Set up logic for starting following

Co-Authored-By: Antonio Scandurra <me@as-cii.com>

crates/workspace/src/pane.rs

@@ -1,5 +1,7 @@
 use super::{ItemHandle, SplitDirection};
 use crate::{Item, Settings, WeakItemHandle, Workspace};
+use anyhow::{anyhow, Result};
+use client::PeerId;
 use collections::{HashMap, VecDeque};
 use gpui::{
     action,
@@ -105,6 +107,13 @@ pub struct Pane {
     active_toolbar_visible: bool,
 }
 
+pub(crate) struct FollowerState {
+    pub(crate) leader_id: PeerId,
+    pub(crate) current_view_id: usize,
+    pub(crate) items_by_leader_view_id:
+        HashMap<usize, (Option<ProjectEntryId>, Box<dyn ItemHandle>)>,
+}
+
 pub trait Toolbar: View {
     fn active_item_changed(
         &mut self,
@@ -313,6 +322,21 @@ impl Pane {
         cx.notify();
     }
 
+    pub(crate) fn set_follow_state(
+        &mut self,
+        follower_state: FollowerState,
+        cx: &mut ViewContext<Self>,
+    ) -> Result<()> {
+        let current_view_id = follower_state.current_view_id as usize;
+        let (project_entry_id, item) = follower_state
+            .items_by_leader_view_id
+            .get(&current_view_id)
+            .ok_or_else(|| anyhow!("invalid current view id"))?
+            .clone();
+        self.add_item(project_entry_id, item, cx);
+        Ok(())
+    }
+
     pub fn items(&self) -> impl Iterator<Item = &Box<dyn ItemHandle>> {
         self.items.iter().map(|(_, view)| view)
     }