iOS viewDidLoad 中创建的局部变量释放时机

如下代码中，string 何时被释放：

- (void)viewDidLoad {
    [super viewDidLoad];
    NSString *string = [[NSString alloc] initWithFormat:@"Test AutoreleasePool"];
}

有很多地方说，在 runloop 进入休眠的时候释放，通过如下实验，可得出其实是在 viewDidLoad 大括号结束后就释放了。

测试方法：创建一个 weak 类型的对象指针，指向我们创建的 string 指向的地址，因为加载 VC 是先加载 viewDidLoad 再加载 viewWillAppear:，所以我们可以在 viewWillAppear: 中查看，这个对象释放被释放。得出的结论是，在viewWillAppear: 中访问时已经是 nil 了。

另一种测试方式，就是在 runloop 进入休眠时，打印一下这个 weak 对象的内容以及地址，发现在发出即将进入休眠通知时，已经释放了。

具体代码如下：

static ViewController *object;

@interface ViewController ()

@property (nonatomic, weak)NSString *myString;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    object = self;

    CFRunLoopObserverContext context = {0, (__bridge void *)(self), NULL, NULL, NULL};
    CFRunLoopObserverRef observer = CFRunLoopObserverCreate(kCFAllocatorDefault, kCFRunLoopAllActivities, YES, 0, &myRunLoopObserver, &context);
    
    NSRunLoop *myRunLoop = [NSRunLoop currentRunLoop];
    CFRunLoopRef cfRunLoop = [myRunLoop getCFRunLoop];
    CFRunLoopAddObserver(cfRunLoop, observer, kCFRunLoopDefaultMode);
    
    NSString *string = [[NSString alloc] initWithFormat:@"Test AutoreleasePool"];
    NSLog(@"string Text: %@", string);
    NSLog(@"string Text p: %p", string);

    
    self.myString = string;
    NSLog(@"testLabel Text: %@", self.myString);
    NSLog(@"string Text p: %p", self.myString);
    
    [NSMachPort port];
}

void myRunLoopObserver(CFRunLoopObserverRef observer, CFRunLoopActivity activity, void *info)
{
    switch(activity)
    {
        case kCFRunLoopEntry:
            NSLog(@"run loop entry");
            break;
        case kCFRunLoopBeforeTimers:
            NSLog(@"run loop before timers");
            break;
        case kCFRunLoopBeforeSources:
            NSLog(@"run loop before sources");
            break;
        case kCFRunLoopBeforeWaiting:
            NSLog(@"run loop before waiting -----------------------");
            NSLog(@"testLabel Text: %@", object.myString);
            NSLog(@"string Text p: %p", object.myString);
            NSLog(@"run loop before waiting -----------------------");
            break;
        case kCFRunLoopAfterWaiting:
            NSLog(@"run loop after waiting -----------------------");
            NSLog(@"testLabel Text: %@", object.myString);
            NSLog(@"string Text p: %p", object.myString);
            NSLog(@"run loop after waiting -----------------------");
            break;
        case kCFRunLoopExit:
            NSLog(@"run loop exit");
            break;
        default:
            break;
    }
}

- (void)viewWillAppear:(BOOL)animated {
    NSLog(@"viewWillAppear myString Text: %@", self.myString);
    NSLog(@"viewWillAppear myString Text: %p", self.myString);
}

具体运行结果如下：

2018-01-30 14:10:04.221671+0800   string Text: Test AutoreleasePool
2018-01-30 14:10:04.221805+0800   string Text p: 0x608000240930
2018-01-30 14:10:04.221929+0800   testLabel Text: Test AutoreleasePool
2018-01-30 14:10:04.222028+0800   string Text p: 0x608000240930
2018-01-30 14:10:04.222179+0800   viewWillAppear myString Text: (null)
2018-01-30 14:10:04.222271+0800   viewWillAppear myString Text: 0x0
2018-01-30 14:10:04.254927+0800   run loop before timers
2018-01-30 14:10:04.255037+0800   run loop before sources
2018-01-30 14:10:04.255182+0800   run loop before timers
2018-01-30 14:10:04.255279+0800   run loop before sources
2018-01-30 14:10:04.255396+0800   run loop before timers
2018-01-30 14:10:04.255475+0800   run loop before sources
2018-01-30 14:10:04.256013+0800   run loop before timers
2018-01-30 14:10:04.256115+0800   run loop before sources
2018-01-30 14:10:04.256213+0800   run loop before timers
2018-01-30 14:10:04.256313+0800   run loop before sources
2018-01-30 14:10:04.275536+0800   run loop before timers
2018-01-30 14:10:04.275646+0800   run loop before sources
2018-01-30 14:10:04.275754+0800   run loop before timers
2018-01-30 14:10:04.275847+0800   run loop before sources
2018-01-30 14:10:04.275943+0800   run loop before waiting -----------------------
2018-01-30 14:10:04.276025+0800   testLabel Text: (null)
2018-01-30 14:10:04.276107+0800   string Text p: 0x0
2018-01-30 14:10:04.276194+0800   run loop before waiting -----------------------
2018-01-30 14:10:04.290160+0800   run loop after waiting -----------------------
2018-01-30 14:10:04.290312+0800   testLabel Text: (null)
2018-01-30 14:10:04.290407+0800   string Text p: 0x0
2018-01-30 14:10:04.290495+0800   run loop after waiting -----------------------