Question

我有一个带有搜索栏的列表视图，我可以在列表视图中搜索一个项目，然后单击该项目并导航到该项目的详细信息，但是当我单击后退箭头时，我的HttpResponseMessage上会出现System.NullReferenceException 。有人可以告诉我我可能做错了什么。如果搜索栏为空，则工作正常。

ViewModel

    private async Task GetProjects(string email)
    {
        IsBusy = true;
        ProjectList = new ObservableCollection<ProjectModel>();
        using (HttpClient client = new HttpClient())
        {
            try
            {
                using (HttpResponseMessage response = await client.GetAsync("http://example/api/GetProject/email=" + email + "/"))
                {
                    if (response.IsSuccessStatusCode)
                    {
                        using (HttpContent content = response.Content)
                        {
                            var textresponse = await content.ReadAsStringAsync();
                            var json = JsonConvert.DeserializeObject<List<ProjectModel>>(textresponse);

                            foreach (var t in json)
                            {
                                if (t.pjtIsActive == 1)
                                {
                                    ProjectList.Add(new ProjectModel
                                    {
                                        ..............
                                    });
                                }
                            }
                            IsBusy = false;
                        }
                    }
                    else
                    {

                    }
                }
            }
            catch (Exception)
            {
                IsBusy = false;
            }
        }
    }
private ICommand _searchCommand;
        public ICommand SearchCommand
        {
            get
            {
                return _searchCommand ?? (_searchCommand = new Command<string>
                    (async (text) =>
                {
                    if (text.Length >= 1)
                    {
                        ProjectList.Clear();
                        await GetProjects(EmailAddress);
                        var projectSearch = ProjectList.Where(c => c.pjtName.ToLower().StartsWith(text.ToLower()) || c.ClientName.ToLower().StartsWith(text.ToLower()) || c.ContractorName.ToLower().StartsWith(text.ToLower()) || c.pjtNumber.ToLower().StartsWith(text.ToLower())).ToList();

                        ProjectList.Clear();
                        foreach (var item in projectSearch)
                            ProjectList.Add(item);
                    }
                    else
                    {
                        GetProjects(EmailAddress);
                    }

                }));
            }
        }

private ICommand _projectDetailsCommand;
            public ICommand ProjectDetailsCommand=> _projectDetailsCommand?? (_projectDetailsCommand= new Command(async (object obj) => {
                var item = (obj as ProjectModel);
                ProjectModel project = new ProjectModel();
                ...........
                Navigation.PushAsync(new Project_Details(project));

            }));

内容页面

protected override void OnAppearing()
        {
            BindingContext = new Project_View_ViewModel(Navigation);
            base.OnAppearing();
        }

Answer 1

您应该在构造函数中调用绑定上下文，并在OnAppearing中刷新所需的数据，

 private Project_View_ViewModel bindingv;

    public Project_View()
    {
        try
        {
            InitializeComponent();
            bindingv = new Project_View_ViewModel(Navigation);
            BindingContext = bindingv;
        }
        catch (Exception ex)
        {
            Logger.Log(ex);
        }
    }

    protected async override void OnAppearing()
    {
        base.OnAppearing();
        try
        {
            if (bindingv != null)
            {
                await bindingv.GetProjects();
            }
        }
        catch (Exception ex)
        {
            Logger.Log(ex);
        }
    }

在搜索栏中键入后使用后退箭头时，列表视图未填充

1 个答案: