Xamarin Forms 选项卡式页面选项卡初始化

Question

I am developing a tabbed Page in Xamarin that consist of 3 tabs ( Tab1= "Home", Tab2= "Chat",Tab3="Menu" )

When the App starts ( MainPage = new TabbedPage() ) it initializes ( InitializeComponent(); ) all the tabs together at first.

My question is that if there is a way to initialize each component alone whenever the tab is pressed?

Answer 1

In my experience, what takes most of the time isn't loading the XAML, its a combination of "measure and layout" , then "loading data" (eg the items of a list).

This example shows how to defer "measure and layout". It is as simple as setting IsVisible="False" on any XAML you want to defer. Then when it is needed, set IsVisible="True" . I show the simplest case, where there is no data binding.

Deferring data loading depends on your data. The simplest technique is to start all lists ( ListView , CollectionView ) with ZERO ELEMENTS. When the data is needed, add the elements. I do not show that below. See How to load data when a tab is clicked .

NOTE: ON Android , InitializeComponent can be sped up further if you have Visual Studio 2019 Enterprise . In your MyProjectName.Android project, in Project Properties / Android Options (for Configuration "Release" ) / Code Generation and Runtime, check "AOT Compilation" . and "Enable Startup Tracing". You may want to also check "Use LLVM Optimiziing Compiler". These options may increase app bundle size. Test load time on an actual device - emulator may give misleading results as to what combination loads fastest.

TabbedPage1.xaml:

<TabbedPage xmlns="http://xamarin.com/schemas/2014/forms"
            xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
            xmlns:local="clr-namespace:TabbedPage1.Views"
            x:Class="TabbedPage1.TabbedPage1">
    <local:StartPage Title="Start" />
    <local:SlowPage Title="Slow Load" />
</TabbedPage>

TabbedPage1.xaml.cs:

using Xamarin.Forms;

namespace TabbedPage1
{
    public partial class TabbedPage1 : TabbedPage
    {
        public TabbedPage1()
        {
            InitializeComponent();
        }
    }
}

SlowPage.xaml:

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="TabbedPage1.Views.SlowPage">
    <ContentPage.Content>
        <StackLayout x:Name="MyContents" IsVisible="False">
            <Label Text="This text should appear after a 3 second delay."
                VerticalOptions="CenterAndExpand" 
                HorizontalOptions="CenterAndExpand" />
        </StackLayout>
    </ContentPage.Content>
</ContentPage>

SlowPage.xaml.cs:

using System.Threading.Tasks;
using Xamarin.Forms;

namespace TabbedPage1.Views
{
    public partial class SlowPage : ContentPage
    {
        public SlowPage()
        {
            InitializeComponent();
        }

        protected override async void OnAppearing()
        {
            base.OnAppearing();

            if (!MyContents.IsVisible) {
                // Simulate a page that is slow to layout or load.
                await Task.Delay(3000);
                
                MyContents.IsVisible = true;
            }
        }
    }
}

StartPage.xaml:

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="TabbedPage1.Views.StartPage">
    <ContentPage.Content>
        <StackLayout>
            <Label Text="Welcome!" FontSize="32"
                VerticalOptions="CenterAndExpand" 
                HorizontalOptions="CenterAndExpand" />
        </StackLayout>
    </ContentPage.Content>
</ContentPage>

StartPage.xaml.cs:

using Xamarin.Forms;

namespace TabbedPage1.Views
{
    public partial class StartPage : ContentPage
    {
        public StartPage()
        {
            InitializeComponent();
        }
    }
}