WKWebView causes my view controller to leak

Question

My view controller displays a WKWebView. I installed a message handler, a cool Web Kit feature that allows my code to be notified from inside the web page:

override func viewDidAppear(animated: Bool) {
    super.viewDidAppear(animated)
    let url = // ...
    self.wv.loadRequest(NSURLRequest(URL:url))
    self.wv.configuration.userContentController.addScriptMessageHandler(
        self, name: "dummy")
}

func userContentController(userContentController: WKUserContentController,
    didReceiveScriptMessage message: WKScriptMessage) {
        // ...
}

So far so good, but now I've discovered that my view controller is leaking - when it is supposed to be deallocated, it isn't:

deinit {
    println("dealloc") // never called
}

It appears that merely installing myself as a message handler causes a retain cycle and hence a leak!

Solution 1

Correct as usual, King Friday. It turns out that the WKUserContentController retains its message handler. This makes a certain amount of sense, since it could hardly send a message to its message handler if its message handler had ceased to exist. It's parallel to the way a CAAnimation retains its delegate, for example.

However, it also causes a retain cycle, because the WKUserContentController itself is leaking. That doesn't matter much on its own (it's only 16K), but the retain cycle and leak of the view controller are bad.

My workaround is to interpose a trampoline object between the WKUserContentController and the message handler. The trampoline object has only a weak reference to the real message handler, so there's no retain cycle. Here's the trampoline object:

class LeakAvoider : NSObject, WKScriptMessageHandler {
    weak var delegate : WKScriptMessageHandler?
    init(delegate:WKScriptMessageHandler) {
        self.delegate = delegate
        super.init()
    }
    func userContentController(userContentController: WKUserContentController,
        didReceiveScriptMessage message: WKScriptMessage) {
            self.delegate?.userContentController(
                userContentController, didReceiveScriptMessage: message)
    }
}

Now when we install the message handler, we install the trampoline object instead of self:

self.wv.configuration.userContentController.addScriptMessageHandler(
    LeakAvoider(delegate:self), name: "dummy")

It works! Now deinit is called, proving that there is no leak. It looks like this shouldn't work, because we created our LeakAvoider object and never held a reference to it; but remember, the WKUserContentController itself is retaining it, so there's no problem.

For completeness, now that deinit is called, you can uninstall the message handler there, though I don't think this is actually necessary:

deinit {
    println("dealloc")
    self.wv.stopLoading()
    self.wv.configuration.userContentController.removeScriptMessageHandlerForName("dummy")
}

Solution 2

The leak is caused by userContentController.addScriptMessageHandler(self, name: "handlerName") which will keep a reference to the message handler self.

To prevent leaks, simply remove the message handler via userContentController.removeScriptMessageHandlerForName("handlerName") when you no longer need it. If you add the addScriptMessageHandler at viewDidAppear, its a good idea to remove it in viewDidDisappear.

Solution 3

The solution posted by matt is just what's needed. Thought I'd translate it to objective-c code

@interface WeakScriptMessageDelegate : NSObject<WKScriptMessageHandler>

@property (nonatomic, weak) id<WKScriptMessageHandler> scriptDelegate;

- (instancetype)initWithDelegate:(id<WKScriptMessageHandler>)scriptDelegate;

@end

@implementation WeakScriptMessageDelegate

- (instancetype)initWithDelegate:(id<WKScriptMessageHandler>)scriptDelegate
{
    self = [super init];
    if (self) {
        _scriptDelegate = scriptDelegate;
    }
    return self;
}

- (void)userContentController:(WKUserContentController *)userContentController didReceiveScriptMessage:(WKScriptMessage *)message
{
    [self.scriptDelegate userContentController:userContentController didReceiveScriptMessage:message];
}

@end

Then make use of it like this:

WKUserContentController *userContentController = [[WKUserContentController alloc] init];    
[userContentController addScriptMessageHandler:[[WeakScriptMessageDelegate alloc] initWithDelegate:self] name:@"name"];

Solution 4

I've also noted that you also need to remove the message handler(s) during teardown, otherwise the handler(s) will still live on (even if everything else about the webview is deallocated):

WKUserContentController *controller = 
self.webView.configuration.userContentController;

[controller removeScriptMessageHandlerForName:@"message"];

Solution 5

Details

• Swift 5.1
• Xcode 11.6 (11E708)

Solution

based on Matt's answer

protocol ScriptMessageHandlerDelegate: class {
    func userContentController(_ userContentController: WKUserContentController, didReceive message: WKScriptMessage)
}

class ScriptMessageHandler: NSObject, WKScriptMessageHandler {

    deinit { print("____ DEINITED: \(self)") }
    private var configuration: WKWebViewConfiguration!
    private weak var delegate: ScriptMessageHandlerDelegate?
    private var scriptNamesSet = Set<String>()

    init(configuration: WKWebViewConfiguration, delegate: ScriptMessageHandlerDelegate) {
        self.configuration = configuration
        self.delegate = delegate
        super.init()
    }

    func deinitHandler() {
        scriptNamesSet.forEach { configuration.userContentController.removeScriptMessageHandler(forName: $0) }
        configuration = nil
    }
    
    func registerScriptHandling(scriptNames: [String]) {
        for scriptName in scriptNames {
            if scriptNamesSet.contains(scriptName) { continue }
            configuration.userContentController.add(self, name: scriptName)
            scriptNamesSet.insert(scriptName)
        }
    }

    func userContentController(_ userContentController: WKUserContentController,
                               didReceive message: WKScriptMessage) {
        delegate?.userContentController(userContentController, didReceive: message)
    }
}

