项目常见崩溃9(陆续更新)

循环引用是一个大家熟悉有陌生的东西, 今天的崩溃也和循环引用有点关系.

崩溃堆栈

Thread 0 (crashed)
 0  Hago!__38-[HGSettingViewController viewDidLoad]_block_invoke [MFSettingViewController.m : 94 + 0x0]
    Found by: given as instruction pointer in context
 1  Hago!__38-[HGSettingViewController viewDidLoad]_block_invoke [MFSettingViewController.m : 93 + 0x3]
    Found by: previous frame's frame pointer
 2  libdispatch.dylib + 0x1795
    Found by: previous frame's frame pointer
 3  libdispatch.dylib + 0x1781
    Found by: previous frame's frame pointer
 4  libdispatch.dylib + 0x5d03
    Found by: previous frame's frame pointer
 5  CoreFoundation!__CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ + 0x7
    Found by: previous frame's frame pointer
 6  CoreFoundation!__CFRunLoopRun + 0x34f
    Found by: previous frame's frame pointer
 7  CoreFoundation!CFRunLoopRunSpecific + 0x1d5
    Found by: previous frame's frame pointer
 8  CoreFoundation!CFRunLoopRunInMode + 0x67
    Found by: previous frame's frame pointer
 9  GraphicsServices!GSEventRunModal + 0x4f
    Found by: previous frame's frame pointer
10  UIKit + 0x72a51
    Found by: previous frame's frame pointer
11  Hago!main [main.mm : 16 + 0xf]
    Found by: previous frame's frame pointer
12  libdyld.dylib + 0x34e9
    Found by: previous frame's frame pointer

定位到崩溃所在的93,94行代码

@implementation HGSettingViewController
{
    NSUInteger _totalFileSize;
}

- (void)dealloc {
    [[NSNotificationCenter defaultCenter] removeObserver:self];
}

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    
    self.title = PKLocalizedString(@"Setting");
    [self.view addSubview:self.tableView];
    [self.view addSubview:self.signoutButton];
    WEAKIFYSELF;
    [FileModel getCachedSize:^(NSUInteger size, NSUInteger count) {
        STRONGIFYSELF
        self->_totalFileSize = size;
        NSString *totalFileSize = [[NSString stringWithFormat:@"%.2fM", size/ 1024./1024.] stringByReplacingOccurrencesOfString:@"." withString:PKLocalizedString(@".")];
        self.cacheSizeLabel.text = totalFileSize;
        [self.tableView reloadData];
    }];
    self.remindSwitch = [[HGSwitch alloc] initWithFrame:CGRectZero];
    self.remindSwitch.on = [HGConfigModel loadPrivateUInt64:isAlertPushKey default:YES];//[MFPushModel isOn] ? @"已开启" : @"已关闭";
    self.remindSwitch.onTintColor = [UIColor colorWithValue:0xF6D859];
    [self.remindSwitch addTarget:self action:@selector(onRemindSwitchButton:) forControlEvents:UIControlEventTouchUpInside];
    
}

我们看到94行的代码确实有些辣眼睛, 这里使用了self->的语法, 但指针之不能为空的, 所以当self为nil, 这里就崩溃了, 但为什么self会为nil, 我们还可以再dig一下, 这里92行getCachedSize是一个异步的方法, 在block回调前, VC可能就已经被pop了, 这时候self就是nil了.
解决的办法:

解决的办法1: 在STRONGIFYSELF后判断下

if (!self) { return; } 

使用如下的宏

#define CHECK_STRONGIFY_SELF \
    __strong __typeof(self) self = weak_self; \
    if (!self) { return; }

有人说, 直接使用_totalFileSize不就行了?
Xcode会报warning, 对于有强迫症的人是绝对不能忍受的. 而且, 这会造成VC延时释放, 这几乎成了很多疑难杂症的万恶之源! VC都释放了, block里的操作还有什么意义? 只有极少数情况是需要在VC释放了继续处理block的, 而且通常是设计不合理造成的.

Block implicitly retains 'self'; explicitly mention 'self' to indicate this is intended behavior

有人说, 使用block里用weak, 不用strong

weak_self->_totalFileSize = size;

也不行, 会直接编译不过.

Dereferencing a __weak pointer is not allowed due to possible null value caused by race condition, assign it to strong variable first

解决办法2: 把成员变成属性

[FileModel getCachedSize:^(NSUInteger size, NSUInteger count) {
        STRONGIFYSELF
        self.totalFileSize = size;
        NSString *totalFileSize = [[NSString stringWithFormat:@"%.2fM", size / 1024. / 1024.] stringByReplacingOccurrencesOfString:@"." withString:PKLocalizedString(@".")];
        self.cacheSizeLabel.text = totalFileSize;
        [self.tableView reloadData];
    }];