I need to run some code to get a user's username and department when they first connect to my Blazor Server Side application. I could just do this using OnInitialized() but that appears to only work on the one page in which it was placed. Users will likely be sent separate links to different pages though and I don't want to have to place this code on every page. I discovered that I can place code in my main layout and it will run no matter what page I start on but it runs on every page change and it doesn't allow me to run things asynchronously so that's not ideal. I'm looking for something like a Global.asax but in Blazor if that makes sense.
Edit: Turns out I can run things asynchronously in my layout! I just needed to create a code block like any other razor page. Makes sense. Though It's still weird that we have to put this type of code in the layout. It just doesn't feel right.
This is what I do:
Create a state object (class) that can be injected where needed. This is somewhat like session, but can also have global events. See here.
Add it to IoC in Startup.cs. Background info here
services.AddScoped<MyState>();
Initialize it in MainLayout.razor or elsewhere:
if (MyState.User == null)
{
MyState.User = authService.User;
}
Instantiate in pages/components as needed:
[Inject]
public MyState myState { get; set; }
...
myObj.CreatedBy = myState.User.UserName;
Related
I have an app with four main pages, switched through a tab bar (no "back" button).
One page has a lot of content (ScrollView) and takes quite a few seconds until it's rendered. I handle that by showing a "loading" overlay while the work is done. But for that specific page I'd like to keep the view alive, so that when the user switches to another page and comes back later, the page is ready without loading everything again.
I'm not sure how to do that in MvvmCross, though.
I did read the documentation and from what I understood the View Presenter would be the right way to do it, since the docs say:
"Another kind of presentation changes your app can request through
hints includes clearing / modifying the BackStack, changing a root
while maintaining the existent views, … possibilities are really
endless. Once again your app is king here!"
I guess I would need to create a custom MvxPresentationHint for that, but I don't quite get it :(
How or rather where would I access and store/load the View?
I'm generally still quite unfamiliar with MvvmCross (how it works under the hood) and especially customization of Mvx classes, even though I've been using it for a while.
Any explanation and preferably code examples beyond what's written in the documentation would be extremely appreciated!
It isn't meaningful to attempt to "store" a view in MVVM. The XF view is a representation of what will be created with native (e.g. "Android" or "iOS") widgets. Creating and measuring/laying out those native widgets is what is slow. MVVM View Presenter won't speed up that logic.
Instead of "store", you need "keep alive":
For a ContentPage called MyPage, when you create it, store it in a static variable. Then re-use that variable. If you never need more than one of these, you can store it in the class itself.
Modify the "code behind", MyPage.xaml.cs:
public partial class MyPage : ContentPage
{
// Singleton Pattern.
private static MyPage _it;
public static MyPage It {
get {
if (_it == null)
_it = new MyPage();
return _it;
}
}
// "private", because calling this directly defeats the purpose. Instead, use `MyPage.It`.
private MyPage()
{
InitializeComponent();
}
}
To create it, whereever you would put:
new MyPage()
instead put this:
MyPage.It
For instance, you might do PushAsync(MyPage.It);
This will always return the SAME INSTANCE of MyPage. So once it has been created, it keeps its state.
IMPORTANT: Note that the constructor is only called ONCE. Any code that needs to be done each time the page appears, put in override .. OnAppearing() method.
LIMITATION: Views "expect" to be part of the visual hierarchy when they are manipulated. If you attempt to alter the page or its view model while it is not on the screen, you may encounter problems. Those are beyond the scope of this answer - create a new StackOverflow question with the details of any problem you encounter.
In PrismForms we got the problem, that the NavigationStack is empty after navigating to a new page. That means after using the hardware back-button on the SecondPage, the app is closed. Although the back-arrow in the header on Android isnt shown. If looking closely you can see the back-arrow for a short moment after the page is switched. I guess thats before the NavigationStack gets cleared.
To the first page we navigate with the following command in OnInitialized() in our App.xaml.cs which derives from PrismApplication.
NavigationService.NavigateAsync("NavigationPage/StartPage");
(If only Navigating to „StartPage“ here, the Stack doesnt get cleared.)
That has do to with PageNavigationService.ProcessNavigationForNavigationPage(...) calling
bool clearNavStack = GetClearNavigationPageNavigationStack(currentPage); and PageNavigationService.ProcessNavigationForContentPage(...) not.
