Navigation in Xamarin.forms: buffer null - xamarin.forms

Question

In a Xamarin.Forms app, I have a button on a Page (Page_1) that sends me to another Page (Page_2) with the statement
await Navigation.PushAsync (new Page_2 ());
On this Page, I have several ImageButtons that all return me to Page_1 when clicked with the statement
await Navigation.PopAsync ();
This works the first time around, but the second time I press the button on the Page_1, I go to the Page_2, then when I click on an ImageButton the application quits with the error message:
System.ArgumentNullException
Message = Buffer cannot be null.
Parameter name: buffer
How can I make the Navigation.NavigationStack still contain the Page_1?
Thanks a lot for your help.
Add for Deczaloth : this is the partial code of the first page :
private async void btnModifImage_Clicked(object sender, EventArgs e)
{
string imgLocation;
// affichage et choix : gallerie ou internet
imgLocation = await DisplayActionSheet("Emplacement", "Annuler", null, imageLocation);
if (imgLocation == "Gallerie")
{
// chargement image depuis gallerie
imageChanged = true;
article_nomcat = pickerCat.Title;
article_unit = pickerUnit.Title;
try
{
FileData fileData = await CrossFilePicker.Current.PickFile();
//
var filepath = fileData.FilePath;
//
if (fileData == null)
return; // user canceled file picking
newimageByte = fileData.DataArray;
resizedImageByte = await ImageResizer.ResizeImage(newimageByte, 100, 100);
if (resizedImageByte != null)
{
imageChanged = true;
var stream1 = new MemoryStream(resizedImageByte);
ImageArticle.Source = ImageSource.FromStream(() => stream1);
}
}
catch (Exception ex)
{
imageChanged = false;
}
}
else
{
App.ImageName = name.Text;
await Navigation.PushAsync(new ImageBingPage());
}
}
and the partial code of the second page ImageBingPage :
private async void img1_Clicked(object sender, EventArgs e)
{
byte[] img1Byte = await DownloadImageAsync(img1Path);
App.imgByte = img3Byte;
await Navigation.PopAsync();
}
And the biginning of the second page :
protected override void OnAppearing()
{
base.OnAppearing();
SearchImage();
base.OnDisappearing();
}
public void SearchImage()
{
List<Images> imageList = new List<Images>();
SearchResult bingResult = BingImageSearch(App.ImageName);
var res = JsonConvert.DeserializeObject<ResponseModel>(bingResult.jsonResult);
for (int i = 0; i < res.value.Count; i++)
{
imageList.Add(new Images { ImagePath = res.value[i].contentUrl });
}
img1.Source = ImageSource.FromUri(new Uri(imageList[0].ImagePath));
img1Path = imageList[0].ImagePath;
img2.Source = ImageSource.FromUri(new Uri(imageList[1].ImagePath));
img2Path = imageList[1].ImagePath;
img3.Source = ImageSource.FromUri(new Uri(imageList[2].ImagePath));
img3Path = imageList[2].ImagePath;
}
static SearchResult BingImageSearch(string searchQuery)
{
var uriQuery = baseURl + "?q=" + Uri.EscapeDataString(searchQuery);
WebRequest request = HttpWebRequest.Create(uriQuery);
request.Headers["Ocp-Apim-Subscription-Key"] = APIKey;
HttpWebResponse response = (HttpWebResponse)request.GetResponseAsync().Result;
string json = new StreamReader(response.GetResponseStream()).ReadToEnd();
var searchResult = new SearchResult();
searchResult.jsonResult = json;
searchResult.relevantHeaders = new Dictionary<String, String>();
foreach (String header in response.Headers)
{
if (header.StartsWith("BingAPIs-") || header.StartsWith("X-MSEdge-"))
searchResult.relevantHeaders[header] = response.Headers[header];
}
return searchResult;
}
async Task<byte[]> DownloadImageAsync(string imageUrl)
{
try
{
using (var httpResponse = await _httpClient.GetAsync(imageUrl))
{
if (httpResponse.StatusCode == HttpStatusCode.OK)
{
return await httpResponse.Content.ReadAsByteArrayAsync();
}
else
{
//Url is Invalid
return null;
}
}
}
catch (Exception e)
{
//Handle Exception
return null;
}
}
Add 2 for Deczaloth : You're right: the stack is not empty (see the two attached screenshots). So where does the problem come from?
