因此，与此recently posted question类似，我在将Amazon的AWS Obj-C库与我的Swift应用程序集成时遇到问题。我有NSOperation使用Transfer Utility library处理文件上传到S3，其中包括对后台文件传输的支持。在最近发布我们的应用程序后，我一直看到代码中的一些崩溃，当应用程序返回到前台时，它会重新启动进度处理程序。代码改编自their Obj-C example：
- (void)viewDidLoad {
[super viewDidLoad];
...
AWSS3TransferUtility *transferUtility = [AWSS3TransferUtility defaultS3TransferUtility];
[transferUtility
enumerateToAssignBlocksForUploadTask:^(AWSS3TransferUtilityUploadTask *uploadTask, __autoreleasing AWSS3TransferUtilityUploadProgressBlock *uploadProgressBlockReference, __autoreleasing AWSS3TransferUtilityUploadCompletionHandlerBlock *completionHandlerReference) {
NSLog(@"%lu", (unsigned long)uploadTask.taskIdentifier);
// Use `uploadTask.taskIdentifier` to determine what blocks to assign.
*uploadProgressBlockReference = // Reassign your progress feedback block.
*completionHandlerReference = // Reassign your completion handler.
}
downloadTask:^(AWSS3TransferUtilityDownloadTask *downloadTask, __autoreleasing AWSS3TransferUtilityDownloadProgressBlock *downloadProgressBlockReference, __autoreleasing AWSS3TransferUtilityDownloadCompletionHandlerBlock *completionHandlerReference) {
NSLog(@"%lu", (unsigned long)downloadTask.taskIdentifier);
// Use `downloadTask.taskIdentifier` to determine what blocks to assign.
*downloadProgressBlockReference = // Reassign your progress feedback block.
*completionHandlerReference = // Reassign your completion handler.
}];
}
到我的Swift版本，在尝试取消引用EXC_BAD_ACCESS KERN_INVALID_ADDRESS时与newProgressPointer崩溃：
// Swift 2.3
class AttachmentQueue: NSOperationQueue {
...
/**
Recreates `UploadOperation` instances for any that were backgrounded by the user leaving the
app.
*/
func addBackgroundedOperations() {
let transferUtility = AWSS3TransferUtility.defaultS3TransferUtility()
transferUtility.enumerateToAssignBlocksForUploadTask({ (task, progress, completion) -> Void in
guard let operation = UploadOperation(task: task, oldProgressPointer: progress, oldCompletionPointer: completion) else { return }
self.addOperation(operation)
}, downloadTask: nil)
}
}
/// An `UploadOperation` is an `NSOperation` that is responsible for uploading an attachment asset
/// file (photo or video) to Amazon S3. It leans on `AWSS3TransferUtility` to get the actual
/// uploading done.
class UploadOperation: AttachmentOperation {
...
/// An `AutoreleasingUnsafeMutablePointer` to the upload progress handler block.
typealias UploadProgressPointer = AutoreleasingUnsafeMutablePointer<(@convention(block) (AWSS3TransferUtilityTask, NSProgress) -> Void)?>
/// An `AutoreleasingUnsafeMutablePointer` to the upload completion handler block.
typealias UploadCompletionPointer = AutoreleasingUnsafeMutablePointer<(@convention(block) (AWSS3TransferUtilityUploadTask, NSError?) -> Void)?>
/**
A convenience initializer to be used to re-constitute an `AWSS3TransferUtility` upload task that
has been moved to the background. It should be called from `.enumerateToAssignBlocksForUploadTask()`
when the app comes back to the foreground and is responsible for re-hooking-up its progress and
completion handlers.
- parameter task: The `AWSS3TransferUtilityTask` that needs re-hooking-up.
- parameter oldProgressPointer: An `AutoreleasingUnsafeMutablePointer` to the original progress handler.
- parameter oldCompletionPointer: An `AutoreleasingUnsafeMutablePointer` to the original completion handler.
*/
convenience init?(task: AWSS3TransferUtilityUploadTask, oldProgressPointer: UploadProgressPointer, oldCompletionPointer: UploadCompletionPointer) {
self.init(attachment: nil) // Actual implementation finds attachment record
// Re-connect progress handler
var progressBlock: AWSS3TransferUtilityProgressBlock = self.uploadProgressHandler
let newProgressPointer = UploadProgressPointer(&progressBlock)
print("newProgressPointer", newProgressPointer)
print("newProgressPointer.memory", newProgressPointer.memory) // Throws EXC_BAD_ACCESS KERN_INVALID_ADDRESS
oldProgressPointer.memory = newProgressPointer.memory
// Re-connect completion handler
var completionBlock: AWSS3TransferUtilityUploadCompletionHandlerBlock = self.uploadCompletionHandler
let newCompletionPointer = UploadCompletionPointer(&completionBlock)
oldCompletionPointer.memory = newCompletionPointer.memory
}
/**
Handles file upload progress. `AWSS3TransferUtility` calls this repeatedly while the file is
uploading.
