iOS

addScriptMessageHandler 内存泄露

今天使用 addScriptMessageHandler 向 WKWebView 注入方法给 js 调用时发现有内存泄露问题。

出现问题的代码

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WKWebViewConfiguration *webViewConfiguration = [[WKWebViewConfiguration alloc] init];
WKUserContentController *userContentController = [[WKUserContentController alloc] init];
[userContentController addScriptMessageHandler:self name:@"nativeProcess"];
webViewConfiguration.userContentController= userContentController;
_webView = [[WKWebView alloc] initWithFrame:CGRectMake(0, 0, self.view.bounds.size.width, self.view.bounds.size.height) configuration:webViewConfiguration];

原因

调用 addScriptMessageHandler 时 userContentController 会 retain self。而 self 又间接 retain userContentController，形成了循环引用。

解决方案

搜了下，网上已经有解决方案了。后来在 cordova-plugin-wkwebview-engine 里也看到处理这个问题。这里记录下3个解决方案。

方案1

在适当的时候调用 removeScriptMessageHandlerForName 方法。这个方案缺点时是不好找到适当的时间点，比如该 viewController 被其他地方 dismiss 这时候就不好处理。

方案2

新建一个类来代理 self，这样 userContentController 就 retain 这个新类的对象，新类对象只是弱引用 self，这样循环引用就解开了。

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@interface CDVWKWeakScriptMessageHandler : NSObject <WKScriptMessageHandler>

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

- (instancetype)initWithScriptMessageHandler:(id<WKScriptMessageHandler>)scriptMessageHandler;

@end
@implementation CDVWKWeakScriptMessageHandler

- (instancetype)initWithScriptMessageHandler:(id<WKScriptMessageHandler>)scriptMessageHandler
{
    self = [super init];
    if (self) {
        _scriptMessageHandler = scriptMessageHandler;
    }
    return self;
}

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

@end
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// CDVWKWebViewEngine.m
CDVWKWeakScriptMessageHandler *weakScriptMessageHandler = [[CDVWKWeakScriptMessageHandler alloc] initWithScriptMessageHandler:self];

WKUserContentController* userContentController = [[WKUserContentController alloc] init];
[userContentController addScriptMessageHandler:weakScriptMessageHandler name:CDV_BRIDGE_NAME];

WKWebViewConfiguration* configuration = [self createConfigurationFromSettings:settings];
configuration.userContentController = userContentController;

WKWebView* wkWebView = [[WKWebView alloc] initWithFrame:self.engineWebView.frame configuration:configuration];
self.engineWebView = wkWebView;

方案3

该方案跟方案2基本一样，不过新类集成 NSProxy，并把所有接收到的方法都转发给 self。该方案使用于所有类似场景，更具通用性。

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//
//  WeakProxy.h
//
//  Created by Ashoka on 2020/5/30.
//

#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

@interface WeakProxy : NSProxy

+ (instancetype)weakProxy:(id)object;

@property (nonatomic, weak) id object;

@end

NS_ASSUME_NONNULL_END

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//
//  WeakProxy.m
//
//  Created by Ashoka on 2020/5/30.
//

#import "WeakProxy.h"

@implementation WeakProxy

+ (instancetype)weakProxy:(id)object {
    return [[WeakProxy alloc] initWithObject:object];
}

- (instancetype)initWithObject:(id)object {
    self.object = object;
    return self;
}

- (NSMethodSignature *)methodSignatureForSelector:(SEL)sel {
    return [self.object methodSignatureForSelector:sel];
}

- (void)forwardInvocation:(NSInvocation *)invocation {
    [invocation invokeWithTarget:self.object];
}

@end

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WKWebViewConfiguration *webViewConfiguration = [[WKWebViewConfiguration alloc] init];
WKUserContentController *userContentController = [[WKUserContentController alloc] init];
[userContentController addScriptMessageHandler:(id<WKScriptMessageHandler>)[WeakProxy weakProxy:self] name:@"nativeProcess"];
webViewConfiguration.userContentController= userContentController;
_webView = [[WKWebView alloc] initWithFrame:CGRectMake(0, 0, self.view.bounds.size.width, self.view.bounds.size.height) configuration:webViewConfiguration];