I have a custom WkWebView where I set the height of the view to the size of the html content.
This works great when I initialize it, but the problem comes when I change the source of the WkWebView to a shorter html.
I've already encountered this problem in the android version of my app and I fixed that by setting the HeightRequest to 0 before EvaluateJavaScriptAsync. In that way the view will always get bigger.
I tried the same thing with iOS but it keeps the highest content I had .
So for example :
If I set the source of the WkWebView to a html file that is 200px height and change it to a html file that is 1000px, it works fine and I can see all the content. BUT, if I try to go back to my 200px html file, I get a 800px blank space underneath and keep the 1000px height.
The goal is to always have the height of the WKWebView to adapt to the height of its content.
Here is the current version of the custom render
namespace MyNamespace.iOS.CustomControl
{
public class AutoHeightWebViewRenderer : WkWebViewRenderer
{
protected override void OnElementChanged(VisualElementChangedEventArgs e)
{
try
{
base.OnElementChanged(e);
if (NativeView != null)
{
var webView = (WKWebView)NativeView;
NavigationDelegate = new ExtendedWKWebViewDelegate(this);
}
}
catch (Exception ex)
{
//Log the Exception
}
}
}
class ExtendedWKWebViewDelegate : WKNavigationDelegate
{
AutoHeightWebViewRenderer webViewRenderer;
public ExtendedWKWebViewDelegate(AutoHeightWebViewRenderer _webViewRenderer = null)
{
webViewRenderer = _webViewRenderer ?? new AutoHeightWebViewRenderer();
}
public override async void DidFinishNavigation(WKWebView webView, WKNavigation navigation)
{
try
{
var _webView = webViewRenderer.Element as AutoHeightWebView;
if (_webView != null)
{
_webView.HeightRequest = 0d;
await Task.Delay(300);
var result = await _webView.EvaluateJavaScriptAsync("(function(){return document.body.scrollHeight;})()");
_webView.HeightRequest = Convert.ToDouble(result);
}
}
catch (Exception ex)
{
//Log the Exception
}
}
}
}
EDIT 1 :
To be clearer, I'm able to change the height of the webview, but I dont know the height because it always returned the height of the largest html displayed so far. Nomatter if I use _webView.EvaluateJavaScriptAsync("(function(){return document.body.scrollHeight;})()") or webView.ScrollView.ContentSize.Height.
EDIT 2 :
Here a little sample to help understand the problem. I got two buttons and my custom webview (initialize with a 40 HeightRequest empty html).
The first button set the Source of the webview to a 70px long HTML. The second one set the Source to a 280px long HTML.
In this example, I click on the first button than the second one and finaly back on the first button again. You see the webview getting bigger on the first 2 clicks. But then then webview should get shrunk when I choose back the first html (passing from 280px to 70px) but it keeps the 280px long.
First button (70px long)
Second button (280px long)
Back to the first button (should be 70px long instead of 280px).
The problem occured on both simulator and iOS device.
You can change the Frame of webView to change the height.
public override async void DidFinishNavigation(WKWebView webView, WKNavigation navigation)
{
try
{
var _webView = webViewRenderer.Element as WebView;
if (_webView != null)
{
webView.Frame = new CGRect(webView.Frame.Location, new CGSize(webView.Frame.Size.Width, 0));
var a = webView.ScrollView.ContentSize.Height;
var b = await _webView.EvaluateJavaScriptAsync("(function(){return document.body.scrollHeight;})()");
Console.WriteLine(a);
Console.WriteLine(b);
CGRect tempRect = webView.Frame;
// for test
webView.Frame = new CGRect(tempRect.Location.X, tempRect.Location.Y, tempRect.Size.Width, float.Parse(b));
}
}
catch (Exception ex)
{
//Log the Exception
}
}
I had the same problem and the CustomNavigationDelegate would also only show the same large size, which was the Device Display size.
I found that I had set this on the init part of the XAML and code behind, which somehow overrides the later content-based calculation.
See my fix here: https://stackoverflow.com/a/62409859/3443564
Am trying to load ItemSource of a picker when the picker is focused.
