To provide some context, I'm writing a Xamarin.Forms application and utilizing data binding with INotifyPropertyChanged. Currently I have an inventory counter displayed on a button. The text on this button displays the bounded "Count" variable (e.g Current Inventory: 35). When I press the button, I push a screen onto the navigation stack which allows me to edit this "Count" variable. I use the class implementation like this
public class UserInventory : INotifyPropertyChanged
{
private int count = 0;
// Declare the event
public event PropertyChangedEventHandler PropertyChanged;
public int Count
{
get => Preferences.Get(nameof(Count),0);
set
{
if (count == value || value <1)
return;
Preferences.Set(nameof(Count), value);
//count = value;
//Application.Current.Properties["Count"] = count;
OnPropertyChanged(nameof(Count));
//OnPropertyChanged(nameof(displayName));
}
}
public UserInventory()
{
}
void OnPropertyChanged(string count)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(count));
}
}
I add this class in Xaml according to the tutorial on the Xamarin <ContentPage.BindingContext>
<local:UserInventory />
</ContentPage.BindingContext>
So the variables are bounded correctly and I have no issues seeing updates on the current page or when I push new pages. The issue is when I swipe back on iOS the previous screen with the button "Current Inventory: 35" does not update to reflect the new changes. If I push that screen the changes are reflected.
Is there anyway to ensure the bounded data is updated when you go back (PopAsync()) ?
Try overriding page's OnAppearing() method and call OnPropertyChanged from there.
Assuming 'UserInventory' the binded VM.....
public partial class Page1:ContentPage
{
public Page1()
{
InitializeComponent();
VM = (UserInventory)BindingContext;
}
public UserInventory VM { get; }
protected override void OnAppearing()
{
VM.Notify();
base.OnAppearing();
}
}
.
public class UserInventory: INotifyPropertyChanged
{
........
public void Notify()
{
OnPropertyChanged(nameof(Count));
}
}
I tried to use the back navigation by overriding OnBackButtonPressed, but somehow it wasn't get called at all. I am using the ContentPage and the latest 1.4.2 release.
Alright, after many hours I figured this one out. There are three parts to it.
#1 Handling the hardware back button on android. This one is easy, override OnBackButtonPressed. Remember, this is for a hardware back button and android only. It will not handle the navigation bar back button. As you can see, I was trying to back through a browser before backing out of the page, but you can put whatever logic you need in.
protected override bool OnBackButtonPressed()
{
if (_browser.CanGoBack)
{
_browser.GoBack();
return true;
}
else
{
//await Navigation.PopAsync(true);
base.OnBackButtonPressed();
return true;
}
}
#2 iOS navigation back button. This one was really tricky, if you look around the web you'll find a couple examples of replacing the back button with a new custom button, but it's almost impossible to get it to look like your other pages. In this case I made a transparent button that sits on top of the normal button.
[assembly: ExportRenderer(typeof(MyAdvantagePage), typeof
(MyAdvantagePageRenderer))]
namespace Advantage.MyAdvantage.MobileApp.iOS.Renderers
{
public class MyAdvantagePageRenderer : Xamarin.Forms.Platform.iOS.PageRenderer
{
public override void ViewWillAppear(bool animated)
{
base.ViewWillAppear(animated);
if (((MyAdvantagePage)Element).EnableBackButtonOverride)
{
SetCustomBackButton();
}
}
private void SetCustomBackButton()
{
UIButton btn = new UIButton();
btn.Frame = new CGRect(0, 0, 50, 40);
btn.BackgroundColor = UIColor.Clear;
btn.TouchDown += (sender, e) =>
{
// Whatever your custom back button click handling
if (((MyAdvantagePage)Element)?.
CustomBackButtonAction != null)
{
((MyAdvantagePage)Element)?.
CustomBackButtonAction.Invoke();
}
};
NavigationController.NavigationBar.AddSubview(btn);
}
}
}
Android, is tricky. In older versions and future versions of Forms once fixed, you can simply override the OnOptionsItemselected like this
public override bool OnOptionsItemSelected(IMenuItem item)
{
// check if the current item id
// is equals to the back button id
if (item.ItemId == 16908332)
{
// retrieve the current xamarin forms page instance
var currentpage = (MyAdvantagePage)
Xamarin.Forms.Application.
Current.MainPage.Navigation.
