Feature/view lifecycle (#798)

* Adds iOS callback for viewWillAppear/didAppear e.t.c.

* Adds in screen events on Android

* Fixed issue with rightButtons getting ignored in preference for empty Screen buttons

* Android screen lifecycle

This commit is contains a few changes and enhancements to the PR made by simon:

* Use getEventEmitter instead of sendNavigatorEvent
* Dispatch willDisappear and didDisappear events when pushing screens
* Dispatch willAppear and didAppear events when popping screen

android/app/src/main/java/com/reactnativenavigation/screens/Screen.java

 import android.widget.RelativeLayout;
 
 import com.facebook.react.bridge.Callback;
+import com.reactnativenavigation.NavigationApplication;
 import com.reactnativenavigation.animation.VisibilityAnimator;
 import com.reactnativenavigation.events.ContextualMenuHiddenEvent;
 import com.reactnativenavigation.events.Event;
     public abstract void setOnDisplayListener(OnDisplayListener onContentViewDisplayedListener);
 
     public void show() {
+        NavigationApplication.instance.getEventEmitter().sendNavigatorEvent("willAppear", screenParams.getNavigatorEventId());
+        NavigationApplication.instance.getEventEmitter().sendNavigatorEvent("didAppear", screenParams.getNavigatorEventId());
         screenAnimator.show(screenParams.animateScreenTransitions);
     }
 
     public void show(boolean animated) {
+        NavigationApplication.instance.getEventEmitter().sendNavigatorEvent("willAppear", screenParams.getNavigatorEventId());
+        NavigationApplication.instance.getEventEmitter().sendNavigatorEvent("didAppear", screenParams.getNavigatorEventId());
         screenAnimator.show(animated);
     }
 
     public void show(boolean animated, Runnable onAnimationEnd) {
+        NavigationApplication.instance.getEventEmitter().sendNavigatorEvent("willAppear", screenParams.getNavigatorEventId());
+        NavigationApplication.instance.getEventEmitter().sendNavigatorEvent("didAppear", screenParams.getNavigatorEventId());
         setStyle();
         screenAnimator.show(animated, onAnimationEnd);
     }
 
     public void hide(boolean animated, Runnable onAnimatedEnd) {
+        NavigationApplication.instance.getEventEmitter().sendNavigatorEvent("willDisappear", screenParams.getNavigatorEventId());
+        NavigationApplication.instance.getEventEmitter().sendNavigatorEvent("didDisappear", screenParams.getNavigatorEventId());
         screenAnimator.hide(animated, onAnimatedEnd);
     }
 

android/app/src/main/java/com/reactnativenavigation/screens/ScreenStack.java

                                           @Nullable final Screen.OnDisplayListener onDisplay) {
         nextScreen.setVisibility(View.INVISIBLE);
         addScreen(nextScreen, layoutParams);
+        NavigationApplication.instance.getEventEmitter().sendNavigatorEvent("willDisappear", previousScreen.getNavigatorEventId());
         nextScreen.setOnDisplayListener(new Screen.OnDisplayListener() {
             @Override
             public void onDisplay() {
                     @Override
                     public void run() {
                         if (onDisplay != null) onDisplay.onDisplay();
+                        NavigationApplication.instance.getEventEmitter().sendNavigatorEvent("didDisappear", previousScreen.getNavigatorEventId());
                         parent.removeView(previousScreen);
                     }
                 });
 
     private void readdPrevious(Screen previous) {
         previous.setVisibility(View.VISIBLE);
+        NavigationApplication.instance.getEventEmitter().sendNavigatorEvent("willAppear", previous.getNavigatorEventId());
+        NavigationApplication.instance.getEventEmitter().sendNavigatorEvent("didAppear", previous.getNavigatorEventId());
         parent.addView(previous, 0);
     }
 
         isStackVisible = true;
         stack.peek().setStyle();
         stack.peek().setVisibility(View.VISIBLE);
+        NavigationApplication.instance.getEventEmitter().sendNavigatorEvent("willAppear", stack.peek().getNavigatorEventId());
+        NavigationApplication.instance.getEventEmitter().sendNavigatorEvent("didAppear", stack.peek().getNavigatorEventId());
     }
 
     public void hide() {
+        NavigationApplication.instance.getEventEmitter().sendNavigatorEvent("willDisappear", stack.peek().getNavigatorEventId());
+        NavigationApplication.instance.getEventEmitter().sendNavigatorEvent("didDisappear", stack.peek().getNavigatorEventId());
         isStackVisible = false;
         stack.peek().setVisibility(View.INVISIBLE);
     }

ios/RCCViewController.m

 #import <React/RCTRootView.h>
 #import "RCCManager.h"
 #import <React/RCTConvert.h>
+#import <React/RCTEventDispatcher.h>
 #import "RCCExternalViewControllerProtocol.h"
 #import "RCTHelpers.h"
 #import "RCCTitleViewHelper.h"
     }
 }
 
+- (void)sendScreenChangedEvent:(NSString *)eventName
+{
+    if ([self.view isKindOfClass:[RCTRootView class]]){
+        
+        RCTRootView *rootView = (RCTRootView *)self.view;
+        
+        if (rootView.appProperties && rootView.appProperties[@"navigatorEventID"]) {
+            
+            [[[RCCManager sharedInstance] getBridge].eventDispatcher sendAppEventWithName:rootView.appProperties[@"navigatorEventID"] body:@
+             {
+                 @"type": @"ScreenChangedEvent",
+                 @"method": eventName
+             }];
+        }
+    }
+}
+
+- (void)viewDidAppear:(BOOL)animated
+{
+    [super viewDidAppear:animated];
+    [self sendScreenChangedEvent:@"didAppear"];
+}
+
 - (void)viewWillAppear:(BOOL)animated
 {
     [super viewWillAppear:animated];
-    
+    [self sendScreenChangedEvent:@"willAppear"];
     [self setStyleOnAppear];
 }
 
+- (void)viewDidDisappear:(BOOL)animated
+{
+    [super viewDidDisappear:animated];
+    [self sendScreenChangedEvent:@"didDisappear"];
+}
+
 - (void)viewWillDisappear:(BOOL)animated
 {
     [super viewWillDisappear:animated];
-    
+    [self sendScreenChangedEvent:@"willDisappear"];
     [self setStyleOnDisappear];
 }