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。
- 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。
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