元素未在 Xamarin Forms Android 自定义渲染器中显示

Question

使用 Xamarin Forms 自定义渲染器，我需要在实时相机视图（视频）上叠加十字准线图像。我让 iOS 渲染器工作，它用于 BoxView。 Android 然而对我来说是相当新的。所以在 Android BoxView 渲染器中，我从一些简单的东西开始，只是尝试为初学者显示一个 ImageView 或一个 EditText 控件。

尽管进行了大量谷歌搜索，但仍在努力解决这个问题。

这是我的代码。它执行，但没有控件（图像）出现在屏幕上。

共享代码：

using Xamarin.Forms;

namespace MyApp.Views
{
    public class CameraBoxView : BoxView
    // Uses custom renderer CameraBoxViewRenderer.cs
    {}
}

ANDROID 渲染器：

using System;

using Xamarin.Forms;
using Xamarin.Forms.Platform.Android;

using MyApp.Views;
using MyApp.Droid.Camera;

using Android.Content;
using Android.Widget;
using Android.Views;


[assembly:ExportRenderer (typeof(CameraBoxView), typeof(CameraBoxViewRenderer))]

namespace MyApp.Droid.Camera
{
    public class CameraBoxViewRenderer : BoxRenderer
    {
        private Context context;
        private ImageView imageView;
        private EditText editText;

        public CameraBoxViewRenderer ( Context context ) : base ( context )
        {
            this.context = context;
        }

        protected override void OnElementChanged ( ElementChangedEventArgs<BoxView> e )
        {
            base.OnElementChanged ( e );

            if ( e.OldElement != null || Element == null)
                return;

            CameraBoxView box = Element as CameraBoxView;

            imageView = new ImageView ( context );
            imageView.ContentDescription = "my image";
            imageView.SetImageResource ( Resource.Drawable.myimage );
            imageView.LayoutParameters = new LinearLayout.LayoutParams ( ViewGroup.LayoutParams.MatchParent,ViewGroup.LayoutParams.MatchParent );
            imageView.SetScaleType ( ImageView.ScaleType.FitCenter );
            imageView.Visibility = ViewStates.Visible;
            this.AddView ( imageView );

            //editText = new EditText ( context );
            //editText.ContentDescription = "edit_text";
            //editText.LayoutParameters = new LinearLayout.LayoutParams ( ViewGroup.LayoutParams.MatchParent,ViewGroup.LayoutParams.MatchParent );
            //editText.Visibility = ViewStates.Visible;
            //editText.Hint = "Hello world!";
            //this.AddView ( editText );
        }
    }
}

Answer 1

添加以下代码解决了问题。感谢 Adam Kemp.

        protected override void OnLayout ( bool changed, int l, int t, int r, int b )
        {
            base.OnLayout ( changed, l, t, r, b );

            GetChildAt ( 0 ).Layout ( 0, 0, r-l, b-t );
        }

        protected override void OnMeasure ( int widthMeasureSpec, int heightMeasureSpec )
        {
            base.OnMeasure ( widthMeasureSpec, heightMeasureSpec );

            GetChildAt (0).Measure ( widthMeasureSpec, heightMeasureSpec );
        }

元素未在 Xamarin Forms Android 自定义渲染器中显示

Element not displaying in Xamarin Forms Android custom renderer

android

xamarin.forms

custom-renderer