I have a question about Prism Navigation related on Xamarin.Forms.
Let's say that I have 3 pages, the Navigation stack looks like Navigation/ViewA/ViewB/ViewC. When navigate from ViewA to ViewB, I have a List to pass as parameter, so basically in ViewB I will use OnNavigatedTo method to get the parameters and set the data to some bindable properties.
Then, from ViewB to ViewC, I also need to pass that same parameter, so this parameter is kind of going though the 3 views.
Then problem happened, when I go back from ViewC to ViewB, ViewB will still call OnNavigatedTo to get the parameters, but this time, since it's navigated from ViewC, so ViewC did not have the code to pass that parameter to ViewB.
Then, ViewB cannot get the parameter, then if by that time when to go to ViewC again, ViewC will not have the data that it needs to bind to the bindable properties.
So, my question is: Do I have to pass the same parameter again from ViewC to ViewB? If that's the resolution, then wouldn't it be a little bit stupid to do so? Any solution that we can keep the state of the previous page and when we go back from ViewC, everything is just there?
Thanks and look forward to the solution or any insights on this.
I only reproduce the issue and I know I can pass back the parameter again, but that will definitely not be an ideal solution for this problem.
Let's see. In ViewA:
private async void Navigate()
{
var parameter = new NavigationParameter();
parameter.Add("SomeData", list);
await _navigationService.NavigateAsync("ViewB", parameter);
}
In View B:
public void OnNavigatedTo(NavigationParameter paramters)
{
// This code will be null when going back from ViewC
var list = parameters["SomeData"] as List<string>();
this.SomeBindableData = list;
}
ViewB used the same method as ViewA's navigate, and the problem occured when coming back from ViewC.
I would like an ideal solution for this problem. And please do help to check this by some guy from Prism team, I believe this is something needs to be handled.
While the View is in the Stack it won't be disposed or anything, so its state will be kept. Here you just have to know if your View is initialized or not:
In ViewB:
private bool _isInitialized;
public void OnNavigatedTo(NavigationParameter paramters)
{
if (_isInitialized)
return;
// This code will be null when going back from ViewC
var list = parameters["SomeData"] as List<string>();
this.SomeBindableData = list;
_isInitialized = true;
}
Related
I have an issue with handling popups. I have implemented ILifeSpanHandler and OnBeforeBrowse (amoungst others) from the IRequestHandler.
How do I know in the ILifeSpanHandler what URL is being called? I am unable to get it in either the OnAfterCreated or OnBeforePopup. Currently I see it first in OnBeforeBrowse.
I have no code as my question is a "How to". In OnBeforePopup I have checked targetUrl however it seems to be there for decoration as I have read that it is not implemented anyway. I have also looked at the browner/chromiumWebBrowser objects, browser and newBroswer seem to be nothing. One would expect in OnAfterCreated chromiumWebBrowser would return an object but it is nothing in my case.
I am testing with the following
Public Sub OnAfterCreated(chromiumWebBrowser As IWebBrowser, browser As IBrowser) Implements ILifeSpanHandler.OnAfterCreated
Try
Debug.Print(vbNewLine)
Debug.Print("OnAfterCreated")
Debug.Print(String.Concat("OnAfterCreated - MainFrame.Url "), browser.MainFrame.Url)
Debug.Print("OnAfterCreated")
Debug.Print(vbNewLine)
Catch ex As Exception
End Try
End Sub
And I have the following
Public Function OnBeforePopup(chromiumWebBrowser As IWebBrowser, browser As IBrowser, frame As IFrame, targetUrl As String, targetFrameName As String, targetDisposition As WindowOpenDisposition, userGesture As Boolean, popupFeatures As IPopupFeatures, windowInfo As IWindowInfo, browserSettings As IBrowserSettings, ByRef noJavascriptAccess As Boolean, ByRef newBrowser As IWebBrowser) As Boolean Implements ILifeSpanHandler.OnBeforePopup
Try
Debug.Print(vbNewLine)
Debug.Print("OnBeforePopup")
Debug.Print(String.Concat("OnBeforePopup - targetUrl "), targetUrl)
Debug.Print(String.Concat("OnBeforePopup - browser.MainFrame.Url "), browser.MainFrame.Url)
Debug.Print(String.Concat("OnBeforePopup - chromiumWebBrowser.Address "), chromiumWebBrowser.Address)
Debug.Print("OnBeforePopup")
Debug.Print(vbNewLine)
Catch ex As Exception
End Try
Return False
End Function
I have seen different approaches in handling popups using ILifeSpanHandler interface. One approach that I've seen also here in Stack Overflow and was accepted as the correct answer to that particular question is to return true in the OnBeforePopup implementation of ILifeSpanHandler then pass the targetURL argument to a handler that creates the popup.
