General Phidget Programming

This page presents the general concepts needed to write code for a Phidget.

By this point, you should have installed the drivers for your operating system and the libraries for your specific programming language.

The Basic Functions

To use your Phidget within code, you'll want to:

1. Create a Phidget software object, which gives you access to the functions specific to that device
2. Open the Phidget using the object
3. Detect when a Phidget is attached (plugged in) by using the object
4. Use functions that the object provides, like turning on LEDs, reading sensors, triggering events on data change, etc
5. Close the object, when you are done

Small code snippets are provided for each step below. C++ and Java were selected because Java is a relatively high-level language and C++ is a relatively low level language, thereby showing how specific each language API really is. So, the most useful resource for the actual functions would be the API for your specific language. This page is a high-level introduction, by design.

Creating a Software Object

Phidget devices are controlled using software objects. All software device objects have a common API and set of functions that allow you to open it, close it, and set a few listeners to general events such as attach (plug in), detach (unplug), and errors.

But when you create an actual software object, it is a software object specific to your device.

For example, in Java:

   // Create a new Accelerometer object
   AccelerometerPhidget device = new AccelerometerPhidget();

   // Create a new RFID device object
   RFIDPhidget device = new RFIDPhidget();

Or in C:

    // Create a new Accelerometer object
    CPhidgetAccelerometerHandle device = 0;
    CPhidgetAccelerometer_create(&device);

    // Create a new RFID device object
    CPhidgetRFIDHandle device = 0;
    CPhidgetRFID_create(&device);

Each software object has an API and available functions which are specific to that device. For example, the RFID device API includes a function to turn on the RFID antenna. The accelerometer device API includes a function to set the sensitivity on each axis.

Opening the Phidget

Phidgets can either be opened when attached directly to a computer, or they can be opened remotely using the Phidget Webservice. This section deals primarily with opening Phidgets directly.

Once you have created your software object for your specific type of device, you can call the open() function in your language on that object. For example, in Java:

    device.open();

Or in C:

    CPhidget_open((CPhidgetHandle) device, -1);

All specific language calls can be found in the API documentation located on each individual language page.

The open() function in any language opens the software object for use, not the hardware itself. Having the software "open" before the hardware means that the software can capture all events, including multiple attach (plug in) and detach (unplug) events for one open() call.

Details for Open()

Open will return immediately once called, because it can be called even if the Phidget to be used is not attached to the system. This is known as an asynchronous call. It’s important to understand that most calls on a Phidget will fail if they are calls when the Phidget is not attached - in fact the only calls that are allowed on a detached Phidget are close(), waitForAttachment() and getAttached().

Open is also pervasive. This means that once open has been called, it will constantly try to stay attached to a Phidget. Even if the Phidget is unplugged from the computer and then plugged back in, you will simply get a Detach event, and then an Attach event. It’s a good idea to handle the Detach event in order to avoid calling the Phidget after it has detached.

The different types of open(), such as openAny(), openRemote(), etc. can be used with parameters to try and get the first device it can find, open based on its serial number, or even open across the network. The API overview lists all of the available modes that open provides, and you can find even more detail in the API for your specific language. If there are more than one of the same type of Phidget attached to a computer, and you use open() with no serial number, there is no way of knowing which Phidget will be opened first.

Once a Phidget is opened by an application, it cannot be opened again in another application until closed by the first. When open and attached in software, no other programs or instances can read data from or change the Phidget. The one exception is if the Phidget is controlled only over the network with the Phidget Webservice, and not directly. Then, you can use multiple remote control programs.

Attaching the Phidget

Physically, attaching a Phidget means plugging it in. In your code, you can detect an attachment either with an event in event-driven programming, or waiting for it, in logic programming.

Event Attachment

For example, to use an event to detect attachment in Java:

  // After creating a Phidget object called "device":
  device.addAttachListener(new AttachListener() {
      public void attached(AttachEvent ae) {
          System.out.println("A new device has been plugged in!");
      }
  });

Or to use an event to detect attachment in C:

  int AttachHandler (CPhidgetHandle device, void *userData) {
      printf("A new device has been plugged in!");
      return 0;
  }

  // .....Then, in the main code after creating a Phidget object "device":
  CPhidget_set_OnAttach_Handler((CPhidgetHandle) device, AttachHandler, NULL);

Both of the code snippets above do the same thing. The function AttachHandler(...) is called automatically when a device is plugged in.

You will want to attach events (via addAttachListener() above, for example) before you open the Phidget object. Otherwise, triggered events may be lost.

This method for using events to detect attachment can be expanded to other events and more complex control flow. Where possible, all example code downloadable from the specific language pages shows event-driven programming.

Wait for Attachment

Waiting for attachment is a straightforward process. Your code does not handle events, it simply waits for a device to be plugged in before moving on and doing something else.

