets can be controlled and communicated with over your network- either wirelessly or over ethernet.

			[[Best Phidgets Practices]] - Good programming habits that will save you from common problems when writing code for your Phidgets.

← Older revision		Revision as of 17:20, 13 January 2017
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	[[Phidget Programming Basics]] - Here you can find the basic concepts to help you get started with making your own programs that use Phidgets.		[[Phidget Programming Basics]] - Here you can find the basic concepts to help you get started with making your own programs that use Phidgets.

	[[Data ~~Rate~~ / Change Trigger]] - Learn about these two properties that control how much data comes in from your sensors.		[[Data Interval/Change Trigger]] - Learn about these two properties that control how much data comes in from your sensors.

	[[Using Multiple Phidgets]] - It can be difficult to figure out how to use more than one Phidget in your program. This page will guide you through the steps.		[[Using Multiple Phidgets]] - It can be difficult to figure out how to use more than one Phidget in your program. This page will guide you through the steps.

← Older revision		Revision as of 19:13, 13 January 2017
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	[[Logging, Exceptions, and Errors]] - Learn about all the tools you can use to debug your program.		[[Logging, Exceptions, and Errors]] - Learn about all the tools you can use to debug your program.

	[[~~Using Phidgets on the~~ Network]] - Phidgets can be controlled and communicated with over your network- either wirelessly or over ethernet.		[[Phidget Network Service]] - Phidgets can be controlled and communicated with over your network- either wirelessly or over ethernet.