feat(GetToken): Add application_secret method to GetToken trait.

This makes decoupling Authenticator and individual flows easier while
allowing for refreshing tokens.

src/device.rs

@@ -55,6 +55,9 @@ impl<
     fn api_key(&mut self) -> Option<String> {
         None
     }
+    fn application_secret(&self) -> ApplicationSecret {
+        self.application_secret.clone()
+    }
 }
 
 impl<FD, C> DeviceFlow<FD, C>

src/installed.rs

@@ -61,10 +61,8 @@ where
     })
 }
 
-impl<
-        FD: FlowDelegate + 'static + Send + Clone,
-        C: hyper::client::connect::Connect + 'static,
-    > GetToken for InstalledFlow<FD, C>
+impl<FD: FlowDelegate + 'static + Send + Clone, C: hyper::client::connect::Connect + 'static>
+    GetToken for InstalledFlow<FD, C>
 {
     fn token<'b, I, T>(
         &mut self,
@@ -79,6 +77,9 @@ impl<
     fn api_key(&mut self) -> Option<String> {
         None
     }
+    fn application_secret(&self) -> ApplicationSecret {
+        self.appsecret.clone()
+    }
 }
 
 pub struct InstalledFlow<FD: FlowDelegate, C: hyper::client::connect::Connect + 'static> {
@@ -99,11 +100,8 @@ pub enum InstalledFlowReturnMethod {
     HTTPRedirect(u16),
 }
 
-impl<
-        'c,
-        FD: 'static + FlowDelegate + Clone + Send,
-        C: 'c + hyper::client::connect::Connect,
-    > InstalledFlow<FD, C>
+impl<'c, FD: 'static + FlowDelegate + Clone + Send, C: 'c + hyper::client::connect::Connect>
+    InstalledFlow<FD, C>
 {
     /// Starts a new Installed App auth flow.
     /// If HTTPRedirect is chosen as method and the server can't be started, the flow falls

src/service_account.rs

@@ -16,7 +16,7 @@ use std::error;
 use std::sync::{Arc, Mutex};
 
 use crate::storage::{hash_scopes, MemoryStorage, TokenStorage};
-use crate::types::{GetToken, StringError, Token};
+use crate::types::{ApplicationSecret, GetToken, StringError, Token};
 
 use futures::stream::Stream;
 use futures::{future, prelude::*};
@@ -358,6 +358,12 @@ where
         )
     }
 
+    /// Returns an empty ApplicationSecret as tokens for service accounts don't need to be
+    /// refreshed (they are simply reissued).
+    fn application_secret(&self) -> ApplicationSecret {
+        Default::default()
+    }
+
     fn api_key(&mut self) -> Option<String> {
         None
     }

src/types.rs

@@ -205,6 +205,10 @@ pub trait GetToken {
         I: Iterator<Item = &'b T>;
 
     fn api_key(&mut self) -> Option<String>;
+
+    /// Return an application secret with at least token_uri, client_secret, and client_id filled
+    /// in. This is used for refreshing tokens without interaction from the flow.
+    fn application_secret(&self) -> ApplicationSecret;
 }
 
 /// Represents a token as returned by OAuth2 servers.