But the data is not loaded on 1st focus.
here is the code sample
List<object> itmSrc;
Picker picker = new Picker();
itmSrc = Controls[i].ItemSource;
picker.Focused += BindItemSourceOnFocus;
public void BindItemSourceOnFocus(object sender, FocusEventArgs e)
{
var p = e.VisualElement as Picker;
p.ItemsSource = itmSrc;
}
If any other different approach is possible, let me know.
You can do it adding items on an async method, or another thread. Load the data on view focus is just transferring the issue to another place, and it gives a bad user experience at all.
If you run a code block inside a Task.Run(), for example, this code will be executed on another thread, and the interface should not hang on data loading.
Something like this:
public class MyPage : ContentPage
{
List<object> itmSrc;
Picker picker;
public MyPage()
{
// Your stuff goes here
itmSrc = new List<object>();
picker = new Picker();
StackLayout content = new StackLayout();
content.Crindren.Add(picker);
this.Content = content;
Task.Run(() => LoadData());
}
private void LoadData()
{
// Get your data from anywhere and put it on the itemSrc from here.
// Then...
picker.ItemsSource = itmSrc;
}
}
I hope it helps.
I have a Prism based Xamarin Forms app that contains an edit page that is wrapped in a Navigation page so there is a back button at top left on both Android and iOS. To avoid the user accidentally losing an edit in progress by accidentally clicking the back button (in particular on Android) we want to prompt them to confirm that they definitely want to cancel.
Thing is, this seems like something that is not baked in to Xamarin forms. You can override OnBackButtonPressed in a navigation page, but that only gets called for the hardware/software back button on Android. There are articles detailing techniques to intercept the actual arrow button at the top left on Android (involving overriding OnOptionsItemSelected in the Android MainActivity), but on iOS I'm not sure it is even possible.
So I can't help but wonder if I am going about this the wrong way? Should I not be intercepting the top left / hardware / software back button in this way? Seems like a pretty common thing to do (e.g. press back when editing a new contact in the android built in Contacts app and you get a prompt) but it really feels like I am fighting the system here somehow.
There are previous questions around this, most relevant appears to be How to intercept Navigation Bar Back Button Clicked in Xamarin Forms? - but I am looking for some broad brush suggestions for an approach here. My objective is to show the user a page with the <- arrow at top left for Android, "Cancel" for iOS but I would like to get some views about the best way to go about it that does not involve me fighting against prism / navigation pages / xamarin forms and (where possible) not breaking the various "best practices" on Android and iOS.
After going down the same path as you and being told not to prevent users from going back, I decided on showing an alert after they tap the back button (within ContentPage.OnDisappearing()) that says something like Would you like to save your work?.
If you go with this approach, be sure to use Application.MainPage.DisplayAlert() instead of just this.DisplayAlert() since your ContentPage might not be visible at that point.
Here is how I currently handle saving work when they click the back button (I consolidated a good bit of code and changed some things):
protected override async void OnDisappearing() {
base.OnDisappearing();
// At this point the page is gone or is disappearing, but all properties are still available
#region Auto-save Check and Execution
/*
* Checks to see if any edits have been made and if a save is not in progress, if both are true, it asks if they want to save, if yes, it checks for validation errors.
* If it finds them, it marks it as such in the model before saving the model to the DB and showing an alert stating what was done
*/
if(!_viewModel.WorkIsEdited || _viewModel.SaveInProgress) { //WorkIsEdited changes if they enter/change data or focus on certain elements such as a Picker
return;
}
if(!await Application.Current.MainPage.DisplayAlert("ALERT", "You have unsaved work! Would you like to save now?", "Yes", "No")) {
return;
}
if(await _viewModel.SaveClaimErrorsOrNotAsync()) { //The return value is whether validation succeeds or not, but it gets saved either way
App.SuccessToastConfig.Message = "Work saved successfully. Try saving it yourself next time!";
UserDialogs.Instance.Toast(App.SuccessToastConfig);
} else if(await Application.Current.MainPage.DisplayAlert("ERROR", "Work saved successfully but errors were detected. Tap the button to go back to your work.", "To Work Entry", "OK")) {
await Task.Delay(200); //BUG: On Android, the alert above could still be displayed when the page below is pushed, which prevents the page from displaying //BUG: On iOS 10+ currently the alerts are not fully removed from the view hierarchy when execution returns (a fix is in the works)
await Application.Current.MainPage.Navigation.PushAsync(new WorkPage(_viewModel.SavedWork));
}
#endregion
}
What you ask for is not possible. The back button tap cannot be canceled on iOS even in native apps. You can do some other tricks like having a custom 'back' button, but in general you shouldn't do that - you should instead have a modal dialog with the Done and Cancel buttons (or something similar).
