Detect trackpad pinch and expose SDL_PINCH custom event

Represented PRs: libsdl-org#4, libsdl-org#5

README.md

@@ -11,6 +11,8 @@ This repository is Fractal's fork of SDL, with a few modifications. We forked SD
 
 - Allow `Command+W` to passthrough to the application on macOS, instead of being captured by the "Close Window" shortcut
 
+- Enable capturing macOS pinch gestures using Cocoa, to enable pinch-to-zoom on macOS trackpad devices
+
 - Created a GitHub Actions workflow, `build-and-publish-sdl.yml` to build, test and publish on Windows, macOS and Linux Ubuntu
 
 ## Development

include/SDL_events.h

@@ -139,6 +139,7 @@ typedef enum
     SDL_DOLLARGESTURE   = 0x800,
     SDL_DOLLARRECORD,
     SDL_MULTIGESTURE,
+    SDL_PINCH,
 
     /* Clipboard events */
     SDL_CLIPBOARDUPDATE = 0x900, /**< The clipboard changed */
@@ -503,6 +504,19 @@ typedef struct SDL_MultiGestureEvent
 } SDL_MultiGestureEvent;
 
 
+/**
+ *  \brief Finger Pinch Event (event.pinch.*)
+ */
+typedef struct SDL_PinchEvent
+{
+    Uint32 type;        /**< ::SDL_PINCH */
+    Uint32 timestamp;   /**< In milliseconds, populated using SDL_GetTicks() */
+    float magnification; /***< The magnification factor */
+    float scroll_amount; /***< Equivalent vertical scroll factor */
+    Uint32 windowID;    /**< The window underneath the pinch, if any */
+} SDL_PinchEvent;
+
+
 /**
  * \brief Dollar Gesture Event (event.dgesture.*)
  */
@@ -626,6 +640,7 @@ typedef union SDL_Event
     SDL_MultiGestureEvent mgesture;         /**< Gesture event data */
     SDL_DollarGestureEvent dgesture;        /**< Gesture event data */
     SDL_DropEvent drop;                     /**< Drag and drop event data */
+    SDL_PinchEvent pinch;                   /**< Trackpad pinch event data */
 
     /* This is necessary for ABI compatibility between Visual C++ and GCC.
        Visual C++ will respect the push pack pragma and use 52 bytes (size of

src/events/SDL_touch.c

@@ -475,4 +475,19 @@ SDL_TouchQuit(void)
     SDL_GestureQuit();
 }
 
+int
+SDL_SendPinch(SDL_Window* window, float magnification, float scroll_amount)
+{
+    int posted = 0;
+    if (SDL_GetEventState(SDL_PINCH) == SDL_ENABLE) {
+        SDL_Event event;
+        event.pinch.type = SDL_PINCH;
+        event.pinch.magnification = magnification;
+        event.pinch.scroll_amount = scroll_amount;
+        event.pinch.windowID = window ? SDL_GetWindowID(window) : 0;
+        posted = (SDL_PushEvent(&event) > 0);
+    }
+    return posted;
+}
+
 /* vi: set ts=4 sw=4 expandtab: */

src/events/SDL_touch_c.h

@@ -51,6 +51,8 @@ extern int SDL_SendTouch(SDL_TouchID id, SDL_FingerID fingerid, SDL_Window * win
 extern int SDL_SendTouchMotion(SDL_TouchID id, SDL_FingerID fingerid, SDL_Window * window,
                                float x, float y, float pressure);
 
+extern int SDL_SendPinch(SDL_Window* window, float magnification, float scroll_amount);
+
 /* Remove a touch */
 extern void SDL_DelTouch(SDL_TouchID id);
 

src/video/cocoa/SDL_cocoawindow.h

@@ -106,6 +106,7 @@ typedef enum
 -(void) touchesMovedWithEvent:(NSEvent *) theEvent;
 -(void) touchesEndedWithEvent:(NSEvent *) theEvent;
 -(void) touchesCancelledWithEvent:(NSEvent *) theEvent;
+-(void) magnifyWithEvent:(NSEvent *) theEvent;
 
 /* Touch event handling */
 -(void) handleTouches:(NSTouchPhase) phase withEvent:(NSEvent*) theEvent;

src/video/cocoa/SDL_cocoawindow.m

@@ -910,7 +910,7 @@ - (void)windowDidFailToEnterFullScreen:(NSNotification *)aNotification
 
     isFullscreenSpace = NO;
     inFullscreenTransition = NO;
-    
+
     [self windowDidExitFullScreen:nil];
 }
 
@@ -926,7 +926,7 @@ - (void)windowDidEnterFullScreen:(NSNotification *)aNotification
         pendingWindowOperation = PENDING_OPERATION_NONE;
         [self setFullscreenSpace:NO];
     } else {
-        /* Unset the resizable flag. 
+        /* Unset the resizable flag.
            This is a workaround for https://bugzilla.libsdl.org/show_bug.cgi?id=3697
          */
         SetWindowStyle(window, [nswindow styleMask] & (~NSWindowStyleMaskResizable));
@@ -967,16 +967,16 @@ when returning to windowed mode from a space (instead of using a pending
 - (void)windowDidFailToExitFullScreen:(NSNotification *)aNotification
 {
     SDL_Window *window = _data->window;
-    
+
     if (window->is_destroying) {
         return;
     }
 
     SetWindowStyle(window, (NSWindowStyleMaskTitled|NSWindowStyleMaskClosable|NSWindowStyleMaskMiniaturizable|NSWindowStyleMaskResizable));
-    
+
     isFullscreenSpace = YES;
     inFullscreenTransition = NO;
-    
+
     [self windowDidEnterFullScreen:nil];
 }
 
@@ -1403,6 +1403,17 @@ - (void)touchesCancelledWithEvent:(NSEvent *) theEvent
     [self handleTouches:NSTouchPhaseCancelled withEvent:theEvent];
 }
 
+- (void)magnifyWithEvent:(NSEvent *) theEvent
+{
+    // The scroll amount is relative to the magnification (a percentage)
+    //     and the current height of the window.
+    //     Visit `Cocoa_HandleMouseWheel` to understand the division by 80
+    // Ming and Suriya did some manual testing to find that dividing by 2
+    //     yields a more accurate zoom feel, so that's where the 2 comes from
+    CGFloat scroll_amount = [theEvent magnification] * _data->window->h / (80 * 2);
+    SDL_SendPinch(NULL, [theEvent magnification], scroll_amount);
+}
+
 - (void)handleTouches:(NSTouchPhase) phase withEvent:(NSEvent *) theEvent
 {
     NSSet *touches = [theEvent touchesMatchingPhase:phase inView:nil];
@@ -1760,7 +1771,7 @@ - (BOOL)acceptsFirstMouse:(NSEvent *)theEvent
     if (!(window->flags & SDL_WINDOW_OPENGL)) {
         return 0;
     }
-    
+
     /* The rest of this macro mess is for OpenGL or OpenGL ES windows */
 #if SDL_VIDEO_OPENGL_ES2
     if (_this->gl_config.profile_mask == SDL_GL_CONTEXT_PROFILE_ES) {
@@ -2257,7 +2268,7 @@ - (BOOL)acceptsFirstMouse:(NSEvent *)theEvent
 
         NSArray *contexts = [[data->nscontexts copy] autorelease];
         for (SDLOpenGLContext *context in contexts) {
-            /* Calling setWindow:NULL causes the context to remove itself from the context list. */            
+            /* Calling setWindow:NULL causes the context to remove itself from the context list. */
             [context setWindow:NULL];
         }
         [data->nscontexts release];