Device Factories & Providers

Device Factories

Iternally, all devices are provisioned via a Device Factory - all of the classes in com.diozero.api and com.diozero.devices include a constructor parameter that allows a device factory to be specified. If a device factory isn’t specified it will be auto-detected using DeviceFactoryHelper.getNativeDeviceFactory().

The device factory concept is particularly useful for GPIO expansion boards and Analog-to-Digital converters - the expansion board’s device factory instance can be used to provision the device instead of the host board. For example, some boards like the Raspberry Pi provide no analog input pins; attempting to create an AnalogInputDevice such as an LDR using the Raspberry Pi default native device factory would result in a runtime error (UnsupportedOperationException). The diozero implementation of Analog to Digital Converter expansion devices such as the MCP3008 have been implemented as analog input device factories hence can be used in the constructor of analog input devices like LDRs:

try (AnalogInputDeviceFactoryInterface ain_df = new McpAdc(McpAdc.Type.MCP3008, chipSelect);
		LDR ldr = new LDR(ain_df, pin, r1)) {
	System.out.println(ldr.getUnscaledValue());
}

In this example, the ain_df McpAdc object handles all of the logic for interfacing with the MCP3008 ADC chip as well as implementing AnalogInputDeviceFactoryInterface to enable it to provision AnalogInputDevice instances.

Repeating the previous example of controlling an LED when you press a button but with all devices connected via an MCP23017 GPIO expansion board:

try (MCP23017 mcp23017 = new MCP23017(12);
		Button button = new Button(mcp23017, 1, GpioPullUpDown.PULL_UP);
		LED led = new LED(mcp23017, 2)) {
	button.whenPressed(nanoTime -> led.on());
	button.whenReleased(nanoTime -> led.off());
	SleepUtil.sleepSeconds(10);
}

Providers

The Device Factory section introd To maximise the portability across the broad spectrum of boards and devices, diozero has the concept of providers that are base native device factory

The built-in provider is designed to be portable across different boards.

Providers

Unless you are implementing a new provider or device factory you shouldn’t need to use any of the interfaces or helper classes (within the com.diozero.internal package).

pigpio

Uses the excellent pigpio C library to provide fully optimised GPIO / SPI and I²C support for all Raspberry Pi models. Makes use of the pigpioj library to access pigpio.

Make sure that pigpio is installed:

sudo apt update && sudo apt -y install pigpio

The pigpioj library has two mechanisms for interfacing with pigpio:

1. JNI - default mode of operation; requires root access; the pigpiod daemon process must be stopped
2. Sockets - activate when the environment variable PIGPIOD_HOST is set; does not require root access

The pigpioj project provides additional details.

Provider Lookup

The class DeviceFactoryHelper encapsulates the logic for accessing the configured service provider. Interfaces for implementing a new service provider are in the com.diozero.internal.provider package. Developing a new service provider is relatively straightforward given the provided APIs and base classes.

The device native service provider library is resolved in the following order:

1. System property diozero.devicefactory, e.g. -Ddiozero.devicefactory=com.diozero.internal.provider.pigpio.PigpioJDeviceFactory
2. Service definition file on the classpath, file: /META-INF/services/com.diozero.internal.spi.NativeDeviceFactoryInterface. For example the one for Firmata provider
3. The built-in default device factory