If you use xamarin forms that code it is work.
CrossPlatform source
public class CoolContentPage : ContentPage
{
public Action CustomBackButtonAction { get; set; }
public static readonly BindableProperty EnableBackButtonOverrideProperty =
BindableProperty.Create(nameof(EnableBackButtonOverride), typeof(bool), typeof(CoolContentPage), false);
public bool EnableBackButtonOverride{
get { return (bool)GetValue(EnableBackButtonOverrideProperty); }
set { SetValue(EnableBackButtonOverrideProperty, value); }
}
}
}
Android source
public override bool OnOptionsItemSelected(IMenuItem item)
{
if (item.ItemId == 16908332)
{
var currentpage = (CoolContentPage)
Xamarin.Forms.Application.
Current.MainPage.Navigation.
NavigationStack.LastOrDefault();
if (currentpage?.CustomBackButtonAction != null)
{
currentpage?.CustomBackButtonAction.Invoke();
return false;
}
return base.OnOptionsItemSelected(item);
}
else
{
return base.OnOptionsItemSelected(item);
}
}
public override void OnBackPressed()
{
var currentpage = (CoolContentPage)
Xamarin.Forms.Application.
Current.MainPage.Navigation.
NavigationStack.LastOrDefault();
if (currentpage?.CustomBackButtonAction != null)
{
currentpage?.CustomBackButtonAction.Invoke();
}
else
{
base.OnBackPressed();
}
}
iOS source
public override void ViewWillAppear(bool animated)
{
base.ViewWillAppear(animated);
if (((CoolContentPage)Element).EnableBackButtonOverride)
{
SetCustomBackButton();
}
}
private void SetCustomBackButton()
{
var backBtnImage = UIImage.FromBundle("iosbackarrow.png");
backBtnImage = backBtnImage.ImageWithRenderingMode
(UIImageRenderingMode.AlwaysTemplate);
var backBtn = new UIButton(UIButtonType.Custom)
{
HorizontalAlignment =
UIControlContentHorizontalAlignment.Left,
TitleEdgeInsets =
new UIEdgeInsets(11.5f, 15f, 10f, 0f),
ImageEdgeInsets =
new UIEdgeInsets(1f, 8f, 0f, 0f)
};
backBtn.SetTitle("Back", UIControlState.Normal);
backBtn.SetTitleColor(UIColor.White, UIControlState.Normal);
backBtn.SetTitleColor(UIColor.LightGray, UIControlState.Highlighted);
backBtn.Font = UIFont.FromName("HelveticaNeue", (nfloat)17);
backBtn.SetImage(backBtnImage, UIControlState.Normal);
backBtn.SizeToFit();
backBtn.TouchDown += (sender, e) =>
{
// Whatever your custom back button click handling
if(((CoolContentPage)Element)?.
CustomBackButtonAction != null)
{
((CoolContentPage)Element)?.
