objective-c – 为什么这个ObjC块在发布时不释放它被捕获的引用？包括失败的单元测试

2023年6月30日 252次阅读

我遇到了一个问题,即在对象和块的所有引用都设置为nil之后,块中捕获的对象似乎不会被释放.

为了说明这个问题,我把这个非常简单的单元测试放在一起,它应该通过但不是：

/* 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？

最佳答案 您的captureBlock正在创建一个自动释放的对象(可能通过捕获,但可能是块本身).如果你把@autoreleasepool放在它周围,它会做你想要的：

  @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中放置@autoreleasepool(在创建exp之后和waitForExpectations之前……).对于要验证对象在池耗尽时解除分配的任何测试,您可以执行此操作.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];
}
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