Swift code showing how to set the opacity of a MGLSymbolStyleLayer based on a feature property

MGL_MATCH for icon opacity

private func applyOpacity(style: MGLStyle) {
//        NSExpression *iconOpacityStyle =
        //                [NSExpression expressionWithFormat:@"MGL_MATCH(%@, %@, %f, %f)", selectedAttributeName, selectedAttributeValue,
        //                                                   selectedIconOpacity, deselectedIconOpacity];

        // works for the Nav V5 style to fade away all the road shields that aren't for I-280
        if let symbolLayer = style.layer(withIdentifier: "road-number-shield-navigation") as? MGLSymbolStyleLayer {
            let selectedAttributeName = "ref"
            let selectedAttributeValue = "280"
            let selectedIconOpacity = NSExpression(forConstantValue: NSNumber(1.0))
            let deselectedIconOpacity = NSExpression(forConstantValue: NSNumber(0.1))
            let iconOpacityExpression = NSExpression(format: "MGL_MATCH(\(selectedAttributeName), %@, %@, %@)", selectedAttributeValue, selectedIconOpacity, deselectedIconOpacity)
            symbolLayer.iconOpacity = iconOpacityExpression
        }

        // my version for Snap with a placeholder layer identifier and a set value for the place_id field from the attributes they provided
        if let symbolLayer = style.layer(withIdentifier: "my-snap-poi-layer") as? MGLSymbolStyleLayer {
            let selectedAttributeName = "place_id"
            let selectedAttributeValue = "3fc061dc-9993-11e8-bcbe-fffaaefe62e0"
            let selectedIconOpacity = NSExpression(forConstantValue: NSNumber(1.0))
            let deselectedIconOpacity = NSExpression(forConstantValue: NSNumber(0.1))
            let iconOpacityExpression = NSExpression(format: "MGL_MATCH(\(selectedAttributeName), %@, %@, %@)", selectedAttributeValue, selectedIconOpacity, deselectedIconOpacity)
            symbolLayer.iconOpacity = iconOpacityExpression
        }
    }