vintasoft barcode .net sdk 15: File Loading and Saving in Objective-C

Making QR Code ISO/IEC18004 in Objective-C 15: File Loading and Saving

CHAPTER 15: File Loading and Saving
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you can pass an autoreleased NSData instead without worrying about cleaning it up. Here is an NSData object that will hold an ordinary C string, which is just a sequence of bytes, and print out the data:
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const char *string = "Hi there, this is a C string!"; NSData *data = [NSData dataWithBytes: string length: strlen(string) + 1]; NSLog (@"data is %@", data);
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Here are the results:
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data is <48692074 68657265 2c207468 69732069 73206120 43207374 72696e67 2100>
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That s, uh, special. But, if you have an ASCII chart handy (you can find one by firing up the terminal and typing the command man ascii), you can see that this chunk of hexadecimal is actually our string. 0x48 is H , 0x69 is i , and so on. The -length method gives us the number of bytes, and the -bytes method gives us a pointer to the beginning of the string. Notice the + 1 in the +dataWithBytes: call That s to include the trailing zero-byte that C strings need. Also notice the 00 at the end of the results of the NSLog. By including the zerobyte, we can use the %s format specifier to print the string:
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NSLog (@"%d byte string is '%s'", [data length], [data bytes]);
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which results in the following output:
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30 byte string is 'Hi there, this is a C string!' NSData objects are immutable. Once you create them, that s it. You can use them, but you can t
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change them. NSMutableData, though, lets you add and remove bytes from the data s contents.
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Writing and Reading Property Lists
Now that you have seen all of the property list classes, what can we do with them The collection property list classes (NSArray, NSDictionary) have a method called -writeToFile: atomically:, which writes the property lists to files. NSString and NSData also have a writeToFile:atomically: method, but it just writes out strings or blobs of data. So, we could load up an array with strings and then save it:
NSArray *phrase; phrase = [NSArray arrayWithObjects: @"I", @"seem", @"to", @"be", @"a", @"verb", nil]; [phrase writeToFile: @"/tmp/verbiage.txt" atomically: YES];
CHAPTER 15: File Loading and Saving
Now, look at the file /tmp/verbiage.txt, and you should see something like this:
< xml version="1.0" encoding="UTF-8" > <!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd"> <plist version="1.0"> <array> <string>I</string> <string>seem</string> <string>to</string> <string>be</string> <string>a</string> <string>verb</string> </array> </plist>
This code, while a bit verbose, is exactly what we tried to save: an array of strings. These property list files can be arbitrarily complex, with arrays of dictionaries containing arrays of strings and numbers and dates. Xcode also includes a property list editor, so you can poke around plist files and modify them. If you look around the operating system, you ll find lots of property list files, like all of your preference files in Library/Preferences in your home directory and system configuration files like those in /System/Library/LaunchDaemons.
NOTE
Some property list files, especially the preferences files, are stored in a compressed binary format. You can convert these files to something human-readable using the plutil command: plutil -convert xml1 filename.plist.
Now that we have our verbiage.txt file sitting on disk, we can read it in with +arrayWithContentsOfFile: method, like this:
NSArray *phrase2 = [NSArray arrayWithContentsOfFile: @"/tmp/verbiage.txt"]; NSLog (@"%@", phrase2);
And our output happily matches what we saved earlier:
( I, seem, to, be, a, verb )
CHAPTER 15: File Loading and Saving
NOTE
Did you notice the word atomically in our writeToFile: method Are these calls radioactive Nope. The atomically: argument, which takes a BOOL, tells Cocoa whether it should save the contents of the file in a temporary file first and, later, swap this temporary file with the original file when the file save is successful. This argument is a safety mechanism if something bad happens during the save, you won t clobber the original file. But that safety does come at a price: you re consuming double the disk space while the save happens since the original file is still there. Unless you re saving huge files that might fill the user s hard drives, you should save your files atomically.
If you can boil your data down to property list types, you can use these very convenient calls to save stuff to disk and read it back later. When you re playing around with new ideas or bootstrapping a new project, you can use these conveniences to get your programs up and running quickly. Even if you just want to save a blob of data to disk and you re not using objects at all, you can use NSData to ease the work. Just wrap your data in an NSData object and call writeToFile:atomically: on the NSData object. One downside to these functions is that they don t return any error information. If you can t load a file, you ll just get a nil pointer back from the method, without any idea of what went wrong.
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