使用 nil 必需属性保存上下文时，Core Data 自定义访问器崩溃

Question

阅读 this excellent article 关于 Swift 中自定义访问器的内容后，我重构了 NSDecimalNumber 以使用新的 Decimal 类型。我有一个相当复杂的模型，几天来一切正常，但如果我在必需的（非可选的）属性为零时保存托管对象上下文，我现在看到 EXC_BAD_ACCESS 崩溃， baseAmount 就我而言。直接使用NSDecimalNumber时，重构前保存对象会失败，但不会崩溃。

不确定这是否重要，但我正在使用 Magical Record 的功能保存上下文：NSManagedObjectContext.mr_default().mr_saveToPersistentStoreAndWait()

关于如何修改 baseAmount 访问器以实现与 Core Data 提供的相同的失败保存行为以使其不会崩溃的任何想法？

这是我的 NSManagedObject 子类的相关部分，后面是堆栈跟踪：

extension DFTransaction {

    @NSManaged private var primitiveBaseAmount: NSDecimalNumber

    var baseAmount: Decimal {
        get {
            willAccessValue(forKey: "baseAmount")
            defer { didAccessValue(forKey: "baseAmount") }
            return primitiveBaseAmount.decimalValue
        }
        set {
            willChangeValue(forKey: "baseAmount")
            defer { didChangeValue(forKey: "baseAmount") }
            primitiveBaseAmount = NSDecimalNumber(decimal: newValue)
        }
    }
}


[appname]`@objc DFTransaction.baseAmount.getter:
0x100343cd0 <+0>:   stp    x29, x30, [sp, #-16]!
0x100343cd4 <+4>:   mov    x29, sp
0x100343cd8 <+8>:   sub    sp, sp, #96               ; =96 
0x100343cdc <+12>:  stur   x0, [x29, #-32]
0x100343ce0 <+16>:  stur   x8, [x29, #-40]
0x100343ce4 <+20>:  bl     0x10051ca64               ; symbol stub for: objc_retain
0x100343ce8 <+24>:  sub    x8, x29, #24              ; =24 
0x100343cec <+28>:  ldur   x30, [x29, #-32]
0x100343cf0 <+32>:  str    x0, [sp, #48]
0x100343cf4 <+36>:  mov    x0, x30
0x100343cf8 <+40>:  bl     0x100343da4               ; [appname].DFTransaction.baseAmount.getter : __C.Decimal at DFTransaction.swift:64
0x100343cfc <+44>:  ldur   w9, [x29, #-24]
0x100343d00 <+48>:  ldurh  w10, [x29, #-20]
0x100343d04 <+52>:  ldurh  w11, [x29, #-18]
0x100343d08 <+56>:  ldurh  w12, [x29, #-16]
0x100343d0c <+60>:  ldurh  w13, [x29, #-14]
0x100343d10 <+64>:  ldurh  w14, [x29, #-12]
0x100343d14 <+68>:  ldurh  w15, [x29, #-10]
0x100343d18 <+72>:  ldurh  w16, [x29, #-8]
0x100343d1c <+76>:  ldurh  w17, [x29, #-6]
0x100343d20 <+80>:  ldur   x0, [x29, #-32]
0x100343d24 <+84>:  str    w9, [sp, #44]
0x100343d28 <+88>:  str    w10, [sp, #40]
0x100343d2c <+92>:  str    w11, [sp, #36]
0x100343d30 <+96>:  str    w12, [sp, #32]
0x100343d34 <+100>: str    w13, [sp, #28]
0x100343d38 <+104>: str    w14, [sp, #24]
0x100343d3c <+108>: str    w15, [sp, #20]
0x100343d40 <+112>: str    w16, [sp, #16]
0x100343d44 <+116>: str    w17, [sp, #12]
0x100343d48 <+120>: bl     0x10051ca58               ; symbol stub for: objc_release
0x100343d4c <+124>: ldr    w9, [sp, #44]
0x100343d50 <+128>: ldur   x8, [x29, #-40]
->  0x100343d54 <+132>: str    w9, [x8]    //EXC_BAD_ACCESS
0x100343d58 <+136>: ldr    w10, [sp, #40]
0x100343d5c <+140>: strh   w10, [x8, #4]
0x100343d60 <+144>: ldr    w11, [sp, #36]
0x100343d64 <+148>: strh   w11, [x8, #6]
0x100343d68 <+152>: ldr    w12, [sp, #32]
0x100343d6c <+156>: strh   w12, [x8, #8]
0x100343d70 <+160>: ldr    w13, [sp, #28]
0x100343d74 <+164>: strh   w13, [x8, #10]
0x100343d78 <+168>: ldr    w14, [sp, #24]
0x100343d7c <+172>: strh   w14, [x8, #12]
0x100343d80 <+176>: ldr    w15, [sp, #20]
0x100343d84 <+180>: strh   w15, [x8, #14]
0x100343d88 <+184>: ldr    w16, [sp, #16]
0x100343d8c <+188>: strh   w16, [x8, #16]
0x100343d90 <+192>: ldr    w17, [sp, #12]
0x100343d94 <+196>: strh   w17, [x8, #18]
0x100343d98 <+200>: mov    sp, x29
0x100343d9c <+204>: ldp    x29, x30, [sp], #16
0x100343da0 <+208>: ret

Answer 1

我在文章中提到 "Primitive accessors are always nullable Objective-C reference types"。这是一个不受 isOptional 设置影响的事实，正如您在这些崩溃中所发现的那样。

因此，原始访问器必须这样声明：

@NSManaged private var primitiveBaseAmount: NSDecimalNumber?

除此之外，您已将 baseAmount 访问器声明为 Decimal（非可选），因此您的自定义 get 实现必须在遇到一个 null 原始值，例如 return primitiveBaseAmount?.decimalValue ?? 0.

使用 nil 必需属性保存上下文时，Core Data 自定义访问器崩溃

Core Data custom accessor crash when saving context with nil required attribute

ios

swift

core-data

magicalrecord