File system（文件系统）
显示原文
LVGL has a 'File system' abstraction module that enables you to attach
any type of file system. A file system is identified by an assigned
drive letter. For example, if an SD card is associated with the letter
'S', a file can be reached using "S:path/to/file.txt".
LVGL有一个“文件系统”抽象模块，可以让你连接任何类型的文件系统。文件系统通过分配的驱动器号来识别。
例如，如果SD卡与字母 'S' 关联，可以使用 "S:path/to/file.txt" 来访问文件。
Ready to use drivers（准备使用驱动程序）
显示原文
LVGL contains prepared drivers for the API of POSIX, standard C, Windows, and FATFS. Learn more here.
Adding a driver（添加驱动程序）
Registering a driver（注册驱动）
显示原文
To add a driver, a lv_fs_drv_t needs to be initialized like below.
The lv_fs_drv_t needs to be static, global or dynamically allocated
and not a local variable.
static lv_fs_drv_t drv; /*Needs to be static or global*/
lv_fs_drv_init(&drv); /*Basic initialization*/
drv.letter = 'S'; /*An uppercase letter to identify the drive */
drv.cache_size = my_cache_size; /*Cache size for reading in bytes. 0 to not cache.*/
drv.ready_cb = my_ready_cb; /*Callback to tell if the drive is ready to use */
drv.open_cb = my_open_cb; /*Callback to open a file */
drv.close_cb = my_close_cb; /*Callback to close a file */
drv.read_cb = my_read_cb; /*Callback to read a file */
drv.write_cb = my_write_cb; /*Callback to write a file */
drv.seek_cb = my_seek_cb; /*Callback to seek in a file (Move cursor) */
drv.tell_cb = my_tell_cb; /*Callback to tell the cursor position */
drv.dir_open_cb = my_dir_open_cb; /*Callback to open directory to read its content */
drv.dir_read_cb = my_dir_read_cb; /*Callback to read a directory's content */
drv.dir_close_cb = my_dir_close_cb; /*Callback to close a directory */
drv.user_data = my_user_data; /*Any custom data if required*/
lv_fs_drv_register(&drv); /*Finally register the drive*/
Any of the callbacks can be NULL to indicate that operation is not
supported.
添加驱动程序时，需要像下面这样初始化一个 lv_fs_drv_t 类型的变量。
lv_fs_drv_t 变量需要是静态的、全局的或者动态分配的，不能是局部变量。
static lv_fs_drv_t drv; /* 需要是静态的或全局的 */
lv_fs_drv_init(&drv); /* 基本初始化 */
drv.letter = 'S'; /* 用一个大写字母来标识驱动器 */
drv.cache_size = my_cache_size; /* 读取缓存大小（以字节为单位）。0 表示不进行缓存。*/
drv.ready_cb = my_ready_cb; /* 通知驱动器是否可以使用的回调函数 */
drv.open_cb = my_open_cb; /* 打开文件的回调函数 */
drv.close_cb = my_close_cb; /* 关闭文件的回调函数 */
drv.read_cb = my_read_cb; /* 读取文件的回调函数 */
drv.write_cb = my_write_cb; /* 写入文件的回调函数 */
drv.seek_cb = my_seek_cb; /* 在文件中寻找（移动游标）的回调函数 */
drv.tell_cb = my_tell_cb; /* 获取游标位置的回调函数 */
drv.dir_open_cb = my_dir_open_cb; /* 打开目录以读取其中内容的回调函数 */
drv.dir_read_cb = my_dir_read_cb; /* 读取目录内容的回调函数 */
drv.dir_close_cb = my_dir_close_cb; /* 关闭目录的回调函数 */
drv.user_data = my_user_data; /* 如有需要，可设置任意自定义数据 */
lv_fs_drv_register(&drv); /* 最后注册驱动程序 */
任何回调函数都可以为 NULL，表示不支持该操作。
Implementing the callbacks（实现回调）
Open callback（打开回调）
显示原文
The prototype of open_cb looks like this:
void * (*open_cb)(lv_fs_drv_t * drv, const char * path, lv_fs_mode_t mode);
path is the path after the drive letter (e.g. "S:path/to/file.txt" -> "path/to/file.txt").
mode can be LV_FS_MODE_WR or LV_FS_MODE_RD to open for writes or reads.
