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japarson / maui

.NET MAUI is the .NET Multi-platform App UI, a framework for building native device applications spanning mobile, tablet, and desktop.
https://devblogs.microsoft.com/dotnet/introducing-net-multi-platform-app-ui/
MIT License
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Picker Attribute "SelectedIndex" Not being respected on page load on Android? #318

Open japarson opened 5 months ago

japarson commented 5 months ago

Description

I have a Picker that is attached to an enum for a "list type" property, with a "SelectedIndex=0" attribute on the picker, which I believe means that I want that index of the enum to be selected first, as a 'default'. This works great on the windows app, but when running on Android, the picker control is empty. Then, I'm able to force something into the control by manually changing the "selected index" in the .xaml from say, 0 to 1, which I think then triggers the event, and updates in the app. But obviously since I think the point of SelectedIndex is to default something in, then, well, it should default something in.

I found a workaround for this by grabbing the picker control in the constructor in the code-behind for the page, and setting it's SelectedIndex property there, which fixes it. But I presume that the xaml property should work without needing to do that.

Steps to Reproduce

  1. Use Picker Control in a ContentPage
  2. Set the "SelectedIndex="0"" value on the Picker
  3. See that on that page loading, nothing is populated on Android, but is on the Windows App

Did you find any workaround?

Yes, as described above - "I found a workaround for this by grabbing the picker control in the constructor in the code-behind for the page, and setting it's SelectedIndex property there, which fixes it."

Investigation

Investigated this for a few hours. It seems that in XamlNode.cs the properties of Picker have SelectedIndex above the Binding PickerItems.

It first then iterates into SelectedIndex, eventually it comes in BindableObject.cs in SetValueCore, where property.CoerceValue is executed against the selectedIndex from the xaml (1 in my example). It is coerced to -1 because there are no items yet, and after that it is not loaded from the XamlNode again.

The next property after SelectedIndex is the PickerItems Binding.

I have yet to figure out what determines the order in which properties are listed, so that I can change the behavior. As seen in the screenshot other properties x:name and textColor are applied after the Binding PickerItems.

Files to Investigate

Checklist - [X] Modify `src/Controls/src/Core/Picker/Picker.cs` ✓ https://github.com/japarson/maui/commit/0df0b640d416e6f80d995765fbb10e95446c0919 [Edit](https://github.com/japarson/maui/edit/sweep/picker_attribute_selectedindex_not_being/src/Controls/src/Core/Picker/Picker.cs#L232-L237) - [X] Running GitHub Actions for `src/Controls/src/Core/Picker/Picker.cs` ✓ [Edit](https://github.com/japarson/maui/edit/sweep/picker_attribute_selectedindex_not_being/src/Controls/src/Core/Picker/Picker.cs#L232-L237) - [X] Modify `src/Controls/src/Xaml/XamlNode.cs` ✓ https://github.com/japarson/maui/commit/c84fc59c7670bd08f4687a83dcd05a29c20f6c0f [Edit](https://github.com/japarson/maui/edit/sweep/picker_attribute_selectedindex_not_being/src/Controls/src/Xaml/XamlNode.cs#L153-L188) - [X] Running GitHub Actions for `src/Controls/src/Xaml/XamlNode.cs` ✓ [Edit](https://github.com/japarson/maui/edit/sweep/picker_attribute_selectedindex_not_being/src/Controls/src/Xaml/XamlNode.cs#L153-L188)
sweep-ai[bot] commented 5 months ago

🚀 Here's the PR! #320

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Step 1: 🔎 Searching

I found the following snippets in your repository. I will now analyze these snippets and come up with a plan.

Some code snippets I think are relevant in decreasing order of relevance (click to expand). If some file is missing from here, you can mention the path in the ticket description. https://github.com/japarson/maui/blob/7639a2cf24ac3e5a17a08fc1eb0103883659d57d/src/Compatibility/ControlGallery/src/Core/GalleryPages/VisualStateManagerGalleries/PickerDisabledStatesGallery.xaml#L45-L60 https://github.com/japarson/maui/blob/7639a2cf24ac3e5a17a08fc1eb0103883659d57d/src/Compatibility/Core/src/iOS/Renderers/PickerRenderer.cs#L161-L195 https://github.com/japarson/maui/blob/7639a2cf24ac3e5a17a08fc1eb0103883659d57d/src/Controls/src/Core/Picker/Picker.cs#L232-L261 https://github.com/japarson/maui/blob/7639a2cf24ac3e5a17a08fc1eb0103883659d57d/src/Controls/src/Xaml/XamlNode.cs#L153-L188

Step 2: ⌨️ Coding

--- 
+++ 
@@ -232,8 +232,10 @@

        static object CoerceSelectedIndex(BindableObject bindable, object value)
        {
-           var picker = (Picker)bindable;
-           return picker.Items == null ? -1 : ((int)value).Clamp(-1, picker.Items.Count - 1);
+           var picker = (Picker)bindable;
+           if (picker.Items == null || !picker.Items.Any())
+               return value; // Do not coerce if Items are not set yet
+           return ((int)value).Clamp(-1, picker.Items.Count - 1);
        }

        void OnItemDisplayBindingChanged(BindingBase oldValue, BindingBase newValue)

Ran GitHub Actions for 0df0b640d416e6f80d995765fbb10e95446c0919:

--- 
+++ 
@@ -172,6 +172,21 @@
        public NameScopeRef NameScopeRef { get; set; }

        public override void Accept(IXamlNodeVisitor visitor, INode parentNode)
+       Dictionary deferredProperties = new Dictionary();
+
+       public void DeferProperty(XmlName propertyName, object value)
+       {
+           deferredProperties[propertyName] = value;
+       }
+
+       public void ApplyDeferredProperties()
+       {
+           foreach (var kvp in deferredProperties)
+           {
+               Properties[kvp.Key] = (INode)kvp.Value;
+           }
+           deferredProperties.Clear();
+       }
        {
            if (visitor.VisitingMode == TreeVisitingMode.TopDown && !SkipVisitNode(visitor, parentNode))
                visitor.Visit(this, parentNode);
@@ -210,6 +225,19 @@
        {
            var clone = new ElementNode(XmlType, NamespaceURI, NamespaceResolver, LineNumber, LinePosition)
            {
+           if (!SkipChildren(visitor, this, parentNode))
+           {
+               // Check if ItemsSource or Items is being set and apply deferred SelectedIndex if any
+               if (Properties.ContainsKey(XmlName.xItemsSource) || Properties.ContainsKey(XmlName.xItems))
+               {
+                   ApplyDeferredProperties();
+               }
+
+               foreach (var node in Properties.Values.ToArray())
+                   node.Accept(visitor, this);
+               foreach (var node in CollectionItems.ToArray())
+                   node.Accept(visitor, this);
+           }
                IgnorablePrefixes = IgnorablePrefixes
            };
            foreach (var kvp in Properties)

Ran GitHub Actions for c84fc59c7670bd08f4687a83dcd05a29c20f6c0f:

Step 3: 🔁 Code Review

I have finished reviewing the code for completeness. I did not find errors for sweep/picker_attribute_selectedindex_not_being.

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