[英]Swift 3 - Working with UnsafePointer<CFDictionary>?
I'm currently reading Chris Adamson's "Learning Core Audio" and try to follow along in Swift 3 (instead of Objective-C ). 我目前正在阅读Chris Adamson的“学习核心音频”并尝试在Swift 3 (而不是Objective-C )中继续学习。
The first code example makes use of AudioTool
to gather information about an audio-file. 第一个代码示例使用
AudioTool
来收集有关音频文件的信息。 My Swift 3 version looks like this: 我的Swift 3版本看起来像这样:
import Foundation
import AudioToolbox
func main() {
let arguments = CommandLine.arguments
guard arguments.count > 1 else {
print("Usage: CAMetaData /full/path/to/audiofile")
return
}
// Get filepath out of cmd-arguments
let audiofilePath = NSString(string: arguments[1]).expandingTildeInPath
// Load audio file into appropriate data structure
let audioURL = NSURL(fileURLWithPath: audiofilePath)
var audiofile: AudioFileID? = nil
var possibleError = noErr
possibleError = AudioFileOpenURL(audioURL, AudioFilePermissions.readPermission, 0, &audiofile)
assert(possibleError == noErr)
// Get size of metadata dictionary
var outDataSize: UInt32 = 0
possibleError = AudioFileGetPropertyInfo(audiofile!, kAudioFilePropertyInfoDictionary, &outDataSize, nil)
assert(possibleError == noErr)
// Get metadata
var outDataPointer: UnsafePointer<CFDictionary>? = nil
possibleError = AudioFileGetProperty(audiofile!, kAudioFilePropertyInfoDictionary, &outDataSize, &outDataPointer)
assert(possibleError == noErr)
// How to use this outDataPointer?
let outData = outDataPointer!.pointee as NSDictionary
dump(outData)
// No CFRelease necessary - Swift takes care of that
// Close connection to audiofile
possibleError = AudioFileClose(audiofile!)
assert(possibleError == noErr)
}
main()
Everything seems to work great (all assertions/ AudioToolbox
-API call pass). 一切似乎都很好(所有断言/
AudioToolbox
调用传递)。 Now I'm asking myself how I'm able to display the data stored inside the outDataPointer
. 现在我问自己如何能够显示存储在
outDataPointer
的数据。
This is how I understand the situation: outDataPointer
holds an optional with associated type UnsafePointer<CFDictionary>
. 这就是我理解这种情况的方法:
outDataPointer
包含一个带有关联类型UnsafePointer<CFDictionary>
。 I'm able to verify that outDataPointer
is not nil
, therefore accessing the associated value won't crash my program. 我能够验证
outDataPointer
不是nil
,因此访问相关值不会导致我的程序崩溃。 outDataPointer!.pointee
should give me the CFDictionary
pointed to by the associated value behind outDataPointer
. outDataPointer!.pointee
应该给我一个由outDataPointer
背后的关联值指向的CFDictionary
。 CFDictionary
is castable to NSDictionary
. CFDictionary
可以转换为NSDictionary
。
Sadly dumping the underlaying data prints 遗憾的是,倾销了下层数据
- __NSAtom #0
__NSAtom#0
to the console. 到控制台。 Quite not what I expected (information about the audiofile).
不是我所期望的(关于audiofile的信息)。 How can I get this data out of my
outDataPointer
variable? 如何从
outDataPointer
变量中获取此数据?
Swift's CFDictionary
isn't itself a data structure; Swift的
CFDictionary
本身不是一个数据结构; it's a pointer to a data structure, and it is equivalent to Objective-C's CFDictionaryRef
. 它是指向数据结构的指针,它等同于Objective-C的
CFDictionaryRef
。 In other words, it behaves like a Swift class
, not a struct
. 换句话说,它的行为类似于Swift
class
,而不是struct
。
The value written into outDataPointer
is not a pointer to a CFDictionary
; 写入
outDataPointer
的值不是指向CFDictionary
的指针; it is a CFDictionary
. 这是一个
CFDictionary
。 You're dereferencing it one too many times, causing the data stored in the dictionary to be treated as a pointer to a dictionary. 您将其解除引用次数太多次,导致存储在字典中的数据被视为指向字典的指针。 On my system, the resulting memory address was
0x001dffffc892e2f1
, which Objective-C treats as a tagged pointer , resulting in the NSAtom
message. 在我的系统上,生成的内存地址为
0x001dffffc892e2f1
,Objective-C将其视为标记指针 ,从而生成NSAtom
消息。
To fix the problem, declare outDataPointer
as a CFDictionary?
要解决此问题,
outDataPointer
声明为CFDictionary?
instead of an UnsafePointer<CFDictionary>?
而不是
UnsafePointer<CFDictionary>?
: :
// Get metadata
var outDataPointer: CFDictionary? = nil
possibleError = AudioFileGetProperty(audiofile!, kAudioFilePropertyInfoDictionary, &outDataSize, &outDataPointer)
assert(possibleError == noErr)
let outData = outDataPointer! as NSDictionary
dump(outData)
Output: 输出:
▿ 1 key/value pair #0
▿ (2 elements)
- .0: approximate duration in seconds #1
- super: __NSCFString
- super: NSMutableString
- super: NSString
- super: NSObject
- .1: 187.387 #2
- super: NSString
- super: NSObject
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