在 xamarin.forms 中的按钮单击中调用异步任务 [英] Calling Async task in button click in xamarin.forms

问题描述

我有 xamarin.forms 应用程序包含一个列表视图，它将从 Rest API 加载值.它工作正常.我在列表视图上方有按钮.当我单击按钮时，将再次放置列表视图 API 调用，并且列表视图应该更新.但是卡在这个更新部分.我没有使用 MVVM 模式.列表视图列表部分是一个异步任务.当按钮单击时，我再次调用异步任务，但应用程序崩溃了.是否是由于单击按钮再次调用异步任务?任何帮助表示赞赏.

I have xamarin.forms app contains a listview which will load values from Rest API.Which is working fine.I have button just above the listview.When I click on the button, the listview API call will be placed again and the listview should update. But stuck at this update part.I am not using MVVM pattern.The listview listing portion is an async Task.I am calling the async task again when the button click, but App gets crash. Is it due to calling the async task again from button click? Any help is appreciated.

这是我的代码.

 namespace app
    {
        public partial class List : ContentPage
        {   
            PendingWeekRange pendingWeekRange = new PendingWeekRange();
            public TimeSheetList()
            {
                InitializeComponent();        
                Task.Run(async () =>
                {
                    await LoadScreenItems();
                });           
            }    
            async Task LoadScreenItems()
            {
               await Task.Run(async () => {               
                    try
                    {                 
                          // Doing some stuff

                            await loadTimeSheetList();               
                    }
                    catch (Exception)
                    {

                    }
                });
            }  
            async Task loadTimeSheetList()
            {
                await Task.Run(() => {  +  string postdataForPendingList = "{\"date\":\"" + "1" + "\"}";
                APICall callForAPICallResult = new APICall("/API/ListMobile/ListForApproval", postdataForList, loadingIndicator);       
                    try
                    {                                        
                        List<ListData> resultObjForPendingTimeSheetList = callForAPICallResult<List<ListData>>();
                        if (resultObjForPendingTimeSheetList != null)
                        {

                            TimesheetList.ItemsSource = resultObjForPendingTimeSheetList;
                            screenStackLayout.VerticalOptions = LayoutOptions.FillAndExpand;
                            TimesheetList.IsVisible = true;
                        }
                        else
                        {

                        }
                    }
                    catch (Exception)
                    {                    
                    }
                });          
            }
         async   void Button_Tapped(object sender, EventArgs e)
            {
                try
                {            
                      // Calling my listview again. After calling app gets crash                
                  Task.Run(async () => await loadTimeSheetList());                           
                }
                catch (Exception ex) { }
            } 
        }
    }

推荐答案

在解决问题之前的一些事情.你的异步/等待都错了，尽管 异步编程

A few things before getting to the problem. You've got async/await all wrong, go though Async Programming

Task.Run 在不同的线程上运行传递的操作，如果您在此线程上更改 UI 元素，您的应用肯定会(相信我的话)崩溃.

Task.Run runs the passed action on a different thread, if you make changes to UI elements on this thread, your app will definitely(take my word) crash.

如果您想在页面启动时进行异步调用，请使用 OnAppearing 方法(如果您只想调用一次，请维护一个标志)

If you want to make async call at page launch, make use of OnAppearing method (if you only want to call once, maintain a flag)

不要频繁更改列表视图的ItemsSource，只需清除并添加项目即可.

Do not change the ItemsSource of a list view frequently, just clear and add items to it.

namespace app
{
    public partial class List : ContentPage
    {   
        PendingWeekRange pendingWeekRange = new PendingWeekRange();

        private ObservableCollection<ListData> TimesheetObservableCollection = new ObservableCollection<ListData>();
        public TimeSheetList()
        {
            InitializeComponent();          
            TimesheetList.ItemsSource = TimesheetObservableCollection;
        }
        protected override async OnAppearing()
        {
            // flag for first launch?
            await LoadScreenItems();

        }
        async Task LoadScreenItems()
        {     
            try
            {                 
                    // Doing some stuff
                    TimesheetObservableCollection.Clear();
                    TimesheetObservableCollection.AddRange(await GetTimeSheetList());
            }
            catch (Exception)
            {
                //handle exception
            }
        }  
        async Task<List<ListData>> GetTimeSheetList()
        {
            string postdataForPendingList = "{\"date\":\"" + "1" + "\"}";
            APICall callForAPICallResult = new APICall("/API/ListMobile/ListForApproval", postdataForList, loadingIndicator);       
            try
            {                                        
                return callForAPICallResult<List<ListData>>();
            }
            catch (Exception) 
            { 
                // handle exception
            }        
        }
        async void Button_Tapped(object sender, EventArgs e)
        {
            try
            {
                // Calling my listview again. After calling app gets crash                
                TimesheetObservableCollection.Clear();
                TimesheetObservableCollection.AddRange(await GetTimeSheetList());
            }
            catch (Exception ex) { }
        } 
    }
}

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