image_view: Rework model loading and page navigation

Rework the model loading to not care about the edges.
Now we only care about where we are in the model and
the number of buffer pages. This algorithm handles
the edges without special casing.

src/widgets/image_view.rs

@@ -29,7 +29,7 @@ use anyhow::{bail, Context};
 use ashpd::desktop::wallpaper;
 use ashpd::WindowIdentifier;
 use once_cell::sync::Lazy;
-use std::cell::RefCell;
+use std::cell::{Cell, RefCell};
 
 use crate::file_model::LpFileModel;
 use crate::thumbnail::Thumbnail;
@@ -38,7 +38,7 @@ use crate::widgets::LpImagePage;
 
 // Maximum number of pages to load
 // at any given time
-const N_PAGES: u32 = 3;
+const N_PAGES: u32 = 2;
 
 mod imp {
     use super::*;
@@ -51,6 +51,7 @@ mod imp {
 
         pub model: RefCell<Option<LpFileModel>>,
         pub filename: RefCell<Option<String>>,
+        pub prev_index: Cell<u32>,
     }
 
     #[glib::object_subclass]
@@ -187,42 +188,27 @@ impl LpImageView {
 
         if let Some(model) = imp.model.borrow().as_ref() {
             let index = model.index_of(file).unwrap() as u32;
+            log::debug!("Currently at file {index} in the directory");
             imp.filename.replace(util::get_file_display_name(file));
             carousel.append(&LpImagePage::from_file(file));
 
-            // Here we need to check if we're at the start or end of our directory.
+            for i in 1..=N_PAGES {
-            // If we are, we try to add two files to the other side. Otherwise,
+                if let Some(file) = model.item(index + i).and_then(|o| o.downcast().ok()) {
-            // add one file on each end of the current file.
+                    carousel.append(&LpImagePage::from_file(&file))
-            let iterations = if index == 0 || index == model.n_items() - 1 {
-                2
-            } else {
-                1
-            };
-
-            let mut next_file = file.clone();
-            for i in 0..iterations {
-                log::debug!("Loop to find next files: {}", i);
-                if let Some(f) = model.next(&next_file) {
-                    log::debug!("Adding next image URI: {}", f.uri());
-                    carousel.append(&LpImagePage::from_file(&f));
-                    next_file = f;
-                } else {
-                    // Return early if there are no files in this direction;
-                    break;
                 }
             }
 
-            let mut prev_file = file.clone();
+            for i in 1..=N_PAGES {
-            for i in 0..iterations {
+                if let Some(file) = index
-                log::debug!("Loop to find prior files: {}", i);
+                    .checked_sub(i)
-                if let Some(f) = model.previous(&prev_file) {
+                    .and_then(|i| model.item(i))
-                    log::debug!("Adding previous image URI: {}", f.uri());
+                    .and_then(|o| o.downcast().ok())
-                    carousel.prepend(&LpImagePage::from_file(&f));
+                {
-                    prev_file = f;
+                    carousel.prepend(&LpImagePage::from_file(&file))
-                } else {
-                    break;
                 }
             }
+
+            imp.prev_index.set(index);
         }
     }
 
@@ -257,33 +243,62 @@ impl LpImageView {
     }
 
     #[template_callback]
-    fn page_changed_cb(&self, index: u32, carousel: &adw::Carousel) {
+    fn page_changed_cb(&self, _index: u32, carousel: &adw::Carousel) {
         let imp = self.imp();
         let b = imp.model.borrow();
         let model = b.as_ref().unwrap();
         let current = self.current_page().and_then(|p| p.file()).unwrap();
+
+        let model_index = model.index_of(&current).unwrap() as u32;
+        let prev_index = imp.prev_index.get();
+
+        if model_index != prev_index {
             imp.filename.replace(util::get_file_display_name(&current));
             self.notify("filename");
             self.update_action_state(&current);
 
             // We've moved forward
-        if index + 1 == N_PAGES {
+            if let Some(diff) = model_index.checked_sub(prev_index) {
-            if let Some(ref next) = model.next(&current) {
+                for i in 0..diff {
-                log::debug!("Next URI: {}", next.uri());
+                    if prev_index
-                // Remove the page at index 0, add a new page at the end
+                        .checked_sub(N_PAGES)
+                        .and_then(|r| r.checked_sub(i))
+                        .is_some()
+                    {
                         carousel.remove(&carousel.nth_page(0));
-                carousel.append(&LpImagePage::from_file(next));
+                    }
+
+                    let s = prev_index + N_PAGES + i + 1;
+                    if s <= model.n_items() {
+                        if let Some(ref file) =
+                            model.item(s).and_then(|o| o.downcast::<gio::File>().ok())
+                        {
+                            carousel.append(&LpImagePage::from_file(file));
+                        }
+                    }
                 }
             }
 
             // We've moved backward
-        if index == 0 {
+            if let Some(diff) = prev_index.checked_sub(model_index) {
-            if let Some(ref prev) = model.previous(&current) {
+                for i in 0..diff {
-                log::debug!("Previous URI: {}", prev.uri());
+                    let s = prev_index + N_PAGES + i + 1;
-                // Remove the page at the front, add a new page at the back
+                    if s <= model.n_items() {
                         carousel.remove(&carousel.nth_page(carousel.n_pages() - 1));
-                carousel.prepend(&LpImagePage::from_file(prev));
                     }
+
+                    if let Some(ref file) = prev_index
+                        .checked_sub(N_PAGES)
+                        .and_then(|d| d.checked_sub(i + 1))
+                        .and_then(|d| model.item(d))
+                        .and_then(|i| i.downcast::<gio::File>().ok())
+                    {
+                        carousel.prepend(&LpImagePage::from_file(file));
+                    }
+                }
+            }
+
+            imp.prev_index.set(model_index);
         }
     }