objective-c 写入字典到CSV文件但不覆盖原数据

在Objective-C中，如果你想将字典写入CSV文件并且避免覆盖已有数据，你可以采用追加（appending）模式打开文件，而不是直接覆盖。这里是一个基本的步骤：

1. 首先，你需要导入Foundation框架，因为它包含了`NSFileManager`和`NSString`等用于文件操作的类。

#import <Foundation/Foundation.h>

2. 定义一个函数来写入CSV数据，假设你已经有了一个字典数组 (`NSArray<NSDictionary *> *dicts`) 和 CSV文件路径 (`NSString *filePath`):

- (void)appendDictsToCSV:(NSArray<NSDictionary *> *)dicts filePath:(NSString *)filePath {
    NSError *error = nil;
    
    // 创建一个CSV字符串
    NSMutableString *csvString = [NSMutableString string];
    for (NSDictionary *dict in dicts) {
        NSArray *keys = dict.allKeys;
        for (int i = 0; i < keys.count; i++) {
            NSString *key = keys[i];
            id value = dict[key];
            if (i > 0) csvString = [csvString stringByAppendingString:@","];

            // 对非字符串值进行转换（例如数字）
            NSString *formattedValue = value == [NSNull null] ? @"" : [value description];
            csvString = [csvString stringByAppendingFormat:@"%@%@", key, formattedValue];
        }
        csvString = [csvString stringByAppendingString:@"\n"];
    }

    // 追加到文件
    NSOutputStream *outputStream = [[NSOutputStream alloc] initToFileAtPath:filePath append:YES];
    if (!outputStream) {
        NSLog(@"Failed to open output stream: %@", error.localizedDescription);
        return;
    }

    const char *utf8Data = [csvString cStringUsingEncoding:NSUTF8StringEncoding];
    NSInteger bytesWritten = [csvString lengthOfBytesUsingEncoding:NSUTF8StringEncoding];

    if ([outputStream open]) {
        uint8_t buffer[4096];
        while (bytesWritten > 0) {
            NSInteger written = [outputStream write:&utf8Data bytes:bytesWritten];
            if (written <= 0) {
                NSLog(@"Error writing to file: %@", error.localizedDescription);
                break;
            }
            utf8Data += written;
            bytesWritten -= written;
        }
        [outputStream close];
    } else {
        NSLog(@"Failed to open file for appending: %@", error.localizedDescription);
    }
}

3. 调用这个函数，并传入字典数组和文件路径:

NSArray<NSDictionary *> *myDictionaries = ...; // 你的字典数组
NSString *filePath = @"path_to_your_file.csv";
[self appendDictsToCSV:myDictionaries filePath:filePath];