Make drag and drop optional (fixes OleInitialize failure #1255) (#1524)

Co-authored-by: Osspial <osspial@gmail.com>

src/platform/windows.rs

@@ -117,6 +117,14 @@ pub trait WindowBuilderExtWindows {
 
     /// This sets `WS_EX_NOREDIRECTIONBITMAP`.
     fn with_no_redirection_bitmap(self, flag: bool) -> WindowBuilder;
+
+    /// Enables or disables drag and drop support (enabled by default). Will interfere with other crates
+    /// that use multi-threaded COM API (`CoInitializeEx` with `COINIT_MULTITHREADED` instead of
+    /// `COINIT_APARTMENTTHREADED`) on the same thread. Note that winit may still attempt to initialize
+    /// COM API regardless of this option. Currently only fullscreen mode does that, but there may be more in the future.
+    /// If you need COM API with `COINIT_MULTITHREADED` you must initialize it before calling any winit functions.
+    /// See https://docs.microsoft.com/en-us/windows/win32/api/objbase/nf-objbase-coinitialize#remarks for more information.
+    fn with_drag_and_drop(self, flag: bool) -> WindowBuilder;
 }
 
 impl WindowBuilderExtWindows for WindowBuilder {
@@ -137,6 +145,12 @@ impl WindowBuilderExtWindows for WindowBuilder {
         self.platform_specific.no_redirection_bitmap = flag;
         self
     }
+
+    #[inline]
+    fn with_drag_and_drop(mut self, flag: bool) -> WindowBuilder {
+        self.platform_specific.drag_and_drop = flag;
+        self
+    }
 }
 
 /// Additional methods on `MonitorHandle` that are specific to Windows.

src/platform_impl/windows/event_loop.rs

@@ -82,7 +82,7 @@ lazy_static! {
 pub(crate) struct SubclassInput<T: 'static> {
     pub window_state: Arc<Mutex<WindowState>>,
     pub event_loop_runner: EventLoopRunnerShared<T>,
-    pub file_drop_handler: FileDropHandler,
+    pub file_drop_handler: Option<FileDropHandler>,
 }
 
 impl<T> SubclassInput<T> {

src/platform_impl/windows/mod.rs

@@ -14,11 +14,23 @@ pub use self::icon::WinIcon as PlatformIcon;
 use crate::event::DeviceId as RootDeviceId;
 use crate::icon::Icon;
 
-#[derive(Clone, Default)]
+#[derive(Clone)]
 pub struct PlatformSpecificWindowBuilderAttributes {
     pub parent: Option<HWND>,
     pub taskbar_icon: Option<Icon>,
     pub no_redirection_bitmap: bool,
+    pub drag_and_drop: bool,
+}
+
+impl Default for PlatformSpecificWindowBuilderAttributes {
+    fn default() -> Self {
+        Self {
+            parent: None,
+            taskbar_icon: None,
+            no_redirection_bitmap: false,
+            drag_and_drop: true,
+        }
+    }
 }
 
 unsafe impl Send for PlatformSpecificWindowBuilderAttributes {}

src/platform_impl/windows/window.rs

@@ -70,8 +70,9 @@ impl Window {
         //
         // done. you owe me -- ossi
         unsafe {
+            let drag_and_drop = pl_attr.drag_and_drop;
             init(w_attr, pl_attr, event_loop).map(|win| {
-                let file_drop_handler = {
+                let file_drop_handler = if drag_and_drop {
                     use winapi::shared::winerror::{OLE_E_WRONGCOMPOBJ, RPC_E_CHANGED_MODE, S_OK};
 
                     let ole_init_result = ole2::OleInitialize(ptr::null_mut());
@@ -80,7 +81,11 @@ impl Window {
                     if ole_init_result == OLE_E_WRONGCOMPOBJ {
                         panic!("OleInitialize failed! Result was: `OLE_E_WRONGCOMPOBJ`");
                     } else if ole_init_result == RPC_E_CHANGED_MODE {
-                        panic!("OleInitialize failed! Result was: `RPC_E_CHANGED_MODE`");
+                        panic!(
+                            "OleInitialize failed! Result was: `RPC_E_CHANGED_MODE`. \
+                            Make sure other crates are not using multithreaded COM library \
+                            on the same thread or disable drag and drop support."
+                        );
                     }
 
                     let file_drop_runner = event_loop.runner_shared.clone();
@@ -99,7 +104,9 @@ impl Window {
                         ole2::RegisterDragDrop(win.window.0, handler_interface_ptr),
                         S_OK
                     );
-                    file_drop_handler
+                    Some(file_drop_handler)
+                } else {
+                    None
                 };
 
                 let subclass_input = event_loop::SubclassInput {