KVO on WKWebView

indoxan

I'm trying out Key-Value-Observing with WKWebView in objc_util.

class WebView(ui.View):

    def __init__(self):
        self.create_webview()

    @objc_util.on_main_thread
    def create_webview(self):
        self.webview = objc_util.ObjCClass("WKWebView").alloc().initWithFrame_(...)
        self.objc_instance.addSubview_(self.webview)

class View(ui.View):

    def __init__(self):

        self.webview = WebView()
        self.add_subview(self.webview)

        observer = objc_util.create_objc_class("observer", methods=[self.observeValueForKeyPath_ofObject_change_context_])

        self.webview.webview.addObserver_forKeyPath_options_context_(observer, "canGoBack", 0, None)
        self.webview.webview.addObserver_forKeyPath_options_context_(observer, "canGoForward", 0, None)
        ...

    def observeValueForKeyPath_ofObject_change_context_(_self, _cmd, _path, _obj, _change, _context):
        ...

I'm using something like this, but observeValueForKeyPath_ofObject_change_context_ doesn't seem to get called. Is there something wrong?

indoxan

@vivek101 Thank you for your welcome and for answering my question.

I tried the solution you showed, but it still seems that observeValueForKeyPath_ofObject_change_context_ is not called.
Also, when I specified a non-existent name in keyPath of addObserver, nothing happened as well.

I'm not familiar with Objective-C and there are many things I don't understand, but I have the following questions:

I understand that in create_objc_class the superclass defaults to NSObject.
When using observeValueForKeyPath_ofObject_change_context_ as a class method, shouldn't I use the classmethod argument of create_objc_class?

indoxan

I also tried programs like:

class WebView(ui.View):

    def __init__(self):
        self.create_webview()

    @objc_util.on_main_thread
    def create_webview(self):
        self.webview = objc_util.ObjCClass("WKWebView").alloc().initWithFrame_(...)
        self.objc_instance.addSubview_(self.webview)

class View(ui.View):

    def __init__(self):

        self.webview = WebView()
        self.add_subview(self.webview)

        observer_cls = objc_util.create_objc_class("observer_cls", methods=[self.observeValueForKeyPath_ofObject_change_context_])
        observer = objc_util.ObjCClass("observer_cls").alloc()

        self.webview.webview.addObserver_forKeyPath_options_context_(observer, "canGoBack", 0, None)
        self.webview.webview.addObserver_forKeyPath_options_context_(observer, "canGoForward", 0, None)
        ...

    def observeValueForKeyPath_ofObject_change_context_(_self, _cmd, _path, _obj, _change, _context):
            ...

observer is used as an instance of observer_cls.
But in this case an error occurs. _objc_exception.py looks like this:

Exception ignored on calling ctypes callback function: <bound method View.observeValueForKeyPath_ofObject_change_context_ of <__main__.View object at 0x116bbc180>>
TypeError: View.observeValueForKeyPath_ofObject_change_context_() takes 6 positional arguments but 7 were given

cvp

@indoxan Perhaps could you find some help/examples in @Mikael anchors which uses observer....

JonB

@indoxan you were on the right track with your second try. But you need to use
(self, _obj, _cmd, ...)

The python self gets eliminated, but the objc method needs to have two hidden arguments, _obj, _cmd. The error says that the system tried to call with 7 arguments, but your method (when converted to a function) only takes 6.

def observeValueForKeyPath_ofObject_change_context_(_self, _cmd, _path, _obj, _change, _context):
            
Should be

def observeValueForKeyPath_ofObject_change_context_(self, _self, _cmd, _path, _obj, _change, _context):

JonB

@indoxan another problem might be that you aren't creating an instance, you are creating a class in your original code. The second code called alloc but not init.

You should use create_ObjC_classs outside of the class, at the start of your script:

MyObserver = create_objc_class(...)

Then, inside init:

self.observer = MyObserver.alloc().init()

It is important that both your instance and class continue to exist in python after the scope ends, otherwise your object maybe garbage collected, which can cause crashes.

indoxan

@vivek101 @cvp @JonB

I was able to solve this problem by using ObjCDelegate of @mikael pythonista-achors for the time being. I will try other methods as well.

Thank you for your kind reply!

cvp

@indoxan 👍