Simplify http demo and add new download_image eframe demo

eframe/examples/download_image.rs

@@ -0,0 +1,82 @@
+#![cfg_attr(not(debug_assertions), windows_subsystem = "windows")] // hide console window on Windows in release
+
+use eframe::{egui, epi};
+use poll_promise::Promise;
+
+fn main() {
+    let options = eframe::NativeOptions::default();
+    eframe::run_native(Box::new(MyApp::default()), options);
+}
+
+#[derive(Default)]
+struct MyApp {
+    /// `None` when download hasn't started yet.
+    promise: Option<Promise<ehttp::Result<egui::TextureHandle>>>,
+}
+
+impl epi::App for MyApp {
+    fn name(&self) -> &str {
+        "Download and show an image with eframe/egui"
+    }
+
+    fn update(&mut self, ctx: &egui::Context, frame: &epi::Frame) {
+        let promise = self.promise.get_or_insert_with(|| {
+            // Begin download.
+            // We download the image using `ehttp`, a library that works both in WASM and on native.
+            // We use the `poll-promise` library to communicate with the UI thread.
+            let ctx = ctx.clone();
+            let frame = frame.clone();
+            let (sender, promise) = Promise::new();
+            let request = ehttp::Request::get("https://picsum.photos/seed/1.759706314/1024");
+            ehttp::fetch(request, move |response| {
+                frame.request_repaint(); // wake up UI thread
+                let texture = response.and_then(|response| parse_response(&ctx, response));
+                sender.send(texture); // send the results back to the UI thread.
+            });
+            promise
+        });
+
+        egui::CentralPanel::default().show(ctx, |ui| match promise.ready() {
+            None => {
+                ui.add(egui::Spinner::new()); // still loading
+            }
+            Some(Err(err)) => {
+                ui.colored_label(egui::Color32::RED, err); // something went wrong
+            }
+            Some(Ok(texture)) => {
+                let mut size = texture.size_vec2();
+                size *= (ui.available_width() / size.x).min(1.0);
+                size *= (ui.available_height() / size.y).min(1.0);
+                ui.image(texture, size);
+            }
+        });
+    }
+}
+
+fn parse_response(
+    ctx: &egui::Context,
+    response: ehttp::Response,
+) -> Result<egui::TextureHandle, String> {
+    let content_type = response.content_type().unwrap_or_default();
+    if content_type.starts_with("image/") {
+        let image = load_image(&response.bytes).map_err(|err| err.to_string())?;
+        Ok(ctx.load_texture("my-image", image))
+    } else {
+        Err(format!(
+            "Expected image, found content-type {:?}",
+            content_type
+        ))
+    }
+}
+
+fn load_image(image_data: &[u8]) -> Result<egui::ColorImage, image::ImageError> {
+    use image::GenericImageView as _;
+    let image = image::load_from_memory(image_data)?;
+    let size = [image.width() as _, image.height() as _];
+    let image_buffer = image.to_rgba8();
+    let pixels = image_buffer.as_flat_samples();
+    Ok(egui::ColorImage::from_rgba_unmultiplied(
+        size,
+        pixels.as_slice(),
+    ))
+}