android onAppear

lib/android/app/build.gradle

     testCompile 'junit:junit:4.12'
     testCompile 'org.robolectric:robolectric:3.3'
     testCompile 'org.assertj:assertj-core:2.5.0'
+    testCompile 'com.squareup.assertj:assertj-android:1.1.1'
     testCompile 'org.mockito:mockito-core:2.7.5'
 }

lib/android/app/src/main/java/com/reactnativenavigation/viewcontrollers/ViewController.java

 import android.support.annotation.NonNull;
 import android.support.annotation.Nullable;
 import android.view.View;
+import android.view.ViewTreeObserver;
 
 import com.reactnativenavigation.utils.StringUtils;
 
-public abstract class ViewController {
+public abstract class ViewController implements ViewTreeObserver.OnGlobalLayoutListener {
 
 	private final Activity activity;
 	private final String id;
 	public View getView() {
 		if (view == null) {
 			view = createView();
+			view.getViewTreeObserver().addOnGlobalLayoutListener(this);
 		}
 		return view;
 	}
 	public ViewController findControllerById(String id) {
 		return isSameId(id) ? this : null;
 	}
+
+	public void onAppear() {
+	}
+
+	@Override
+	public void onGlobalLayout() {
+		if (getView().isShown())
+			onAppear();
+	}
 }

lib/android/app/src/test/java/com/reactnativenavigation/viewcontrollers/ViewControllerTest.java

 
 import android.app.Activity;
 import android.view.View;
+import android.view.ViewParent;
 
 import com.reactnativenavigation.BaseTest;
 import com.reactnativenavigation.mocks.SimpleViewController;
 
+import org.assertj.android.api.Assertions;
 import org.junit.Test;
+import org.robolectric.Shadows;
 
 import static org.assertj.core.api.Java6Assertions.assertThat;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.spy;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
 
 public class ViewControllerTest extends BaseTest {
 
 		assertThat(uut.findControllerById("456")).isNull();
 		assertThat(uut.findControllerById("uut")).isEqualTo(uut);
 	}
+
+	@Test
+	public void onAppear_WhenVisibleOnScreen() throws Exception {
+		ViewController spy = spy(uut);
+		spy.getView().getViewTreeObserver().dispatchOnGlobalLayout();
+		Assertions.assertThat(spy.getView()).isNotShown();
+		verify(spy, times(0)).onAppear();
+
+		Shadows.shadowOf(spy.getView()).setMyParent(mock(ViewParent.class));
+		spy.getView().getViewTreeObserver().dispatchOnGlobalLayout();
+		Assertions.assertThat(spy.getView()).isShown();
+
+		verify(spy, times(1)).onAppear();
+	}
+
+
 }
+