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 = 1, TypeName = "type1" });
            Types.Add(new MyType { TypeId = 2, TypeName = "type2" });
            Types.Add(new MyType { TypeId = 3, TypeName = "type3" });
            Types.Add(new MyType { TypeId = 4, 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 = -1;     
            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();
        }