Open Radar

 
Starting on iOS 13.5.1, an app which contains a WKWebView may consume CPU usage even when backgrounded.

Example code:

- (void)viewDidLoad {
  [super viewDidLoad];
 
  _webView = [[WKWebView alloc] initWithFrame:CGRectMake(0, 0, 400, 400)];
  _webView.navigationDelegate = self;
  [self.view addSubview:_webView];
 
  [self doJS];  
}

- (void)doJS {
  [_webView evaluateJavaScript:@"setTimeout({console.log('text')}, 10000);" completionHandler:^(id _Nullable arg, NSError * _Nullable error) {
    NSLog(@"Ran some JS!");
  }];
 
  dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
    [self doJS];
  });
}

Steps to reproduce:
1. Create a blank app and add the provided code to the ViewController.
2. Launch the app and then background it.

Expected behavior:
The app ceases using the battery and CPU.

Observed behavior:
The webview continues executing JS, using up both the battery and CPU.

After around 30 seconds of backgrounding on iOS 13.5.1, the following errors show up:

[ProcessSuspension] Background task expired while holding WebKit ProcessAssertion (isMainThread? 1).
2020-06-16 17:00:07.314584-0700 FoundationExperiments[1685:89122] [ProcessSuspension] 0x2802523d0 - WKProcessAssertionBackgroundTaskManager: Ignored request to start a new background task because the application is already in the background
2020-06-16 17:00:07.317894-0700 FoundationExperiments[1685:89122] [ProcessSuspension] 0x2802523d0 - WKProcessAssertionBackgroundTaskManager: Ignored request to start a new background task because the application is already in the background

Testing the provided code on iOS 13.3.1, the JS runs while backgrounded for ~15 seconds before suspending.