feat(backend_detect): add portal/screencopy availability pre-checks

Add check_portal_available() and check_screencopy_available() to
probe each backend independently before committing. This enables
smarter fallback logic and better diagnostics when no backend is
found. Includes Chinese documentation comments.

src/backend_detect.rs

@@ -6,18 +6,25 @@ use wayland_client::{Connection, Dispatch, QueueHandle};
 
 use crate::args::Args;
 
+// 屏幕捕获后端类型
+
 /// Capture backend to use for screen capture.
+/// 屏幕捕获后端枚举
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum CaptureBackend {
     /// wlroots wlr-screencopy protocol (Sway, Hyprland, etc.)
+    /// wlroots 合成器的 wlr-screencopy 协议（适用于 Sway、Hyprland 等）
     WlrScreencopy,
     /// xdg-desktop-portal with PipeWire (KWin/KDE, GNOME, etc.)
+    /// XDG 桌面门户 + PipeWire 方式（适用于 KWin/KDE、GNOME 等）
     PortalPipeWire,
 }
 
 /// Minimal dispatch type for listing Wayland globals during backend detection.
+/// 用于后端检测期间列举 Wayland 全局对象的最小化分发类型（无需实际处理事件）
 struct RegistryLs;
 
+// 为 RegistryLs 实现 Wayland 注册表事件分发（空实现，仅需类型满足 trait 约束）
 impl Dispatch<WlRegistry, GlobalListContents> for RegistryLs {
     fn event(
         _state: &mut Self,
@@ -30,37 +37,49 @@ impl Dispatch<WlRegistry, GlobalListContents> for RegistryLs {
     }
 }
 
-/// Detect which capture backend to use.
-///
-/// Priority:
-/// 1. Explicit `--backend` override from CLI args
-/// 2. Auto-detect by checking for `zwlr_screencopy_manager_v1` in Wayland globals
-///
-/// The detection Wayland connection is dropped before returning so the actual
-/// capture backend can create its own connection without holding two simultaneously.
-pub fn detect_backend(args: &Args) -> Result<CaptureBackend> {
-    // 1. Check explicit override
-    if let Some(ref backend) = args.backend {
-        return match backend.as_str() {
-            "portal" => {
-                tracing::info!("Backend override: Portal/PipeWire");
-                Ok(CaptureBackend::PortalPipeWire)
-            }
-            "screencopy" => {
-                tracing::info!("Backend override: wlr-screencopy");
-                Ok(CaptureBackend::WlrScreencopy)
-            }
-            other => {
-                anyhow::bail!(
-                    "Unknown backend '{}'. Use 'screencopy' or 'portal'.",
-                    other
-                );
+// 通过 D-Bus 检测 XDG Desktop Portal 的 ScreenCast 接口是否可用
+// 尝试创建 Screencast proxy，如果 Portal 服务未运行则返回 false
+fn check_portal_available() -> bool {
+    use ashpd::desktop::screencast::Screencast;
+
+    let rt = match tokio::runtime::Runtime::new() {
+        Ok(rt) => rt,
+        Err(e) => {
+            tracing::warn!("Failed to create tokio runtime for portal check: {e}");
+            return false;
+        }
+    };
+
+    rt.block_on(async {
+        let proxy = match Screencast::new().await {
+            Ok(p) => p,
+            Err(e) => {
+                tracing::info!("Portal not available: {e}");
+                return false;
             }
         };
-    }
 
-    // 2. Auto-detect: check if zwlr_screencopy_manager_v1 is available
-    tracing::info!("Auto-detecting capture backend...");
+        // Verify the portal actually exposes ScreenCast capabilities,
+        // not just that the D-Bus service is running.
+        match proxy.available_source_types().await {
+            Ok(types) if !types.is_empty() => {
+                tracing::info!("Portal ScreenCast available (source types: {types:?})");
+                true
+            }
+            Ok(types) => {
+                tracing::info!("Portal ScreenCast proxy exists but no source types available ({types:?})");
+                false
+            }
+            Err(e) => {
+                tracing::info!("Portal ScreenCast available_source_types query failed: {e}");
+                false
+            }
+        }
+    })
+}
+
+// 通过 Wayland globals 检测 wlr-screencopy 协议是否可用
+fn check_screencopy_available() -> Result<bool> {
     let conn = Connection::connect_to_env()?;
     let (globals, _queue) = registry_queue_init::<RegistryLs>(&conn)?;
 
