emscripten: Attempt at adding clipboard support.

This is not usable as-is, do not merge.

Reference Issue #13785.

src/video/emscripten/SDL_emscriptenclipboard.c

@@ -0,0 +1,164 @@
+/*
+  Simple DirectMedia Layer
+  Copyright (C) 1997-2025 Sam Lantinga <slouken@libsdl.org>
+
+  This software is provided 'as-is', without any express or implied
+  warranty.  In no event will the authors be held liable for any damages
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely, subject to the following restrictions:
+
+  1. The origin of this software must not be misrepresented; you must not
+     claim that you wrote the original software. If you use this software
+     in a product, an acknowledgment in the product documentation would be
+     appreciated but is not required.
+  2. Altered source versions must be plainly marked as such, and must not be
+     misrepresented as being the original software.
+  3. This notice may not be removed or altered from any source distribution.
+*/
+#include "SDL_internal.h"
+
+#ifdef SDL_VIDEO_DRIVER_EMSCRIPTEN
+
+#include "SDL_emscriptenvideo.h"
+#include "SDL_emscriptenclipboard.h"
+#include "../SDL_clipboard_c.h"
+#include "../../events/SDL_events_c.h"
+
+void Emscripten_InitClipboard(SDL_VideoDevice *device)
+{
+    MAIN_THREAD_EM_ASM_INT({
+        // !!! FIXME: we make sure `Module['SDL3']` exists like this all over. Can we move this to the "main" startup code?
+        if (typeof(Module['SDL3']) === 'undefined') {
+            Module['SDL3'] = {};
+        }
+
+        var SDL3 = Module['SDL3'];
+        SDL3.clipboard = {};
+        SDL3.clipboard.contents = [];
+        SDL3.clipboard.polling_id = -1;
+
+        // we don't touch the clipboard until the app explicitly requests it, because it pops
+        //  up a permissions dialog and might result in ongoing overhead. So we call this
+        //  function when we finally need to make contact with the outside world.
+        SDL3.clipboard.prepare = function() {
+            SDL3.clipboard.refresh = function() {
+                navigator.clipboard.read().then((clipboardItems) => {
+                    SDL3.clipboard.contents = [];
+                    for (clipboardItem of clipboardItems) {
+                        for (type of clipboardItem.types) {
+                            clipboardItem.getType(type).then((blob) => {
+                                blob.arrayBuffer().then((buffer) => {
+                                    console.log("Got a new clipboard buffer for type '" + type + "'");
+                                    SDL3.clipboard.contents[type] = new Uint8Array(buffer);
+                                });
+                            });
+                        }
+                    }
+                }).catch((err) => { /*console.error(err);*/ /* oh well, no clipboard update for you. */ });
+            };
+
+            SDL3.clipboard.refresh();  // get this started right now.
+
+            // There's a new "clipboardchanged" event proposed (as of September 2025). Try to use it.
+            if ('onclipboardchange' in navigator.clipboard) {
+                SDL3.clipboard.eventHandlerClipboardChange = function(event) {
+                    SDL3.clipboard.refresh();
+                };
+                navigator.clipboard.addEventListener('clipboardchange', SDL3.clipboard.eventHandlerClipboardChange);
+            } else {  // fall back to polling once per second. Gross.
+                SDL3.clipboard.polling_id = setInterval(SDL3.clipboard.refresh, 1000);
+            }
+
+            SDL3.clipboard.prepare = function() {};   // turn this into a no-op now that we're ready.
+        };
+    });
+}
+
+const char **Emscripten_GetTextMimeTypes(SDL_VideoDevice *device, size_t *num_mime_types)
+{
+    static const char *text_plain = "text/plain";
+    *num_mime_types = 1;
+    return &text_plain;
+}
+
+bool Emscripten_SetClipboardData(SDL_VideoDevice *device)
+{
+    MAIN_THREAD_EM_ASM({ var SDL3 = Module['SDL3']; SDL3.clipboard.contents = []; SDL3.clipboard.pending_contents = []; });
+
+    for (size_t i = 0; i < device->num_clipboard_mime_types; i++) {
+        const char *mime_type = device->clipboard_mime_types[i];
+        size_t clipboard_data_size = 0;
+        const void *clipboard_data = device->clipboard_callback(device->clipboard_userdata, mime_type, &clipboard_data_size);
+        if (clipboard_data && (clipboard_data_size > 0)) {
+            MAIN_THREAD_EM_ASM({
+                var mime_type = UTF8ToString($0);
+                var clipboard_data = $1;
+                var clipboard_data_size = $2;
+                var ui8array = new Uint8Array(Module.HEAPU8.subarray(clipboard_data, clipboard_data + clipboard_data_size));  // !!! FIXME: I _think_ this makes a copy of the data, not just a view into the heap.
+                var SDL3 = Module['SDL3'];
+                SDL3.clipboard.contents[mime_type] = ui8array;
+                SDL3.clipboard.pending_contents[mime_type] = new Blob([ui8array], { type: mime_type} );
+            }, mime_type, clipboard_data, clipboard_data_size);
+        }
+    }
+
+    MAIN_THREAD_EM_ASM({
+        var SDL3 = Module['SDL3'];
+        const clipboardItem = new ClipboardItem(SDL3.clipboard.pending_contents);
+        SDL3.clipboard.pending_contents = undefined;
+        navigator.clipboard.write([clipboardItem]).then(() => {
+            //console.log("We set the clipboard!");
+        }).catch((err) => { console.error(err); /* oh well, ignore it. */ });
+    });
+
+    return true;  // this is an async call, just pretend it worked, even if it didn't.
+}
+
+void *Emscripten_GetClipboardData(SDL_VideoDevice *device, const char *mime_type, size_t *length)
+{
+    if (!Emscripten_HasClipboardData(device, mime_type)) {
+        return NULL;
+    }
+
+    const size_t buflen = (size_t) MAIN_THREAD_EM_ASM_INT({ return Module['SDL3'].clipboard.contents[UTF8ToString($0)].byteLength; }, mime_type);
+    void *retval = SDL_malloc(buflen + 1);
+    if (retval) {
+        MAIN_THREAD_EM_ASM({
+            var ui8array = Module['SDL3'].clipboard.contents[UTF8ToString($0)];
+            var heapu8 = new Uint8Array(Module.HEAPU8.buffer, $1, $2);
+            heapu8.set(ui8array);
+        }, mime_type, retval, buflen);
+        ((char *)retval)[buflen] = '\0';  // make sure it's null-terminated.
+    }
+
+    return retval;
+}
+
+bool Emscripten_HasClipboardData(SDL_VideoDevice *device, const char *mime_type)
+{
+    return MAIN_THREAD_EM_ASM_INT({
+        var SDL3 = Module['SDL3'];
+        SDL3.clipboard.prepare();
+        return (SDL3.clipboard.contents[UTF8ToString($0)] !== undefined) ? 1 : 0;
+    }, mime_type);
+}
+
+void Emscripten_QuitClipboard(SDL_VideoDevice *device)
+{
+    MAIN_THREAD_EM_ASM({
+        var SDL3 = Module['SDL3'];
+        if (SDL3.clipboard.eventHandlerClipboardChange !== undefined) {
+            navigator.clipboard.removeEventListener('clipboardchange', SDL3.clipboard.eventHandlerClipboardChange);
+            SDL3.clipboard.eventHandlerClipboardChange = undefined;
+        }
+        if (SDL3.clipboard.polling_id != -1) {
+            clearInterval(SDL3.clipboard.polling_id);
+        }
+        SDL3.clipboard = undefined;
+    });
+}
+
+#endif // SDL_VIDEO_DRIVER_EMSCRIPTEN