The return value is a pointer to a file object that describes the
opened file or NULL if there were any issues (e.g. the file wasn't
found). The returned file object will be passed to other file system
related callbacks. (see below)
open_cb 的原型如下：
void * (*open_cb)(lv_fs_drv_t * drv, const char * path, lv_fs_mode_t mode);
path 是驱动器字母后的路径（例如"S:path/to/file.txt" -> "path/to/file.txt"）。
mode 可以是 LV_FS_MODE_WR 或 LV_FS_MODE_RD，用于进行写入或读取打开。
返回值是指向描述打开文件的 文件对象 的指针，如果存在任何问题（例如文件未找到），则返回 NULL。
返回的文件对象将传递给其他与文件系统相关的回调函数（见下文）。
Other callbacks（其他回调）
显示原文
The other callbacks are quite similar. For example write_cb looks
like this:
lv_fs_res_t (*write_cb)(lv_fs_drv_t * drv, void * file_p, const void * buf, uint32_t btw, uint32_t * bw);
For file_p, LVGL passes the return value of open_cb, buf is
the data to write, btw is the Bytes To Write, bw is the actually
written bytes.
For a template of these callbacks see lv_fs_template.c.
其他的回调函数非常相似。例如， write_cb 的结构如下：
lv_fs_res_t (*write_cb)(lv_fs_drv_t * drv, void * file_p, const void * buf, uint32_t btw, uint32_t * bw);
对于 file_p 参数，LVGL 传递了 open_cb 的返回值； buf 是要写入的数据； btw 是要写入的字节数； bw 是实际写入的字节数。
关于这些回调函数的模板，请参考 lv_fs_template.c。
Usage example（使用示例）
显示原文
The example below shows how to read from a file:
lv_fs_file_t f;
lv_fs_res_t res;
res = lv_fs_open(&f, "S:folder/file.txt", LV_FS_MODE_RD);
if(res != LV_FS_RES_OK) my_error_handling();
uint32_t read_num;
uint8_t buf[8];
res = lv_fs_read(&f, buf, 8, &read_num);
if(res != LV_FS_RES_OK || read_num != 8) my_error_handling();
lv_fs_close(&f);
The mode in lv_fs_open() can be LV_FS_MODE_WR to open for writes
only or LV_FS_MODE_RD | LV_FS_MODE_WR for both
This example shows how to read a directory's content. It's up to the
driver how to mark directories in the result but it can be a good
practice to insert a '/' in front of each directory name.
lv_fs_dir_t dir;
lv_fs_res_t res;
res = lv_fs_dir_open(&dir, "S:/folder");
if(res != LV_FS_RES_OK) my_error_handling();
char fn[256];
while(1) {
res = lv_fs_dir_read(&dir, fn, sizeof(fn));
if(res != LV_FS_RES_OK) {
my_error_handling();
break;
}
/*fn is empty, if not more files to read*/
if(strlen(fn) == 0) {
break;
}
printf("%s\n", fn);
}
lv_fs_dir_close(&dir);
以下示例显示如何从文件中读取：
lv_fs_file_t f;
lv_fs_res_t res;
res = lv_fs_open(&f, "S:folder/file.txt", LV_FS_MODE_RD);
if(res != LV_FS_RES_OK) my_error_handling();
uint32_t read_num;
uint8_t buf[8];
res = lv_fs_read(&f, buf, 8, &read_num);
if(res != LV_FS_RES_OK || read_num != 8) my_error_handling();
lv_fs_close(&f);
在 lv_fs_open() 中的模式可以是 LV_FS_MODE_WR ，用于仅打开写入，或 LV_FS_MODE_RD | LV_FS_MODE_WR 用于两者都可以
该示例显示如何读取目录的内容。如何标记结果中的目录是由驱动程序决定的，但在每个目录名称前面加上 '/' 可能是一个很好的做法。
lv_fs_dir_t dir;
lv_fs_res_t res;
res = lv_fs_dir_open(&dir, "S:/folder");
if(res != LV_FS_RES_OK) my_error_handling();
char fn[256];
while(1) {
res = lv_fs_dir_read(&dir, fn, sizeof(fn));
if(res != LV_FS_RES_OK) {
my_error_handling();
break;
}
/*fn为空，如果没有更多文件可读取*/
if(strlen(fn) == 0) {
break;
}
printf("%s\n", fn);
}
lv_fs_dir_close(&dir);
Use drives for images（使用图像驱动程序）
显示原文
Image objects can be opened from files too (besides variables stored in the compiled program).