@@ -73,14 +92,73 @@ pub fn detect_backend(args: &Args) -> Result<CaptureBackend> {
     // Drop the Wayland connection explicitly before returning.
     // The screencopy path creates its own connection. Holding two connections
     // simultaneously is wasteful and may cause issues on some compositors.
+    // 显式释放检测用的 Wayland 连接，避免与后续捕获后端同时占用两个连接
     drop(conn);
 
-    if has_screencopy {
-        tracing::info!("Detected wlr-screencopy support → using WlrScreencopy backend");
-        Ok(CaptureBackend::WlrScreencopy)
-    } else {
-        tracing::info!("No wlr-screencopy support → using Portal/PipeWire backend");
-        Ok(CaptureBackend::PortalPipeWire)
+    Ok(has_screencopy)
+}
+
+/// Detect which capture backend to use.
+/// 检测应使用哪种屏幕捕获后端
+///
+/// Priority:
+/// 优先级：
+/// 1. Explicit `--backend` override from CLI args
+///    用户通过 `--backend` 命令行参数显式指定
+/// 2. Auto-detect: check wlr-screencopy (Wayland) and Portal (D-Bus) respectively
+///    自动检测：分别通过 Wayland globals 和 D-Bus 检测两个后端
+///
+/// Both backends are checked independently. If neither is available, returns an error.
+/// 两个后端独立检测，都不可用时返回错误。
+pub fn detect_backend(args: &Args) -> Result<CaptureBackend> {
+    // 1. Check explicit override
+    // 步骤 1：检查用户是否通过命令行参数显式指定了后端
+    if let Some(ref backend) = args.backend {
+        return match backend.as_str() {
+            "portal" => {
+                tracing::info!("Backend override: Portal/PipeWire");
+                Ok(CaptureBackend::PortalPipeWire)
+            }
+            "screencopy" => {
+                tracing::info!("Backend override: wlr-screencopy");
+                Ok(CaptureBackend::WlrScreencopy)
+            }
+            other => {
+                // 未知后端名称，返回错误
+                anyhow::bail!(
+                    "Unknown backend '{}'. Use 'screencopy' or 'portal'.",
+                    other
+                );
+            }
+        };
+    }
+
+    // 2. Auto-detect: check both backends independently
+    // 步骤 2：自动检测 — 分别检测两个后端的可用性
+    tracing::info!("Auto-detecting capture backend...");
+
+    // 检测 wlr-screencopy（通过 Wayland globals）
+    let has_screencopy = check_screencopy_available()?;
+    // 检测 Portal（通过 D-Bus）
+    let has_portal = check_portal_available();
+
+    // 根据检测结果选择后端，screencopy 优先（性能更好、延迟更低）
+    match (has_screencopy, has_portal) {
+        (true, _) => {
+            tracing::info!("Detected wlr-screencopy support → using WlrScreencopy backend");
+            Ok(CaptureBackend::WlrScreencopy)
+        }
+        (false, true) => {
+            tracing::info!("No wlr-screencopy, Portal available → using Portal/PipeWire backend");
+            Ok(CaptureBackend::PortalPipeWire)
+        }
+        (false, false) => {
+            anyhow::bail!(
+                "No supported capture backend found. \
+                 Install a wlroots compositor (for wlr-screencopy) \
+                 or xdg-desktop-portal (for Portal/PipeWire)."
+            );
+        }
     }
 }
 
@@ -88,6 +166,7 @@ pub fn detect_backend(args: &Args) -> Result<CaptureBackend> {
 mod tests {
     use super::*;
 
+    // 测试辅助函数：构造指定后端参数的 Args 实例
     fn make_args(backend: Option<&str>) -> Args {
         Args {
             output: "test.mp4".to_string(),
@@ -104,6 +183,7 @@ mod tests {
         }
     }
 
+    // 测试：显式指定 portal 后端
     #[test]
     fn explicit_portal_backend() {
         let args = make_args(Some("portal"));
@@ -112,6 +192,7 @@ mod tests {
         assert_eq!(result.unwrap(), CaptureBackend::PortalPipeWire);
     }
 
+    // 测试：显式指定 screencopy 后端
     #[test]
     fn explicit_screencopy_backend() {
         let args = make_args(Some("screencopy"));
@@ -120,6 +201,7 @@ mod tests {
         assert_eq!(result.unwrap(), CaptureBackend::WlrScreencopy);
     }
 
+    // 测试：无效的后端名称应返回错误
     #[test]
     fn invalid_backend_name_returns_error() {
         let args = make_args(Some("magic"));