For example, in Java you wait for attachment on a created and open software object (called device) like this

  // Wait until a device is plugged in
  device.waitForAttachment();

Or in C (again, device has been created and opened) :

  int result;
  // Wait up to 10000 ms for a device to be plugged in
  if((result = CPhidget_waitForAttachment((CPhidgetHandle) device, 10000))) {
      // No attachment, error
  }
  // Successful attachment

So, unlike the event model above, a Phidget software object should be open before waiting for a device to be plugged in.

Do Things with the Phidget

After you have a properly created Phidget software object, you can actually call function to turn LEDs on, change output state, read data from sensors, etc.

The thing you probably want to do with your Phidget is read data from its sensors or inputs. This might be, say, a sensor plugged in to a 1018 - Interface Kit 8/8/8 used in the code snippets below. You can do this either by detecting changes via event driven code, or polling for new values via logic code.

Details about data handling:

• When a Phidget is opened, its initial state will be read before it is marked as attached. This allows polling of many properties -- including some data -- even during the Attach event, and anytime afterwards.

• If you are handling data using events as described below, the data event functions will fire when the device is plugged in and its initial state is read.

• Some properties have default values, but these should not be trusted. Remember: always set, don’t rely on defaults. Trying to read an uninitialized value with no default will result in an Exception.

• Often Phidgets will retain their last state unless power is lost. This can give surprising results as the previous state may not always be what you expect. For example, if you open an InterfaceKit and set an output, this output may stay set even after the Phidget is closed.

Capture Data Change with Events

To capture data changes in sensors or inputs as they happen, you need to use event driven code.

Like defining an event function that fires when the Phidget is plugged in, you can create functions that automatically run when, for example, a sensor value or input value changes.

For example, for an Interface Kit, you can create a function that gets called when a sensor changes. You would do this before the Phidget software object has been opened.

In Java, this would look like:

  // After creating a Phidget object called "device":
  device.addSensorChangeListener(new SensorChangeListener() {
      public void sensorChanged(SensorChangeEvent sensorEvent) {
          System.out.println("New Value: " + sensorEvent.getValue());
      }
  });

Or to use an event to detect attachment in C:

  int SensorChangeHandler (CPhidgetHandle device, void *userData, int boardIndex, int newValue) {
      printf("New Value %d from sensor at location %d\n", newValue, boardIndex);
      return 0;
  }

  // .....Then, in the main code after creating a Phidget object "device":
  CPhidget_set_OnSensorChange_Handler((CPhidgetHandle) device, SensorChangeHandler, NULL);

Poll for Data Change

To poll for sensor data, or output state, you usually want to look for a get...Value or get...State function available in the API for your device. Then, you simply set up a loop that get the value of a sensor continuously.

To poll your software object, the object must be open. This is in contrast to the event-driven method above, where all event functions are declared and attached before opening the object.

These code snippets assume device is an Interface Kit 8/8/8. For example, in Java, for the Sensor at location 5 on the board:

  int val;
  for (int i = 0; i < 10; i++) {
    val = device.getSensorValue(5);
    System.out.println("Value: " + val);
  }

Or, in C, for the Sensor at location 5 on the Interface Kit board:

  int val;
  for (int i = 0; i < 10; i++) {
    CPhidgetInterfaceKit_getSensorValue(phid, 5, &val);
    printf("Value: %d\n", val);
  }

Learning Everything You Can Do

The things you can do with your particular Phidget are many and varied, so we only include general concepts on this page.

You can go one of two places for more information on what functions are available for your specific device. We provide both documentation on the raw API for each programming language as well as a language-independent description of the calls for each device.

• Read the raw API list on your your specific programming language page, in the Getting Started section.
• Read the high-level API description for your hardware on your specific Phidget device product page.

Close the Phidget

When you are finished with the Phidget software object at the end of your program, you should close and (in some languages) delete it.

For example, in Java:

  device.close();
  device = null;

Or, in C:

    CPhidget_close((CPhidgetHandle) device);
    CPhidget_delete((CPhidgetHandle) device);

The close() call removes the lock that open put on the Phidget. Make sure to close your object, so other software can use the Phidget!

The close() function also makes sure the thread associated with the Phidget close properly. Any outstanding writes will block close() until they complete, because writes are guaranteed to complete (unless a device is detached).

Also note that a device should be put into a known state before calling close. For example, if a motor controller is driving a motor and close is called, it will continue to drive the motor even though the application has exited. This may or may not be what you want.

Using Multiple Phidgets

Each Phidget has a unique serial number. Using this serial number, you can use a specific open call to open by serial number.

For example, in Java, this would be:

    device.open(SerialNumber);

Or in C:

    CPhidget_open((CPhidgetHandle) device, serialNumber);

For each event, you can then check the serial number of the device handed through the event, and act accordingly.

For example, in Java, your attach event handler might look like this:

    detachHandler = new DetachListener() { 
        public void detached(DetachEvent event) {
            int serialNumber = ((Phidget)event.getSource()).getSerialNumber();
            // Do something according to serialNumber
    }    }

Or in C:

    int AttachHandler(CPhidgetHandle device, void *userptr) {
	int serialNo;
	CPhidget_getSerialNumber(device, &serialNo);
         // Do something according to serialNumber
    }

Using Phidgets over a Network

Control of a Phidget over a network uses the Phidget Webservice.

