__weak与__unsafe__unretained关键词

ownership 为__weak 的使用，对象的生命周期

- (void)test_weak_ownership {
    id obj1 = [NSObject new];
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-unsafe-retained-assign"
    // 这里会有警告, 用上面的会消除警告
    __weak id obj2 = [NSObject new];
    // obj1 输出正确的结果,obj2输出nil
    NSLog(@"obj1:%@, obj2:%@", obj1, obj2);
#pragma clang diagnostic pop
}
- (void)test_weak_ownership2 {
    id obj1 = [NSObject new];
    __weak id obj2 = obj1;
    NSLog(@"obj2:%@", obj2); // 一切正常
}

类似的__unsafe__unretained

- (void)test_unsafe_unretained_ownership {
    id obj1 = [NSObject new];
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-unsafe-retained-assign"
    // 这里会有警告, 用上面的会消除警告
    __unsafe_unretained id obj2 = [NSObject new];
    // obj2 野指针,极大概率会崩溃
    NSLog(@"obj1:%@, obj2:%@", obj1, obj2);
#pragma clang diagnostic pop
}
- (void)test_unsafe_unretained_ownership2 {
    id obj1 = [NSObject new];
    // 这里会有警告, 用上面的会消除警告
    __unsafe_unretained id obj2 = obj1;
    NSLog(@"obj2:%@", obj2); // 一切正常
}