CustomBackButtonAction.Invoke();
}
};
backBtn.Frame = new CGRect(
0,
0,
UIScreen.MainScreen.Bounds.Width / 4,
NavigationController.NavigationBar.Frame.Height);
var btnContainer = new UIView(
new CGRect(0, 0,
backBtn.Frame.Width, backBtn.Frame.Height));
btnContainer.AddSubview(backBtn);
var fixedSpace =
new UIBarButtonItem(UIBarButtonSystemItem.FixedSpace)
{
Width = -16f
};
var backButtonItem = new UIBarButtonItem("",
UIBarButtonItemStyle.Plain, null)
{
CustomView = backBtn
};
NavigationController.TopViewController.NavigationItem.LeftBarButtonItems = new[] { fixedSpace, backButtonItem };
}
using in xamarin forms
public Page2()
{
InitializeComponent();
if (EnableBackButtonOverride)
{
this.CustomBackButtonAction = async () =>
{
var result = await this.DisplayAlert(null, "Go back?" Yes go back", "Nope");
if (result)
{
await Navigation.PopAsync(true);
}
};
}
}
I am picking a photo from photo library and i get the following
AlbumPath:
assets-library://asset/asset.JPG?id=106E99A1-4F6A-45A2-B320-B0AD4A8E8473&ext=JPG
Path:
/Users/myname/Library/Developer/CoreSimulator/Devices/21CB035B-A738-4F74-B121-2DB2A6B5372A/data/Containers/Data/Application/3081A323-98AF-4EF6-95B9-29D4C2CD8425/Documents/temp/IMG_20170408_111143.jpg
How do i assign this image to a button ? I tried the following.
var file = await CrossMedia.Current.PickPhotoAsync(
new Plugin.Media.Abstractions.PickMediaOptions
{
});
Button btn = new Button();
btn.Image = (Xamarin.Forms.FileImageSource)ImageSource.FromFile(file.Path);
No image is displayed on the button. Any help help is appreciated.
Thank you.
Once you deploy the application the app will not have access to your mac. For the application to use the picture it will need to be a bundled resource within the iOS application. Below is a simple example of using images within an iOS app. You'll definitely want to consider just adding a gesture listener to your image instead of making it a button though. If you try to just use the image on a button you'll have to do some styling adjustments to get it to look clean.
Image Setup
Put image in iOS Resource folder
Make sure bundledresource is selected from image properties.
Image Button
public class ImageButton : ContentPage
{
private Button _imageBtn = new Button { Image = "icon.png", Text = "Sample button" };
public ImageButton()
{
Content = new StackLayout
{
Children =
{
_imageBtn
}
};
}
}
Image with TapGesture
public class ImageButton : ContentPage
{
private Image _imageBtn = new Image { Source = "icon.png" };
private TapGestureRecognizer _imageTap = new TapGestureRecognizer();
public ImageButton()
{
_imageBtn.GestureRecognizers.Add(_imageTap);
Content = new StackLayout
{
Children =
{
_imageBtn
}
};
_imageTap.Tapped += (s, e) =>
{
// handle the tap
};
}
}
How do you switch between pages in Xamarin Forms?
My main page is a ContentPage and I don't want to switch to something like a Tabbed Page.
I've been able to pseudo-do it by finding parents of the controls that should trigger the new page until I find the ContentPage and then swap out the Content with controls for a new page. But this seems really sloppy.
In the App class you can set the MainPage to a Navigation Page and set the root page to your ContentPage:
public App ()
{
// The root page of your application
MainPage = new NavigationPage( new FirstContentPage() );
}
Then in your first ContentPage call:
Navigation.PushAsync (new SecondContentPage ());
Xamarin.Forms supports multiple navigation hosts built-in:
NavigationPage, where the next page slide in,
TabbedPage, the one you don't like
CarouselPage, that allows for switching left and right to next/prev pages.
On top of this, all pages also supports PushModalAsync() which just push a new page on top of the existing one.
At the very end, if you want to make sure the user can't get back to the previous page (using a gesture or the back hardware button), you can keep the same Page displayed and replace its Content.
The suggested options of replacing the root page works as well, but you'll have to handle that differently for each platform.
