从子类TextView中创建一个定制的Andr​​oid UI元素时避免一个NullReferenceException [英] Avoiding a NullReferenceException when creating a custom Android UI element from subclassed TextView

问题描述

一对夫妇在这里首创的 - 使用MonoDroid的第一款Android应用程序，并第一次（我有很多与C＃.NET体验）。

A couple of firsts here - first Android app and first time using MonoDroid (I've got lots of experience with C# .NET).

在我的用户界面我想提请围绕一个TextView边框，发现一个帖子上SO（<一个href=\"http://stackoverflow.com/questions/2026873/android-way-to-appear-bordered-text-on-the-textview\">2026873)所推荐子类的TextView。我也发现了另一篇文章（<一个href=\"http://stackoverflow.com/questions/2695646/declaring-a-custom-android-ui-element-using-xml\">2695646)与使用XML声明定制的Andr​​oid UI元素一些额外的信息。（注：在本例中发布所有code是在Java中，必须翻译成C＃/ MonoDroid的环境）

In my user interface I want to draw a border around a TextView and found a post on SO (2026873) that recommended subclassing TextView. I also found another post (2695646) with some additional info on declaring a custom Android UI element using XML. (Note: All code in the example posts were in Java, had to translate into the C#/MonoDroid environment.)

当我运行仿真器中的code我得到一个System.NullReferenceException：对象不设置到对象的实例。

When I run the code in the emulator I get a System.NullReferenceException: Object reference not set to an instance of an object.

下面是我出的现成活动1 code和code的子类的TextView。

Here's my out-of-the-box Activity1 code and the code for the subclassed TextView.

namespace MBTA
{
    [Activity(Label = "MBTA", MainLauncher = true, Icon = "@drawable/icon")]
    public class Activity1 : Activity
    {
        protected override void OnCreate(Bundle bundle)
        {
            base.OnCreate(bundle);
            SetContentView(Resource.Layout.Main);
        }
    }

    public class BorderedTextView : TextView
    {
        public BorderedTextView(Context context, IAttributeSet attrs, int defStyle) : base(context, attrs, defStyle) { }
        public BorderedTextView(Context context, IAttributeSet attrs) : base(context, attrs) { }
        public BorderedTextview(Context context) : base(context) { }

        protected override void OnDraw (Android.Graphics.Canvas canvas)
        {
            base.OnDraw (canvas);

            Rect rect = new Rect();
            Paint paint = new Paint();

            paint.SetStyle(Android.Graphics.Paint.Style.Stoke);
            paint.Color = Android.Graphics.Color.White;
            paint.StrokeWidth = 3;

            GetLocalVisibleRect(rect);
            canvas.DrawRect(rect, paint);
        }
    }
}

我Main.axml布局如下：

My Main.axml layout is as follows:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res/MBTA"     
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent">
    <LinearLayout
        android:orientation="horizontal"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:layout_weight="1">
        <MBTA.BorderedTextView
            android:text="DATE"
            android:textSize="15pt"
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:gravity="center_horizontal|center_vertical"
            android:layout_weight="1"/>
    </LinearLayout>
</LinearLayout>

和我attrs.xml文件如下（与它的BuildAction的设置为AndroidResource）：

And my attrs.xml file is as follows (with its BuildAction set to AndroidResource):

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <declare-styleable name="BorderedTextView">
        <attr name="android:text"/>
    <attr name="android:textSize"/>
    <attr name="android:layout_width"/>
    <attr name="android:layout_height"/>
    <attr name="android:gravity"/>
    <attr name="android:layout_weight"/>
    </declare-styleable>
</resources>

先谢谢了。

推荐答案

单为Android产生了Java的 Android的调用包装使用由Android和布局的XML文件。

Mono for Android lowercases the namespace name when generating the Java Android Callable Wrappers for use by Android and layout XML files.

结果（如的previous答案），你需要使用 mbta.BorderedTextView ，而不是 MBTA.BorderedTextView 。

Consequently (as noted in the previous answer), you need to use mbta.BorderedTextView, not MBTA.BorderedTextView.

Mono的为Android文档铁饼在布局使用自定义的意见，其中涉及本场景。

The Mono for Android documentation discuses using custom views in a layout, which addresses this scenario.

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