Edit for Jack : I have modified my code to simplify the problem:
private async void img1_Clicked(object sender, EventArgs e)
{
//byte[] img1Byte = await DownloadImageAsync(imgPath[0]);
//App.imgByte = img1Byte;
await Navigation.PopAsync();
}
private async void img2_Clicked(object sender, EventArgs e)
{
//byte[] img2Byte = await DownloadImageAsync(imgPath[1]);
//App.imgByte = img2Byte;
await Navigation.PopAsync();
}
And now I can do 3 round trips between the ArticleEntry page and the ImageBingPage page, but after, the app crashes anyway.
Edit for Jack : there is the code on the page ArticleEntry :
protected override async void OnAppearing()
{
base.OnAppearing();
if (App.AddArticle == true)
{
ToolbarItems.Remove(btnDelete);
}
//
_article = (Article)BindingContext;
old_article_ID = _article.ID;
old_article_name = _article.Name;
article_nomcat = _article.NomCat;
article_unit = _article.UniteMesure;
//
if (_article.ID == 0)
{
name.Focus();
}
//
if (pickerCat.Title != null & pickerCat.Title != "choisissez une catégorie")
{
_article.NomCat = pickerCat.Title;
pickerCat.TitleColor = Color.Black;
}
else
{
if (_article.NomCat != null)
{
pickerCat.Title = _article.NomCat;
pickerCat.SelectedItem = _article.NomCat;
pickerCat.TitleColor = Color.Black;
}
else
{
pickerCat.Title = "choisissez une catégorie";
pickerCat.TitleColor = Color.Gray;
}
}
//
if (pickerUnit.Title != null & pickerUnit.Title != "choisissez une unité de mesure")
{
_article.UniteMesure = pickerUnit.Title;
pickerUnit.TitleColor = Color.Black;
}
else
{
if (_article.UniteMesure != null)
{
pickerUnit.Title = _article.UniteMesure;
pickerUnit.SelectedItem = _article.UniteMesure;
pickerUnit.TitleColor = Color.Black;
}
else
{
pickerUnit.Title = "choisissez une unité de mesure";
pickerUnit.TitleColor = Color.Gray;
}
}
//
if (pickerListeAchats.Title != null & pickerListeAchats.Title != "choisissez une liste d'achats")
{
//_article.UniteMesure = pickerUnit.Title;
pickerListeAchats.TitleColor = Color.Black;
}
//
ReadAllCategorieList dbcategorie = new ReadAllCategorieList();
DB_CategorieList = dbcategorie.GetAllCategorie(); //Get all DB Categorie
// trier
DB_CategorieList_Sorted = DB_CategorieList.OrderBy(i => i.NomCat).ToList();
//
// rempliir la liste du picker Cat
string[] pickerListCat;
List<string> pkListCat = new List<string>();
foreach (var item in DB_CategorieList_Sorted)
{
string element = item.NomCat;
//pickerCat.Items.Add(element);
pkListCat.Add(element);
}
pickerListCat = pkListCat.ToArray();
pickerCat.ItemsSource = pickerListCat;
//
// rempliir la liste du picker Unit
ReadAllUnitList dbunit = new ReadAllUnitList();
DB_UnitList = dbunit.GetAllUnit();
// trier
DB_UnitList_Sorted = DB_UnitList.OrderBy(i => i.UniteMesure).ToList();
//
List<string> pickerListUnit = new List<string>();
foreach (var item in DB_UnitList_Sorted)
{
string element = item.UniteMesure;
pickerListUnit.Add(element);
}
pickerUnit.ItemsSource = pickerListUnit;
//
// remplir la liste du picker ListeAchats
ReadAllListeAchatsList dblisteachats = new ReadAllListeAchatsList();
DB_ListeAchatsList = dblisteachats.GetAllListeAchats();
// trier
DB_ListeAchatsList_Sorted = DB_ListeAchatsList.OrderBy(i => i.NomListe).ToList();
