WKWebView 导致我的视​​图控制器泄漏

Question

我的视图控制器显示一个 WKWebView。我安装了一个消息处理程序，这是一个很酷的 Web Kit 功能，它允许我的代码从网页内部得到通知：

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) {
        // ...
}

到目前为止一切顺利，但现在我发现我的视图控制器正在泄漏 - 当它应该被释放时，它不是：

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

似乎仅仅将自己安装为消息处理程序会导致保留周期，从而导致泄漏！

Answer 1

像往常一样正确，星期五国王。事实证明，WKUserContentController 保留了它的消息处理程序。这在一定程度上是有道理的，因为如果它的消息处理程序不再存在，它几乎不能向它的消息处理程序发送消息。例如，它类似于 CAAnimation 保留其委托的方式。

但是，它也会导致一个保留循环，因为 WKUserContentController 本身正在泄漏。这本身并不重要（它只有 16K），但是视图控制器的保留周期和泄漏是不好的。

我的解决方法是在 WKUserContentController 和消息处理程序之间插入一个蹦床对象。 trampoline 对象只有对真实消息处理程序的弱引用，因此没有保留周期。这是蹦床对象：

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)
    }
}

现在当我们安装消息处理程序时，我们安装蹦床对象而不是self：

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

有效！现在调用deinit，证明没有泄漏。看起来这应该行不通，因为我们创建了 LeakAvoider 对象并且从未持有对它的引用；但请记住，WKUserContentController 本身会保留它，所以没有问题。

为了完整起见，现在调用了deinit，您可以在那里卸载消息处理程序，尽管我认为这实际上没有必要：

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

Answer 2

泄漏是由userContentController.addScriptMessageHandler(self, name: "handlerName") 引起的，它将保留对消息处理程序self 的引用。

为防止泄漏，只需在不再需要时通过userContentController.removeScriptMessageHandlerForName("handlerName") 删除消息处理程序即可。如果您在viewDidAppear 添加addScriptMessageHandler，最好将其删除到viewDidDisappear。

Answer 3

matt 发布的解决方案正是我们所需要的。以为我会把它翻译成objective-c代码

@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

然后像这样使用它：

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

Answer 4

我还注意到，您还需要在拆卸期间删除消息处理程序，否则处理程序仍将继续存在（即使有关 webview 的所有其他内容都已解除分配）：

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

[controller removeScriptMessageHandlerForName:@"message"];

Answer 5

详情

• 斯威夫特 5.1
• Xcode 11.6 (11E708)

解决方案

基于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)
    }
}

完整样本

不要忘记在此处粘贴解决方案代码

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

添加 Info.plist 传输安全设置

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

Answer 6

基本问题：WKUserContentController 拥有对添加到其中的所有 WKScriptMessageHandler 的强引用。您必须手动删除它们。

由于这仍然是 Swift 4.2 和 iOS 11 的问题，我想提出一个解决方案，它使用与持有 UIWebView 的视图控制器分开的处理程序。这样视图控制器可以正常地取消初始化并告诉处理程序进行清理。

这是我的解决方案：

UIViewController：

import UIKit
import WebKit

class MyViewController: JavascriptMessageHandlerDelegate {

    private let javascriptMessageHandler = JavascriptMessageHandler()

    private lazy var webView: WKWebView = WKWebView(frame: .zero, configuration: self.javascriptEventHandler.webViewConfiguration)

    override func viewDidLoad() {
        super.viewDidLoad()

        self.javascriptMessageHandler.delegate = self

        // TODO: Add web view to the own view properly

        self.webView.load(URLRequest(url: myUrl))
    }

    deinit {
        self.javascriptEventHandler.cleanUp()
    }
}

// MARK: - JavascriptMessageHandlerDelegate
extension MyViewController {
    func handleHelloWorldEvent() {

    }
}

处理程序：

import Foundation
import WebKit

protocol JavascriptMessageHandlerDelegate: class {
    func handleHelloWorld()
}

enum JavascriptEvent: String, CaseIterable {
    case helloWorld
}

class JavascriptMessageHandler: NSObject, WKScriptMessageHandler {

    weak var delegate: JavascriptMessageHandlerDelegate?

    private let contentController = WKUserContentController()

    var webViewConfiguration: WKWebViewConfiguration {
        for eventName in JavascriptEvent.allCases {
            self.contentController.add(self, name: eventName.rawValue)
        }

        let config = WKWebViewConfiguration()
        config.userContentController = self.contentController

        return config
    }

    /// Remove all message handlers manually because the WKUserContentController keeps a strong reference on them
    func cleanUp() {
        for eventName in JavascriptEvent.allCases {
            self.contentController.removeScriptMessageHandler(forName: eventName.rawValue)
        }
    }

    deinit {
        print("Deinitialized")
    }
}

// MARK: - WKScriptMessageHandler
extension JavascriptMessageHandler {
    func userContentController(_ userContentController: WKUserContentController, didReceive message: WKScriptMessage) {
        // TODO: Handle messages here and call delegate properly
        self.delegate?.handleHelloWorld()
    }
}

Answer 7

@matt 完美描述了视图控制器泄漏的原因，我建议使用指向 self 的弱指针并将其用作函数参数。

required init?(coder: NSCoder) {     
    super.init(coder: coder)
            
    self.weakSelf = self
}
...
webView.configuration.userContentController.add(weakSelf, name: "dummy")
...

private weak var weakSelf: WKScriptMessageHandler!

这解决了释放视图控制器的问题，但是如果你看一下Instruments->Leaks ;), webView 对象退出并保留计数=1。我做了一些研究并意识到传递给函数的引用类型（强或弱）并不重要，重要的是 - 你必须调用：

webView.configuration.userContentController.removeScriptMessageHandler(forName: "dummy")

我建议在 viewWillDisappear() 方法中这样做。