NavigationStack.LastOrDefault();
// check if the page has subscribed to
// the custom back button event
if (currentpage?.CustomBackButtonAction != null)
{
// invoke the Custom back button action
currentpage?.CustomBackButtonAction.Invoke();
// and disable the default back button action
return false;
}
// if its not subscribed then go ahead
// with the default back button action
return base.OnOptionsItemSelected(item);
}
else
{
// since its not the back button
//click, pass the event to the base
return base.OnOptionsItemSelected(item);
}
}
However, if you are using FormsAppCompatActivity, then you need to add onto your OnCreate in MainActivity this to set your toolbar:
Android.Support.V7.Widget.Toolbar toolbar = this.FindViewById<Android.Support.V7.Widget.Toolbar>(Resource.Id.toolbar);
SetSupportActionBar(toolbar);
But wait! If you have too old a version of .Forms or too new version, a bug will come up where toolbar is null. If this happens, the hacked together way I got it to work to make a deadline is like this. In OnCreate in MainActivity:
MobileApp.Pages.Articles.ArticleDetail.androdAction = () =>
{
Android.Support.V7.Widget.Toolbar toolbar = this.FindViewById<Android.Support.V7.Widget.Toolbar>(Resource.Id.toolbar);
SetSupportActionBar(toolbar);
};
ArticleDetail is a Page, and androidAction is an Action that I run on OnAppearing if the Platform is Android on my page. By this point in your app, toolbar will no longer be null.
Couple more steps, the iOS render we made above uses properties that you need to add to whatever page you are making the renderer for. I was making it for my MyAdvantagePage class that I made, which implements ContentPage . So in my MyAdvantagePage class I added
public Action CustomBackButtonAction { get; set; }
public static readonly BindableProperty EnableBackButtonOverrideProperty =
BindableProperty.Create(
nameof(EnableBackButtonOverride),
typeof(bool),
typeof(MyAdvantagePage),
false);
/// <summary>
/// Gets or Sets Custom Back button overriding state
/// </summary>
public bool EnableBackButtonOverride
{
get
{
return (bool)GetValue(EnableBackButtonOverrideProperty);
}
set
{
SetValue(EnableBackButtonOverrideProperty, value);
}
}
Now that that is all done, on any of my MyAdvantagePage I can add this
:
this.EnableBackButtonOverride = true;
this.CustomBackButtonAction = async () =>
{
if (_browser.CanGoBack)
{
_browser.GoBack();
}
else
{
await Navigation.PopAsync(true);
}
};
That should be everything to get it to work on Android hardware back, and navigation back for both android and iOS.
You are right, in your page class override OnBackButtonPressed and return true if you want to prevent navigation. It works fine for me and I have the same version.
protected override bool OnBackButtonPressed()
{
if (Condition)
return true;
return base.OnBackButtonPressed();
}
Depending on what exactly you are looking for (I would not recommend using this if you simply want to cancel back button navigation), OnDisappearing may be another option:
protected override void OnDisappearing()
{
//back button logic here
}
OnBackButtonPressed() this will be called when a hardware back button is pressed as in android. This will not work on the software back button press as in ios.
Additional to Kyle Answer
Set
Inside YOURPAGE
public static Action SetToolbar;
YOURPAGE OnAppearing
if (Device.RuntimePlatform == Device.Android)
{
SetToolbar.Invoke();
}
MainActivity
YOURPAGE.SetToolbar = () =>
{
Android.Support.V7.Widget.Toolbar toolbar =
this.FindViewById<Android.Support.V7.Widget.Toolbar>(Resource.Id.toolbar);
SetSupportActionBar(toolbar);
};
I use Prism libray and for handle the back button/action I extend INavigatedAware interface of Prism on my page and I implement this methods:
public void OnNavigatedFrom(INavigationParameters parameters)
{
if (parameters.GetNavigationMode() == NavigationMode.Back)
{
//Your code
}
}
public void OnNavigatedTo(INavigationParameters parameters)
{
}
Method OnNavigatedFrom is raised when user press back button from Navigation Bar (Android & iOS) and when user press Hardware back button (only for Android).
For anyone still fighting with this issue - basically you cannot intercept back navigation cross-platform. Having said that there are two approaches that effectively solve the problem:
Hide the NavigationPage back button with NavigationPage.ShowHasBackButton(this, false) and push a modal page that has a custom Back/Cancel/Close button
Intercept the back navigation natively for each platform. This is a good article that does it for iOS and Android: https://theconfuzedsourcecode.wordpress.com/2017/03/12/lets-override-navigation-bar-back-button-click-in-xamarin-forms/
For UWP you are on your own :)
Edit:
Well, not anymore since I did it :) It actually turned out to be pretty easy – there is just one back button and it’s supported by Forms so you just have to override ContentPage’s OnBackButtonPressed:
protected override bool OnBackButtonPressed()
{
if (Device.RuntimePlatform.Equals(Device.UWP))
{
OnClosePageRequested();
return true;
}
else
{
base.OnBackButtonPressed();
return false;
}
}
async void OnClosePageRequested()
{
var tdvm = (TaskDetailsViewModel)BindingContext;
if (tdvm.CanSaveTask())
{
var result = await DisplayAlert("Wait", "You have unsaved changes! Are you sure you want to go back?", "Discard changes", "Cancel");
if (result)
{
tdvm.DiscardChanges();
await Navigation.PopAsync(true);
}
}
else
{
await Navigation.PopAsync(true);
}
}
protected override bool OnBackButtonPressed()
{
base.OnBackButtonPressed();
return true;
}
base.OnBackButtonPressed() returns false on click of hardware back button.
