We Keep Coding

How can I pop a navigation page when a value changes in a static class?

So I have a bit of a weird problem.
I've implemented a camera preview class (largely following this code here: https://learn.microsoft.com/en-us/samples/xamarin/xamarin-forms-samples/customrenderers-view/) but have added a button at the bottom to take a picture. This involves the use of both some xamarin forms code and some xamarin android code.
However, the CapturePage is only put on the stack when the user announces that they want to take a photo, and after the photo has been taken, I want to pop the Capture page to go back to the main screen. Currently, I have a static boolean value in the overall project that is changed from false to true when a capture has occurred. Is there some way to get my code in Main.xaml.cs to wait on this value changing, then pop whatever is on top of the navigation stack? Is this a use for a property changed? See code below:
The code in Project.Droid that handles the capturing and saving of the image:
void OnCapButtonClicked(object sender, EventArgs e)
{
// capButton.capture is an instance of Android.Hardware.Camera
capButton.capture.TakePicture(null, null, this);
// stop the preview when the capture happens
CameraInfoContainer.isPreviewing = false;
}
public void OnPictureTaken(byte[] data, Camera camera)
{
var filepath = string.Empty;
var clientInstanceId = Guid.NewGuid().ToString();
var saveLoc = Android.OS.Environment.GetExternalStoragePublicDirectory(Android.OS.Environment.DirectoryPictures);
filepath = System.IO.Path.Combine(saveLoc.AbsolutePath, clientInstanceId + ".jpg");
try
{
System.IO.File.WriteAllBytes(filepath, data);
//mediascan adds the saved image into the gallery
var mediaScanIntent = new Intent(Intent.ActionMediaScannerScanFile);
mediaScanIntent.SetData(Android.Net.Uri.FromFile(new File(filepath)));
Forms.Context.SendBroadcast(mediaScanIntent);
}
catch (Exception e)
{
System.Console.WriteLine(e.ToString());
}
// CameraInfoContainer is a static class in Project NOT in Project.Droid
CameraInfoContainer.savedCapture = filepath;
CameraInfoContainer.capType = CaptureType.Photo;
CameraInfoContainer.captureComplete = true; // here is where I set the value (in Project)
}
Now the code in Project that pushes the capture page on the stack and that I want to trigger when the capture has happened:
// this method puts the capture page on the stack and starts the whole process
private async Task ExecuteNewCapture()
{
var cp = new CapturePage();
var np = new NavigationPage(cp);
await Navigation.PushModalAsync(np, true);
}
// this is the method that I want to trigger when a photo is taken (in Project/Main.xaml.cs)
private async Task Complete(string fileLoc)
{
await Navigation.PopModalAsync();
}

Answer is in a comment from Junior Jiang. Ended up using Xamarin.Forms.MessagingCenter to get done what I needed.

Loading screen for GoogleMaps activity

I'm trying to implement a loading screen since GoogleMaps sometimes takes ~2 seconds to load. I tried using a RelativeLayout and putting the ImageView above the map fragment and making it ViewStates.Gone once the map loads. However, I noticed that the loading is happening on base.OnCreateBundle and not on SetUpMap() and now I'm clueless how to implement it.
private GoogleMap mMap;
protected override void OnCreate(Bundle bundle)
{
base.OnCreate(bundle);
SetContentView(Resource.Layout.Map);
mSplash = FindViewById<ImageView>(Resource.Id.splash);
SetUpMap();
}
private void SetUpMap()
{
if (mMap == null)
{
FragmentManager.FindFragmentById<MapFragment(Resource.Id.map).GetMapAsync(this);
}
}
public void OnMapReady(GoogleMap googleMap)
{
mSplash.Visibility = ViewStates.Gone;
mMap = googleMap;
mMap.MapType = GoogleMap.MapTypeSatellite;
(...)
}

If I'm not mistaken, OnMapReady is meant to be used to verify that Google Play Services is installed and active on the device. As the documentation puts it:
If Google Play services is not installed on the device, the user will
be prompted to install it, and the onMapReady(GoogleMap) method will
only be triggered when the user has installed it and returned to the
app.
OnMapReady Documentation
Alternatively, the OnMapLoaded function is meant to be called after the map has finished loading its tiles.
Called when the map has finished rendering. This will only be called
once. You must request another callback if you want to be notified
again.
OnMapLoaded Documentation
You can implement the OnMapLoaded function by first setting the callback once you have a reference to the map
mMap.setOnMapLoadedCallback(this);
Finish by simply overriding the OnMapLoaded function and handling it accordingly
#Override
public void onMapLoaded() {
if (mMap != null) {
mSplash.Visibility = ViewStates.Gone;
}
}
Remember that if you need to load the map again and need to display another splash screen, you will first need to set the callback again, as onMapLoaded is only called once per callback.

