製作 choropleth 地圖

「choropleth 地圖」是一種主題地圖，其中的行政區會根據資料值加上顏色或陰影。您可以使用樣式工廠函式，根據資料設定地圖樣式，其中每個行政區都與一個範圍內的數值相關聯。以下地圖範例為美國各州的分級著色圖。

在本例中，資料是由州/省的地點 ID 組成。樣式工廠函式會根據狀態的地點 ID 雜湊值，為每個狀態加上條件式上色。

1. 如果尚未建立新的地圖 ID 和地圖樣式，請按照「開始使用」一文中的步驟進行。請務必啟用 行政區層級 1 地圖項目圖層。

2. 在地圖初始化時，取得行政區第一級地圖項目圖層的參照。在美國境內，這些行政層級對應到各州。

   Java

   private FeatureLayer areaLevel1Layer;
   
   @Override
   public void onMapReady(GoogleMap map) {
     areaLevel1Layer = map.getFeatureLayer(new FeatureLayerOptions.Builder()
       .featureType(FeatureType.ADMINISTRATIVE_AREA_LEVEL_1)
       .build());
   
     // Apply style factory function to ADMINISTRATIVE_AREA_LEVEL_1 layer.
     styleAreaLevel1Layer();
   }

   Kotlin

   private var areaLevel1Layer: FeatureLayer? = null
   
   override fun onMapReady(googleMap: GoogleMap) {
     // Get the ADMINISTRATIVE_AREA_LEVEL_1 feature layer.
     areaLevel1Layer = googleMap.getFeatureLayer(FeatureLayerOptions.Builder()
       .featureType(FeatureType.ADMINISTRATIVE_AREA_LEVEL_1)
       .build())
   
     // Apply style factory function to ADMINISTRATIVE_AREA_LEVEL_1 layer.
     styleAreaLevel1Layer()
   }

3. 建立樣式工廠函式，並套用至 行政區層級 1 地圖項目圖層。以下範例將函式套用至代表美國各州的多邊形。

   Java

   private void styleAreaLevel1Layer() {
     FeatureLayer.StyleFactory styleFactory = (Feature feature) -> {
       if (feature instanceof PlaceFeature) {
         PlaceFeature placeFeature = (PlaceFeature) feature;
   
         // Return a hueColor in the range [-299,299]. If the value is
         // negative, add 300 to make the value positive.
         int hueColor = placeFeature.getPlaceId().hashCode() % 300;
         if (hueColor < 0) {
           hueColor += 300;
         }
   
         return new FeatureStyle.Builder()
           // Set the fill color for the state based on the hashed hue color.
           .fillColor(Color.HSVToColor(150, new float[] {hueColor, 1, 1}))
           .build();
       }
       return null;
     };
   
     // Apply the style factory function to the feature layer.
     areaLevel1Layer.setFeatureStyle(styleFactory);
   }

   Kotlin

   private fun styleAreaLevel1Layer() {
     val styleFactory = FeatureLayer.StyleFactory { feature: Feature ->
         if (feature is PlaceFeature) {
             val placeFeature: PlaceFeature = feature as PlaceFeature
   
             // Return a hueColor in the range [-299,299]. If the value is
             // negative, add 300 to make the value positive.
             var hueColor: Int = placeFeature.getPlaceId().hashCode() % 300
             if (hueColor < 0) {
                 hueColor += 300
             }
             return@StyleFactory FeatureStyle.Builder()
             // Set the fill color for the state based on the hashed hue color.
               .fillColor(Color.HSVToColor(150, floatArrayOf(hueColor.toFloat(), 1f, 1f)))
               .build()
         }
         return@StyleFactory null
     }
   
     // Apply the style factory function to the feature layer.
     areaLevel1Layer?.setFeatureStyle(styleFactory)
   }