Setting a default page in the Xamarin.Forms shell menu

问题

Using the Xaminals example, I'm trying to set the default startup page. Here's the relevant code from the xaml:

<FlyoutItem Route="animals" x:Name="shellAnimals"
            Title="Animals"
            FlyoutDisplayOptions="AsMultipleItems">
    <Tab Title="Domestic" x:Name="shellDomestic"
         Route="domestic"
         Icon="paw.png">
        <ShellContent Route="cats" x:Name="shellCats"
                      Style="{StaticResource DomesticShell}"
                      Title="Cats"
                      Icon="cat.png"
                      ContentTemplate="{DataTemplate views:CatsPage}" />
        <ShellContent Route="dogs" x:Name="shellDogs"
                      Style="{StaticResource DomesticShell}"
                      Title="Dogs"
                      Icon="dog.png"
                      ContentTemplate="{DataTemplate views:DogsPage}" />
    </Tab>
    <ShellContent Route="monkeys" x:Name="shellMonkeys"
                  Style="{StaticResource MonkeysShell}"
                  Title="Monkeys"
                  Icon="monkey.png"
                  ContentTemplate="{DataTemplate views:MonkeysPage}" />
    <ShellContent Route="elephants" x:Name="shellElephants"
                  Style="{StaticResource ElephantsShell}"
                  Title="Elephants"
                  Icon="elephant.png"
                  ContentTemplate="{DataTemplate views:ElephantsPage}" />
    <ShellContent Route="bears" x:Name="shellBears"
                  Style="{StaticResource BearsShell}"
                  Title="Bears"
                  Icon="bear.png"
                  ContentTemplate="{DataTemplate views:BearsPage}" />
</FlyoutItem>

In the code behind I can successfully set the default startup page.

For example, the dogs page:

shellAnimals.CurrentItem.CurrentItem = shellDogs;

But on the bears page:

// This works, but I don't like the indexing. Will give trouble on dynamic menu's
shellAnimals.CurrentItem = shellAnimals.Items[3];

// This doesn't work, but I don't understand why. It gives a null reference exception in the Xamarin Shell code, on the PushAsync
shellAnimals.CurrentItem = shellBears;

// This shows the bears page at startup, but when I click on an item in the listview, I also get a null reference error
CurrentItem = shellBears;

Why doesn't the second/third snippet work?
How can I set the default page to the bears page without using the indexer?

Can't find anything on this subject except Set default tab on Xamarin Forms Shell but that doesn't answer it.

回答1:

Why doesn't the second/third snippet work?

The second snippet:

shellAnimals.CurrentItem is a variable which type is ShellSection while your shellBears is kind of ShellContent.

The third snippet:

When you use CurrentItem directly in the shell class, that is Shell.CurrentItem and it's a ShellItem.

How can I set the default page to the bears page without using the indexer?

In your Xaml:

    <ShellSection x:Name="shellBears" Title="Bears"
                      Icon="bear.png">
        <ShellContent Route="bears"
                      Style="{StaticResource BearsShell}"                                                   
                      ContentTemplate="{DataTemplate views:BearsPage}" />
    </ShellSection >

And in code behind:

    shellAnimals.CurrentItem = shellBears;

回答2:

You can set the current page in XAML, as per the documentation Set-the-current-flyoutitem. This would then mean your Shell tag, you would add `CurrentItem="{x:Reference shellBears}"1.

Additionally, I see in your second code behind snippet you are missing another .CurrentItem step when compared to the dogs example that you say works. Can you try shellAnimals.CurrentItem.CurrentItem = shellBears;

来源：https://stackoverflow.com/questions/59248713/setting-a-default-page-in-the-xamarin-forms-shell-menu

标签

xamarin.forms

xamarin.shell