CliRunner: replace start hook by dispatch hook

The `CliRunner` lets a custom binary add processing that should happen
before running a command. This patch replaces that by a hook that can
do processing before and/or after. I thought I would want to use this
for adding telemetry (such as timings) to our custom binary at
Google. I ended up adding that logging outside of `CliRunner::run()`
instead, so it gives a more accurate timing of the whole invocation. I
think this patch is still an improvement, as it's more generic than
the start hook.

cli/src/cli_util.rs

@@ -3656,11 +3656,14 @@ pub struct CliRunner {
     commit_template_extensions: Vec<Arc<dyn CommitTemplateLanguageExtension>>,
     operation_template_extensions: Vec<Arc<dyn OperationTemplateLanguageExtension>>,
     dispatch_fn: CliDispatchFn,
-    start_hook_fns: Vec<CliDispatchFn>,
+    dispatch_hook_fns: Vec<CliDispatchHookFn>,
     process_global_args_fns: Vec<ProcessGlobalArgsFn>,
 }
 
-type CliDispatchFn = Box<dyn FnOnce(&mut Ui, &CommandHelper) -> Result<(), CommandError>>;
+pub type CliDispatchFn = Box<dyn FnOnce(&mut Ui, &CommandHelper) -> Result<(), CommandError>>;
+
+type CliDispatchHookFn =
+    Box<dyn FnOnce(&mut Ui, &CommandHelper, CliDispatchFn) -> Result<(), CommandError>>;
 
 type ProcessGlobalArgsFn = Box<dyn FnOnce(&mut Ui, &ArgMatches) -> Result<(), CommandError>>;
 
@@ -3682,7 +3685,7 @@ impl CliRunner {
             commit_template_extensions: vec![],
             operation_template_extensions: vec![],
             dispatch_fn: Box::new(crate::commands::run_command),
-            start_hook_fns: vec![],
+            dispatch_hook_fns: vec![],
             process_global_args_fns: vec![],
         }
     }
@@ -3779,8 +3782,14 @@ impl CliRunner {
         self
     }
 
-    pub fn add_start_hook(mut self, start_hook_fn: CliDispatchFn) -> Self {
+    /// Add a hook that gets called when it's time to run the command. It is
-        self.start_hook_fns.push(start_hook_fn);
+    /// the hook's responsibility to call the given inner dispatch function to
+    /// run the command.
+    pub fn add_dispatch_hook<F>(mut self, dispatch_hook_fn: F) -> Self
+    where
+        F: FnOnce(&mut Ui, &CommandHelper, CliDispatchFn) -> Result<(), CommandError> + 'static,
+    {
+        self.dispatch_hook_fns.push(Box::new(dispatch_hook_fn));
         self
     }
 
@@ -3940,10 +3949,15 @@ impl CliRunner {
         let command_helper = CommandHelper {
             data: Rc::new(command_helper_data),
         };
-        for start_hook_fn in self.start_hook_fns {
+        let dispatch_fn = self.dispatch_hook_fns.into_iter().fold(
-            start_hook_fn(ui, &command_helper)?;
+            self.dispatch_fn,
-        }
+            |old_dispatch_fn, dispatch_hook_fn| {
-        (self.dispatch_fn)(ui, &command_helper)
+                Box::new(move |ui: &mut Ui, command_helper: &CommandHelper| {
+                    dispatch_hook_fn(ui, command_helper, old_dispatch_fn)
+                })
+            },
+        );
+        (dispatch_fn)(ui, &command_helper)
     }
 
     #[must_use]