creating a trail with paths

[zoff99]

i am trying to add a "tail" to some GPS movements on a map.
i am using path. but i cant seem to find out how to just add a new segment to the front and remove the last segment at the end.
so i got this silly hack now, always removing the full path and creating a new one.
this works but it flickers, and also is ugly.

any help on using the path API properly?

points is:

val points = mutableListOf<Pair<Double, Double>>()
a list of asNormalizedWebMercator gps coords of the last movement of the "person"

// HACK - HACK - HACK
// HACK - HACK - HACK
GlobalViewModels.osm.state.removeAllPaths()
GlobalViewModels.osm.state.addPath(id = "" + (path_global_hackish_id_cur + 1) + ":::trail:::" + fpubkey) {
    addPoints(points)
}
path_global_hackish_id_cur++
if (path_global_hackish_id_cur > 1000)
{
    path_global_hackish_id_cur = 1
}
// HACK - HACK - HACK
// HACK - HACK - HACK

[p-lr]

Instead of removing the path then adding it back, you can use updatePath, which is more efficient and is designed for this purpose. This api allows you to change the PathData.
For adding points to the tail, it's easy. Something like:

// making the base path
val pathBuilder = mapState.makePathDataBuilder()
pathBuild.addPoints(allPoints)
val pathData = pathBuider.build()

// adding the path
mapState.addPath("id", pathData)

// adding a points to the tail
pathBuilder.addPoints(tailPoints)
val newPathData = pathBuilder.build()

// updating the path (more efficient than removing and adding it)
mapState.updatePath("id", newPathData)

Now PathDataBuilder doesn't have a "addFirst" or equivalent api. But it doesn't matter. Your app already has all the points in memory. Creating a new PathDataBuilder and adding all points to it is cheap.

Also, if you ever need to display only a part of a path, the updatePath api allows you do that efficiently. Use the offset and count parameter. It only draws the path starting from offset, and count is the number of points drawn starting from that offset. FYI:

/**
 * Updates some properties of a previously added path (using [addPath]).
 *
 * @param id The unique identifier of the path
 * @param pathData The points of the path. The [PathDataBuilder] which was originally used to create
 * the path can be used to build new [PathData] instances with additional points.
 * @param width The width of the path, in [Dp]
 * @param color The color of the path
 * @param offset The number of points to skip from the beginning of the path
 * @param count The number of points to draw after [offset]
 * @param cap The cap style at the start and end of the path
 * @param simplify By default, the path is simplified depending on the scale to improve performance.
 * Higher values increase the simplification effect, while a value of 0f effectively disables path
 * simplification. Sensible values a typically in the range [0.5f..2f]. Default value is 1f.
 * @param zIndex A path with larger zIndex will be drawn on top of paths with smaller zIndex.
 * When paths have the same zIndex, the more recently added path is drawn on top of the others.
 * @param clickable Controls whether the path is clickable.
 * @param pattern The dash pattern. By default, no dash effect is applied.
 */
fun MapState.updatePath(
    id: String,
    pathData: PathData? = null,
    visible: Boolean? = null,
    width: Dp? = null,
    color: Color? = null,
    fillColor: Color? = null,
    offset: Int? = null,
    count: Int? = null,
    cap: Cap? = null,
    simplify: Float? = null,
    clickable: Boolean? = null,
    zIndex: Float? = null,
    pattern: List<PatternItem>? = null
)