Description
TL;DR
The current mechanism of implementing additional interfaces to add support for optional characteristics is non-conducive to supporting dynamic sets of optional characteristics.
Background and current workaround
In my fork of Openhab-homekit, I support low battery notification. However, whether or not an accessory actually has low-battery notification support is based on the items configuration. As a workaround, in order to avoid creating two classes for sensors that potentially have battery status reporting support, I create a faux-battery notification implementation that always returns batteryLow=false).
Limitations of providing substitute static-values
Such workarounds are not possible when adding a characteristic that fundamentally changes the presentation of an accessory in homekit. For example, with the thermostat, the addition of optional characteristics Characteristic.CoolingThresholdTemperature and Characteristic.HeatingThresholdTemperature causes the UI to show a temperature range, rather than a single temperature, when the AUTO TargetCoolingHeatingState is active.
Proposal and way forward
I propose that accessories classes only specify getters, setters, and subscribers for their base class.
Then, for optional characteristics, I propose that the accessory has an overridable getter which gets some optional characteristic, with the default value being none:
@Override
protected Optional<HeatingThresholdCharacteristicSupport> heatingThresholdTemperatureCharacteristic() {
heatingThresholdItem.map(item -> {
new HeatingThresholdCharacteristicSupport() {
@Override
CompletableFuture<Double> getHeatingThresholdTemperature() {
...
}
...
}
});
}
The HeatingThermostat trait can be modified such that the interface methods are lifted out in to HeatingThresholdCharacteristicSupport, retaining backwards compatibility (and potentially marking as deprecated for removal in next minor release of HAP-Java). After such a lift, the interface would be effectively empty:
package com.beowulfe.hap.accessories.thermostat;
import com.beowulfe.hap.HomekitCharacteristicChangeCallback;
import java.util.concurrent.CompletableFuture;
@Deprecated
public interface HeatingThermostat extends BasicThermostat, HeatingThresholdCharacteristicSupport {
}