This approach is very unideal because you are destroying the link between the page that actually opened the popup and the popup itself. If you access via JavaScript the opener property of the window object inside the popup you would notice that it is null. And the page that opened the popup would never know that the popup was actually opened because returning true cancels the creation.
The other approach is to let Cef create the popup and the programmer just decides whether to show the browser as a popup window or a child to control (like in tabbed browsing). This is error-free and almost ideal. But the problem with this approach is that you are not able to listen to events such as FrameLoadStart, FrameLoadEnd, AddressChanged, TitleChanged, etc.
One approach that is tagged experimental by the Cef developers is to return a new IWebBrowser instance via newWebBrowser out parameter. This has so many many side effects. The page that opened the popup would, of course, recognize the popup as his although it was not the original browser (IBrowser) that it created. The page may just ignore it like btcclicks.com and in that case, there'd be no problem. But there are websites like drops.xyz that is so particular with his stuff and will discard everything that is not originally his. So this is a problem.
So what is the correct approach?
The ChromeWebBrowser control
Now I'm going to share with you an undocumented approach in handling popups. Speaking of ChromeWebBrowser control, it is very much of help that we know how it creates the webbrowser which, in reality, it doesn't. The control just hosts the webbrowser window handle. It has a private field called managedCefBrowserAdapter (ManagedCefBrowserAdapter class) that handles the actual creation of the web browser. ChromiumWEbBrowser implements the IWebBrowserInternal that has a method OnAfterBrowserCreated with a single parameter of type IBrowser. The control then invokes browser.GetHost().GetWindowHandle() to get the actual window handle (HWND) of the webbrowser it is being hosted. It is quite good.
The problem of the ChromeWebBrowser is that it won't have a constructor that accepts an IBrowser as an argument. It only has constructor that accepts HtmlString, string and IRequestContext arguments. These control waits for the
invocation of OnHandleCreated (a base class override) where it calls the managedCefBrowserAdapter.CreateBrowser after which it waits till its implementation of IWebBrowserInternal's OnAfterBrowserCreated is invoked.
Again, what is the approach that works?
Now, this approach that actually works is a product of long series of trial and error. One caution though is that I don't know why and how it works but I know it works.
First, I did not use ChromeWebBrowser. But I copied its code omitting the part where it creates .net control. In this case, I am targeting the browser's window handle (HWND) to be host by any object that exposes a HWND. Obviously I created a class (NativeCefWebBrowser) that uses the modified code. The ChromeWebBrowser orignal constructors were still there untouched becuase they are used to the create the parent webrowser. But I added one constructor that accept the following arguments: ICefBrowserParent parent (an interface I've created and IBrowser browser that receives the browser argument in the ILifeSpanHandler's OnBeforePopup. I also added a public method AttachBrowser that has a single parameter IBrowser that recieves the IBrowser argument in the ILifeSpanHandler's OnAfterCreated. It the browser that will be kept by CefNativeWebBrowser class.
Why didn't I keep the browser instance received form ILifeSpanHandler.OnBeforePopup but used the instance received from ILifeSpanHandler.OnAfterCreated when they are the same browser instance? This is one of those parts that I don't know why. One thing I noticed is that when I called browser.GetHost().GetWindowHandle() during ILiffeSpanHandler.OnBeforePopup, the first window handle I received was the different compared to when I invoked the method during ILifeSpanHandler.OnAfterCreatd. Because of that, I store the browser instance from the latter that I passed to the NativeCefWebBrowser.AttachBrowser for its safekeeping.
