Swift: Downcast导致应用程序崩溃，为什么？

Question

我有以下代码片段在某些设备上崩溃：

Crashed: com.apple.root.default-qos
EXC_BAD_ACCESS KERN_INVALID_ADDRESS at 0x00000000cbd

代码：

var obj:AnyObject = command.arguments[0] as AnyObject!
var theData:AnyObject = obj["getContactImagesByEmails"] as AnyObject!

if let contactImagesByEmails:AnyObject = obj["emails"]{
   if contactImagesByEmails is Array<String>{
 /*line 176*/  let array:Array<String> = 
                  contactImagesByEmails as Array<String> // CRASH happens here

       results = WmSqliteImagesModel.getInstance.getImagesByEmailAsWmContactImage(array) as Dictionary<String,AnyObject>
            }
        }

完整堆栈跟踪

Thread : Crashed: com.apple.root.default-qos
0  libswiftCore.dylib             0x0000000100559794 swift_unknownRetain + 32
1  MyApp                       0x000000010017c8a0 MyApp.Plugin.(getContactImagesByEmails (MyApp.Plugin) -> (ObjectiveC.CDVInvokedUrlCommand) -> ()).(closure #1) (Plugin.swift:176)
2  MyApp                       0x000000010017c8a0 MyApp.Plugin.(getContactImagesByEmails (MyApp.Plugin) -> (ObjectiveC.CDVInvokedUrlCommand) -> ()).(closure #1) (Plugin.swift:176)
3  MyApp                       0x00000001001790b0 partial apply forwarder for reabstraction thunk helper from @callee_owned () -> (@unowned ()) to @callee_owned (@in ()) -> (@out ()) with unmangled suffix "125" (Plugin.swift:62)
4  MyApp                       0x0000000100179120 partial apply forwarder for reabstraction thunk helper from @callee_owned (@in ()) -> (@out ()) to @callee_owned () -> (@unowned ()) with unmangled suffix "128" (Plugin.swift:62)
5  libdispatch.dylib              0x00000001937e13ac _dispatch_call_block_and_release + 24
6  libdispatch.dylib              0x00000001937e136c _dispatch_client_callout + 16
7  libdispatch.dylib              0x00000001937ed40c _dispatch_root_queue_drain + 1152
8  libdispatch.dylib              0x00000001937ee75c _dispatch_worker_thread3 + 108
9  libsystem_pthread.dylib        0x00000001939bd2e4 _pthread_wqthread + 816

Plugin.swift:176 指向：

let array:Array<String> = contactImagesByEmails as Array<String>

我错过了什么吗？我认为这段代码应该是安全的。

如果contactImagesByEmails is Array<String>returnstrue，为什么contactImagesByEmails as Array<String>失败了？

请帮忙，

[编辑]

command 的类型为 CDVInvokedUrlCommand

@interface CDVInvokedUrlCommand : NSObject {
    NSString* _callbackId;
    NSString* _className;
    NSString* _methodName;
    NSArray* _arguments;
}

Answer 1

我不知道这是否真的可以解决问题，但在 2 if 和后面的 let 中有一点冗余。可以简单的写成：

if let contactImagesByEmails = obj["emails"] as? Array<String> {
    results = WmSqliteImagesModel.getInstance.getImagesByEmailAsWmContactImage(contactImagesByEmails) as Dictionary<String,AnyObject>
}

此外，在调用 getImagesByEmailAsWmContactImage

时结合可选绑定和可选向下转换会更安全

if let results = WmSqliteImagesModel.getInstance.getImagesByEmailAsWmContactImage(contactImagesByEmails) as? Dictionary<String,AnyObject> {
    ...
}

最后，您确定 getInstance 是属性而不是方法吗？不应该是：

if let results = WmSqliteImagesModel.getInstance().getImagesByEmailAsWmContactImage(contactImagesByEmails) as? Dictionary<String,AnyObject> {
    ...
}

Answer 2

也许尝试另一种方法来获取转换值：

if let contactImagesByEmails = obj["emails"] as? [String] {
       results = WmSqliteImagesModel.getInstance.getImagesByEmailAsWmContactImage(contactImagesByEmails) as Dictionary<String,AnyObject>
    }

我也想知道你的崩溃是否不是因为在你的原始代码中命名你的变量 "array"...

Answer 3

通过减少对 AnyObject（当然不是 AnyObject!）的引用来逐步简化此过程。编译器可能允许通过一些非法的东西，因为当你开始退出 AnyObject 时，你说的是 "I know exactly what I'm doing; don't check this."

您希望此代码类似于：

let obj = command.arguments[0]
if let contactImagesByEmails = obj["emails"] as? [String] {
    results = WmSqliteImagesModel.getInstance.getImagesByEmailAsWmContactImage(array) as? [String: AnyObject]
}

最后results会是[String:AnyObject]?

这里的关键点是你应该尽可能多地删除对 AnyObject 的引用，并且你应该使用 if-let-as? 来确定类型是否符合你的期望。

事实上 command.arguments 充满了您必须进行类型检查的各种类型的东西，这是非常危险的，并且是严重设计问题的标志。如果那里真的有多种类型，你应该使用枚举而不是像 [String:AnyObject] 这样的随机事物进行类型检查。这就是枚举的明确用途。（如果这是从 ObjC 桥接；枚举在这里可能是不可能的，但即使在 ObjC 中，正确的答案也很少是 NSArray 填充异构类型。你创建一个 class 来保存它们而不是枚举。）

Swift: Downcast导致应用程序崩溃，为什么？

Swift: Downcast causes application to crash, why?

ios

cordova

swift