WinRT ViewModel DataBind为异步方法 [英] WinRT ViewModel DataBind to async method
问题描述
async ，并且这一直到ViewModel，其中Property getter不能被标记为... 问题
-
我将一个文件夹中的所有XML文件反序列化为
列表< Profile>中。此方法（必须）标记为async。public static async任务<列表< Profile>> GetAllProfiles（）
{
DataContractSerializer ser = new DataContractSerializer（typeof（Profile））;
StorageFolder文件夹= await ApplicationData.Current.RoamingFolder.CreateFolderAsync（Profiles，CreationCollisionOption.OpenIfExists）;
列表<个人资料> profiles = new List< Profile>（）;
foreach（var f in await folder.GetFilesAsync（））
{
var fs = await f.OpenStreamForReadAsync（）;
profiles.Add（（Profile）ser.ReadObject（fs））;
fs.Dispose（）;
}
返回配置文件;
}
理想解决方案1
-
我的ViewModel中的binding属性然后将 调用这样的静态方法
public async任务< ObservableCollection< string>>列出
{
get
{
返回新的ObservableCollection< string>（GetAllProfiles（））Select（p => p.Name））;
}
}
-
但是无法将属性标记为
async
理想解决方案2
public ObservableCollection< string>列出
{
get
{
返回新的ObservableCollection< string>（（GetAllProfiles（）。Result）.Select（p => p.Name））;
}
}
- 但这不会执行（它阻止在
await folder.GetFilesAsync（）调用某些原因）
< h2>当前解决方案
调用 async 加载 NotifyPropertyChanged（Lists）调用：
public ViewModel（）
{
Initialize（）;
}
public async void Initialize（）
{
_profiles = await Profile.GetAllProfiles（）;
NotifyPropertyChanged（Lists）;
}
私人列表<个人资料> _profiles;
public ObservableCollection< string>列出
{
get
{
if（_profiles！= null）
返回新的ObservableCollection< string>（_ profiles.Select（p => p.Name） ）;
else
返回null;
}
}
问题
有更好的方法吗？
有没有一个我还没有发现的模式/方法？
编辑
在执行非UI代码时出现问题的根源，您不能依赖NotifyPropertyChanged来执行一些线程同步的东西。 - 方法Initialize必须等待，并且ctors不能异步，所以这是模式是无用的。
public MyClass ）
{
Initialize（）;
}
public async void Initialize（）
{
_profiles = await Profile.GetAllProfiles（）;
}
private ObservableCollection<个人资料> _profiles;
public ObservableCollection< string>列出
{
get
{
return _profiles; //这将永远是空的
}
}
属性不能异步，因此此解决方案将无法正常工作。 Task.Result等待完成任务，但这是阻止您的UI线程，其中I / O操作的异步回调返回，因此您的应用程序死锁，因为从未调用回调。你的解决方案真的是最好的方法。可以改进。
- 您应该将_profiles字段设置为ObservableCollection，因此您每次访问列表时都不需要将列表转换为OC。
- 由于您正在执行可能需要花费大量时间的I / O操作 - 您应该在进行过程中启用某种进度指示器。
- 在某些情况下，您可能希望Lists属性变得懒惰，并且只能在第一次访问时调用Init方法。
I am deserializing a list of objects from an XML file, and would like to bind to the actual content of those objects in my View, passing over a ViewModel. The problem is that file operations are async and this bubbles all the way up to the ViewModel, where Property getters cannot be marked as such...
Problem
I deserialize all XML files in a folder to
Profileobjects and store them in aList<Profile>. This method (has to be) markedasync.public static async Task<List<Profile>> GetAllProfiles() { DataContractSerializer ser = new DataContractSerializer(typeof(Profile)); StorageFolder folder = await ApplicationData.Current.RoamingFolder.CreateFolderAsync("Profiles", CreationCollisionOption.OpenIfExists); List<Profile> profiles = new List<Profile>(); foreach (var f in await folder.GetFilesAsync()) { var fs = await f.OpenStreamForReadAsync(); profiles.Add((Profile)ser.ReadObject(fs)); fs.Dispose(); } return profiles; }
Ideal solution 1
The binding property in my ViewModel would then ideally call that static method like this
public async Task<ObservableCollection<string>> Lists { get { return new ObservableCollection<string>(GetAllProfiles().Select(p => p.Name)); } }BUT Properties cannot be marked
async
Ideal solution 2
public ObservableCollection<string> Lists
{
get
{
return new ObservableCollection<string>((GetAllProfiles().Result).Select(p => p.Name));
}
}
- BUT this never executes (it blocks in the
await folder.GetFilesAsync()call for some reason)
Current solution
Calls an async Initialize() method that loads the result of the GetProfiles() function in a variable, and then makes a NotifyPropertyChanged("Lists") call:
public ViewModel()
{
Initialize();
}
public async void Initialize()
{
_profiles = await Profile.GetAllProfiles();
NotifyPropertyChanged("Lists");
}
private List<Profile> _profiles;
public ObservableCollection<string> Lists
{
get
{
if (_profiles != null)
return new ObservableCollection<string>(_profiles.Select(p => p.Name));
else
return null;
}
}
Question
Is there a better way? Is there a pattern/method that I haven't yet discovered?
Edit
The root of the problem appears when doing non-UI code, and you cannot rely on the NotifyPropertyChanged to do some thread-synchronization stuff. -- The method Initialize has to be awaited and ctors cannot be async, so essentialy this is pattern is useless.
public MyClass()
{
Initialize();
}
public async void Initialize()
{
_profiles = await Profile.GetAllProfiles();
}
private ObservableCollection<Profile> _profiles;
public ObservableCollection<string> Lists
{
get
{
return _profiles; // this will always be null
}
}
Properties can't be async so this solution will not work as you mentioned. Task.Result waits for the task to complete, but this is blocking your UI thread where the I/O operation's async callback returns, so you are deadlocking your application, since the callback is never called. Your solution really is the best way. It could be improved though.
- You should make the _profiles field an ObservableCollection, so you would not need to convert the List to the OC every time the list is accessed.
- Since you are performing an I/O operation that can take arbitrary amount of time - you should enable some sort of a progress indicator while it is in progress.
- In some cases you might want the Lists property to be lazier and only call the Init method the first time it is accessed.
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