//
List<string> pickerListAchat = new List<string>();
foreach (var item in DB_ListeAchatsList_Sorted)
{
string element = item.NomListe;
pickerListAchat.Add(element);
}
pickerListeAchats.ItemsSource = pickerListAchat;
//
//
imageByte = _article.ImageArt;
//
// convertir le byte[] en bitmapimage (si != 0)
if (_article.ImageArt != null)
{
if (imageChanged)
{
var stream1 = new MemoryStream(newimageByte);
ImageArticle.Source = ImageSource.FromStream(() => stream1);
}
else
{
var stream1 = new MemoryStream(imageByte);
ImageArticle.Source = ImageSource.FromStream(() => stream1);
}
}
else
{
if (imageChanged == false)
{
imageByte = ImageDataFromResource("MemoCourses.Assets.Images.appareil_photo.png");
var stream1 = new MemoryStream(imageByte);
ImageArticle.Source = ImageSource.FromStream(() => stream1);
}
}
//
if (App.imgByte != null)
{
resizedImageByte = await ImageResizer.ResizeImage(App.imgByte, 100, 100);
if (resizedImageByte != null)
{
imageChanged = true;
var stream1 = new MemoryStream(resizedImageByte);
ImageArticle.Source = ImageSource.FromStream(() => stream1);
}
}
//
base.OnDisappearing();
App.AddArticle = false;
}
Code changed based on Jack's comments
protected override async void OnAppearing()
{
base.OnAppearing();
if (App.AddArticle == true)
{
ToolbarItems.Remove(btnDelete);
}
//
_article = (Article)BindingContext;
old_article_ID = _article.ID;
old_article_name = _article.Name;
article_nomcat = _article.NomCat;
article_unit = _article.UniteMesure;
//
if (_article.ID == 0)
{
name.Focus();
}
//
if (pickerCat.Title != null & pickerCat.Title != "choisissez une catégorie")
{
_article.NomCat = pickerCat.Title;
pickerCat.TitleColor = Color.Black;
}
else
{
if (_article.NomCat != null)
{
pickerCat.Title = _article.NomCat;
pickerCat.SelectedItem = _article.NomCat;
pickerCat.TitleColor = Color.Black;
}
else
{
pickerCat.Title = "choisissez une catégorie";
pickerCat.TitleColor = Color.Gray;
}
}
//
if (pickerUnit.Title != null & pickerUnit.Title != "choisissez une unité de mesure")
{
_article.UniteMesure = pickerUnit.Title;
pickerUnit.TitleColor = Color.Black;
}
else
{
if (_article.UniteMesure != null)
{
pickerUnit.Title = _article.UniteMesure;
pickerUnit.SelectedItem = _article.UniteMesure;
pickerUnit.TitleColor = Color.Black;
}
else
{
pickerUnit.Title = "choisissez une unité de mesure";
pickerUnit.TitleColor = Color.Gray;
}
}
//
if (pickerListeAchats.Title != null & pickerListeAchats.Title != "choisissez une liste d'achats")
{
pickerListeAchats.TitleColor = Color.Black;
}
//
ReadAllCategorieList dbcategorie = new ReadAllCategorieList();
DB_CategorieList = dbcategorie.GetAllCategorie(); //Get all DB Categorie
// trier
DB_CategorieList_Sorted = DB_CategorieList.OrderBy(i => i.NomCat).ToList();
//
// rempliir la liste du picker Cat
string[] pickerListCat;
List<string> pkListCat = new List<string>();
foreach (var item in DB_CategorieList_Sorted)
{
string element = item.NomCat;
pkListCat.Add(element);
}
pickerListCat = pkListCat.ToArray();
pickerCat.ItemsSource = pickerListCat;
//
// rempliir la liste du picker Unit
ReadAllUnitList dbunit = new ReadAllUnitList();