If your project has been set up as a PCL forms project (and very likely as Shared Forms as well but I haven't tried that) there is a class App.cs that looks like this:
public class App
{
public static Page GetMainPage ()
{
AuditorDB.Model.Extensions.AutoTimestamp = true;
return new NavigationPage (new LoginPage ());
}
}
you can modify the GetMainPage method to return a new TabbedPaged or some other page you have defined in the project
From there on you can add commands or event handlers to execute code and do
// to show OtherPage and be able to go back
Navigation.PushAsync(new OtherPage());
// to show AnotherPage and not have a Back button
Navigation.PushModalAsync(new AnotherPage());
// to go back one step on the navigation stack
Navigation.PopAsync();
Push a new page onto the stack, then remove the current page. This results in a switch.
item.Tapped += async (sender, e) => {
await Navigation.PushAsync (new SecondPage ());
Navigation.RemovePage(this);
};
You need to be in a Navigation Page first:
MainPage = NavigationPage(new FirstPage());
Switching content isn't ideal as you have just one big page and one set of page events like OnAppearing ect.
If you do not want to go the previous page i.e. do not let the user go back to the login screen once authorization is done, then you can use;
App.Current.MainPage = new HomePage();
If you want to enable back functionality, just use
Navigation.PushModalAsync(new HomePage())
Seems like this thread is very popular and it will be sad not to mention here that there is an alternative way - ViewModel First Navigation. Most of the MVVM frameworks out there using it, however if you want to understand what it is about, continue reading.
All the official Xamarin.Forms documentation is demonstrating a simple, yet slightly not MVVM pure solution. That is because the Page(View) should know nothing about the ViewModel and vice versa. Here is a great example of this violation:
// C# version
public partial class MyPage : ContentPage
{
public MyPage()
{
InitializeComponent();
// Violation
this.BindingContext = new MyViewModel();
}
}
// XAML version
<?xml version="1.0" encoding="utf-8"?>
<ContentPage
xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:viewmodels="clr-namespace:MyApp.ViewModel"
x:Class="MyApp.Views.MyPage">
<ContentPage.BindingContext>
<!-- Violation -->
<viewmodels:MyViewModel />
</ContentPage.BindingContext>
</ContentPage>
If you have a 2 pages application this approach might be good for you. However if you are working on a big enterprise solution you better go with a ViewModel First Navigation approach. It is slightly more complicated but much cleaner approach that allow you to navigate between ViewModels instead of navigation between Pages(Views). One of the advantages beside clear separation of concerns is that you could easily pass parameters to the next ViewModel or execute an async initialization code right after navigation. Now to details.
(I will try to simplify all the code examples as much as possible).
1. First of all we need a place where we could register all our objects and optionally define their lifetime. For this matter we can use an IOC container, you can choose one yourself. In this example I will use Autofac(it is one of the fastest available). We can keep a reference to it in the App so it will be available globally (not a good idea, but needed for simplification):
public class DependencyResolver
{
static IContainer container;
public DependencyResolver(params Module[] modules)
{
var builder = new ContainerBuilder();
if (modules != null)
foreach (var module in modules)
builder.RegisterModule(module);
container = builder.Build();
}
public T Resolve<T>() => container.Resolve<T>();
public object Resolve(Type type) => container.Resolve(type);
}
public partial class App : Application
{
public DependencyResolver DependencyResolver { get; }
// Pass here platform specific dependencies
public App(Module platformIocModule)
{
InitializeComponent();
DependencyResolver = new DependencyResolver(platformIocModule, new IocModule());
MainPage = new WelcomeView();
}
/* The rest of the code ... */
}
2.We will need an object responsible for retrieving a Page (View) for a specific ViewModel and vice versa. The second case might be useful in case of setting the root/main page of the app. For that we should agree on a simple convention that all the ViewModels should be in ViewModels directory and Pages(Views) should be in the Views directory. In other words ViewModels should live in [MyApp].ViewModels namespace and Pages(Views) in [MyApp].Views namespace. In addition to that we should agree that WelcomeView(Page) should have a WelcomeViewModel and etc. Here is a code example of a mapper:
public class TypeMapperService
{
public Type MapViewModelToView(Type viewModelType)
{
var viewName = viewModelType.FullName.Replace("Model", string.Empty);
var viewAssemblyName = GetTypeAssemblyName(viewModelType);
var viewTypeName = GenerateTypeName("{0}, {1}", viewName, viewAssemblyName);
return Type.GetType(viewTypeName);
}
public Type MapViewToViewModel(Type viewType)
{
var viewModelName = viewType.FullName.Replace(".Views.", ".ViewModels.");
var viewModelAssemblyName = GetTypeAssemblyName(viewType);
var viewTypeModelName = GenerateTypeName("{0}Model, {1}", viewModelName, viewModelAssemblyName);
return Type.GetType(viewTypeModelName);
}
string GetTypeAssemblyName(Type type) => type.GetTypeInfo().Assembly.FullName;
string GenerateTypeName(string format, string typeName, string assemblyName) =>
string.Format(CultureInfo.InvariantCulture, format, typeName, assemblyName);
}
3.For the case of setting a root page we will need sort of ViewModelLocator that will set the BindingContext automatically:
public static class ViewModelLocator
{
public static readonly BindableProperty AutoWireViewModelProperty =
BindableProperty.CreateAttached("AutoWireViewModel", typeof(bool), typeof(ViewModelLocator), default(bool), propertyChanged: OnAutoWireViewModelChanged);
public static bool GetAutoWireViewModel(BindableObject bindable) =>
(bool)bindable.GetValue(AutoWireViewModelProperty);
public static void SetAutoWireViewModel(BindableObject bindable, bool value) =>
bindable.SetValue(AutoWireViewModelProperty, value);
static ITypeMapperService mapper = (Application.Current as App).DependencyResolver.Resolve<ITypeMapperService>();
static void OnAutoWireViewModelChanged(BindableObject bindable, object oldValue, object newValue)
{
var view = bindable as Element;
var viewType = view.GetType();
var viewModelType = mapper.MapViewToViewModel(viewType);
var viewModel = (Application.Current as App).DependencyResolver.Resolve(viewModelType);
view.BindingContext = viewModel;
}
}
// Usage example
<?xml version="1.0" encoding="utf-8"?>
<ContentPage
xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:viewmodels="clr-namespace:MyApp.ViewModel"
viewmodels:ViewModelLocator.AutoWireViewModel="true"
x:Class="MyApp.Views.MyPage">
</ContentPage>
4.Finally we will need a NavigationService that will support ViewModel First Navigation approach:
public class NavigationService
{
TypeMapperService mapperService { get; }
public NavigationService(TypeMapperService mapperService)
{
this.mapperService = mapperService;
}
protected Page CreatePage(Type viewModelType)
{
Type pageType = mapperService.MapViewModelToView(viewModelType);
if (pageType == null)
{
throw new Exception($"Cannot locate page type for {viewModelType}");
}
return Activator.CreateInstance(pageType) as Page;
}
protected Page GetCurrentPage()
{
var mainPage = Application.Current.MainPage;
if (mainPage is MasterDetailPage)
{
return ((MasterDetailPage)mainPage).Detail;
}
// TabbedPage : MultiPage<Page>
// CarouselPage : MultiPage<ContentPage>
if (mainPage is TabbedPage || mainPage is CarouselPage)
{
return ((MultiPage<Page>)mainPage).CurrentPage;
}
return mainPage;
}
public Task PushAsync(Page page, bool animated = true)
{
var navigationPage = Application.Current.MainPage as NavigationPage;
return navigationPage.PushAsync(page, animated);
}
public Task PopAsync(bool animated = true)
{
var mainPage = Application.Current.MainPage as NavigationPage;
return mainPage.Navigation.PopAsync(animated);
}
public Task PushModalAsync<TViewModel>(object parameter = null, bool animated = true) where TViewModel : BaseViewModel =>
InternalPushModalAsync(typeof(TViewModel), animated, parameter);
public Task PopModalAsync(bool animated = true)
{
var mainPage = GetCurrentPage();
if (mainPage != null)
return mainPage.Navigation.PopModalAsync(animated);
throw new Exception("Current page is null.");
}
async Task InternalPushModalAsync(Type viewModelType, bool animated, object parameter)
{
var page = CreatePage(viewModelType);
var currentNavigationPage = GetCurrentPage();
if (currentNavigationPage != null)
{
await currentNavigationPage.Navigation.PushModalAsync(page, animated);
}
else
{
throw new Exception("Current page is null.");
