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Guide to using GeoJsonDataSource (#2977)
Co-authored-by: Bart Louwers <[email protected]>
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| # Using a GeoJSON Source | ||
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| This guide will teach you how to use [`GeoJsonSource`](https://maplibre.org/maplibre-native/android/api/-map-libre%20-native%20-android/org.maplibre.android.style.sources/-geo-json-source/index.html) by deep diving into [GeoJSON](https://geojson.org/) file format. | ||
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| ## Goals | ||
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| After finishing this documentation you should be able to: | ||
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| 1. Understand how `Style`, `Layer`, and `Source` interact with each other. | ||
| 2. Explore building blocks of GeoJSON data. | ||
| 3. Use GeoJSON files in constructing `GeoJsonSource`s. | ||
| 4. Update data at runtime. | ||
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| ## 1. Styles, Layers, and Data source | ||
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| - A style defines the visual representation of the map such as colors and appearance. | ||
| - Layers control how data should be presented to the user. | ||
| - Data sources hold actual data and provides layers with it. | ||
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| Styles consist of collections of layers and a data source. Layers reference data sources. Hence, they require a unique source ID when you construct them. | ||
| It would be meaningless if we don't have any data to show, so we need know how to supply data through a data source. | ||
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| Firstly, we need to understand how to store data and pass it into a data source; therefore, we will discuss GeoJSON in the next session. | ||
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| ## 2. GeoJSON | ||
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| [GeoJSON](https://geojson.org/) is a JSON file for encoding various geographical data structures. | ||
| It defines several JSON objects to represent geospatial information. Typicalle the`.geojson` extension is used for GeoJSON files. | ||
| We define the most fundamental objects: | ||
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| - `Geometry` refers to a single geometric shape that contains one or more coordinates. These shapes are visual objects displayed on a map. A geometry can be one of the following six types: | ||
| - Point | ||
| - MultiPoint | ||
| - LineString | ||
| - MultilineString | ||
| - Polygon | ||
| - MultiPolygon | ||
| - `Feautue` is a compound object that combines a single geometry with user-defined attributes, such as name, color. | ||
| - `FeatureCollection` is set of features stored in an array. It is a root object that introduces all other features. | ||
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| A typical GeoJSON structure might look like: | ||
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| ```json | ||
| { | ||
| "type": "Feature", | ||
| "geometry": { | ||
| "type": "Point", | ||
| "coordinates": [125.6, 10.1] | ||
| }, | ||
| "properties": { | ||
| "name": "Dinagat Islands" | ||
| } | ||
| } | ||
| ``` | ||
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| So far we learned describing geospatial data in GeoJSON files. We will start applying this knowledge into our map applications. | ||
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| ## 3. GeoJsonSource | ||
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| As we discussed before, map requires some sort data to be rendered. We use different sources such as Vector, Raster and GeoJSON. | ||
| We will focus exclusively on `GeoJsonSource` and will not address other sources. | ||
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| `GeoJsonSource` is a type of source that has a unique `String` ID and GeoJSON data. | ||
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| There are several ways to construct a `GeoJsonSource`: | ||
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| - Locally stored files such as assets and raw folders | ||
| - Remote services | ||
| - Raw string parsed into FeatureCollections objects | ||
| - Geometry, Feature, and FeatureCollection objects that map to GeoJSON Base builders | ||
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| A sample `GeoJsonSource`: | ||
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| ```kotlin | ||
| --8<-- "MapLibreAndroidTestApp/src/main/java/org/maplibre/android/testapp/activity/style/CollectionUpdateOnStyleChange.kt:setupLayer" | ||
| ``` | ||
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| Note that you can not simply show data on a map. Layers must reference them. Therefore, you create a layer that gives visual appearance to it. | ||
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| ### Creating GeoJSON sources | ||
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| There are various ways you can create a `GeoJSONSource`. Some of the options are shown below. | ||
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| ```kotlin title="Loading from local files with assets folder file" | ||
| --8<-- "MapLibreAndroidTestApp/src/main/java/org/maplibre/android/testapp/activity/style/NoStyleActivity.kt:setup" | ||
| ``` | ||
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| ```kotlin title="Loading with raw folder file" | ||
| --8<-- "MapLibreAndroidTestApp/src/main/java/org/maplibre/android/testapp/activity/style/RuntimeStyleActivity.kt:source" | ||
| ``` | ||
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| ```kotlin title="Parsing inline JSON" | ||
| --8<-- "MapLibreAndroidTestApp/src/main/java/org/maplibre/android/testapp/utils/ResourceUtils.kt:readRawResource" | ||
| ``` | ||
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| ```kotlin title="Loading from remote services" | ||
| --8<-- "MapLibreAndroidTestApp/src/main/java/org/maplibre/android/testapp/activity/style/HeatmapLayerActivity.kt:createEarthquakeSource" | ||
| ``` | ||
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| ```kotlin | ||
| --8<-- "MapLibreAndroidTestApp/src/main/java/org/maplibre/android/testapp/activity/style/HeatmapLayerActivity.kt:constants" | ||
| ``` | ||
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| ```kotlin title="Parsing string with the fromJson method of FeatureCollection" | ||
| --8<-- "MapLibreAndroidTestApp/src/main/java/org/maplibre/android/testapp/activity/turf/MapSnapshotterWithinExpression.kt:fromJson" | ||
| ``` | ||
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| ```kotlin title="Creating Geometry, Feature, and FeatureCollections from scratch" | ||
| --8<-- "MapLibreAndroidTestApp/src/main/java/org/maplibre/android/testapp/activity/feature/QuerySourceFeaturesActivity.kt:JsonObject" | ||
| ``` | ||
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| Note that the GeoJSON objects we discussed earlier have classes defined in the MapLibre SDK. | ||
| Therefore, we can either map JSON objects to regular Java/Kotlin objects or build them directly. | ||
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| ## 4. Updating data at runtime | ||
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| The key feature of `GeoJsonSource`s is that once we add one, we can set another set of data. | ||
| We achieve this using `setGeoJson()` method. For instance, we create a source variable and check if we have not assigned it, then we create a new source object and add it to style; otherwise, we set a different data source: | ||
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| ```kotlin | ||
| --8<-- "MapLibreAndroidTestApp/src/main/java/org/maplibre/android/testapp/activity/style/ZoomFunctionSymbolLayerActivity.kt:createFeatureCollection" | ||
| ``` | ||
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| ```kotlin | ||
| --8<-- "MapLibreAndroidTestApp/src/main/java/org/maplibre/android/testapp/activity/style/ZoomFunctionSymbolLayerActivity.kt:updateSource" | ||
| ``` | ||
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| See [this guide](styling/animated-symbol-layer.md) for an advanced example that showcases random cars and a passenger on a map updating their positions with smooth animation. | ||
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| ## Summary | ||
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| GeoJsonSources have their pros and cons. They are most effective when you want to add additional data to your style or provide features like animating objects on your map. | ||
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| However, working with large datasets can be challenging if you need to manipulate and store data within the app; in such cases, it’s better to use a remote data source. |