以Xamarin形式等待MessagingCenter

Question

我尝试使用来自我的MessagingCenter的ViewModel来实现MVVM。我获得了以下错误，因为多个线程接收到相同的消息"ClearStackLayout“，并且不等待回调的结束：

索引超出了数组的界限。

以下是我的视图代码：

public partial class LibraryChoicePage : DefaultBackgroundPage {

        private Object thisLock = new Object();

        public LibraryChoicePage() {
            InitializeComponent();

            /* ClearStackLayout */
            MessagingCenter.Subscribe<LibraryChoiceViewModel>(this, "ClearStackLayout", (sender) => {
                lock (thisLock) {
                    this._choices.Children.Clear();
                }
            });

            /* AddToStackLayout */
            MessagingCenter.Subscribe<LibraryChoiceViewModel, View>(this, "AddToStackLayout", (sender, arg) => {
                lock (thisLock) {
                    this._choices.Children.Add(arg);
                }
            });

        }

    }

Answer 1

首先，总是在UI线程上调用StackLayout.Children.Clear|Add。iOS不喜欢将UIView子视图从主UI线程中移除，并且会引发异常，甚至可能导致本机崩溃。

这就是我如何serialized消息传递调用的方式：

var semaphone = new SemaphoreSlim(1);
MessagingCenter.Subscribe<object>(this, "ClearStackLayout",  async (sender) =>
{
    await semaphone.WaitAsync();
    Device.BeginInvokeOnMainThread(() =>
    {
        _choices.Children.Clear();
    });
    semaphone.Release();
});

MessagingCenter.Subscribe<object, View>(this, "AddToStackLayout", async (sender, arg) =>
{
    await semaphone.WaitAsync();
    Device.BeginInvokeOnMainThread(() =>
    {
        _choices.Children.Add(arg);
    });
    semaphone.Release();
});

注意：try/finally应该包装SemaphoreSlim.Release和catch，以执行添加/清除故障所需的任何恢复程序代码。

UIUnit并行测试方法：

Random random = new Random();
var tasks = new List<Task>();
for (int i = 0; i < 50; i++)
{
    if (random.NextDouble() > .1)
        tasks.Add(Task.Factory.StartNew(() => { AddLayout(); }));
    else
        tasks.Add(Task.Factory.StartNew(() => { ClearLayout(); }));
}
var completed = Task.Factory.ContinueWhenAll(tasks.ToArray(), (messagecenterTasks) => { 
    foreach (var task in messagecenterTasks)
    {
        if (task.Status == TaskStatus.Faulted)
        {
            D.WriteLine("Faulted:");
            D.WriteLine($"  {task.Exception.Message}");
        }
    }
}).Wait(1000);
if (!completed)
    D.WriteLine("Some tasks did not complete in time allocated");

注意: AddLayout/ClearLayout是MessageCenter.Send of AddToStackLayout和ClearStackLayout__的方法包装器。