ImmersiveMode.java

// previous imports
  . . . 

+ import android.os.Handler;

public class ImmersiveMode extends ReactContextBaseJavaModule {
// let's declare a handler for our UI thread;
+  Handler uiHandler;
  
  . . . 
  
  
  // our constructor
  public class ImmersiveMode extends ReactContextBaseJavaModule {
    super(reactContext);
+   // instantiate a handler, tell it to construct a Handler object 
+   // for our UI thread using reactContext.getMainLooper();
+   this.uiHandler = new Handler(reactContext.getMainLooper());
  }

   @ReactMethod
+  public void enterImmersiveMode() {
+    // ^^ let's give our fail() method a more optimistic name
+    // Handlers have a post() method that schedules a runnable on its thread
+    uiHandler.post(runnableEnterImmersiveMode)
  }
  
  // let's take our previous logic from fail() and stick it in a runnable
+  private Runnable runnableEnterImmersiveMode = new Runnable() {
+    // implement the required run() method of Runnable
+    @Override
+    public void run() {
+      // use the setSystemUIFlags method and the flags we defined for immersive mode
+      // our runnable will execute this code on the thread of our choosing
+      setSystemUIFlags(
+        View.SYSTEM_UI_FLAG_IMMERSIVE
+        | View.SYSTEM_UI_FLAG_LAYOUT_STABLE
+        | View.SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION
+        | View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN
+        | View.SYSTEM_UI_FLAG_HIDE_NAVIGATION
+        | View.SYSTEM_UI_FLAG_FULLSCREEN
+      );
+    }
+  };
  
  . . . 
  
}