DB_UnitList = dbunit.GetAllUnit();
// trier
DB_UnitList_Sorted = DB_UnitList.OrderBy(i => i.UniteMesure).ToList();
//
List<string> pickerListUnit = new List<string>();
foreach (var item in DB_UnitList_Sorted)
{
string element = item.UniteMesure;
pickerListUnit.Add(element);
}
pickerUnit.ItemsSource = pickerListUnit;
//
// remplir la liste du picker ListeAchats
ReadAllListeAchatsList dblisteachats = new ReadAllListeAchatsList();
DB_ListeAchatsList = dblisteachats.GetAllListeAchats();
// trier
DB_ListeAchatsList_Sorted = DB_ListeAchatsList.OrderBy(i => i.NomListe).ToList();
//
List<string> pickerListAchat = new List<string>();
foreach (var item in DB_ListeAchatsList_Sorted)
{
string element = item.NomListe;
pickerListAchat.Add(element);
}
pickerListeAchats.ItemsSource = pickerListAchat;
//
//
imageByte = _article.ImageArt;
//
// convertir le byte[] en bitmapimage (si != 0)
if (_article.ImageArt != null)
{
if (imageGallery)
{
var stream1 = new MemoryStream(resizedImageByte);
ImageArticle.Source = ImageSource.FromStream(() => stream1);
}
else
{
if (App.imgByte == null)
{
// image depuis la base SQLite
var stream1 = new MemoryStream(imageByte);
ImageArticle.Source = ImageSource.FromStream(() => stream1);
}
else
{
// image depuis internet
var stream1 = new MemoryStream(App.imgByte);
ImageArticle.Source = ImageSource.FromStream(() => stream1);
}
}
}
else
{
if (imageChanged == false)
{
// image par défaut
imageByte = ImageDataFromResource("MemoCourses.Assets.Images.appareil_photo.png");
var stream1 = new MemoryStream(imageByte);
ImageArticle.Source = ImageSource.FromStream(() => stream1);
}
}
//
if (App.imgByte != null & imageInternet == true)
{
resizedImageByte = await ImageResizer.ResizeImage(App.imgByte, 100, 100);
if (resizedImageByte != null)
{
var stream1 = new MemoryStream(resizedImageByte);
ImageArticle.Source = ImageSource.FromStream(() => stream1);
}
}
//
base.OnDisappearing();
App.AddArticle = false;
imageGallery = false;
imageInternet = false;
}

Answer 1

The problem is not caused by the NavigationStack.
1.You should not call base.OnDisappearing(); in OnAppearing:
protected override void OnAppearing()
{
base.OnAppearing();
SearchImage();
//remove this line
//base.OnDisappearing();
}
2.You get a java.io.IOException: Cleartext HTTP traffic to aardbeienzo.com not permitted which you does not have permission to load url start with http:
Go to Android project --> Properties --> Assemblyinfo.cs, add UsesCleartextTraffic = true:
// Add some common permissions, these can be removed if not needed
[assembly: UsesPermission(Android.Manifest.Permission.Internet)]
[assembly: UsesPermission(Android.Manifest.Permission.WriteExternalStorage)]
[assembly: Application(UsesCleartextTraffic = true)]
Update:
private async void img1_Clicked(object sender, EventArgs e)
{
byte[] img1Byte = await DownloadImageAsync(img1Path);
//App.imgByte = img3Byte;
App.imgByte = img1Byte;
await Navigation.PopAsync();
}
Update 2:
Generally, If you use Navigation push/pop correctly, it will not cause the error. Pay attention to the OnAppearing method of Page and check the code there.
The correct code has been updated in the question by Marcel Delhaye.