macos - NSTextField 在 Swift 中绑定(bind)为 double，如何允许空白？

Question

我正在使用 NSTextField，我已通过 Storyboard 中的绑定(bind)将其绑定(bind)到整数。 Swift 代码如下所示:

@objc dynamic var quantity: Int = 0

这对于验证数字非常有用，如果尝试输入无效的内容(例如字母)，我会收到一条有用的消息。

我遇到的问题是，当您在未填写任何内容的情况下跳出字段时，应用程序会崩溃。根据错误消息，最相关的似乎是:

2019-03-09 16:29:46.813928+0000 ViewWebSocketLearning[34535:1969855] [General] [<ViewWebSocketLearning.OrderFormViewController 0x103131820> setNilValueForKey]: could not set nil as the value for the key show.
2019-03-09 16:29:46.825146+0000 ViewWebSocketLearning[34535:1969855] [General] (
    0   CoreFoundation                      0x00007fff3d7b6ded __exceptionPreprocess + 256
    1   libobjc.A.dylib                     0x00007fff69882720 objc_exception_throw + 48
    2   CoreFoundation                      0x00007fff3d7b6c1f +[NSException raise:format:] + 201
    3   Foundation                          0x00007fff3fbe8dbb -[NSObject(NSKeyValueCoding) setNilValueForKey:] + 81
    4   Foundation                          0x00007fff3fac1450 -[NSObject(NSKeyValueCoding) setValue:forKey:] + 331
    5   Foundation                          0x00007fff3faec38a -[NSObject(NSKeyValueCoding) setValue:forKeyPath:] + 271
    6   AppKit                              0x00007fff3af0bf03 -[NSBinder _setValue:forKeyPath:ofObject:mode:validateImmediately:raisesForNotApplicableKeys:error:] + 473
    7   AppKit                              0x00007fff3af0bccf -[NSBinder setValue:forBinding:error:] + 236
    8   AppKit                              0x00007fff3b54c376 -[NSValueBinder _applyObjectValue:forBinding:canRecoverFromErrors:handleErrors:typeOfAlert:discardEditingCallback:otherCallback:callbackContextInfo:didRunAlert:] + 225
    9   AppKit                              0x00007fff3b54c698 -[NSValueBinder applyDisplayedValueHandleErrors:typeOfAlert:canRecoverFromErrors:discardEditingCallback:otherCallback:callbackContextInfo:didRunAlert:error:] + 544
    10  AppKit                              0x00007fff3b54c81a -[NSValueBinder _applyDisplayedValueIfHasUncommittedChangesWithHandleErrors:typeOfAlert:discardEditingCallback:otherCallback:callbackContextInfo:didRunAlert:error:] + 105
    11  AppKit                              0x00007fff3b053e4e -[NSValueBinder validateAndCommitValueInEditor:editingIsEnding:errorUserInterfaceHandled:] + 460
    12  AppKit                              0x00007fff3b053c5a -[_NSBindingAdaptor _validateAndCommitValueInEditor:editingIsEnding:errorUserInterfaceHandled:bindingAdaptor:] + 175
    13  AppKit                              0x00007fff3b053b8d -[_NSBindingAdaptor validateAndCommitValueInEditor:editingIsEnding:errorUserInterfaceHandled:] + 240
    14  AppKit                              0x00007fff3af83e2b -[NSTextField textShouldEndEditing:] + 368
    15  AppKit                              0x00007fff3af44cc9 -[NSTextView(NSSharing) resignFirstResponder] + 499
    16  AppKit                              0x00007fff3ada2522 -[NSWindow _realMakeFirstResponder:] + 258
    17  AppKit                              0x00007fff3b058001 -[NSTextView(NSPrivate) _giveUpFirstResponder:] + 263

我将此解释为 NSTextField 绑定(bind)不接受 nil 条目，这显然是由于将该字段留空所致。

如何防止此异常发生？对于我的应用程序，在处理其他事情时将该字段留空是完全可以的。

Answer 1

通过将 NSTextFields 与绑定(bind)一起使用，Cocoa 使用键值编码方法 (KVC)，例如 setValue:forKey: Storyboard/XIB 中的绑定(bind)设置始终与 NSObject 子类对象( Controller )绑定(bind)。非对象值经过特殊处理:

If your key-value coding compliant object receives a setValue:forKey: message with nil passed as the value for a non-object property, the default implementation has no appropriate, generalized course of action. It therefore sends itself a setNilValueForKey: message, which you can override. The default implementation of setNilValueForKey: raises an NSInvalidArgumentException exception, but you can provide an appropriate, implementation-specific behavior.

https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/KeyValueCoding/HandlingNon-ObjectValues.html#//apple_ref/doc/uid/10000107i-CH5-SW1

在您的情况下 OrderFormViewController 没有重写 setNilValueForKey: 方法，并且引发了异常。

实现setNilValueForKey:将解决所有问题。

PS:使用 NSNumberFormatter 或绑定(bind)到 NSNumber 对象也可以解决该问题。