UISplitViewController在iOS 9中是否存在保留周期錯誤？

Question

在下面的示例中，我將展示一個UIViewController作為其子UIStackViewController ：

UIViewController *splitViewParentVC = UIViewController.new;

UIViewController *masterVC = UIViewController.new;
UIViewController *detailVC = UIViewController.new;

UISplitViewController *splitViewController = [[UISplitViewController alloc] init];
splitViewController.viewControllers = @[masterVC, detailVC];

[splitViewParentVC addChildViewController:splitViewController];
[splitViewParentVC.view addSubview:splitViewController.view];
[splitViewController didMoveToParentViewController:splitViewParentVC];
splitViewController.view.frame = splitViewParentVC.view.bounds;
splitViewController.view.autoresizingMask = UIViewAutoresizingFlexibleHeight | UIViewAutoresizingFlexibleWidth;

__weak UISplitViewController *wSplitViewController = splitViewController;

[self presentViewController:splitViewParentVC animated:YES completion:nil];

dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
    [self dismissViewControllerAnimated:YES completion:^{
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
            if (wSplitViewController) {
                NSLog(@"the split view controller has leaked");
            } else {
                NSLog(@"the split view controller didn't leak");
            }
        });
    }];
});

在iOS 9和9.1中，上面的代碼將打印the split view controller has leaked ，表明UIStackViewController已泄漏（更重要的是，它也泄漏了其主視圖和詳細視圖控制器）。

Answer 1

是的，Apple員工已確認 iOS 9中存在保留周期錯誤。

我已經測試過iOS 8.4中不存在保留周期，但在iOS 9.0和9.1中確實存在。 從iOS 9.2（在iOS 9.2模擬器上的Xcode 7.2 beta 2中測試）看，漏洞似乎已得到修復。我已經整理了一個示例項目，以便輕松確認UISplitViewController是否導致自身泄漏（只需運行它並檢查控制台）輸出）。

這還測試了允許取消分配主視圖控制器和詳細視圖控制器的嘗試。 可以看出，即使從UISplitViewController.viewControllers數組屬性中刪除了主視圖控制器， UISplitViewController似乎仍然保留了它。

以下是示例項目中的代碼：

- (void)viewDidLoad {
    [super viewDidLoad];

    [self testSplitViewControllerRetainCycleWithCompletion:^{
        [self testManuallyFreeingUpMasterAndDetailViewControllers];
    }];
}

- (void)testSplitViewControllerRetainCycleWithCompletion:(void (^)())completion {

    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{

        UIViewController *splitViewParentVC = UIViewController.new;

        UIViewController *masterVC = UIViewController.new;
        UIViewController *detailVC = UIViewController.new;

        UISplitViewController *splitViewController = [[UISplitViewController alloc] init];
        splitViewController.viewControllers = @[masterVC, detailVC];
        splitViewController.preferredDisplayMode = UISplitViewControllerDisplayModeAllVisible;
        splitViewController.preferredPrimaryColumnWidthFraction = 0.3125; // 320 / 1024
        splitViewController.minimumPrimaryColumnWidth = 100;

        [splitViewParentVC addChildViewController:splitViewController];
        [splitViewParentVC.view addSubview:splitViewController.view];
        [splitViewController didMoveToParentViewController:splitViewParentVC];
        splitViewController.view.frame = splitViewParentVC.view.bounds;
        splitViewController.view.autoresizingMask = UIViewAutoresizingFlexibleHeight | UIViewAutoresizingFlexibleWidth;

        __weak UISplitViewController *wSplitViewController = splitViewController;
        __weak UIViewController *wMaster = masterVC;
        __weak UIViewController *wDetail = detailVC;

        [self presentViewController:splitViewParentVC animated:YES completion:nil];

        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
            [self dismissViewControllerAnimated:YES completion:^{
                dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
                    if (wSplitViewController) {
                        NSLog(@"the split view controller has leaked");
                    } else {
                        NSLog(@"the split view controller didn't leak");
                    }
                    if (wMaster) {
                        NSLog(@"the master view controller has leaked");
                    } else {
                        NSLog(@"the master view controller didn't leak");
                    }
                    if (wDetail) {
                        NSLog(@"the detail view controller has leaked");
                    } else {
                        NSLog(@"the detail view controller didn't leak");
                    }

                    completion();
                });
            }];
        });
    });
}

- (void)testManuallyFreeingUpMasterAndDetailViewControllers {

    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{

        UIViewController *splitViewParentVC = UIViewController.new;

        UIViewController *masterVC = UIViewController.new;
        UIViewController *detailVC = UIViewController.new;

        UISplitViewController *splitViewController = [[UISplitViewController alloc] init];
        splitViewController.viewControllers = @[masterVC, detailVC];
        splitViewController.preferredDisplayMode = UISplitViewControllerDisplayModeAllVisible;
        splitViewController.preferredPrimaryColumnWidthFraction = 0.3125; // 320 / 1024
        splitViewController.minimumPrimaryColumnWidth = 100;

        [splitViewParentVC addChildViewController:splitViewController];
        [splitViewParentVC.view addSubview:splitViewController.view];
        [splitViewController didMoveToParentViewController:splitViewParentVC];
        splitViewController.view.frame = splitViewParentVC.view.bounds;
        splitViewController.view.autoresizingMask = UIViewAutoresizingFlexibleHeight | UIViewAutoresizingFlexibleWidth;

        __weak UIViewController *wMaster = masterVC;
        __weak UIViewController *wDetail = detailVC;

        [self presentViewController:splitViewParentVC animated:YES completion:nil];

        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
            [self dismissViewControllerAnimated:YES completion:nil];

            splitViewController.viewControllers = @[UIViewController.new, UIViewController.new];

            dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
                if (wMaster) {
                    NSLog(@"the master view controller has STILL leaked even after an attempt to free it");
                } else {
                    NSLog(@"the master view controller didn't leak");
                }
                if (wDetail) {
                    NSLog(@"the detail view controller has STILL leaked even after an attempt to free it");
                } else {
                    NSLog(@"the detail view controller didn't leak");
                }
            });
        });
    });
}

更新：從iOS 9.2開始，泄漏似乎已得到修復（在iOS 9.2模擬器上的Xcode 7.2 beta 2中測試過）

Answer 2

據我所知-[UIViewController addChildViewController:]在iOS 9.0~9.1有內存泄漏問題。 所以我認為它不僅僅是UISplitViewController的錯誤。 Snipets如下，

- (void)viewDidLoad {
    [super viewDidLoad];
    MyFirstViewController *vc = [self.storyboard instantiateViewControllerWithIdentifier:@"MyFirstViewController"];
    [self addChildViewController:vc];
    [self.view addSubview:vc.view];
    [vc didMoveToParentViewController:self];
}

如果從當前視圖控制器撤退，你會發現沒有調用MyFirstViewController的dealloc。

一種可能的解決方法是在代碼中使用storyboard的Container View而不是addChildViewController 。 我確認Container View的子視圖控制器將被正確釋放。

另一種解決方法是removeChildViewController: in -(void)viewDidDisappear:(BOOL)animated 。 但是，正如蘋果公司的工作人員所說 ，不推薦這種解決方法。

- (void)viewDidDisappear:(BOOL)animated {
    [super viewDidDisappear:animated];
    NSArray<__kindof UIViewController *> * children = self.childViewControllers;
    for (UIViewController *vc in children) { // not recommended
        [vc willMoveToParentViewController:nil];
        [vc.view removeFromSuperview];
        [vc removeFromParentViewController];
    }
}