feat(FlowrScreen): Observe fragment changes in flowr screen

Author: Hussein Aladeen

flowr/src/main/java/com/fueled/flowr/AbstractFlowrActivity.java

@@ -6,6 +6,7 @@
 import android.os.Bundle;
 import android.support.annotation.ColorInt;
 import android.support.annotation.Nullable;
+import android.support.v4.app.Fragment;
 import android.support.v4.app.FragmentManager;
 import android.support.v7.app.AppCompatActivity;
 
@@ -61,6 +62,11 @@ protected int getDefaultNavigationBarColor() {
         return Color.BLACK;
     }
 
+    @Override
+    public void onCurrentFragmentChanged(@Nullable Fragment currentFragment) {
+        // Do Nothing. No Default implementation is required.
+    }
+
     @SuppressWarnings("WrongConstant")
     @Override
     public void setScreenOrientation(int orientation) {

flowr/src/main/java/com/fueled/flowr/Flowr.java

@@ -509,6 +509,7 @@ public void syncScreenState() {
         }
 
         if (screen != null) {
+            screen.onCurrentFragmentChanged(getCurrentFragment());
             screen.setScreenOrientation(screenOrientation);
             screen.setNavigationBarColor(navigationBarColor);
         }

flowr/src/main/java/com/fueled/flowr/FlowrScreen.java

@@ -1,6 +1,8 @@
 package com.fueled.flowr;
 
 import android.support.annotation.ColorInt;
+import android.support.annotation.Nullable;
+import android.support.v4.app.Fragment;
 import android.support.v4.app.FragmentManager;
 
 /**
@@ -10,11 +12,36 @@
 
 public interface FlowrScreen {
 
+    /**
+     * Return the FragmentManager for interacting with fragments associated
+     * with this screen.
+     */
     FragmentManager getScreenFragmentManager();
 
+    /**
+     * Invoke the {@code onBackPressed} method for this screen.
+     */
     void invokeOnBackPressed();
 
+    /**
+     * Indicates that the fragment currently being displayed has changed.
+     *
+     * @param currentFragment the fragment currently displayed or {@code null} if no fragment
+     *                        is being displayed.
+     */
+    void onCurrentFragmentChanged(@Nullable Fragment currentFragment);
+
+    /**
+     * Specify the screen orientation to be used for this screen.
+     *
+     * @param orientation the screen orientation to be used.
+     */
     void setScreenOrientation(int orientation);
 
+    /**
+     * Specify the navigation bar color to be used for this screen.
+     *
+     * @param color the navigation bar color to be used.
+     */
     void setNavigationBarColor(@ColorInt int color);
 }