Move due date when setting a start date that is later in time. Fixes

dmfs#180

src/org/dmfs/tasks/model/TaskFieldAdapters.java

@@ -34,6 +34,7 @@
 import org.dmfs.tasks.model.contraints.ChecklistConstraint;
 import org.dmfs.tasks.model.contraints.NotAfter;
 import org.dmfs.tasks.model.contraints.NotBefore;
+import org.dmfs.tasks.model.contraints.ShiftIfAfter;
 
 
 /**
@@ -119,7 +120,7 @@ public final class TaskFieldAdapters
 	/**
 	 * Adapter for the start date of a task.
 	 */
-	public final static TimeFieldAdapter DTSTART = (TimeFieldAdapter) _DTSTART.addContraint(new NotAfter(DUE));
+	public final static TimeFieldAdapter DTSTART = (TimeFieldAdapter) _DTSTART.addContraint(new ShiftIfAfter(DUE));
 
 	/**
 	 * Adapter for the completed date of a task.

src/org/dmfs/tasks/model/contraints/ShiftIfAfter.java

@@ -0,0 +1,56 @@
+/*
+ * Copyright (C) 2013 Marten Gajda <marten@dmfs.org>
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ * 
+ */
+
+package org.dmfs.tasks.model.contraints;
+
+import org.dmfs.tasks.model.ContentSet;
+import org.dmfs.tasks.model.adapters.TimeFieldAdapter;
+
+import android.text.format.Time;
+
+
+/**
+ * Ensure a time is not after a specific reference time. If that would be the case, the reference time is updated. 
+ * 
+ * @author Marten Gajda <marten@dmfs.org>
+ */
+public class ShiftIfAfter extends AbstractConstraint<Time>
+{
+	private final TimeFieldAdapter mTimeAdapter;
+
+
+	public ShiftIfAfter(TimeFieldAdapter adapter)
+	{
+		mTimeAdapter = adapter;
+	}
+
+
+	@Override
+	public Time apply(ContentSet currentValues, Time oldValue, Time newValue)
+	{
+		Time notAfterThisTime = mTimeAdapter.get(currentValues);
+		if (notAfterThisTime != null && newValue != null)
+		{
+			if (newValue.after(notAfterThisTime))
+			{
+				mTimeAdapter.set(currentValues, newValue);
+			}
+		}
+		return newValue;
+	}
+
+}