- parameter task: The `AWSS3TransferUtilityTask` for the current upload.
- parameter progress: The `NSProgress` object for the current upload.
*/
private func uploadProgressHandler(task: AWSS3TransferUtilityTask, progress: NSProgress) {
// We copy the `completedUnitCount` to operation but it would be nicer if we could just
// reference the one passed to us instead of having two separate instances
self.progress.completedUnitCount = progress.completedUnitCount
// Calculate file transfer rate using an exponential moving average, as per https://stackoverflow.com/a/3841706/171144
let lastRate = self.transferRate
let averageRate = Double(progress.completedUnitCount) / (NSDate.timeIntervalSinceReferenceDate() - self.uploadStartedAt!)
self.transferRate = self.smoothingFactor * lastRate + (1 - self.smoothingFactor) * averageRate;
progress.setUserInfoObject(self.transferRate, forKey: NSProgressThroughputKey)
}
/**
Handles file upload completion. `AWSS3TransferUtility` calls this when the file has finished
uploading or is aborted due to an error.
- parameter task: The `AWSS3TransferUtilityTask` for the current upload.
- parameter error: An instance of `NSError` if the upload failed.
*/
private func uploadCompletionHandler(task: AWSS3TransferUtilityUploadTask, error: NSError?) {
...
}
...
}
为什么指针memory在创建后会直接引用无效？
对iOS开发不熟悉且没有使用Obj-C（或其他非内存管理语言）的实际经验，我有点迷失。如果有人能够提供一些非常感激的亮点。
修改
enumerateToAssignBlocksForUploadTask(…)
/**
Assigns progress feedback and completion handler blocks. This method should be called when the app was suspended while the transfer is still happening.
@param uploadBlocksAssigner The block for assigning the upload pregree feedback and completion handler blocks.
@param downloadBlocksAssigner The block for assigning the download pregree feedback and completion handler blocks.
*/
public func enumerateToAssignBlocksForUploadTask(uploadBlocksAssigner: ((AWSS3TransferUtilityUploadTask, AutoreleasingUnsafeMutablePointer<(@convention(block) (AWSS3TransferUtilityTask, NSProgress) -> Void)?>, AutoreleasingUnsafeMutablePointer<(@convention(block) (AWSS3TransferUtilityUploadTask, NSError?) -> Void)?>) -> Void)?, downloadTask downloadBlocksAssigner: ((AWSS3TransferUtilityDownloadTask, AutoreleasingUnsafeMutablePointer<(@convention(block) (AWSS3TransferUtilityTask, NSProgress) -> Void)?>, AutoreleasingUnsafeMutablePointer<(@convention(block) (AWSS3TransferUtilityDownloadTask, NSURL?, NSData?, NSError?) -> Void)?>) -> Void)?)
答案 0 :(得分：1)
我认为你不应该需要大多数（或任何）这些指针。看看Swift example code from AWS，看看它是否没有找到您正在寻找的内容。
按照你的方式创建指针在Swift中是不安全的。这可能是你所需要的更多信息（你不应该努力工作），但这里可能会发生什么（这种解释在这种情况下并不完全正确，但它和＃39;可能发生的事情，因此值得了解）：
progressBlock。progressBlock不再在范围内的任何其他位置访问。progressBlock。我说这感觉不对，因为对闭包的第二个引用应该保持闭包活着，但通常用这种方式用构造函数创建指针是非常危险的。 / p>
（这可能会导致崩溃，因为您无法print @convention(block)关闭;我从未尝试过这样做。它不是尝试打印非常正常。）
无论如何，如果你确实需要做这种事情（我认为你不是这样），你需要按照以下方式去做：
withUnsafeMutablePointer(to: self.uploadProgressHandler) { newProgressPointer in
... newProgressPointer is safe to use in this block ...
}
但是作为一项规则，如果您要转换ObjC代码（不是纯C，而只是ObjC），并且发现您需要创建大量Unsafe个对象，那么您可能会走错了路。大多数ObjC事物都与Swift很好地联系Unsafe。