Show units of effect parameter knobs

[daschuer]

Feature Description

This may be an addition to #9022
Currently only the value is shown. It would be very useful to see the unit is addition like
"-6 dB"
or
"0.2 s"

We already have an unused stub for this:

mixxx/src/effects/backends/effectmanifestparameter.h

Line 37 in b0f19c3

enum class UnitsHint : int {

But I think we should adopt the LV2 system:
http://lv2plug.in/ns/extensions/units#unit

[ronso0]

@daschuer Could you give some hints re "adopt the LV2 system"?

[daschuer]

When you follow the link you find a list of units including their rendering. We currently maintain a different incomplete list in UnitsHint enum.
My proposal is to replace our enum with the LV2 one.

[ronso0]

Yes, understood. My question was rather whether you propose to a) adopt the LV2 map (replacing existing Mixxx enum, change all built-in effects' manifests) or b) somehow map LV2 units to the extended Mixxx list.
The latter sounds complicated and means we'd have two lists / standards. So I'd go with a

Second question that arises: how to translate the enum to dispay strings?
I never did that, and it seems there are a few ways to accomplish this. However, for sake of consistency, I'd use a switch-case function like

mixxx/src/effects/backends/effectmanifestparameter.h

Lines 61 to 66 in f96d168

	static QString LinkTypeToString(LinkType type) {
	switch (type) {
	case LinkType::Linked:
	return QLatin1String("LINKED");
	case LinkType::LinkedLeft:
	return QLatin1String("LINKED_LEFT");

Edit LV2 units are all lowercase, enum style is PascalCase, soo:
break the style rule or have another map LV2 unit > UnitHint enum?

[ronso0]

Though, a QMap looks nicer, but requires a translator:

QMap<UnitsHint, QString> unitStringsMap {
    {UnitsHint::Unknown, ""},
    {UnitsHint::Beat, "Beat"},
    {UnitsHint::Beats, "Beats"},
    {UnitsHint::Bar, "Bar"},
    {UnitsHint::BPM, "BPM"},
    {UnitsHint::Cent, "Cent"},
    {UnitsHint::Hz, "Hz"}
};

const QString UnitsHintToString(const UnitsHint unitHint) {
    return unitStringsMap.find(unitHint);
}

[ronso0]

I'll do a quick test with:

• existing enum class UnitsHint : int extended by LV2 units
• QMap<QString, EffectManifestParameter::UnitsHint> lv2ToUnitsHintEnum
• QMap<EffectManifestParameter::UnitsHint, QString> unitsHintToDisplayString
• const QString UnitsHintToString(const UnitsHint unitHint) { return unitStringsMap.find(unitHint); }