在 Xamarin.Forms 实现密码输入EntryCell

　　在 Xamarin.Forms 中，我们通常使用 TableView 来构建输入表单。Xamarin 为我们提供了 EntryCell 用于输入文本，但是其并不支持密码输入，即密码掩码。这里要对 EntryCell 进行扩展，使其支持密码输入。

　　首先，我们需要在共享项目（多平台项目）中增加控件定义，我们称之为：ExtendedEntryCell

    /// <summary>
    /// An extended entry cell control that allows set IsPassword
    /// </summary>
    public class ExtendedEntryCell : EntryCell
    {

        /// <summary>
        /// The IsPassword property
        /// </summary>
        public static readonly BindableProperty IsPasswordProperty = BindableProperty.Create<ExtendedEntryCell, bool>(p => p.IsPassword, false);

        /// <summary>
        /// Gets or sets IsPassword 
        /// </summary>
        public bool IsPassword
        {
            get { return (bool)GetValue(IsPasswordProperty); }
            set { SetValue(IsPasswordProperty, value); }
        }
    }

我们需要在Android和iOS中加入各自的Render。

Android 平台的 Render：

using Android.Content;
using Android.Text.Method;
using Android.Views;
using Android.Widget;
using WsLink.App.Controls;
using Xamarin.Forms;
using Xamarin.Forms.Platform.Android;
using View = Android.Views.View;

[assembly: ExportRenderer(typeof(ExtendedEntryCell), typeof(ExtendedEntryCellRenderer))]

namespace App.Controls
{
    public class ExtendedEntryCellRenderer : EntryCellRenderer
    {
        protected override View GetCellCore(Cell item, View convertView, ViewGroup parent, Context context)
        {
            var cell = base.GetCellCore(item, convertView, parent, context);
            var textField = (cell as EntryCellView)?.EditText as TextView;

            if (textField != null && textField.TransformationMethod != PasswordTransformationMethod.Instance)
            {
                textField.TransformationMethod = PasswordTransformationMethod.Instance;
            }
            return cell;
        }
    }
}

iOS 平台的 Render 

using UIKit;
using WsLink.App.Controls;
using Xamarin.Forms;
using Xamarin.Forms.Platform.iOS;

[assembly: ExportRenderer(typeof(ExtendedEntryCell), typeof(ExtendedEntryCellRenderer))]

namespace WsLink.App.Controls
{
    public class ExtendedEntryCellRenderer : EntryCellRenderer
    {
        public override UITableViewCell GetCell(Cell item, UITableViewCell reusableCell, UITableView tv)
        {
            var entryCell = (ExtendedEntryCell) item;
            var cell = base.GetCell(item, reusableCell, tv);
            if (cell != null)
            {
                var textField = (UITextField) cell.ContentView.Subviews[0];
                textField.SecureTextEntry = entryCell.IsPassword;
            }
            return cell;
        }
    }
}

使用方法

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:controls="clr-namespace:WsLink.App.Controls"
             x:Class="App.Account.LoginPage" Title="登录">
  <TableView Intent="Form">
    <TableSection>
      <EntryCell Label="账户" Placeholder="账户" Text="{Binding Username}"></EntryCell>
      <controls:ExtendedEntryCell IsPassword="True" Label="密码" Placeholder="密码" Text="{Binding Password}"></controls:ExtendedEntryCell>
    </TableSection>
  </TableView>
</ContentPage>