Windows Phone ContactPicker won't await properly

Although this has been posted before on StackOverflow but i think none of those reflect my issue and none of those solutions work for me either. So i'm developing a Windows Phone app and my workflow is a bit like this:
App starts
ContactPicker opens up
User selects one or multiple contacts
Based on how many contacts he selected, that many PivotItems are added into the Pivot.
My code is as follows:
protected override void OnNavigatedTo(NavigationEventArgs e)
{
// TODO: Prepare page for display here.
// TODO: If your application contains multiple pages, ensure that you are
// handling the hardware Back button by registering for the
// Windows.Phone.UI.Input.HardwareButtons.BackPressed event.
// If you are using the NavigationHelper provided by some templates,
// this event is handled for you.
SelectContacts();
}
private async Task SelectContacts()
{
var picker = new ContactPicker();
picker.DesiredFieldsWithContactFieldType.Add(ContactFieldType.PhoneNumber);
ContactsList = (List<Contact>)await picker.PickContactsAsync();
DisplayContacts();
}
private void DisplayContacts()
{
if (ContactsList != null)
{
foreach (var item in ContactsList)
{
PivotItem pivotItem = new PivotItem();
pivotItem.Header = item.FirstName.ToString();
ContentRoot.Items.Add(pivotItem);
}
}
}
According to me, in SelectContacts() method, the app should wait at the await call and once it gets back the list of contacts, than it should execute the DisplayContacts() method but its not working. I've tried multiple other variations of this code and they aren't working either.

await the SelectContacts() method and add the DisplayContacts() method beneath it. Remove the DisplayContacts() method from SelectContacts()
await SelectContacts();
DisplayContacts();

I don't know the complete reason why but i figured it out that since i was making the PickContactsAsync() call in the OnNavigatedTo() event, that is why it wasn't working as expected. Once i moved the PickContactsAsync() call into the PageLoaded() event handler, it started working as usual.

Prepare data before go to the main view

When the user touch the app icon,
I want do these steps before user go to the main view
Fetch json string from URI
Use JArray.Parse to get the value
After all finish, go to the main view.
The problem is how can I prevent user to go to the main view
and put all the code
I tried to put it in Application_Launching method in the App.xaml.cs file
// Code to execute when the application is launching (eg, from Start)
// This code will not execute when the application is reactivated
private void Application_Launching(object sender, LaunchingEventArgs e)
{
// code here
}
But it doesn't prevent the program to go to the main view before the fetching finished.
And I found that actually in the MainPage.xaml, if I put this code like this
protected override void OnNavigatedTo(NavigationEventArgs e)
{
while(true) {}
// it will prevent the program to go to the main view,
// and will stick with the loading screen until this function reach its end
}
So I think, I can put the all the code here, when I finish the fetch, I will just break the while and it will go to the main view automatically.
And I try, this is the code
protected override void OnNavigatedTo(NavigationEventArgs e)
{
bool isFetchFinished = false;
ObservableCollection<PromoViewModel> Promos = new ObservableCollection<PromoViewModel>();
WebClient client = new WebClient();
client.DownloadStringCompleted += (s, evt) =>
{
if (evt.Error == null)
{
// Retrieve the JSON
string jsonString = evt.Result;
JArray promos = JArray.Parse(jsonString);
foreach (JObject promo in promos)
{
string name = promo["name"].Value<string>();
string description = promo["description"].Value<string>();
string img = promo["image"].Value<string>();
Promos.Add(new PromoViewModel() { Name = name, Description = description, Img = img });
}
isFetchFinished = true;
System.Diagnostics.Debug.WriteLine("finish fetch");
}
};
// run
client.DownloadStringAsync(new Uri("the json url"));
while(true) {
if(isFetchFinished) {
App.ViewModel.LoadData(Promos); // pass value to main view model
break; // after complete, break
}
}
}
I thought it would work, but it was not.
This is what I found,
The WebClient DownloadStringAsync won't run until the OnNavigatedTo function finished.
Because it's still waiting for the while loop to break and reach the end function.
And this
isFetchFinished = true; // will never executed
Resulting infinite loop.
I think I put the fetch code in the wrong method. Where is the right place to put all of this?

Ouch, you are doing it all wrong. First of all, you have to specify the starting page. If you want to download some data before navigating to it, you can create a special "download" page that is actually the first page navigated to when starting the application. And then, once the download is completed, you navigate to your main page. This is actually a replacement for the extended splash screen.
Also, never put while (true) in any UI code, that will simply freeze the application. Besides, if the application is frozen, you never get the chance to "unfreeze" it.

Delay in event in code behind file

In my application I have a button to save some information. However, I would like to have a delay in the code before the last line is executed, so that the user could read the message that shows up before he gets redirected to the new page.
I know that doing this isn't at all an optimal way, but by some reasons (time, for example) I want to do it anyway.
So is it possible and if so, how could I do it?
Thanks in advance!
protected void SaveButton_Click(object sender, EventArgs e) {
// Lots of code not relevant for the problem here
Service service = new Service();
service.SaveMovie(movie);
successMessage.Visible = true;
happyMessage.Text = "The movie was successfully added, now add some genres!";
// Here I want a delay of 2 seconds before the next line is executed...
Response.Redirect(String.Format("~/Edit.aspx?id={0}", movie.MovieID), false);
}

You need to do this on the client side. One alternative is this:
Define a Javascript function in the page called redirect as so:
function redirect(url)
{
setTimeout(function(){window.location.href=url;} ,2000);
}
protected void SaveButton_Click(object sender, EventArgs e)
{
// Lots of code not relevant for the problem here
Service service = new Service();
service.SaveMovie(movie);
successMessage.Visible = true;
happyMessage.Text = "The movie was successfully added, now add some genres!";
// Here I want a delay of 2 seconds before the next line is executed...
ClientScript.RegisterStartupScript(this.GetType(),"somekey","redirect('"+String.Format("~/Edit.aspx?id={0}", movie.MovieID)+"');");
}

This will be easy if you are using Javascript. Use javascript will boost performance
Button_Click
{
string js ="<script type='text/javascript'>setTimeout(function()window.location.href="+String.Format("~/Edit.aspx?id={0}", movie.MovieID)+";} ,2000);</script>"
ScriptManager.RegisterStartupScript(Me.Page, GetType(Page), "js", js, False)
}
possible duplicate of
asp.net delay before response redirect