How to create an instance variable that is shared across multiple operations

Question

I'm making a UITableView which display image created from processed data.

This processing task is too heavy, I decide to declare an array type instance variable of UITableView class and create an operation per one processing task.

Because Every results would be stored into separate memory space identified by index, I thought It is thread-safe.

A Short version example:

class TestOperation: Operation {
  var pi: [Int?];

  override var isExecuting: Bool { ... }

  override var isFinished: Bool { ... }

  init(_ p: inout [Int?]) {
    pi = p;
  }

  override func start() {
    if pi[0] == nil {
      pi[0] = 0;
    } else if pi[1] == nil {
      pi[1] = 1;
    } else {
      pi[2] = 2;
    }
  }
}

class Root {
  var sharedArr: [Int?];
  var queue: OperationQueue;

  init() {
    sharedArr = Array(repeating: nil, count: 3);
    queue = OperationQueue.init();

    for _ in 0...2 {
        let operation = TestOperation.init(&sharedArr);
        queue.addOperation(operation);
    }
  }
}

Root.init();

I expected sharedArr to change, But all I get is array of nil. Is there anything I can try?

DrPhill · Accepted Answer · 2018-09-05T07:12:12.843

Arrays in swift are structs and so passed by value. I see you are passing the array as inout, so you already know this. I have had this problem a couple of times and in the end I cheated.

Cheat 1: pass a reference to Root into TestOperation, and call a method on Root that adds the value to the shared array. You can make this cleaner with a protocol.

Cheat 2: Create a class that wraps the shared array and has methods to update the wrapped array.

In both cases I would pass the index to use into TestOperation instead of deducing it from the array contents. Deductions may be timing dependent.

I know that this is avoiding rather than solving the problem, but sometimes life is too short.

Here is the gist, untested:

protocol TestOperationProtocol: class {
    function setValue(index: Int, value: Int)
}

class TestOperation: Operation {
    weak var owner: TestOperationProtocol
    let index: Int

    override var isExecuting: Bool { ... }

    override var isFinished: Bool { ... }

    init(owner: TestOperationProtocol, index: Int) {
        self.owner = owner
        self.index = index
    }

    override func start() {
        // calculate value
        let value = self.index
        self.owner?.setValue(self.index, value)
    }
}

class Root: TestOperationProtocol {
    var sharedArr: [Int?];
    var queue: OperationQueue;

    init() {
        sharedArr = Array(repeating: nil, count: 3);
        queue = OperationQueue.init();

        for i in 0...sharedArr.count {
            let operation = TestOperation.init(self, index: index);
            queue.addOperation(operation);
        }
    }

    func setValue(index: Int, value: Int) {
        sharedArray[index] = value
    }
}

The protocol allows you to break a dependency cycle. TestOperation depends on TestOperationProtocol and not on Root. Root depends on TestOperation and its Protocol.

Thank you @DrPhill. Your first suggestion sounds really interesting to me, but I can't get fully understand. I am a swift newbie and not familiar with the protocol. If you don't mind, can you give me a little snippet? — snark, Sep 05 '18 at 01:16

How to create an instance variable that is shared across multiple operations

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