objective-c - 在嵌套作用域中将委托(delegate)分配给 NSOpenPanel 会导致 EXC_BAD_ACCESS

Question

考虑以下简化的代码示例。它在 Swift 中呈现，但同样的行为也发生在 Objective-c 中。

import Foundation
import Cocoa
class MainWindow : NSWindow {
    @IBAction func onClick_openFile(sender : AnyObject?) {
        let path = runOpenPanel(false);
        NSLog(path as String)
    }

    @IBAction func onClick_crashyByeBye(sender : AnyObject?) {
        let path = runOpenPanel(true);
        NSLog(path as String)
    }

    private func runOpenPanel(useCrashyDelegate : Bool) -> NSString {
        let openPanel = NSOpenPanel.init()
        openPanel.canChooseDirectories = false
        openPanel.canChooseFiles = true
        openPanel.allowsMultipleSelection = false

        let safeDelegate = MyOpenPanelDelegate.init() //same scope as openPanel.runModal()--works fine
        if (useCrashyDelegate) {
            let crashyDelegate = MyOpenPanelDelegate.init() //falls out of scope before openPanel.runModal() and crashes
            openPanel.delegate = crashyDelegate
        } else {
            openPanel.delegate = safeDelegate
        }

        if (openPanel.runModal() == NSFileHandlingPanelOKButton && openPanel.URLs.count == 1) {
            return openPanel.URLs[0].path!
        }

        return ""
    }
}

class MyOpenPanelDelegate : NSObject, NSOpenSavePanelDelegate { 
    func panel(sender: AnyObject, shouldEnableURL url: NSURL) -> Bool {
        var isDir : ObjCBool = false
        if (NSFileManager.defaultManager().fileExistsAtPath(url.path!, isDirectory: &isDir)) {
            return isDir || (url.path! as NSString).lastPathComponent.lowercaseString == "foo.txt"
        }
        return false
    }
}

当 runOpenPanel 的 useCrashyDelegate 参数为 true 时，crashyDelegate 会在嵌套作用域中实例化，并在调用 openPanel.runModal()。由于打开的面板将 crashyDelegate 指定为其委托(delegate)，因此我预计crashyDelegate 的引用计数会增加。但是，当 useCrashyDelegate 为 true 时，应用程序会崩溃并显示 EXC_BAD_ACCESS。如果 useCrashyDelegate 为 false，则将在与调用 openPanel.runModal() 的调用相同的作用域中实例化的 safeDelegate 分配给 open面板，并且没有 EXC_BAD_ACCESS。

这让我相信 NSOpenPanel 不会增加其委托(delegate)的引用计数。这是预期的行为，还是可能是一个错误？

Answer 1

This is leading me to believe that NSOpenPanel is not incrementing its delegate's reference count. Is this the expected behavior, or might this be a bug?

这是预料之中的。检查属性的类型，你会看到它是 assign (Objective-C) 或 unsafe (Swift)，不保留强引用。这是 Cocoa 代表的常见设计模式。

HTH