Modify Maui File picker. can pick folder,read pick file after restart application.create file under picked folder Only support windows and Android.
基于Maui的文件选择器，可以选择文件夹，并且在程序重启后访问选择的文件或文件夹。
1.pick a video file and read it:
挑选文件并读取：
FilePickResult res = await Picker.PickFileAsync(FilePickOptions.Videos);
using var stream = Picker.OpenPickedFile(res.PlatformPath, "r");
if you target android api29 or lower and granted android.permission.READ_EXTERNAL_STORAGE and android.permission.WRITE_EXTERNAL_STORAGE, or target android api29 higher and granted android.permission.MANAGE_EXTERNAL_STORAGE, or target windows,you can read file like this:
如果目标安卓平台在api29以下并且获得了读写外部存储权限，或者api29以上并获得管理所有文件权限，或者windows平台，可以直接使用file api访问：
ps:i found not all of picked file can get their absloute path,it may return wrong absloute path like :video:29999(which like media uri) or something else,user can use other contentProvider so this value could be very strange.sp you can try catch its exception and use platform path instead
后续发现这个绝对路径返回不一定正确，可能会返回video:1111这种media Uri形式，甚至用户可以选择其他contentproiver返回更为奇怪的路径 ，可以放在try catch快里捕获异常改为用platformPath
using var fs=File.OpenRead(res.FullPath);
2，pick multiFiles:
选择多个文件：
var results = await Picker.PickFilesAsync(FilePickOptions.Videos);
3，pick special type file:
选择特定文件：
var fileOptions = new FilePickOptions()
{
FileTypes = new FilePickerFileType(new Dictionary>
{
{DevicePlatform.Android,new string[]{"image/*"} },
{DevicePlatform.WinUI,new string[]{"*.png", "*.jpg", "*.jpeg", "*.webp","*.gif","*.bmp"} }
}),
};
FilePickResult res = await Picker.PickFileAsync(fileOptions);
4,pick a folder and create file under it:
挑选文件夹，并在其下创建文件：
var folder = await Picker.PickFolderAsync(null);
var res = Picker.CreateFile(folder.PlatformPath, "test.txt");
using var stream=Picker.OpenPickedFile(res.PlatformPath, "w");
using var sw=new StreamWriter(stream );
sw.Write("测试文字");
5，pick a folder and create folder under it:
挑选文件夹并在其下创建文件夹：
var res3 = Picker.CreateFolder(folder.PlatformPath, "testFolder");
var res4 = Picker.CreateFile(res3.PlatformPath, "TestInnerFolder/test.txt");
using var stream = Picker.OpenPickedFile(res4.PlatformPath, "w");
using var sw = new StreamWriter(stream);
sw.Write("测试文字");