src/video/emscripten/SDL_emscriptenclipboard.h

@@ -0,0 +1,41 @@
+/*
+  Simple DirectMedia Layer
+  Copyright (C) 1997-2025 Sam Lantinga <slouken@libsdl.org>
+
+  This software is provided 'as-is', without any express or implied
+  warranty.  In no event will the authors be held liable for any damages
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely, subject to the following restrictions:
+
+  1. The origin of this software must not be misrepresented; you must not
+     claim that you wrote the original software. If you use this software
+     in a product, an acknowledgment in the product documentation would be
+     appreciated but is not required.
+  2. Altered source versions must be plainly marked as such, and must not be
+     misrepresented as being the original software.
+  3. This notice may not be removed or altered from any source distribution.
+*/
+#include "SDL_internal.h"
+
+#ifndef SDL_emscriptenclipboard_h_
+#define SDL_emscriptenclipboard_h_
+
+typedef struct Emscripten_ClipboardData {
+    SDL_ClipboardDataCallback callback;
+    void *userdata;
+    const char **mime_types;
+    size_t mime_count;
+    Uint32 sequence;
+} SDLEmscripten_ClipboardData;
+
+extern void Emscripten_InitClipboard(SDL_VideoDevice *_this);
+extern const char **Emscripten_GetTextMimeTypes(SDL_VideoDevice *_this, size_t *num_mime_types);
+extern bool Emscripten_SetClipboardData(SDL_VideoDevice *_this);
+extern void *Emscripten_GetClipboardData(SDL_VideoDevice *_this, const char *mime_type, size_t *length);
+extern bool Emscripten_HasClipboardData(SDL_VideoDevice *_this, const char *mime_type);
+extern void Emscripten_QuitClipboard(SDL_VideoDevice *_this);
+
+#endif // SDL_emscriptenclipboard_h_

src/video/emscripten/SDL_emscriptenvideo.c

@@ -31,6 +31,7 @@
 #include "SDL_emscriptenframebuffer.h"
 #include "SDL_emscriptenevents.h"
 #include "SDL_emscriptenmouse.h"
+#include "SDL_emscriptenclipboard.h"
 
 #define EMSCRIPTENVID_DRIVER_NAME "emscripten"
 
@@ -184,6 +185,11 @@ static SDL_VideoDevice *Emscripten_CreateDevice(void)
     device->GL_SwapWindow = Emscripten_GLES_SwapWindow;
     device->GL_DestroyContext = Emscripten_GLES_DestroyContext;
 
+    device->GetTextMimeTypes = Emscripten_GetTextMimeTypes;
+    device->SetClipboardData = Emscripten_SetClipboardData;
+    device->GetClipboardData = Emscripten_GetClipboardData;
+    device->HasClipboardData = Emscripten_HasClipboardData;
+
     device->free = Emscripten_DeleteDevice;
 
     Emscripten_ListenSystemTheme();
@@ -392,6 +398,8 @@ bool Emscripten_VideoInit(SDL_VideoDevice *_this)
 
     Emscripten_RegisterGlobalEventHandlers(_this);
 
+    Emscripten_InitClipboard(_this);
+
     // We're done!
     return true;
 }
@@ -407,6 +415,7 @@ static void Emscripten_VideoQuit(SDL_VideoDevice *_this)
     Emscripten_UnregisterGlobalEventHandlers(_this);
     Emscripten_QuitMouse();
     Emscripten_UnlistenSystemTheme();
+    Emscripten_QuitClipboard(_this);
     pumpevents_has_run = false;
     pending_swap_interval = -1;
 }