In the NativeCefWebBrowser(ICefBrowserParent parent, IBrowser browser) contructor, I set the private following fields to true: browsercreated, browserinitialized (chromewebbrwoser orginal fields) and isAttachingBrowser (added private field). You don't call the ManagedCefBrowserAdapter's CreateBrowser in this contructor in instead call its OnAfterBrowserCreated passing the browser instance. You don't much in this constructor as you will wait the ILifeSpanHandler implementor to pass you the browser instance it will receive during its OnAfterCreated method. Take note that when calling the ManagedCefBrowserAdapter's OnAfterBrowserCreated method, ManagedCefBrowserAdapter will still invoke IWebBrowserInternal implementation of OnAfterBrowserCreated that when happens you have to exit immediately when isAttachingBrowser is true as the following code will no sense no more.
After calling the NativeCefWebBrowser(ICefBrowserParent, IBroser) construct, you can normally set event listeners as you will normally do.
And that's it.
The following are parts of the code that I wrote
The ICefBrowserParent interface
public interface ICefBrowserParent
{
IntPtr Handle { get; }
Size ClientSize { get; }
bool Disposing { get; }
bool IsDisposed { get; }
bool InvokeRequired { get; }
IAsyncResult BeginInvoke(Delegate d);
object Invoke(Delegate d);
event EventHandler Resize;
}
As you would notice, the methods, properties and events in this interface are already implemented by the System.Windowns.Forms.Control class. So if you implementing this from class inhering Control class, you would not need to implement this anymore. This interface is only for non-Control class.
class NativeCefWebBrowser
{
public NativeCefWebBrowser(ICefBrowserParent, IBroser)
{
requestContext = browser.GetHost().RequestContext;
this.parent = parent; // added field
HasParent = true; // IWebBrowserInternal. I don't know what's this for
mustSetBounds = true; // added field
browserCreated = true;
isAttachingBrowser = true; // added field
InitializeFieldsAndCefIfRequired();
managedCefBrowserAdapter.OnAfterBrowserCreated(browser);
}
}
ILifeSpanHandler.OnBeforePopup(..., out IWebBrowser newWebBrowser)
{
CefNativeWebBrowser b = new CefNativeWebBrowser
(
parent, // defined else where
browser
);
// Attach event handlers
b.TitleChanged...;
newWebBrowser = b;
}
ILifeSpanHandler.OnAfterCreated(...)
{
((CefNativeWebBrowser)webBrowser).AttachBrowser(browser);
}
I'm using MVVM Light for my application and I have also implemented the INavigationService for going back/for between pages.
So in a common scenario, it's like this
MainPage > Categories > Rounds > DataPage.
In the DataPage, I'm making a request to fetch the results and depending on the result returned from the callback I call the .GoBack() method to pop the current page from the stack and return to Rounds.
What I have noticed is that if I hit first the DataPage and the .GoBack() gets called and then tap on a different round the callback method will be fired twice, and if I go back and in again thrice, and continues like this.
Essentially this means that the .GoBack() will be called again and the navigation gets messed up.
I believe this has to do with not cleaning up the previous VM's, I tried changing this behavior with the UnRegister / Register class from SimpleIOC but no luck.
In the ViewModel class
public void UnsubscribeFromCallBack()
{
this.event -= method;
}
In the .xaml.cs page
protected override void OnDisappearing()
{
base.OnDisappearing();
PageViewModel vm = (this.BindingContext as PageViewModel);
vm.UnSubscribeFromCallback();
}
We are evaluating Grid Gain 6.5.5 at the moment as a potential solution for distribution of compute jobs over a grid.
The problem we are facing at the moment is a lack of a suitable asynchronous notification mechanism that will notify the sender asynchronously upon job completion (or future completion).
The prototype architecture is relatively simple and the core issue is presented in the pseudo code below (the full code cannot be published due to an NDA). *** Important - the code represents only the "problem", the possible solution in question is described in the text at the bottom together with the question.