In order to prevent operation of back button or prevent navigation to previous page. the overriding function should be returned as true. On return true, it stays on the current xamarin form page and state of page is also maintained.
The trick is to implement your own navigation page that inherits from NavigationPage. It has the appropriate events Pushed, Popped and PoppedToRoot.
A sample implementation could look like this:
public class PageLifetimeSupportingNavigationPage : NavigationPage
{
public PageLifetimeSupportingNavigationPage(Page content)
: base(content)
{
Init();
}
private void Init()
{
Pushed += (sender, e) => OpenPage(e.Page);
Popped += (sender, e) => ClosePage(e.Page);
PoppedToRoot += (sender, e) =>
{
var args = e as PoppedToRootEventArgs;
if (args == null)
return;
foreach (var page in args.PoppedPages.Reverse())
ClosePage(page);
};
}
private static void OpenPage(Page page)
{
if (page is IPageLifetime navpage)
navpage.OnOpening();
}
private static void ClosePage(Page page)
{
if (page is IPageLifetime navpage)
navpage.OnClosed();
page.BindingContext = null;
}
}
Pages would implement the following interface:
public interface IPageLifetime
{
void OnOpening();
void OnClosed();
}
This interface could be implemented in a base class for all pages and then delegate it's calls to it's view model.
The navigation page and could be created like this:
var navigationPage = new PageLifetimeSupportingNavigationPage(new MainPage());
MainPage would be the root page to show.
Of course you could also just use NavigationPage in the first place and subscribe to it's events without inheriting from it.
Maybe this can be usefull, You need to hide the back button, and then replace with your own button:
public static UIViewController AddBackButton(this UIViewController controller, EventHandler ev){
controller.NavigationItem.HidesBackButton = true;
var btn = new UIBarButtonItem(UIImage.FromFile("myIcon.png"), UIBarButtonItemStyle.Plain, ev);
UIBarButtonItem[] items = new[] { btn };
controller.NavigationItem.LeftBarButtonItems = items;
return controller;
}
public static UIViewController DeleteBack(this UIViewController controller)
{
controller.NavigationItem.LeftBarButtonItems = null;
return controller;
}
Then call them into these methods:
public override void ViewWillAppear(bool animated)
{
base.ViewWillAppear(animated);
this.AddBackButton(DoSomething);
UpdateFrames();
}
public override void ViewWillDisappear(Boolean animated)
{
this.DeleteBackButton();
}
public void DoSomething(object sender, EventArgs e)
{
//Do a barrel roll
}
Another way around is to use Rg.Plugins.Popup Which allows you to implement nice popup. It uses another NavigationStack => Rg.Plugins.Popup.Services.PopupNavigation.Instance.PopupStack. So your page won't be wrap around the NavigationBar.
In your case I would simply
Create a full page popup with opaque background
Override ↩️ OnBackButtonPressed for Android on ⚠️ParentPage⚠️ with something like this:
protected override bool OnBackButtonPressed()
{
return Rg.Plugins.Popup.Services.PopupNavigation.Instance.PopupStack.Any();
}
Since the back-button affect the usual NavigationStack your parent would pop out whenever the user try to use it while your "popup is showing".
Now what? Xaml what ever you want to properly close your popup with all the check you want.
💥 Problem solved for these targets💥
[x] Android
[x] iOS
[-] Windows Phone (Obsolete. Use v1.1.0-pre5 if WP is needed)
[x] UWP (Min Target: 10.0.16299)
I want extend an System.Windows.Forms.Panel(just inherit) and using a custom ControlDesigner.
I use a very minimalistic ControlDesigner implementation, just overwrite GetHitTest.
The problem is my custom panel instance is not ready to contains child controls any longer.
I play a little bit with AssociatedComponents but without effect. Remove custom designer attribute and it works great.
can someone help me to pin point whats wrong ???
