我想在导航页面中更改后退箭头图像.为此在Android应用程序中我创建了导航页面渲染器,然后使用方法toolbar.SetNavigationIcon和它不工作,但当我使用toolbar.SetLogo其工作正常.

    protected override void OnElementChanged(ElementChangedEventArgs<NavigationPage> e)
    {

        base.OnElementChanged(e);
        toolbar.SetNavigationIcon(Resource.Drawable.arrow); 
        toolbar.SetLogo(Resource.Drawable.arrow);
    }
    public override void OnViewAdded(Android.Views.View child)
    {
        base.OnViewAdded(child);
        if (child.GetType() == typeof(Android.Support.V7.Widget.Toolbar))
        {
            toolbar = (Android.Support.V7.Widget.Toolbar)child;
            toolbar.ChildViewAdded += Toolbar_ChildViewAdded;
        }
    }

我还尝试将图像添加到app：navigationIcon中的apps.con,它在设计器中显示效果很好
my arrow

但是,当我开始我的应用程序时,我有相同的标准箭头图标
enter image description here

解决方法:

如果您的MainActivity是FormsApplicationActivity,您可以参考此示例：

https://github.com/jessejiang0214/ChangeBackIconInXF/tree/master/Droid

如果您的MainActivity类型是FormsAppCompatActivity,则可以自定义PageRenderer并更改工具栏的NavigationIcon.

例如 ：

[assembly: ExportRenderer(typeof(ContentPage), typeof(NavigationPageRendererDroid))]
...
public class NavigationPageRendererDroid : PageRenderer
{
    protected override void OnElementChanged(ElementChangedEventArgs<Xamarin.Forms.Page> e)
    {
        base.OnElementChanged(e);

        var context = (Activity)Xamarin.Forms.Forms.Context;
        var toolbar = context.FindViewById<Android.Support.V7.Widget.Toolbar>(Droid.Resource.Id.toolbar);

        toolbar.NavigationIcon = Android.Support.V4.Content.ContextCompat.GetDrawable(context, Resource.Drawable.Back);
    }
}

用法：

MainPage = new NavigationPage(new MainPage());
...
//When click a button in MainPage, navigate to another page
private async void Button_Clicked(object sender, EventArgs e)
{
    await Navigation.PushAsync(new TestPage());
}

Effect.

更新：

当您使用Navigation.PushAsync()方法导航到另一个页面时,系统将自动更新工具栏的图标,您可以在source code中找到：

protected virtual Task<bool> OnPushAsync(Page view, bool animated)
{
    return SwitchContentAsync(view, animated);
}

Task<bool> SwitchContentAsync(Page page, bool animated, bool removed = false, bool popToRoot = false)
{
    ...
    UpdateToolbar();
    ...
}

void UpdateToolbar()
{
    ...
    bool isNavigated = ((INavigationPageController)Element).StackDepth > 1;
    if (isNavigated)
    {
        ...
        if (NavigationPage.GetHasBackButton(Element.CurrentPage))
        {
             //Set the navigation back  icon < =================== !!! =-=
             var icon = new DrawerArrowDrawable(activity.SupportActionBar.ThemedContext);
             icon.Progress = 1;
             bar.NavigationIcon = icon;
        }
    }
    ...
}

方案：

所以我们别无选择,只能自定义NavigationPageRenderer,覆盖OnPushAsync方法来设置工具栏的图标.

using AToolbar = Android.Support.V7.Widget.Toolbar;
[assembly: ExportRenderer(typeof(CustomNavigationPage), typeof(NavigationPageRendererDroid))] // APPCOMP
...
public class NavigationPageRendererDroid : Xamarin.Forms.Platform.Android.AppCompat.NavigationPageRenderer // APPCOMP
{
    public AToolbar toolbar;
    public Activity context;

    protected override Task<bool> OnPushAsync(Page view, bool animated)
    {
        var retVal = base.OnPushAsync(view, animated);

        context = (Activity)Xamarin.Forms.Forms.Context;
        toolbar = context.FindViewById<Android.Support.V7.Widget.Toolbar>(Droid.Resource.Id.toolbar);

        if (toolbar != null)
        {
            if (toolbar.NavigationIcon != null)
            {
                toolbar.NavigationIcon = Android.Support.V4.Content.ContextCompat.GetDrawable(context, Resource.Drawable.Back);
                //toolbar.SetNavigationIcon(Resource.Drawable.Back);
            }
        }
        return retVal;
    }
}

CustomNavigationPage在PCL中定义：

public class CustomNavigationPage : NavigationPage
{
    public CustomNavigationPage(Page startupPage) : base(startupPage)
    {
    }
}

用法：

public App()
{
    InitializeComponent();

    MainPage = new CustomNavigationPage(new MainPage());
}
...
// In MainPage
private async void Button_Clicked(object sender, EventArgs e)
{
    await Navigation.PushAsync(new TestPage());
}