To use files in image widgets the following callbacks are required:
open
close
read
seek
tell
Image 对象也可以从文件中打开（除了编译程序中存储的变量）。
要在图像小部件中使用文件，需要以下回调函数：
打开
关闭
读取
定位
告诉
Optional file buffering/caching（可选的文件缓冲/缓存）
显示原文
Files will buffer their reads if the corresponding LV_FS_*_CACHE_SIZE
config option is set to a value greater than zero. Each open file will
buffer up to that many bytes to reduce the number of FS driver calls.
Generally speaking, file buffering can be optimized for different kinds
of access patterns. The one implemented here is optimal for reading large
files in chunks, which is what the image decoder does.
It has the potential to call the driver's read fewer
times than lv_fs_read is called. In the best case where the cache size is
>= the size of the file, read will only be called once. This strategy is good
for linear reading of large files but less helpful for short random reads across a file bigger than the buffer
since data will be buffered that will be discarded after the next seek and read.
The cache should be sufficiently large or disabled in that case. Another case where the cache should be disabled
is if the file contents are expected to change by an external factor like with special OS files.
The implementation is documented below. Note that the FS functions make calls
to other driver FS functions when the cache is enabled. i.e., lv_fs_read may call the driver's seek
so the driver needs to implement more callbacks when the cache is enabled.
lv_fs_read (behavior when the cache is enabled)
lv_fs_write (behavior when the cache is enabled)
The part of the cache that coincides with the written content will be updated to reflect the written content.
lv_fs_seek (behavior when the cache is enabled)
The driver's seek will not actually be called unless the whence
is LV_FS_SEEK_END, in which case seek and tell will be called
to determine where the end of the file is.
lv_fs_tell (behavior when the cache is enabled)
The driver's tell will not actually be called.
如果相应的 LV_FS_*_CACHE_SIZE 配置选项设置为大于零的值，文件将缓冲其读取。每个打开的文件将缓冲最多这么多字节，以减少 FS 驱动程序调用的数量。
一般来说，文件缓冲可以针对不同类型的访问模式进行优化。这里实现的方法最适合以块的形式读取大文件，这就是图像解码器的作用。它有可能减少驱动程序的 read 比调用 lv_fs_read 的次数。在缓存大小 >= 文件大小的最佳情况下， read 只会被调用一次。此策略对于大文件的线性读取很有用，但对于跨大于缓冲区的文件的短随机读取帮助不大，因为数据将被缓冲，这些数据将在下一次查找和读取后被丢弃。在这种情况下，缓存应该足够大或禁用。应禁用缓存的另一种情况是，如果文件内容预计会因外部因素（例如特殊操作系统文件）而发生更改。
下面记录了实施情况。请注意，当启用缓存时，FS 函数会调用其他驱动程序 FS 函数。即， lv_fs_read 可能会调用驱动程序 seek，因此驱动程序需要在启用缓存时实现更多回调。
lv_fs_read (启用缓存时的行为)
lv_fs_write (启用缓存时的行为)
缓存中与写入内容一致的部分将被更新以反映写入内容。
lv_fs_seek (启用缓存时的行为)
驱动程序 seek 实际上不会被调用，除非 whence， LV_FS_SEEK_END 在这种情况下 seek ， tell 将调用 来确定文件结尾在哪里。
The driver's seek will not actually be called unless the whence
is LV_FS_SEEK_END, in which case seek and tell will be called
to determine where the end of the file is.
lv_fs_tell (启用缓存时的行为)
-------------------------------------------------
tell实际上不会调用驱动程序。