Add the ability to show specific route leg with color overriding

[ShanMa1991]

Description

In 1.x , we're not dealing with the legIndex as in 0.4. This pr is to port the ability to handle specific legIndex for mapView route line.

The way we specify the leg index and change the route layer color should be like following:

navigationMapView.routeCasingColor = UIColor.red
navigationMapView.trafficUnknownColor = UIColor.blue
navigationMapView.show([route], legIndex: 1)

The color could be override. If we don't specify the leg Index, the whole route would show congestion color. But if we specify the leg index, only the specified legIndex would show the congestion color, while other legs would show the routeCasingColor.

• allow to show specific route leg
• allow to show current route leg during turn-by-turn navigation with overriding color
• update the changeLog

Implementation

When generating the congestion features for the whole route, the CurrentLegAttribute of the feature property would represent whether this leg is the specified leg. If the route leg is not specified, or it is the specified one, it would be true.

Then during the generation of the gradient stops for the route layer, if the CurrentLegAttribute is true, we have congestion color, if not, we're using the routeCasingColor.

Screenshots or Gifs

The following is how we show specific route leg with with

navigationMapView.routeCasingColor = UIColor.red
navigationMapView.trafficUnknownColor = UIColor.blue
navigationMapView.show([route], legIndex: 1)

The following is that during turn-by turn navigation, the current route leg would be specified with the color overriding. The default specified leg would be the current leg, which is the same as in the 0.4.

navigationViewController.navigationMapView.routeCasingColor = UIColor.red
navigationViewController.navigationMapView.trafficUnknownColor = UIColor.blue

[ShanMa1991]

Now the problem is that the layer and source could be added from the FeatureCollection with right color and opacity. But when the navigation start, the route lines become invisible. I'll work on this issue.

[ShanMa1991]

Fix the opacity expression, now it could allow the overriding route leg color. But it has a problem to display the following vanishing vision during navigation. It seems that there's something wrong with the gradient stops which cause the route line fading away at random place

Besides, to make sure both of the vanishing route line and route overriding work, the next step is to update the fractiontravled method. Because we only need to get the current leg display the vanishing feature, the fractiontravled should be calculated on each leg. The other leg's color should only depend on congestion level and opacity.

[ShanMa1991]

So now the problem is caused by several reasons:

1. The line gradient applied to each Feature, not the FeatureCollection. For example, if we have the following gradient stops, as below, and the line is formed with a FeatureCollection, with 2 Feature inside. It will display 2 seperate lines below:

                let gradientStops:[Double: UIColor] =  [
                    0: UIColor.blue,
                    0.5.nextDown: UIColor.blue,
                    0.5: UIColor.red,
                    1: UIColor.red
                ]

2. The func congestionFeatures(legIndex:, roadClassesWithOverriddenCongestionLevels: is creating multiple feature for even one-leg route. And it cause the congestion level and gradient not working as expected. For example, if we do the let routeFeatures = FeatureCollection(features: [Feature(route.shape!)]) and have a gradient at the median as following:

When we have one feature FeatureCollection, we could still see the color presented for the line gradient as below:

The next step is to:

1. fix the func congestionFeatures(legIndex:, roadClassesWithOverriddenCongestionLevels: to make sure the route line source of FeatureCollection is generated with one feature for each route leg.
2. The gradient stops for vanishing route line should only applies to the current route leg, and the fractiontraveled should also be based on the current route leg.

[ShanMa1991]

Due to the FeatureCollection generated whole route line layer couldn't support the line-gradient , as discussed in https://github.com/mapbox/mapbox-maps-ios-internal/issues/396 and https://github.com/mapbox/mapbox-maps-ios-internal/issues/396. the next step is to generate one Feature for each route leg, and a list of Feature would be used to represent the route line. It would allow the vanishing is happening on the current leg, and the overriding of route line style based on leg index.

[MaximAlien]

I've tested with NavigationMapView and with standalone NavigationViewController and it seems to work similarly to what was available in 0.40. It's great to see that fix turned out to be not really complex one.