Add project files.

Gready_Poang.Common/Base/BehaviorBase.cs

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+namespace GreadyPoang.Common;
+
+public class BehaviorBase<T> : Behavior<T> where T : BindableObject
+{
+    public T? AssociatedObject { get; private set; }
+
+    protected override void OnAttachedTo(T bindable)
+    {
+        base.OnAttachedTo(bindable);
+        AssociatedObject = bindable;
+
+        if (bindable.BindingContext != null)
+            BindingContext = bindable.BindingContext;
+
+        bindable.BindingContextChanged += OnBindingContextChanged;
+    }
+
+    protected override void OnDetachingFrom(T bindable)
+    {
+        base.OnDetachingFrom(bindable);
+        bindable.BindingContextChanged -= OnBindingContextChanged;
+        AssociatedObject = null;
+    }
+
+    void OnBindingContextChanged(object? sender, EventArgs e)
+    {
+        if (AssociatedObject != null)
+            BindingContext = AssociatedObject.BindingContext;
+    }
+}