}
await (page.BindingContext as BaseViewModel).InitializeAsync(parameter);
}
}
As you may see there is a BaseViewModel - abstract base class for all the ViewModels where you can define methods like InitializeAsync that will get executed right after the navigation. And here is an example of navigation:
public class WelcomeViewModel : BaseViewModel
{
public ICommand NewGameCmd { get; }
public ICommand TopScoreCmd { get; }
public ICommand AboutCmd { get; }
public WelcomeViewModel(INavigationService navigation) : base(navigation)
{
NewGameCmd = new Command(async () => await Navigation.PushModalAsync<GameViewModel>());
TopScoreCmd = new Command(async () => await navigation.PushModalAsync<TopScoreViewModel>());
AboutCmd = new Command(async () => await navigation.PushModalAsync<AboutViewModel>());
}
}
As you understand this approach is more complicated, harder to debug and might be confusing. However there are many advantages plus you actually don't have to implement it yourself since most of the MVVM frameworks support it out of the box. The code example that is demonstrated here is available on github. There are plenty of good articles about ViewModel First Navigation approach and there is a free Enterprise Application Patterns using Xamarin.Forms eBook which is explaining this and many other interesting topics in detail.
By using the PushAsync() method you can push and PopModalAsync() you can pop pages to and from the navigation stack. In my code example below I have a Navigation page (Root Page) and from this page I push a content page that is a login page once I am complete with my login page I pop back to the root page
~~~ Navigation can be thought of as a last-in, first-out stack of Page objects.To move from one page to another an application will push a new page onto this stack. To return back to the previous page the application will pop the current page from the stack. This navigation in Xamarin.Forms is handled by the INavigation interface
Xamarin.Forms has a NavigationPage class that implements this interface and will manage the stack of Pages. The NavigationPage class will also add a navigation bar to the top of the screen that displays a title and will also have a platform appropriate Back button that will return to the previous page. The following code shows how to wrap a NavigationPage around the first page in an application:
Reference to content listed above and a link you should review for more information on Xamarin Forms, see the Navigation section:
http://developer.xamarin.com/guides/cross-platform/xamarin-forms/introduction-to-xamarin-forms/
~~~
public class MainActivity : AndroidActivity
{
protected override void OnCreate(Bundle bundle)
{
base.OnCreate(bundle);
Xamarin.Forms.Forms.Init(this, bundle);
// Set our view from the "main" layout resource
SetPage(BuildView());
}
static Page BuildView()
{
var mainNav = new NavigationPage(new RootPage());
return mainNav;
}
}
public class RootPage : ContentPage
{
async void ShowLoginDialog()
{
var page = new LoginPage();
await Navigation.PushModalAsync(page);
}
}
//Removed code for simplicity only the pop is displayed
private async void AuthenticationResult(bool isValid)
{
await navigation.PopModalAsync();
}
In App.Xaml.Cs:
MainPage = new NavigationPage( new YourPage());
When you wish to navigate from YourPage to the next page you do:
await Navigation.PushAsync(new YourSecondPage());
You can read more about Xamarin Forms navigation here: https://learn.microsoft.com/en-us/xamarin/xamarin-forms/app-fundamentals/navigation/hierarchical
Microsoft has quite good docs on this.
There is also the newer concept of the Shell. It allows for a new way of structuring your application and simplifies navigation in some cases.
Intro: https://devblogs.microsoft.com/xamarin/shell-xamarin-forms-4-0-getting-started/
Video on basics of Shell: https://www.youtube.com/watch?v=0y1bUAcOjZY&t=3112s
Docs: https://learn.microsoft.com/en-us/xamarin/xamarin-forms/app-fundamentals/shell/
Call:
((App)App.Current).ChangeScreen(new Map());
Create this method inside App.xaml.cs:
public void ChangeScreen(Page page)
{
MainPage = page;
}
In Xamarin we have page called NavigationPage. It holds stack of ContentPages.
NavigationPage has method like PushAsync() and PopAsync(). PushAsync add a page at the top of the stack, at that time that page will become the currently active page. PopAsync() method remove the page from the top of the stack.
