swift - 主线程上的 BlockOperation 的 start()

Question

为什么在主线程上调用具有超过 1 个 block 的 BlockOperation 的 start() 而不是在主线程上调用其 block ？我的第一个测试总是通过，但第二个测试并非每次都通过 - 有时 block 不在主线程上执行

func test_callStartOnMainThread_executeOneBlockOnMainThread() {
    let blockOper = BlockOperation {
        XCTAssertTrue(Thread.isMainThread, "Expect first block was executed on Main Thread")
    }
    blockOper.start()
}
func test_callStartOnMainThread_executeTwoBlockOnMainThread() {
    let blockOper = BlockOperation {
        XCTAssertTrue(Thread.isMainThread, "Expect first block was executed on Main Thread")
    }
    blockOper.addExecutionBlock {
        XCTAssertTrue(Thread.isMainThread, "Expect second block was executed on Main Thread")
    }
    blockOper.start()
}

即使下一个代码也失败

func test_callStartOnMainThread_executeTwoBlockOnMainThread() {
    let asyncExpectation = expectation(description: "Async block executed")
    asyncExpectation.expectedFulfillmentCount = 2
    let blockOper = BlockOperation {
        XCTAssertTrue(Thread.isMainThread, "Expect first block was executed on Main Thread")
        asyncExpectation.fulfill()
    }
    blockOper.addExecutionBlock {
        XCTAssertTrue(Thread.isMainThread, "Expect second block was executed on Main Thread")
        asyncExpectation.fulfill()
    }
    OperationQueue.main.addOperation(blockOper)
    wait(for: [asyncExpectation], timeout: 2.0)
}

Answer 1

正如安德烈亚斯指出的那样，the documentation warns us :

Blocks added to a block operation are dispatched with default priority to an appropriate work queue. The blocks themselves should not make any assumptions about the configuration of their execution environment.

我们启动操作的线程以及队列的maxConcurrentOperationCount行为是在操作级别管理的，而不是在单个执行 block 中管理的一个手术。向现有操作添加 block 与向队列添加新操作不同。操作队列管理操作之间的关系，而不是操作内的 block 之间的关系。

通过让这些 block 做一些需要一点时间的事情可以暴露这个问题。考虑一个等待一秒钟的任务(您通常永远不会 sleep ，但我们这样做只是为了模拟一个缓慢的任务并表现出相关的行为)。我还添加了必要的“兴趣点”代码，以便我们可以在 Instruments 中观看此内容，这使得更容易可视化正在发生的情况:

import os.log
let pointsOfInterest = OSLog(subsystem: Bundle.main.bundleIdentifier!, category: .pointsOfInterest)

func someTask(_ message: String) {
    let id = OSSignpostID(log: pointsOfInterest)
    os_signpost(.begin, log: pointsOfInterest, name: "Block", signpostID: id, "Starting %{public}@", message)
    Thread.sleep(forTimeInterval: 1)
    os_signpost(.end, log: pointsOfInterest, name: "Block", signpostID: id, "Finishing %{public}@", message)
}

然后使用addExecutionBlock:

let queue = OperationQueue()          // you get same behavior if you replace these two lines with `let queue = OperationQueue.main`
queue.maxConcurrentOperationCount = 1

let operation = BlockOperation {
    self.someTask("main block")
}
operation.addExecutionBlock {
    self.someTask("add block 1")
}
operation.addExecutionBlock {
    self.someTask("add block 2")
}
queue.addOperation(operation)

现在，我将其添加到串行操作队列中(因为您永远不会向主队列添加阻塞操作...我们需要保持该队列空闲且响应灵敏)，但是如果您可以在 OperationQueue.main 上手动启动此操作。因此，底线是，虽然 start 将“立即在当前线程中”运行操作，但使用 addExecutionBlock 添加的任何 block 都只会在“适当的线程”上并行运行。工作队列”，不一定是当前线程。

如果我们在 Instruments 中观察这一点，我们可以看到，addExecutionBlock 不仅不一定遵守启动操作的线程，而且也不遵守队列的串行性质，或者， block 并行运行:

显然，如果您将这些 block 添加为单独的操作，那么一切都很好:

for i in 1 ... 3 {
    let operation = BlockOperation {
        self.someTask("main block\(i)")
    }
    queue.addOperation(operation)
}

产量: