Implemented a scroll-listener. Closes #47.

The scroll listener emits an event every time the first visible day has changed.

Author: Jens Claes

README.md

@@ -129,6 +129,7 @@ Use the following interfaces according to your need.
 - `mWeekView.setEmptyViewClickListener()` to get a callback when any empty space is clicked
 - `mWeekView.setEmptyViewLongPressListener()` to get a callback when any empty space is long pressed
 - `mWeekView.setDateTimeInterpreter()` to set your own labels for the calendar header row and header column
+- `mWeekView.setScrolledListener()` to get an event every time the first visible day has changed
 
 Sample
 ----------

library/src/main/java/com/alamkanak/weekview/WeekView.java

@@ -108,6 +108,7 @@ public class WeekView extends View {
     private EmptyViewClickListener mEmptyViewClickListener;
     private EmptyViewLongPressListener mEmptyViewLongPressListener;
     private DateTimeInterpreter mDateTimeInterpreter;
+    private ScrolledListener mScrolledListener;
 
     private final GestureDetector.SimpleOnGestureListener mGestureListener = new GestureDetector.SimpleOnGestureListener() {
 
@@ -412,8 +413,12 @@ private void drawHeaderRowAndEvents(Canvas canvas) {
         }
 
         // Iterate through each day.
+        Calendar oldFirstVisibleDay = mFirstVisibleDay;
         mFirstVisibleDay = (Calendar) mToday.clone();
         mFirstVisibleDay.add(Calendar.DATE, leftDaysWithGaps);
+        if(!mFirstVisibleDay.equals(oldFirstVisibleDay) && mScrolledListener != null){
+            mScrolledListener.onFirstVisibleDayChanged(mFirstVisibleDay, oldFirstVisibleDay);
+        }
         for (int dayNumber = leftDaysWithGaps + 1;
              dayNumber <= leftDaysWithGaps + mNumberOfVisibleDays + 1;
              dayNumber++) {
@@ -951,6 +956,13 @@ public EmptyViewLongPressListener getEmptyViewLongPressListener(){
         return mEmptyViewLongPressListener;
     }
 
+    public void setScrolledListener(ScrolledListener scrolledListener){
+        this.mScrolledListener = scrolledListener;
+    }
+
+    public ScrolledListener getScrolledListener(){
+        return mScrolledListener;
+    }
     /**
      * Get the interpreter which provides the text to show in the header column and the header row.
      * @return The date, time interpreter.
@@ -1418,6 +1430,17 @@ public interface EmptyViewLongPressListener {
         public void onEmptyViewLongPress(Calendar time);
     }
 
+    public interface ScrolledListener {
+        /**
+         * Called when the first visible day has changed.
+         *
+         * (this will also be called during the first draw of the weekview)
+         * @param newFirstVisibleDay The new first visible day
+         * @param oldFirstVisibleDay The old first visible day (is null on the first call).
+         */
+        public void onFirstVisibleDayChanged(Calendar newFirstVisibleDay, Calendar oldFirstVisibleDay);
+    }
+
 
     /////////////////////////////////////////////////////////////////
     //