读取操作后将 dispatch_data_t 转换为 NSString

Question

在我的小型 iOS 应用程序中，我有一个代表直方图的 CSV 文件。文件内容为：

标签0，值0

...

标签N，值N

在我的 ViewController 中，我以这种方式与 Grand Central Dispatch 异步读取文件：

//create the channel with which read the CSV file
dispatch_io_t ch = dispatch_io_create_with_path(DISPATCH_IO_STREAM, [[[NSBundle mainBundle] pathForResource:@"histogram1" ofType:@"csv"] UTF8String], O_RDONLY, 0, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(int error) {

    if(!error){
        NSLog(@"Channel created!");
    }

});

//read the whole file
dispatch_io_read(ch, 0, SIZE_MAX, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(bool done, dispatch_data_t dataRead, int error) {

    if(!error && done){
        NSString *ma = ; //???
        NSLog(@"%@", ma);
    }

});

我想将dataRead（类型dispatch_data_t）转换NSString为提取代表直方图条形的值。

Answer 1

有很多方法可以做到这一点，但您正在寻找的功能是dispatch_data_apply. 假设您正在阅读的字符串是 UTF8，这应该可以工作：

@interface NSString (FromDispatchData)

+ (instancetype)stringFromDispatchData: (dispatch_data_t)data;

@end

@implementation NSString (FromDispatchData)

+ (instancetype)stringFromDispatchData: (dispatch_data_t)data
{
    if (!data)
        return nil;

    NSMutableString* str = [NSMutableString string];

    dispatch_data_apply(data, ^bool(dispatch_data_t region, size_t offset, const void *buffer, size_t size) {
        [str appendString: [[NSString alloc] initWithBytes:buffer length:size encoding: NSUTF8StringEncoding]];
        return true;
    });

    return [[self class] stringWithString: str];
}

@end