业务场景：
APP内加载大文本文件，例如小说阅读，考试刷题等。直接采用webView或者textView加载会造成严重卡顿。这时候我们就需要引用新的API.我们今天主要研究的就是它。

NSInputStream

直接上代码，代码很简单。
重点！！！！！！！子线程加载数据，主线程刷新UI ,避免频繁刷新

- (void)loadFileContentsIntoTextView{
    // 子线程加载 一定要子线程 避免UI卡顿
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_async(queue, ^{
        
        // 文件路径
        NSString *filePath = [[NSBundle mainBundle] pathForResource:@"txt" ofType:@"txt"];
        //通过流打开一个文件
        NSInputStream *inputStream = [[NSInputStream alloc] initWithFileAtPath: filePath];
        [inputStream open];
        NSInteger maxLength = 12800;// 数据分片大小
        uint8_t readBuffer [maxLength];
        //是否已经到结尾标识
        BOOL endOfStreamReached = NO;
        // NOTE: this tight loop will block until stream ends
        while (! endOfStreamReached)
        {
            NSInteger bytesRead = [inputStream read: readBuffer maxLength:maxLength];
            if (bytesRead == 0)
            {//文件读取到最后
                endOfStreamReached = YES;
            }
            else if (bytesRead == -1)
            {//文件读取错误
                endOfStreamReached = YES;
            }
            else
            {
                NSString *readBufferString =[[NSString alloc] initWithBytesNoCopy: readBuffer length: bytesRead encoding: NSUTF8StringEncoding freeWhenDone: NO];
                // 主线程刷新
                dispatch_sync(dispatch_get_main_queue(), ^{
                    [self appendTextToView: readBufferString];
                });
            }
        }
        [inputStream close];
 });
}

// array缓存加载回来的数据
- (void)appendTextToView:(NSString *)str{
    // 过滤空置
    if (kIsEmptyStr(str)) {
        return;
    }
    if (kIsEmptyArr(self.arrText)) {
        self.txtView.text = str;
    }
    // textView频繁刷新会造成卡顿 ，自己掌握刷新时机
    [self.arrText addObject:str];

}