//will be used as an entry point to the grid for each client that will submit jobs to the grid
public class GridClient{
//client node for submission that will be reused
private static Grid gNode = GridGain.start("config xml file goes here");
//provides the functionality of submitting multiple jobs to the grid for calculation
public int sendJobs2Grid(GridJob[] jobs){
Collection<GridCallable<GridJobOutput>> calls = new ArrayList<>();
for (final GridJob job : jobs) {
calls.add(new GridCallable<GridJobOutput>() {
#Override public GridJobOutput call() throws Exception {
GridJobOutput result = job.process();
return result;
}
});
}
GridFuture<Collection<GridJobOutput>> fut = this.gNode.compute().call(calls);
fut.listenAsync(new GridInClosure<GridFuture<Collection<GridJobOutput>>>(){
#Override public void apply(GridFuture<Collection<GridJobOutput>> jobsOutputCollection) {
Collection<GridJobOutput> jobsOutput;
try {
jobsOutput = jobsOutputCollection.get();
for(GridJobOutput currResult: jobsOutput){
//do something with the current job output BUT CANNOT call jobFinished(GridJobOutput out) method
//of sendJobs2Grid class here
}
} catch (GridException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
});
return calls.size();
}
//This function should be invoked asynchronously when the GridFuture is
//will invoke some processing/aggregation of the result for each submitted job
public void jobFinished(GridJobOutput out) {}
}
}
//represents a job type that is to be submitted to the grid
public class GridJob{
public GridJobOutput process(){}
}
Description:
The idea is that a GridClient instance will be used to in order to submit a list/array of jobs to the grid, notify the sender how many jobs were submitted and when the jobs are finished (asynchronously) is will perform some processing of the results. For the results processing part the "GridClient.jobFinished(GridJobOutput out)" method should be invoked.
Now getting to question at hand, we are aware of the GridInClosure interface that can be used with "GridFuture.listenAsync(GridInClosure> lsnr)"
in order to register a future listener.
The problem (if my understanding is correct) is that it is a good and pretty straightforward solution in case the result of the future is to be "processed" by code that is within the scope of the given GridInClosure. In our case we need to use the "GridClient.jobFinished(GridJobOutput out)" which is out of the scope.
Due to the fact that GridInClosure has a single argument R and it has to be of the same type as of GridFuture result it seems impossible to use this approach in a straightforward manner.
If I got it right till now then in order to use "GridFuture.listenAsync(..)" aproach the following has to be done:
GridClient will have to implement an interface granting access to the "jobFinished(..)" method let's name it GridJobFinishedListener.
GridJob will have to be "wrapped" in new class in order to have an additional property of GridJobFinishedListener type.
GridJobOutput will have to be "wrapped" in new class in order to have an addtional property of GridJobFinishedListener type.
When the GridJob will be done in addition to the "standard" result GridJobOutput will contain the corresponding GridJobFinishedListener reference.
Given the above modifications now GridInClosure can be used now and in the apply(GridJobOutput) method it will be possible to call the GridClient.jobFinished(GridJobOutput out) method through the GridJobFinishedListener interface.
So if till now I got it all right it seems a bit clumsy work around so I hope I have missed something and there is a much better way to handle this relatively simple case of asynchronous call back.
Looking forward to any helpful feedback, thanks a lot in advance.
Your code looks correct and I don't see any problems in calling jobFinished method from the future listener closure. You declared it as an anonymous class which always has a reference to the external class (GridClient in your case), therefore you have access to all variables and methods of GridClient instance.
I have upgraded to MVVM Light 5 and I changed my navigation methods from:
Messenger.Default.Send(new NavigateToPageMessage() { PageName = "UserDetailsPage", Parameter = id });
To the following:
_navigationService.NavigateTo(ViewModelLocator.UserDetailsPageKey, id);
The parameter I am passing does not seem to make it's way to the OnNavigatedTo event of the view anymore, the parameter is completely different, am I missing something?
EDIT:
This new method seems to give me the parameter I need:
GlobalNavigation.GetAndRemoveParameter(NavigationContext)
Although now, when the app is tombstoned, I lose that parameter entirely. Before, when the app was restored I would still have that parameter in the NavigatedTo args, this allowed me to re-hit the server with that ID and get fresh data. Why have I lost this capability
use this
protected override void OnNavigatedTo(NavigationEventArgs e)
{
GalaSoft.MvvmLight.Views.NavigationService navigationService = new GalaSoft.MvvmLight.Views.NavigationService();
var param = navigationService.GetAndRemoveParameter(this.NavigationContext);
base.OnNavigatedTo(e);
}
I have this method on a webpart:
private IFilterData _filterData = null;
[ConnectionConsumer("Filter Data Consumer")]
public void GetConnectionInterface(IFilterData filterData)
{
_filterData = filterData;
}
Now, before I can call upon _filterData, I need to know when i can expect it to not be null. When is this?!
Without knowing this, the best I can do is stuff all of my _filterWebpart dependent code into the last lines of OnPreRender and hope for the best.
According to this document, it looks like Load.
http://msdn.microsoft.com/en-us/library/ms366536.aspx