Add basic multi-window support in Cocoa

Prior to this commit, pywebview applications were limited to one webview
window. This fix enables webviews to open multiple windows on the Mac
platform, using familiar HTML/JavaScript techniques.

Requests to open new windows are handled by the WebPolicyDelegate of the
WebView. This means no changes to the existing API or thread model and no
spawning multiple windows in seperate threads. Utilises the multiwindow
capabilities of Cocoa on the main application thread itself.

Until now the BrowserView class has been a strict singleton. This is
changed by converting the :instance class property to a Python list
holding multiple BrowserView instances. [Thanks to jen@haverstack.com
for the idea. :)]. Class property active_instances is set to track the
number of open windows.

All existing API calls are routed to the initial window by calling them
on BrowserView.instances[0].

webview/cocoa.py

@@ -22,12 +22,13 @@
 
 
 class BrowserView:
-    instance = None
+    instances = []
+    active_instances = 0
     app = AppKit.NSApplication.sharedApplication()
 
     class AppDelegate(AppKit.NSObject):
         def applicationDidFinishLaunching_(self, notification):
-            BrowserView.instance.webview_ready.set()
+            BrowserView.instances[0].webview_ready.set()
 
     class WindowDelegate(AppKit.NSObject):
         def display_confirmation_dialog(self):
@@ -51,7 +52,11 @@ def windowShouldClose_(self, notification):
                 return Foundation.NO
 
         def windowWillClose_(self, notification):
-            BrowserView.app.stop_(self)
+            BrowserView.active_instances -= 1
+
+            # Quit app when the last window is destroyed
+            if BrowserView.active_instances <= 0:
+                BrowserView.app.stop_(self)
 
     class BrowserDelegate(AppKit.NSObject):
         def webView_contextMenuItemsForElement_defaultMenuItems_(self, webview, element, defaultMenuItems):
@@ -124,6 +129,34 @@ def webView_decidePolicyForNavigationAction_request_frame_decisionListener_(self
             # Normal navigation, allow
             listener.use()
 
+        def webView_createWebViewWithRequest_(self, webview, request):
+            """
+            This WebPolicyDelegate method is invoked when a link, a script or a
+            user tries to open a new window, i.e. via:
+                *  <a target="_blank" href="url">
+                *  JavaScript -> window.open(url, "_blank")
+                *  Right/Control click -> Open Link in New Window
+                   (Case 3 is unlikely here, since context menu is disabled)
+
+            :param webview: the webview that sent the message
+            :param request: the request to load
+
+            Due to an intentional change in WebKit, :request is always nil, and
+            this method is only expected to create a new window and return its
+            content webview. The proper request is send to the webview later as a
+            location-change navigation request.
+            """
+
+            # :url is intentionally empty as we don't want BrowserView.__init__()
+            # to load any url
+            new_browser = BrowserView(
+                    title='', url='', width=800, height=600,
+                    resizable=True, fullscreen=False,
+                    min_size=(200,100), webview_ready=None
+                    )
+            new_browser.show()
+            return new_browser.webkit
+
 
     class WebKitHost(WebKit.WebView):
         def performKeyEquivalent_(self, theEvent):
@@ -164,7 +197,7 @@ def performKeyEquivalent_(self, theEvent):
                     return handled
 
     def __init__(self, title, url, width, height, resizable, fullscreen, min_size, webview_ready):
-        BrowserView.instance = self
+        BrowserView.instances.append(self)
 
         self._file_name = None
         self._file_name_semaphor = threading.Semaphore(0)
@@ -195,20 +228,27 @@ def __init__(self, title, url, width, height, resizable, fullscreen, min_size, w
         self.window.setDelegate_(self._windowDelegate)
         BrowserView.app.setDelegate_(self._appDelegate)
 
-        self.load_url(url)
+        # Load request only if the url is non-empty
+        if url:
+            self.load_url(url)
 
         # Add the default Cocoa application menu
-        self._add_app_menu()
-        self._add_view_menu()
+        # We need to call these only once, during the app startup
+        if not BrowserView.active_instances:
+            self._add_app_menu()
+            self._add_view_menu()
 
         if fullscreen:
             self.toggle_fullscreen()
 
+        BrowserView.active_instances += 1
+
     def show(self):
         self.window.display()
         self.window.orderFrontRegardless()
-        BrowserView.app.activateIgnoringOtherApps_(Foundation.YES)
-        BrowserView.app.run()
+        if not BrowserView.app.isRunning():
+            BrowserView.app.activateIgnoringOtherApps_(Foundation.YES)
+            BrowserView.app.run()
 
     def destroy(self):
         BrowserView.app.stop_(self)
@@ -392,24 +432,24 @@ def create_window(title, url, width, height, resizable, fullscreen, min_size, co
 
 
 def create_file_dialog(dialog_type, directory, allow_multiple, save_filename):
-    return BrowserView.instance.create_file_dialog(dialog_type, directory, allow_multiple, save_filename)
+    return BrowserView.instances[0].create_file_dialog(dialog_type, directory, allow_multiple, save_filename)
 
 
 def load_url(url):
-    BrowserView.instance.load_url(url)
+    BrowserView.instances[0].load_url(url)
 
 
 def load_html(content, base_uri):
-    BrowserView.instance.load_html(content, base_uri)
+    BrowserView.instances[0].load_html(content, base_uri)
 
 
 def destroy_window():
-    BrowserView.instance.destroy()
+    BrowserView.instances[0].destroy()
 
 
 def toggle_fullscreen():
-    BrowserView.instance.toggle_fullscreen()
+    BrowserView.instances[0].toggle_fullscreen()
 
 
 def get_current_url():
-    return BrowserView.instance.get_current_url()
+    return BrowserView.instances[0].get_current_url()