为什么这个 ObjC 块在释放时不释放其捕获的引用？包括失败的单元测试

Question

我遇到了一个问题，即在块中捕获的对象似乎没有被释放，即使对对象和块的所有引用都已设置为 nil。

为了说明这个问题，我整理了这个非常简单的单元测试，应该通过但没有：

/* Headers */

@interface BlockTestTests : XCTestCase

@end

// A simple class that calls a callback when it's deallocated
@interface Dummy : NSObject

@property (nonatomic, copy) void(^deallocCallback)();

@end

/* Implementation */

@implementation BlockTestTests

- (void)testExample {
    XCTestExpectation *exp = [self expectationWithDescription:@"strong reference should be deallocated when its capturing block is released"];

    Dummy *dummy = [Dummy new];

    dummy.deallocCallback = ^{
        [exp fulfill];
    };

    void(^capturingBlock)() = ^{
        // Captures a strong reference to the dummy
        id capturedStrongReference = dummy;
    };

    capturingBlock = nil;
    dummy = nil;

    // At this point we would expect that all references to the
    // object have been cleared and it should get deallocated.
    // Just to be safe, we even wait 2 seconds, but it never happens...        

    [self waitForExpectationsWithTimeout:2.0 handler:nil];
}

@end

@implementation Dummy

- (void)dealloc {
    _deallocCallback();
}

@end

你能告诉我为什么这个测试失败了吗？

Answer 1

您的capturingBlock 正在创建一个自动释放的对象（可能通过捕获，但也可能是块本身）。如果你在它周围加上@autoreleasepool，它会做你想做的事：

  @autoreleasepool {
    void(^capturingBlock)() = ^{
      // Captures a strong reference to the dummy
      id capturedStrongReference = dummy;
    };
    capturingBlock = nil;
    dummy = nil;
  }

更一致的方法是在整个测试周围放置一个@autoreleasepool（在创建exp 之后和waitForExpectations... 之前）。对于任何要验证对象在池耗尽时已被释放的测试，您都可以这样做。像这样：

- (void)testExample {
  XCTestExpectation *exp = [self expectationWithDescription:@"strong reference should be deallocated when its capturing block is released"];

  @autoreleasepool {
    Dummy *dummy = [Dummy new];

    dummy.deallocCallback = ^{
      [exp fulfill];
    };

    void(^capturingBlock)() = ^{
      // Captures a strong reference to the dummy
      id capturedStrongReference = dummy;
    };
    capturingBlock = nil;
    dummy = nil;
  }

  [self waitForExpectationsWithTimeout:2.0 handler:nil];
}