Add runtime API for setting tile prefetch delta for Source class (#184)

* Add runtime API for setting tile prefetch delta for Source class

* Use Integer as param.

* Update gl native submodule.

MapboxGLAndroidSDK/src/main/java/com/mapbox/mapboxsdk/style/sources/Source.java

@@ -2,6 +2,8 @@ package com.mapbox.mapboxsdk.style.sources;
 
 import androidx.annotation.Keep;
 import androidx.annotation.NonNull;
+import androidx.annotation.Nullable;
+
 import com.mapbox.mapboxsdk.LibraryLoader;
 import com.mapbox.mapboxsdk.utils.ThreadUtils;
 
@@ -68,6 +70,28 @@ public abstract class Source {
     return nativeGetAttribution();
   }
 
+  /**
+   * Retrieve current pre-fetching zoom delta.
+   *
+   * @return current zoom delta or null if not set.
+   */
+  @Nullable
+  public Integer getPrefetchZoomDelta() {
+    return nativeGetPrefetchZoomDelta();
+  }
+
+  /**
+   * Set the tile pre-fetching zoom delta for current source. Pre-fetching makes sure that a low-resolution
+   * tile at the (current_zoom_level - delta) is rendered as soon as possible at the
+   * expense of a little bandwidth.
+   * If delta has not been set or set to null, it will use the value in MapboxMap instance.
+   *
+   * @param delta zoom delta
+   */
+  public void setPrefetchZoomDelta(@Nullable Integer delta) {
+    nativeSetPrefetchZoomDelta(delta);
+  }
+
   /**
    * Internal use
    *
@@ -85,6 +109,14 @@ public abstract class Source {
   @Keep
   protected native String nativeGetAttribution();
 
+  @NonNull
+  @Keep
+  protected native Integer nativeGetPrefetchZoomDelta();
+
+  @NonNull
+  @Keep
+  protected native void nativeSetPrefetchZoomDelta(Integer delta);
+
   public void setDetached() {
     detached = true;
   }

MapboxGLAndroidSDKTestApp/src/androidTest/java/com/mapbox/mapboxsdk/testapp/style/CustomGeometrySourceTest.kt

@@ -14,6 +14,7 @@ import com.mapbox.mapboxsdk.testapp.activity.style.GridSourceActivity.ID_GRID_LA
 import com.mapbox.mapboxsdk.testapp.activity.style.GridSourceActivity.ID_GRID_SOURCE
 import com.mapbox.mapboxsdk.testapp.utils.TestingAsyncUtils
 import org.junit.Assert
+import org.junit.Assert.*
 import org.junit.Test
 
 class CustomGeometrySourceTest : BaseTest() {
@@ -66,4 +67,20 @@ class CustomGeometrySourceTest : BaseTest() {
         }.count() == THREAD_POOL_LIMIT)
     }
   }
+
+  @Test
+  fun sourceZoomDeltaTest() {
+    validateTestSetup()
+    invoke(mapboxMap) { uiController, mapboxMap ->
+      mapboxMap.prefetchZoomDelta = 3
+      mapboxMap.style!!.getSource(ID_GRID_SOURCE)!!.let {
+        assertNull(it.prefetchZoomDelta)
+        it.prefetchZoomDelta = 5
+        assertNotNull(it.prefetchZoomDelta)
+        assertEquals(5, it.prefetchZoomDelta!!)
+        it.prefetchZoomDelta = null
+        assertNull(it.prefetchZoomDelta)
+      }
+    }
+  }
 }