Receiving NSDraggingDestination messages with a WKWebView - objective-c

Question

I'm porting an OS X app which was using WebView to using WKWebView, the new "modern WebKit API" introduced in OS X Yosemite. My previous WebView subclass supported dropping files onto it by first calling [self registerForDraggedTypes:#[NSFilenamesPboardType]] and then simply implementing - (BOOL)performDragOperation:(id < NSDraggingInfo >)sender.
This doesn't work with the new WKWebView, as performDragOperation never gets called, nor do any of the NSDraggingDestination protocol methods that I tried.
I also tried making a parent NSView implement the protocol, and I'm still not getting the messages. Removing the WKWebView from the hierarchy makes the parent NSView receive those messages.
I also tried implementing the WKNavigationDelegate protocol to prevent the default drop behaviour of WKWebView to happen and this didn't change a thing either.
Edit: Upon further inspection (suggested by Scott Kyle / #appden on twitter), a private class WKView that implements the NSDraggingDestination protocol is a subview of the WKWebView. My code should likely try to get the dragging notifications before the WKView sees them and acts on them.

Answer 1

The only solution I found, thanks to Scott Kyle, was to replace the performDragOperation: method defined on WKView with my own implementation using object_getClass, class_getInstanceMethod and method_exchangeImplementations from the Objective-C runtime. Hopefully, in the future, the WKUIDelegate protocol will be extended to support custom hooks into the drag-and-drop protocol as implemented privately by WKWebView.
Here's an example showing how we exchange the implementation of the nested view's performDragOperation: with our own from our WKWebView subclass:
// Override the performDragOperation: method implemented on WKView so that we may get drop notification.
var originalMethod = class_getInstanceMethod(object_getClass(subviews[0]), "performDragOperation:")
var overridingMethod = class_getInstanceMethod(object_getClass(self), "performDragOperation:")
method_exchangeImplementations(originalMethod, overridingMethod)
And then the implementation where we delegate to a dropDelegate object.
override func performDragOperation(sender: NSDraggingInfo) -> Bool {
let myWebView = superview as MyWebView
if let dropDelegate = myWebView.dropDelegate {
return dropDelegate.webView(myWebView, performDragOperation: sender)
}
return false
}

Answer 2

I needed to retain WKWebView's handling of drags, but be able to introspect and modify pasteboards before it received them. Subclassing WKView is not really helpful since all of WKWebView's classes would still use the original, so I used the runtime swizzle proposed earlier + a class extension.
import WebKit
import Foundation
import ObjectiveC
extension WKView {
func shimmedPerformDragOperation(sender: NSDraggingInfo) -> Bool {
var pboard = sender.draggingPasteboard()
if let items = pboard.pasteboardItems {
for item in items {
if let types = item.types? {
for type in types {
if let value = item.stringForType(type.description) {
NSLog("DnD type(\(type)): \(value)")
}
}
}
}
}
return self.shimmedPerformDragOperation(sender) //return pre-swizzled method
}
}
var webview = WKWebView(frame: CGRectZero, configuration: WkWebViewConfiguration())
var wkview = (webview.subviews.first as WKView) // 1 per frame?
var origDnD = class_getInstanceMethod(WKView.self, "performDragOperation:")
var newDnD = class_getInstanceMethod(WKView.self, "shimmedPerformDragOperation:")
method_exchangeImplementations(origDnD, newDnD)
You do need to add a small snippet to your bridging header to extend WKView:
#import WebKit;
#interface WKView : NSView <NSTextInputClient> {
}
- (BOOL)performDragOperation:(id <NSDraggingInfo>)draggingInfo;
#end