From the StartPage to the next we navigate with NavigateAsync("SecondPage")“. Here the described behaviour appears.
For navigation we use a class which wraps the Prism NavigationService. We hold him as a property and get him via Unity in our constructor:
this.PrismNavigation = prismNavigation ?? throw new ArgumentNullException(nameof(prismNavigation));
The methods „NavigateAsync“ and „GoBackAsync“, etc. we just pass through.
This way we want to seperate our ViewModel-Project from references to XamarinForms to later be able to use the same ViewModels for for example a WPF-GUI.
Why is the stack beeing cleared by our own NavigationService? If we register the original Prism NavigationService in App.xaml.cs instead, navigating back works as expected again. We found the point in the framework and could avoid the clearing with a drity hack, but that’s against the navigation-logic implemented in PrismForms, but we don’t understand how to do it the correct way.
Every help appreciated!
We edited a few things to get it working after finding some interesting information by Brian Lagunas in the forlast-post here:
https://github.com/PrismLibrary/Prism/issues/591
Although the topic was about something else, it led to improvements for overwriting the Navigation Service.
Remember that in your viewModels the Navigation Service must be named "navigationService" by convention. Also we switched from just holding the Prism Navigation Service as a parameter to deriving from it as suggested in the link above.
public class MyNavigationService : UnityPageNavigationService
I'm trying to make it so the links that display on the nav bar from the _Layout view change depedning on whether you are logged in. This is using a SessionStateRepository which I use by initializing it in my controllers.
On the layout page I want to be able to check if
state.GetLoginState().email = "anon"
This is so I can change if the login button is there or not. I just can't find a way to use it on a shared view which goes on all pages.
Well, using session state with MVC is a topic of religious debate (for number of good reasons ) and you should definitely read up on that.
But to answer your question, with traditional session class, you can do this
#if (HttpContext.Current.Session["email"] == "anon")
{
<span>Value1</span>
}
else
{
<span>Value2</span>
}
you will have to devise specific implementation on the similar lines based on your repository
i am using custom url scheme in my flex iOS app and it’s working fine when i start my app from a web link..but the issue is when i start my app from start menu in iPad and move to webpage in safari.In page i click a button that redirects it to my app, at that time app call “preinitialize” method more than once..it wary every time, some time it’s 2,3,4 and different one each time..i don't know why it’s behaving like this..can i know the reason please its urgent..
thanks…any help will be appreciated.
Create a flag initialized and set it to true when preinitialize is called. All other calls to this method can be filtered out. Simple example:
private var initialized:Boolean = false;
public function preinitialize():void
{
if (initialized) return;
initialized = true;
}
Next step would be finding the real cause of the multiple calls, but for that we would need to see some code of your app.
I have to design such that whenever user pass a query I process it
using servlet and then call the js page to draw the chart
1> user writes a query on a page
2> the page call the servelt class
public class MyServlet extends Httpservlet implements
DataSourceServlet {..... return data
The user see a beautiful string like this..
google.visualization.Query.setResponse......... /Tiger'},{v:80.0},
{v:false}]}]}});
3> when the user hits on different html page myhtml.js it draws the
chart.
I want the Myservlet class itself call the myhtml.js page and draw
the chart directly.
and want to eliminate the beautiful string
google.visualization.Query.setResponse......... /Tiger'},{v:80.0},
{v:false}]}]}});
from coming on user's browser
What should i do?
I tried using functions to call another page like request
dispatcher(), redirect() calling myhtml.js page directly after
myservlet process the query results.
But i get the result like this
google.visualization.Query.setResponse......... /Tiger'},{v:80.0},
{v:false}]}]}});
and the entire myhtml.js code page below it on the browsers that to
without the chart been draw.
Is there anyway to element the beautiful string from coming on clients
browser
and only show them the chart been drawn ? :)
This is the small tutorial i am following
http://code.google.com/apis/visualization/documentation/dev/dsl_get_started.html
It is a bit difficult to follow, but I think you approach the problem from the wrong side.
As HTTP is essentially a PULL technology it is much easier to have the HTML page call the myhtml.js functionality and request the data from the servlet.
Calling browserside JavaScript from servlets is not really possible without considerable infrastructure which is probably not what you want, i.e. a lean/simple solution.