Putting It Together

User and device actions can be handled by either:

• Letting the program tell you when they happen and then doing something (event driven code)
• Polling for things to happen then doing something (logic code)

The style of programming you choose (and hence the language you might prefer) would depend on what you want to do with the Phidget. The two sections, Event Driven Code and Logic Code below give benefits, drawbacks, and general examples of each style.

The styles can also mix. For example, you can take a defined set of steps at first such as turning on an LED or antenna (logic code) and then doing nothing until an output change event is fired (event code).

With languages that support both styles, you can mix and match. For languages that support only logic code (see the Language Support Categories above) you can only use the logic paradigm style.

Examples in pseudo-code are given below for each style type so you can see how your language choice can affect your code design.

Logic Code

Logic code has use for:

• Simple, single-device applications
• Non-GUI applications (GUIs usually are event driven)
• The user driving the device rather than listening to it

Logic code is relatively easy to design well. For example, using the create, open, attach, do stuff, and close concepts introduced above, logic code to handle a Phidget might be written like this:

Although this design does not explicitly capture every event that fires when data or input changes, by polling the device often enough no data will be lost.

However, logic code cannot handle constant, asynchronous events as cleanly as event driven code can.

These designs can be mixed. So, if you find that in logic code you have a highly complex if loop driving your program, you should consider changing some of it to event driven code. This type of awkward if-loop might look like this:

  Create Device Software Object
  Open Device

  Loop Until Exit Requested {
     if No Device Attached {
         Wait For Attachment until Timeout
         if Wait Timeout Reached {
             break
         } else { 
             Initialize Device
         }
     } else {  // Device Is Attached
         if Device Data Type 1 Changed {
             Do Something
         }
         if Device Data Type 2 Changed {
             Do Something Else
         }
         // ... More data change functions here
     }
     Collect User Input
  }

  Close Device
  Delete Device

On the other hand, you can probably see that if your language does not give the option for events, you can use this structure to mimic what events would enable you to do.

Event Driven Code

Event driven code allows for clean handling of complex, asynchronous programs:

• Handling multiple Phidgets
• Handling active plugging or unplugging of the Phidget (multiple attach and detach events)
• Working behind a GUI, as many GUIs are already event driven
• Capturing all sensor data - or input and output - without constantly polling

Without event driven code, you will need to constantly poll the device to see if any state has changed. If you poll at a slower rate than your input or output changes, you will not capture all data.

However, event driven code is usually not as useful or efficient for:

• Only one open and close event
• Using only one device
• Having the user (or program) put changes onto the device (in contrast to reading data from the device)

Event driven code is relatively hard to design well. It may help to draw out a flowchart, state machine, or at least a pseudo-code outline of your system design and all events you wish to handle before writing code.

The code examples given for each specific language use events if they are supported by the language.

Using the create, open, attach, do stuff, and close concepts introduced above, event code to handle a Phidget might be written like this:

Once you have written this code flow, the actual order of events that occur within the program look something like this:

Note that the device itself initiates the function call, by 'firing' the event. This allows you to update only when events fire, and capture all changes, because the low-level interface is telling you when a change occurs.

Advanced Concepts

Now that you have the basic create, open, attach, do stuff, and close concepts introduced above, there are other useful concepts which will help you design your code to be persistent and stable.

Threading

Due to the use of events, the Phidget library uses threading extensively.

For example:

• Calling open() starts a central thread.
• Closing everything will shut that central thread down (before the final close returns).
• Each device, once attached, starts its own read and write threads.

These threads provide the support to perform your typical Basic Functions:

• Triggering of data events come from the context of a device read thread.
• Attach and detach events come from the context of the central thread.
• The central thread looks for device attaches and detached, keeping track of which devices are attached internally, and sending out attach and detach events to Phidgets and Managers.
• Writes are performed asynchronously by the write thread. The write queue is only 1 deep so calling a write function while there is a write pending will block.

All Phidget libraries are thread safe, so you don’t need to do any locking on the Phidget objects.

Exceptions

Every function can have error. In C/C++ these errors are returned as a non-zero integer return code. These exceptions must be explicitly caught, otherwise the program will simply continue on in an incorrect state.

In languages that support exceptions (e.g., Java, .NET, Python, AS3), errors are returned using exceptions which will leave you no choice but to catch them, or have your program terminate without warning.

These exceptions are the result of fairly common situations. For example, anytime a function is called on a Phidget object where the Phidget is not attached, an exception will be thrown. Even if you make sure the Phidget is attached before beginning a write, unplugging the Phidget in the middle of a write will either (a) throw an exception silently, such as in C, and you will not know the write failed, or (b) throw an exception explicitly, such as in Java, .NET, Python, AS3 and your program will abruptly terminate.

See the General API or the API for your specific programming language for error code documentation.