From 741745433d9c17b8b5541a14059545ccb5ac554a Mon Sep 17 00:00:00 2001
From: kinto <simone.scremin@gmail.com>
Date: Wed, 15 Mar 2017 09:51:44 +0100
Subject: [PATCH] manage preventDefaultAction flag inside the canvas-drag
 viewer event. Trigger the canvas-drag event before default drag happens on
 the viewer

---
 src/viewer.js | 48 +++++++++++++++++++++++++-----------------------
 1 file changed, 25 insertions(+), 23 deletions(-)

diff --git a/src/viewer.js b/src/viewer.js
index cea725a3..8f6d160d 100644
--- a/src/viewer.js
+++ b/src/viewer.js
@@ -2532,20 +2532,16 @@ function onCanvasDblClick( event ) {
 
 function onCanvasDrag( event ) {
     var gestureSettings;
-
-    if ( !event.preventDefaultAction && this.viewport ) {
-        gestureSettings = this.gestureSettingsByDeviceType( event.pointerType );
-        if( !this.panHorizontal ){
-            event.delta.x = 0;
-        }
-        if( !this.panVertical ){
-            event.delta.y = 0;
-        }
-        this.viewport.panBy( this.viewport.deltaPointsFromPixels( event.delta.negate() ), gestureSettings.flickEnabled );
-        if( this.constrainDuringPan ){
-            this.viewport.applyConstraints();
-        }
-    }
+    var canvasDragEventArgs = {
+        tracker: event.eventSource,
+        position: event.position,
+        delta: event.delta,
+        speed: event.speed,
+        direction: event.direction,
+        shift: event.shift,
+        originalEvent: event.originalEvent,
+        preventDefaultAction: event.preventDefaultAction
+    };
     /**
      * Raised when a mouse or touch drag operation occurs on the {@link OpenSeadragon.Viewer#canvas} element.
      *
@@ -2560,17 +2556,23 @@ function onCanvasDrag( event ) {
      * @property {Number} direction - Current computed direction, expressed as an angle counterclockwise relative to the positive X axis (-pi to pi, in radians). Only valid if speed > 0.
      * @property {Boolean} shift - True if the shift key was pressed during this event.
      * @property {Object} originalEvent - The original DOM event.
+     * @property {Boolean} preventDefaultAction - Set to true to prevent default drag behaviour. Default: false.
      * @property {?Object} userData - Arbitrary subscriber-defined object.
      */
-    this.raiseEvent( 'canvas-drag', {
-        tracker: event.eventSource,
-        position: event.position,
-        delta: event.delta,
-        speed: event.speed,
-        direction: event.direction,
-        shift: event.shift,
-        originalEvent: event.originalEvent
-    });
+    this.raiseEvent( 'canvas-drag', canvasDragEventArgs);
+    if ( !canvasDragEventArgs.preventDefaultAction && this.viewport ) {
+        gestureSettings = this.gestureSettingsByDeviceType( event.pointerType );
+        if( !this.panHorizontal ){
+            event.delta.x = 0;
+        }
+        if( !this.panVertical ){
+            event.delta.y = 0;
+        }
+        this.viewport.panBy( this.viewport.deltaPointsFromPixels( event.delta.negate() ), gestureSettings.flickEnabled );
+        if( this.constrainDuringPan ){
+            this.viewport.applyConstraints();
+        }
+    }
 }
 
 function onCanvasDragEnd( event ) {