Xamarin.Android之Spinner的简单探讨

一、前言

今天用了一下Spinner这个控件，主要是结合官网的例子来用的，不过官网的是把数据写在Strings.xml中的，

某种程度上，不是很符合我们需要的，比较多的应该都是从数据库读出来，绑定上去的。下面是官网的例子：

https://developer.xamarin.com/guides/android/user_interface/spinner/

二、来个简单的Demo

新建一个类 MyType.cs

     public class MyType
     {
         public int TypeId { get; set; }

         public string TypeName { get; set; }

         public override string ToString()
         {
             return TypeId + "-" + TypeName;
         }
     }

再来点数据

     public static class DB
     {
         public static List<MyType> Types { get; private set; }

         static DB()
         {
             Types = new List<MyType>();
             Types.Add(new MyType { TypeId = , TypeName = "type1" });
             Types.Add(new MyType { TypeId = , TypeName = "type2" });
             Types.Add(new MyType { TypeId = , TypeName = "type3" });
             Types.Add(new MyType { TypeId = , TypeName = "type4" });
         }
     }

最后写个Adapter，我们自定义的Adapter都需要继承BaseAdapter

     public class MyTypeAdapter : BaseAdapter<MyType>
     {
         private readonly Activity _context;
         private readonly IList<MyType> _types;

         public MyTypeAdapter(Activity context, IList<MyType> types)
         {
             this._context = context;
             this._types = types;
         }

         public override MyType this[int position]
         {
             get
             {
                 return this._types[position];
             }
         }

         public override int Count
         {
             get
             {
                 return this._types.Count;
             }
         }

         public override long GetItemId(int position)
         {
             return position;
         }

         public override View GetView(int position, View convertView, ViewGroup parent)
         {
             View view = convertView;
             if (view == null)
             {
                 view = this._context.LayoutInflater.Inflate(Android.Resource.Layout.SimpleListItem1, null);
             }

             view.FindViewById<TextView>(Android.Resource.Id.Text1).Text = this._types[position].ToString();
             return view;
         }
     }

到这里，会发现有好几个重写的方法！！这些方法是直接通过实现抽象类生成的，然后自己去完善每个方法。

既然都这样了，那就果断去看看这个BaseAdapter是干嘛的

     //
     // Summary:
     //     Common base class of common implementation for an Android.Widget.Adapter that
     //     can be /// used in both Android.Widget.ListView (by implementing the specialized
     //     /// Android.Widget.IListAdapter interface} and Android.Widget.Spinner (by implementing
     //     the /// specialized Android.Widget.ISpinnerAdapter interface.
     //
     // Type parameters:
     //   T:
     //     To be added.
     //
     // Remarks:
     //     ///
     //     Common base class of common implementation for an Android.Widget.Adapter that
     //     can be /// used in both Android.Widget.ListView (by implementing the specialized
     //     /// Android.Widget.IListAdapter interface} and Android.Widget.Spinner (by implementing
     //     the /// specialized Android.Widget.ISpinnerAdapter interface. ///
     //     ///
     //     /// /// [Android Documentation] /// ///
     //     ///
     [DefaultMember("Item")]
     [Register("android/widget/BaseAdapter", DoNotGenerateAcw = true)]
     public abstract class BaseAdapter<T> : BaseAdapter
     {
         [Register(".ctor", "()V", "")]
         public BaseAdapter();
         public BaseAdapter(IntPtr handle, JniHandleOwnership transfer);

         public abstract T this[int position] { get; }

         public override Java.Lang.Object GetItem(int position);
     }

可以看到这个主要就是给ListView和Spinner用的。主要的还是它继承了BaseAdapter这个抽象类(下面的，我是去掉了注释的，方便看)

     public abstract class BaseAdapter : Java.Lang.Object, IListAdapter, ISpinnerAdapter, IAdapter, IJavaObject, IDisposable
     {
         public BaseAdapter();
         protected BaseAdapter(IntPtr javaReference, JniHandleOwnership transfer);
         public abstract int Count { get; }
         public virtual bool HasStableIds { get; }
         public virtual bool IsEmpty { get; }
         public virtual int ViewTypeCount { get; }
         protected override IntPtr ThresholdClass { get; }
         protected override Type ThresholdType { get; }
         public virtual bool AreAllItemsEnabled();
         public virtual View GetDropDownView(int position, View convertView, ViewGroup parent);
         public abstract Java.Lang.Object GetItem(int position);
         public abstract long GetItemId(int position);
         public virtual int GetItemViewType(int position);
         public abstract View GetView(int position, View convertView, ViewGroup parent);
         public virtual bool IsEnabled(int position);
         public virtual void NotifyDataSetChanged();
         public virtual void NotifyDataSetInvalidated();
         public virtual void RegisterDataSetObserver(DataSetObserver observer);
         public virtual void UnregisterDataSetObserver(DataSetObserver observer);

         public static class InterfaceConsts
         {
             public const int IgnoreItemViewType = -;
             public const int NoSelection = int.MinValue;
         }
     }

可以看到，我们在自定义Adapter时，自动实现的方法，有好几个是在这里面的！！！

具体每个方法是干嘛的，就看看API吧！

https://developer.xamarin.com/api/type/Android.Widget.BaseAdapter/

下面来看看我们的Activity：

 [Activity(Label = "SpinnerActivity",MainLauncher =true)]
     public class SpinnerActivity : Activity
     {
         protected override void OnCreate(Bundle savedInstanceState)
         {
             base.OnCreate(savedInstanceState);

             // Create your application here
             SetContentView(Resource.Layout.spinnerlayout);

             var typesSpinner = FindViewById<Spinner>(Resource.Id.typesSpinner);

             typesSpinner.Adapter = new Models.MyTypeAdapter(this, Models.DB.Types);

             typesSpinner.ItemSelected += typesSpinner_ItemSelected;
         }

         private void typesSpinner_ItemSelected(object sender, AdapterView.ItemSelectedEventArgs e)
         {
             Spinner spinner = (Spinner)sender;

             var toast = string.Format(spinner.GetItemAtPosition(e.Position).ToString());

             Toast.MakeText(this, toast, ToastLength.Long).Show();
         }
     }

很简单，下面看看效果图

     

但是呢，我记得在原生的开发中，下面这句应该直接是得到一个Object对象，然后通过强制转化就可以得到对应的Model

spinner.GetItemAtPosition(e.Position)

我试过.net下面的强制转化和object获取在转化，都没办法得到想要的效果

都是这个错。

回想刚才的BaseAdapter类，它继承了Java.Lang.Object

所以就直接让我们的MyType去继承这个试试。可以点出来了！！

再来看看选择后的效果图：

OK，达到想要的效果了！！这样我们就可以得到任何我们想要的属性了！！不用只局限一个.ToString()方法了！

下面是MyType类和typesSpinner_ItemSelected方法

      public class MyType : Java.Lang.Object
     {
         public int TypeId { get; set; }

         public string TypeName { get; set; }

         public override string ToString()
         {
             return TypeId + "-" + TypeName;
         }
     }

         private void typesSpinner_ItemSelected(object sender, AdapterView.ItemSelectedEventArgs e)
         {
             Spinner spinner = (Spinner)sender;

             //var toast = string.Format(spinner.GetItemAtPosition(e.Position).ToString());

             var toast = (Models.MyType)spinner.GetItemAtPosition(e.Position);

             Toast.MakeText(this, toast.TypeName, ToastLength.Long).Show();
         }