Objective-c 块生命周期 ARC

Question

我对 ARC 下的块的生命周期感到困惑。我已经编写了一个单元测试来演示什么让我感到困惑。

- (void)testBlock {
    NSObject *testObject = [[NSObject alloc] init];
    CompletionBlock testBlock = ^{ NSLog(@"%@", testObject); };
    XCTAssertNotNil(testObject, @"testObject should not be nil");

    __weak NSObject *weakTestObject = testObject;
    @autoreleasepool {
        testObject = nil;
    }
    XCTAssertNotNil(weakTestObject, @"testObject should be retained by testBlock");

    @autoreleasepool {
        testBlock = nil;
    }
    //THIS IS THE FAILING TEST CASE
    XCTAssertNil(weakTestObject, @"testObject should have been released when testBlock was released");
}

我猜测这种行为与块在堆栈/堆中的存储方式有关。

更新！

将块设置为 nil 不会像我预期的那样释放块，因为它在堆栈上并且在超出范围之前不会被释放。强制块超出范围修复了我的测试。下面更新了代码。

- (void)testBlock {
    NSObject *testObject = [[NSObject alloc] init];
    __weak NSObject *weakTestObject = testObject;

    @autoreleasepool {
        CompletionBlock testBlock = ^{ NSLog(@"%@", testObject); };
        XCTAssertNotNil(testBlock, @"testBlock should not be nil");
        XCTAssertNotNil(testObject, @"testObject should not be nil");

        testObject = nil;
        XCTAssertNotNil(weakTestObject, @"testObject should be retained by testBlock");
        //testBlock goes out of scope here
    }
    XCTAssertNil(weakTestObject, @"testObject should have been released when testBlock was released");
}

Answer 1

块是在堆栈上创建的，只有在作用域退出时才会被销毁，这很像 C++ 堆栈分配的带有析构函数的对象。这些块免于引用计数，并将忽略 retain/release 消息。只有在被复制（通过Block_copy() 函数或copy 消息）之后，它们才成为可以保留和释放的普通堆分配对象。

在您的示例中，如果您将断言之前的所有代码包装在额外的大括号 { } 中，断言将开始工作，以便断言在块变量的范围结束后执行。