diff --git a/src/view/identifiable.rs b/src/view/identifiable.rs
index 7c4fe0c..bd35979 100644
--- a/src/view/identifiable.rs
+++ b/src/view/identifiable.rs
@@ -6,9 +6,12 @@ use view::View;
 /// [`IdView`]: ../views/struct.IdView.html
 pub trait Identifiable: View + Sized {
     /// Wraps this view into an IdView with the given id.
+    ///
+    /// This is just a shortcut for `IdView::new(id, self)`
     fn with_id(self, id: &str) -> IdView<Self> {
         IdView::new(id, self)
     }
 }
 
+/// Any `View` implements this trait.
 impl<T: View> Identifiable for T {}
diff --git a/src/view/mod.rs b/src/view/mod.rs
index 38fc83e..61484b2 100644
--- a/src/view/mod.rs
+++ b/src/view/mod.rs
@@ -71,7 +71,9 @@ pub use self::boxable::Boxable;
 
 /// Main trait defining a view behaviour.
 pub trait View {
-    /// Called when a key was pressed. Default implementation just ignores it.
+    /// Called when a key was pressed.
+    ///
+    /// Default implementation just ignores it.
     fn on_event(&mut self, Event) -> EventResult {
         EventResult::Ignored
     }
@@ -81,6 +83,8 @@ pub trait View {
     /// If the view is flexible (it has multiple size options), it can try
     /// to return one that fits the given `constraint`.
     /// It's also fine to ignore it and return a fixed value.
+    ///
+    /// Default implementation always return `(1,1)`.
     fn get_min_size(&mut self, constraint: Vec2) -> Vec2 {
         let _ = constraint;
         Vec2::new(1, 1)
@@ -112,6 +116,8 @@ pub trait View {
     /// Finds the view pointed to by the given path.
     ///
     /// Returns None if the path doesn't lead to a view.
+    ///
+    /// Default implementation always return `None`.
     fn find(&mut self, &Selector) -> Option<&mut Any> {
         None
     }
@@ -120,6 +126,8 @@ pub trait View {
     ///
     /// `source` indicates where the focus comes from.
     /// When the source is unclear, `Front` is usually used.
+    ///
+    /// Default implementation always return `false`.
     fn take_focus(&mut self, source: Direction) -> bool {
         let _ = source;
         false
diff --git a/src/view/position.rs b/src/view/position.rs
index ffa2257..88023aa 100644
--- a/src/view/position.rs
+++ b/src/view/position.rs
@@ -29,8 +29,16 @@ impl Position {
     /// and a parent with the absolute coordinates `parent`, drawing the
     /// child with its top-left corner at the returned coordinates will
     /// position him appropriately.
-    pub fn compute_offset(&self, size: Vec2, available: Vec2, parent: Vec2)
-                          -> Vec2 {
+    pub fn compute_offset<S, A, P>(&self, size: S, available: A, parent: P)
+                                   -> Vec2
+        where S: Into<Vec2>,
+              A: Into<Vec2>,
+              P: Into<Vec2>
+    {
+        let available = available.into();
+        let size = size.into();
+        let parent = parent.into();
+
         Vec2::new(self.x.compute_offset(size.x, available.x, parent.x),
                   self.y.compute_offset(size.y, available.y, parent.y))
     }
@@ -67,3 +75,21 @@ impl Offset {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+
+    use vec::Vec2;
+    use super::{Offset, Position};
+
+    #[test]
+    fn test_center() {
+        let center = Position::center();
+        assert_eq!(Vec2::new(2, 1), center.compute_offset((1,1), (5,3), (0,0)));
+        assert_eq!(Vec2::new(2, 0), center.compute_offset((1,3), (5,3), (0,0)));
+        assert_eq!(Vec2::new(1, 1), center.compute_offset((3,1), (5,3), (0,0)));
+        assert_eq!(Vec2::new(0, 1), center.compute_offset((5,1), (5,3), (0,0)));
+        assert_eq!(Vec2::new(0, 0), center.compute_offset((5,3), (5,3), (0,0)));
+        assert_eq!(Vec2::new(0, 0), center.compute_offset((5,3), (3,1), (0,0)));
+    }
+}