Add CastNone trait (#843)

* Add CastNone trait

Closes #842

* Add CastNone to prelude, add dynamic_cast

* Add example for CastNone

* Describe type using turbofish op

glib/src/lib.rs

@@ -29,8 +29,8 @@ pub use self::bytes::Bytes;
 pub use self::closure::{Closure, RustClosure};
 pub use self::error::{BoolError, Error};
 pub use self::object::{
-    BorrowedObject, Cast, Class, InitiallyUnowned, Interface, IsA, Object, ObjectExt, ObjectType,
-    SendWeakRef, WeakRef,
+    BorrowedObject, Cast, CastNone, Class, InitiallyUnowned, Interface, IsA, Object, ObjectExt,
+    ObjectType, SendWeakRef, WeakRef,
 };
 pub use self::signal::{
     signal_handler_block, signal_handler_disconnect, signal_handler_unblock,

glib/src/object.rs

@@ -288,6 +288,79 @@ pub trait Cast: ObjectType {
 
 impl<T: ObjectType> Cast for T {}
 
+// rustdoc-stripper-ignore-next
+/// Convenience trait mirroring `Cast`, implemented on `Option<Object>` types.
+///
+/// # Warning
+/// Inveitably this trait will discard informations about a downcast failure:
+/// you don't know if the object was not of the expected type, or if it was `None`.
+/// If you need to handle the downcast error, use `Cast` over a `glib::Object`.
+///
+/// # Example
+/// ```ignore
+/// let widget: Option<Widget> = list_item.child();
+///
+/// // Without using `CastNone`
+/// let label = widget.unwrap().downcast::<gtk::Label>().unwrap();
+///
+/// // Using `CastNone` we can avoid the first `unwrap()` call
+/// let label = widget.and_downcast::<gtk::Label>().unwrap();
+/// ````
+pub trait CastNone: Sized {
+    type Inner;
+    fn and_downcast<T: ObjectType>(self) -> Option<T>
+    where
+        Self::Inner: CanDowncast<T>;
+    fn and_downcast_ref<T: ObjectType>(&self) -> Option<&T>
+    where
+        Self::Inner: CanDowncast<T>;
+    fn and_upcast<T: ObjectType>(self) -> Option<T>
+    where
+        Self::Inner: IsA<T>;
+    fn and_upcast_ref<T: ObjectType>(&self) -> Option<&T>
+    where
+        Self::Inner: IsA<T>;
+    fn and_dynamic_cast<T: ObjectType>(self) -> Result<T, Self>;
+    fn and_dynamic_cast_ref<T: ObjectType>(&self) -> Option<&T>;
+}
+impl<I: ObjectType + Sized> CastNone for Option<I> {
+    type Inner = I;
+
+    fn and_downcast<T: ObjectType>(self) -> Option<T>
+    where
+        Self::Inner: CanDowncast<T>,
+    {
+        self.and_then(|i| i.downcast().ok())
+    }
+
+    fn and_downcast_ref<T: ObjectType>(&self) -> Option<&T>
+    where
+        Self::Inner: CanDowncast<T>,
+    {
+        self.as_ref().and_then(|i| i.downcast_ref())
+    }
+    fn and_upcast<T: ObjectType>(self) -> Option<T>
+    where
+        Self::Inner: IsA<T>,
+    {
+        self.map(|i| i.upcast())
+    }
+
+    fn and_upcast_ref<T: ObjectType>(&self) -> Option<&T>
+    where
+        Self::Inner: IsA<T>,
+    {
+        self.as_ref().map(|i| i.upcast_ref())
+    }
+    fn and_dynamic_cast<T: ObjectType>(self) -> Result<T, Self> {
+        self.ok_or(None)
+            .and_then(|i| i.dynamic_cast().map_err(|e| Some(e)))
+    }
+    fn and_dynamic_cast_ref<T: ObjectType>(&self) -> Option<&T> {
+        self.as_ref().and_then(|i| i.dynamic_cast_ref())
+    }
+}
+
 // rustdoc-stripper-ignore-next
 /// Marker trait for the statically known possibility of downcasting from `Self` to `T`.
 pub trait CanDowncast<T> {}

glib/src/prelude.rs

@@ -5,6 +5,6 @@
 
 pub use crate::param_spec::ParamSpecBuilderExt;
 pub use crate::{
-    Cast, Continue, IsA, ObjectExt, ObjectType, ParamSpecType, StaticType, StaticTypeExt,
+    Cast, CastNone, Continue, IsA, ObjectExt, ObjectType, ParamSpecType, StaticType, StaticTypeExt,
     StaticVariantType, ToSendValue, ToValue, ToVariant,
 };