Full Sample

Do not forget to paste the Solution code here

import UIKit
import WebKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        let button = UIButton(frame: CGRect(x: 0, y: 0, width: 100, height: 40))
        button.setTitle("WebView", for: .normal)
        view.addSubview(button)
        button.center = view.center
        button.addTarget(self, action: #selector(touchedUpInsed(button:)), for: .touchUpInside)
        button.setTitleColor(.blue, for: .normal)
    }
    
    @objc func touchedUpInsed(button: UIButton) {
        let viewController = WebViewController()
        present(viewController, animated: true, completion: nil)
    }
}

class WebViewController: UIViewController {

    private weak var webView: WKWebView!
    private var scriptMessageHandler: ScriptMessageHandler!
    private let url = URL(string: "http://google.com")!
    deinit {
        scriptMessageHandler.deinitHandler()
        print("____ DEINITED: \(self)")
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        let configuration = WKWebViewConfiguration()
        scriptMessageHandler = ScriptMessageHandler(configuration: configuration, delegate: self)
        let scriptName = "GetUrlAtDocumentStart"
        scriptMessageHandler.registerScriptHandling(scriptNames: [scriptName])

        let jsScript = "webkit.messageHandlers.\(scriptName).postMessage(document.URL)"
        let script = WKUserScript(source: jsScript, injectionTime: .atDocumentStart, forMainFrameOnly: true)
        configuration.userContentController.addUserScript(script)
        
        let webView = WKWebView(frame: .zero, configuration: configuration)
        self.view.addSubview(webView)
        self.webView = webView
        webView.translatesAutoresizingMaskIntoConstraints = false
        webView.topAnchor.constraint(equalTo: view.topAnchor).isActive = true
        webView.leftAnchor.constraint(equalTo: view.leftAnchor).isActive = true
        view.bottomAnchor.constraint(equalTo: webView.bottomAnchor).isActive = true
        view.rightAnchor.constraint(equalTo: webView.rightAnchor).isActive = true
        webView.load(URLRequest(url: url))
    }
}

extension WebViewController: ScriptMessageHandlerDelegate {
    func userContentController(_ userContentController: WKUserContentController, didReceive message: WKScriptMessage) {
        print("received \"\(message.body)\" from \"\(message.name)\" script")
    }
}

Info.plist

add in your Info.plist transport security setting

<key>NSAppTransportSecurity</key>
<dict>
    <key>NSAllowsArbitraryLoads</key>
    <true/>
</dict>

View more solutions

Comments

[matt]

"when you no longer need it" The problem is: when is that? Ideally it would be in your view controller's deinit (Objective-C dealloc), but it is never called because (wait for it) we are leaking! That is the problem that my trampoline solution solves. By the way, this same problem and this same solution continue on into iOS 9.

[siuying]

Its really depends on your use case. Say if you present it via presentViewController, the time is when you dismiss it. When you push it into a nav view controller, the time is when you pop it. It will not be deinit because WKWebView will never call deinit as it is retaining itself.

[siuying]

As I mentioned, if you called addScriptMessageHandler in viewDidAppear, do the opposite removeScriptMessageHandlerForName in viewDidDisapper will work.

[mkto]

can any kind soul translate this to objectivec equivalent codes?

[johan]

@mkto - Posted a obj-c version of the implementation.

[Alexis]

For me deinit actually never gets called unless I remove the script message handler in viewWillDisappear. Additionally now it's LeakAvoider that gets leaked.

[matt]

@SomaMan Yes, I'm still seeing it.

[SomaMan]

Though I find that I do in fact need to explicitly remove the scriptMessageHandler as well, funnily enough

[matt]

@SomaMan Yes, that might be true; the purpose of my trampoline solution is merely to allow the view controller's deinit to be called so that you have a place to do that. My code is here in case you want to play with it: github.com/mattneub/Programming-iOS-Book-Examples/blob/maste‌​r/…

[Adam Johns]

Could this also be solved by using a weak self in addScriptMessageHandler?

[matt]

@AdamJohns try it and see

[Adam Johns]

Just did - doesn't work. I'm not sure why though. If I don't call addScriptMessageHandler then I don't have a leak, so I know that call is what is causing my leak. Not sure how using weak self doesn't solve it since that is the case.

[matt]

I know it doesn't work. That's why I gave an answer that does work.

[Adam Johns]

Still trying to grasp why it doesn't work. If my WKUserContentController retains its message handler (self) which is causing the leak, shouldn't using weak self cause ARC not to increase reference count of my self. So when self's other sole referencer stops pointing to it, it should be released?

[Jonathan Zrake]

Why does the implementation allow the WKUserContentController instance to leak? Or, does it get GC'd if the web view is dead, and there are no more registered javascript message handlers?

[Jonathan Zrake]

Answered my own question: WKUserContentController does not leak, once its handlers are all deregistered.

[Emre Önder]

As I see now web view is deiniting but LeakAvoider object itself leaks. Is there any info about it?

[StackUnderflow]

Quite overkill solution, just call userContentController.removeScriptMessageHandler(String) on clean-up, thats it!

[Philipp Otto]

I agree with the last comment. This solution is just not necessary since it is really easy to fix the retain cycle by removing the handlers you added.

[Philipp Otto]

It would also be useful to put all the WKUserContentController stuff in a separate handler class. So the view controller can deinit normally and then tell the separate handler to clean up as well.

[matt]

@PhilippOtto Obviously, but the question is when.

[Philipp Otto]

@matt: If you use the WKUserContentController in the view controller itself you could use the viewDidDisappear. I prefer using an external handler. I added an answer to this question to show what I mean.

[Patrick]

Deinit is called for me and so I added removeScriptMessageHandler there and it worked.

[trees_are_great]

My deinit still wasn't being called, but that was because I has a text change listener also (not relating to the web view). I removed that listener and it started working again.