Question

我尝试使用我的ViewModel中的MessagingCenter实现MVVM。我获得了以下错误，因为多个线程收到相同的消息＆＃34; ClearStackLayout＆＃34;并且不要等待对方的回调结束：

索引超出了数组的范围。

这是我的观看代码：

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

首要问题是总是在UI线程上调用StackLayout.Children.Clear|Add。 iOS不喜欢从主UI线程中删除UIView子视图并抛出异常甚至可能导致本机崩溃

这就是消息调用的serialized：

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应该包含SemaphoreSlim.Release和catch以执行添加/清除失败所需的任何恢复代码。

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和AddToStackLayout的{{1}}的方法包装。

等待Xamarin Forms中的MessagingCenter

1 个答案: