iOS 内存泄漏三两事

相信大家都有过重写 dealloc 方法来检查某个 view controller 在消失后是否被释放的经历。这几乎是 iOS 中寻找由于引用循环造成内存泄漏最有效的方法了。基本上每次发布，都会做很多次这种事情。不得不说这件事情很无聊，并且很可能会出错。如果我们在日常的开发中, 提前的学习相关的知识, 那该多好？

下面是两个很少见的 UIViewController的属性:

• isBeingDismissed 当一个模态推送出来的 view controller 正在消失的时候, 为: true.
• isMovingFromParentViewController ,当一个 view controller 正在从它的父 view contrlller 中移除的时候(包括从系统的容器试图比如说 UINavigationController), 为true.

如果这两个属性有一个是 true 的话, 这个 view controller 就会自动的被释放掉。我们不知道一个 view contrller 完成内部状态的改变，并且被 ARC 释放掉需要耗费多长的时间。为了简单起见，我们假设它不会超过两秒。

1.现在看看下面的代码(文末会有OC版):

extension UIViewController {
    public func dch_checkDeallocation(afterDelay delay: TimeInterval = 2.0) {
        let rootParentViewController = dch_rootParentViewController

        // We don’t check `isBeingDismissed` simply on this view controller because it’s common
        // to wrap a view controller in another view controller (e.g. in UINavigationController)
        // and present the wrapping view controller instead.
        if isMovingFromParentViewController || rootParentViewController.isBeingDismissed {
            let type = type(of: self)
            let disappearanceSource: String = isMovingFromParentViewController ? "removed from its parent" : "dismissed"

            DispatchQueue.main.asyncAfter(deadline: .now() + delay, execute: { [weak self] in
                assert(self == nil, "\(type) not deallocated after being \(disappearanceSource)")
            })
        }
    }

    private var dch_rootParentViewController: UIViewController {
        var root = self

        while let parent = root.parent {
            root = parent
        }

        return root
    }
}

在延时操作这个闭包中，我们首先通过 [weak self] 来避免这个闭包强引用self。然后通过断言让程序在 self 不为空的时候抛出异常。只有存在循环引用的情况下这个 view controller 才不为空。

现在我们需要做的就是在 viewDidDisappear 中调用这个方法。只要是你需要检查它在消失后是不是被释放掉的 view controller 都需要添加这个方法。

override func viewDidDisappear(_ animated: Bool) {
    super.viewDidDisappear(animated)

    dch_checkDeallocation()
}

如果发声了内存泄漏，我们就会得到下面的断言:

这个时候，我们只需要打开 Xcode 的 Memory Graph Debugger 找到并且解决这些循环引用。

1. 另外在 twitter 上也看到了类似的解决方案。

3.使用国人写的 MLeaksFinder 在每次发生内存泄漏的时候都会弹窗。并且没有代码侵入性，只需要使用 CocosPod 导入就可以了。

4.在使用图片资源的时候，少使用 imageNamed: 方法去获取使用频次不高的图片资源。因为使用 imageNamed:加载的图片资源会一直存在内存里面， 对内存的浪费也是巨大的。

5.上面的方法写了一个 OC 版本的：

.h:

#import <UIKit/UIKit.h>

@interface UIViewController (FindLeaks)


// 默认为 NO
@property (nonatomic) BOOL noCheckLeaks;

@end

.m:

//
//  UIViewController+FindLeaks.m
//  Leaks
//
//  Created by sunny on 2017/8/27.
//  Copyright © 2017年 CepheusSun. All rights reserved.
//

#import "UIViewController+FindLeaks.h"
#import <objc/runtime.h>

static const char *noCheckLeaksKey = "noChechLeaksKey";

@interface NSObject (MethodSwizzling)

+ (void)sy_swizzleInstanceSelector:(SEL)origSelector
                   swizzleSelector:(SEL)swizzleSelector;

@end

@implementation UIViewController (FindLeaks)

#pragma mark - Binding Property
- (BOOL)noCheckLeaks {
    return [objc_getAssociatedObject(self, noCheckLeaksKey) boolValue];
}

- (void)setNoCheckLeaks:(BOOL)noCheckLeaks {
    objc_setAssociatedObject(self, noCheckLeaksKey, [NSNumber numberWithBool:noCheckLeaks], OBJC_ASSOCIATION_RETAIN_NONATOMIC);

}

#pragma mark - Check
+ (void)load {
    
#if DEBUG
    [self sy_swizzleInstanceSelector:@selector(viewDidDisappear:) swizzleSelector:@selector(fl_viewDidDisappear:)];
#endif
}

- (void)fl_viewDidDisappear:(BOOL)animated {
    [self fl_viewDidDisappear:animated];
    if (!self.noCheckLeaks) {
        [self fl_checkDeallocationAfterDelay:2];
    }
}

- (void)fl_checkDeallocationAfterDelay:(NSTimeInterval)delay {
    UIViewController *root = [self fl_rootParentViewController];
    if (self.isMovingFromParentViewController || root.isBeingDismissed) {
        NSString *type = NSStringFromClass([self class]);
        NSString *disappearanceSource = self.isMovingFromParentViewController ? @"removed from its parent" : @"dismissed";
        __weak typeof(self) weakSelf = self;
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delay * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
            NSString *assert = [NSString stringWithFormat:@"%@ not deallocated after being %@",
             type, disappearanceSource];
            NSAssert(weakSelf == nil,assert);
        });
    }
}

- (UIViewController *)fl_rootParentViewController {
    UIViewController *root = self;
    while (root.parentViewController) {
        root = root.parentViewController;
    }
    return root;
}

@end

@implementation NSObject (MethodSwizzling)

+ (void)sy_swizzleInstanceSelector:(SEL)origSelector
                   swizzleSelector:(SEL)swizzleSelector {
    
    Method origMethod = class_getInstanceMethod(self, origSelector);
    Method swizzleMethod = class_getInstanceMethod(self, swizzleSelector);
    
    BOOL isAdd = class_addMethod(self, origSelector, method_getImplementation(swizzleMethod), method_getTypeEncoding(swizzleMethod));
    
    if (!isAdd) {
        method_exchangeImplementations(origMethod, swizzleMethod);
    }else {
        class_replaceMethod(self, swizzleSelector, method_getImplementation(origMethod), method_getTypeEncoding(origMethod));
    }
}


@end

只需要在不需要检查的方法中设置属性为 YES 就好了。