[Designer(typeof(MyPanelDesigner)), ToolboxItem(true)]
public class MyPanel : System.Windows.Forms.Panel
{
// empty except for OnPaint
}
internal class DrawPanelDesigner : ControlDesigner
{
private MyPanel ParentControl
{
get
{
return Control as MyPanel;
}
}
public override System.Collections.ICollection AssociatedComponents
{
get
{
return ParentControl.Controls;
}
}
protected override bool GetHitTest(System.Drawing.Point point)
{
// hit detection for some owner drawed items in OnPaint
point = ParentControl.PointToClient(point);
var item = ParentControl.View.GetItemFromViewPoint(point.X, point.Y, true);
return null != item;
}
You are using the wrong designer. Try inheriting from the ScrollableControlDesigner instead:
internal class DrawPanelDesigner : ScrollableControlDesigner {
public DrawPanelDesigner() {
AutoResizeHandles = true;
}
private MyPanel ParentControl {
get {
return Control as MyPanel;
}
}
protected Pen BorderPen {
get {
Color penColor = Control.BackColor.GetBrightness() < .5 ?
ControlPaint.Light(Control.BackColor) :
ControlPaint.Dark(Control.BackColor);
Pen pen = new Pen(penColor);
pen.DashStyle = DashStyle.Dash;
return pen;
}
}
protected virtual void DrawBorder(Graphics graphics) {
Panel panel = (Panel)Component;
if (panel == null || !panel.Visible) {
return;
}
Pen pen = BorderPen;
Rectangle rc = Control.ClientRectangle;
rc.Width--;
rc.Height--;
graphics.DrawRectangle(pen, rc);
pen.Dispose();
}
protected override void OnPaintAdornments(PaintEventArgs pe) {
Panel panel = (Panel)Component;
if (panel.BorderStyle == BorderStyle.None) {
DrawBorder(pe.Graphics);
}
base.OnPaintAdornments(pe);
}
}
My application using a 'vertical field' manager and i add some text to it using a custonField class.The vertical manager constructor is:
scrollingRegion = new VerticalFieldManager(
USE_ALL_HEIGHT| VERTICAL_SCROLL | VERTICAL_SCROLLBAR|USE_ALL_WIDTH);
I also craete menu using:
protected void makeMenu(Menu menu, int context) {
menu.add(_imageMenuItem);
//super.makeMenu(menu, context);
}
class ImageMenuItem extends MenuItem {
/**
* Creates a new MenuDemoMenuItem object
*/
ImageMenuItem() {
super("Login Screen", 0, 0);
}
public boolean onMenu(int i) {
return false;
}
public void run() {
UiApplication app = (UiApplication) getApplication();
app.pushScreen(new LoginScreen());
}
}
My problem is when I click in the vertical field(which uses a table row manager), the menu is displayed. How can I avoid it? Can any one suggest a solution
if the item is ButtonField you can consume click by define it in the constructor
ButtonField btn = new ButtonField("Login", ButtonField.CONSUME_CLICK);
I have an windows forms application, with a form that holds 2 tabcontrols and a grid. I'd like to catch the pressing of esc key on any on this controls.
The question is : is it a simpler way to do that without subscribing to the keypress event on each control ?
Thanks!
You can Simply Do following.
Implement an IMessageFilter and Handle Key Down event.
Here is the complete Code to hook Escape Key Down.
public class MyKeboardHook:IMessageFilter
{
public const int WM_KEYDOWN = 0x0100;
public const int VK_ESCAPE = 0x1B;
public event EventHandler EscapeKeyDown;
public bool PreFilterMessage(ref Message m)
{
if (m.Msg == WM_KEYDOWN && m.WParam == new IntPtr(VK_ESCAPE))
{
OnEscapeKeyPressed();
}
return false; //Do not Process anything
}
protected virtual void OnEscapeKeyDown()
{
if(this.EscapeKeyDown!=null)
{
EscapeKeyDown(this, EventArgs.Empty);
}
}
}
Now you need to register this. The best place would be in Main
static class Program
{
/// <summary>
/// The main entry point for the application.
/// </summary>
[STAThread]
static void Main()
{
Application.EnableVisualStyles();
Application.SetCompatibleTextRenderingDefault(false);
MyKeboardHook myKeboardHook = new MyKeboardHook();
myKeboardHook.EscapeKeyDown += (e, x) =>
{
MessageBox.Show("Escape Key Pressed");
};
Application.AddMessageFilter(myKeboardHook);
Application.Run(new Form1());
}
}
Subscribe to the event on the form itself.
If the control doesn't handle the event it should bubble up to the form where it will be handled.