In App.Xaml.Cs set like.
MainPage = new NavigationPage( new YourPage());
From YourPage you await Navigation.PushAsync(new newPage()); this method will add newPage at the top of the stack. At this time newPage will be currently active page.
One page to another page navigation in Xamarin.forms using Navigation property Below sample code
void addClicked(object sender, EventArgs e)
{
//var createEmp = (Employee)BindingContext;
Employee emp = new Employee();
emp.Address = AddressEntry.Text;
App.Database.SaveItem(emp);
this.Navigation.PushAsync(new EmployeeDetails());
this.Navigation.PushModalAsync(new EmployeeDetails());
}
To navigate one page to another page with in view cell Below code Xamrian.forms
private async void BtnEdit_Clicked1(object sender, EventArgs e)
{
App.Database.GetItem(empid);
await App.Current.MainPage.Navigation.PushModalAsync(new EmployeeRegistration(empid));
}
Example like below
public class OptionsViewCell : ViewCell
{
int empid;
Button btnEdit;
public OptionsViewCell()
{
}
protected override void OnBindingContextChanged()
{
base.OnBindingContextChanged();
if (this.BindingContext == null)
return;
dynamic obj = BindingContext;
empid = Convert.ToInt32(obj.Eid);
var lblname = new Label
{
BackgroundColor = Color.Lime,
Text = obj.Ename,
};
var lblAddress = new Label
{
BackgroundColor = Color.Yellow,
Text = obj.Address,
};
var lblphonenumber = new Label
{
BackgroundColor = Color.Pink,
Text = obj.phonenumber,
};
var lblemail = new Label
{
BackgroundColor = Color.Purple,
Text = obj.email,
};
var lbleid = new Label
{
BackgroundColor = Color.Silver,
Text = (empid).ToString(),
};
//var lbleid = new Label
//{
// BackgroundColor = Color.Silver,
// // HorizontalOptions = LayoutOptions.CenterAndExpand
//};
//lbleid.SetBinding(Label.TextProperty, "Eid");
Button btnDelete = new Button
{
BackgroundColor = Color.Gray,
Text = "Delete",
//WidthRequest = 15,
//HeightRequest = 20,
TextColor = Color.Red,
HorizontalOptions = LayoutOptions.EndAndExpand,
};
btnDelete.Clicked += BtnDelete_Clicked;
//btnDelete.PropertyChanged += BtnDelete_PropertyChanged;
btnEdit = new Button
{
BackgroundColor = Color.Gray,
Text = "Edit",
TextColor = Color.Green,
};
// lbleid.SetBinding(Label.TextProperty, "Eid");
btnEdit.Clicked += BtnEdit_Clicked1; ;
//btnEdit.Clicked += async (s, e) =>{
// await App.Current.MainPage.Navigation.PushModalAsync(new EmployeeRegistration());
//};
View = new StackLayout()
{
Orientation = StackOrientation.Horizontal,
BackgroundColor = Color.White,
Children = { lbleid, lblname, lblAddress, lblemail, lblphonenumber, btnDelete, btnEdit },
};
}
private async void BtnEdit_Clicked1(object sender, EventArgs e)
{
App.Database.GetItem(empid);
await App.Current.MainPage.Navigation.PushModalAsync(new EmployeeRegistration(empid));
}
private void BtnDelete_Clicked(object sender, EventArgs e)
{
// var eid = Convert.ToInt32(empid);
// var item = (Xamarin.Forms.Button)sender;
int eid = empid;
App.Database.DeleteItem(empid);
}
}
After PushAsync use PopAsync (with this) to remove current page.
await Navigation.PushAsync(new YourSecondPage());
this.Navigation.PopAsync(this);
XAML page add this
<ContentPage.ToolbarItems>
<ToolbarItem Text="Next" Order="Primary"
Activated="Handle_Activated"/>
</ContentPage.ToolbarItems>
on the CS page
async void Handle_Activated(object sender, System.EventArgs e)
{
await App.Navigator.PushAsync(new PAGE());
}