快速将UnsafeMutableRawPointer转换为String（utf8）

Question

The question is simple How to convert UnsafeMutableRawPointer? to String(encoding: utf8) in swift.

I get some data from socket written in obj c, the model class looks like

@interface PTData : NSObject
@property (readonly) dispatch_data_t dispatchData;
@property (readonly) void *data;
@property (readonly) size_t length;
@end

The corresponding obj c code for converting data to String

PTExampleTextFrame *textFrame = (PTExampleTextFrame*)payload.data;
textFrame->length = ntohl(textFrame->length);
NSString *message = [[NSString alloc] initWithBytes:textFrame->utf8text length:textFrame->length encoding:NSUTF8StringEncoding];

Here PTExampleTextFrame is a struct

typedef struct _PTExampleTextFrame {
  uint32_t length;
  uint8_t utf8text[0];
} PTExampleTextFrame; 

But I can't convert the code to swift. But when I tried to convert the code looks like,

let data = Data(bytesNoCopy: payload!.data, count: payload.length,deallocator: .none)

let text = String(bytes: data, encoding: String.Encoding.utf8)

Somehow I got the answer but the text is prefixed with some escape characters.

hi makes \\0\\0\\0\\u{02}jj

long text to \\0\\0L\\u{1D}long text

I didn't get the concept of UnsafeMutableRawPointer and what is the prefixes.

Answer 1

If your obj c code for converting data to String is confirmed to work properly, I recommend you to write an Objective-C extension to work with Swift.

Such as:

PTData+Swift.h

#import <Foundation/Foundation.h>

@class PTData;

NS_ASSUME_NONNULL_BEGIN

@interface PTData (Swift)
@property (readonly) NSString *message;

@end

NS_ASSUME_NONNULL_END

PTData+Swift.m

#import "PTData.h"
#import "PTData+Swift.h"

@implementation PTData (Swift)

- (NSString *)message {
    PTExampleTextFrame *textFrame = (PTExampleTextFrame*)self.data;
    uint32_t length = ntohl(textFrame->length);
    NSString *message = [[NSString alloc] initWithBytes:textFrame->utf8text
                                                 length:length
                                               encoding:NSUTF8StringEncoding];
    return message;
}

@end

{YourProject}-Bridging-Header.h

#import "PTData.h"
#import "PTData+Swift.h"

(You may need to modify #import "PTData.h" to include actual headers.)

With the above extension, you can simply write in Swift:

debugPrint(payload.message)

---

Or else, if you dare insist on writing a Swift equivalent of your obj c code for converting data to String , your Swift code shown is a little bit too short.

extension PTData {
    var textFrameMessage: String {
        let textFrame = self.data.assumingMemoryBound(to: PTExampleTextFrame.self)
        let length = Int(textFrame.pointee.length.bigEndian)
        let utf8text = self.data
            .advanced(by: MemoryLayout<UInt32>.size) // <- Sadly enough, `MemoryLayout.offset(of:)` does not work as expected for `utf8text`
            .assumingMemoryBound(to: UInt8.self)
        let bytes = UnsafeBufferPointer(start: utf8text, count: length)
        let message = String(bytes: bytes, encoding: .utf8)! // <- This may crash, if the message is not encoded in UTF-8
        return message
    }
}

debugPrint(payload.textFrameMessage)

---

I have tested above codes with minimum estimated data, but it may be different than the actual data you receive. Please try and tell me if you find something wrong.