将自定义设计时属性添加到 NetworkImageView

Volley NetworkImageView 增加了标准 ImageView 的其他几个属性。但是，这些属性只能在代码中设置。以下是如何创建扩展类的示例，该扩展类将从 XML 布局文件中获取属性并将它们应用于 NetworkImageView 实例。

在~/res/xml 目录中，添加名为 attrx.xml 的文件：

<resources>
    <declare-styleable name="MoreNetworkImageView">
        <attr name="defaultImageResId" format="reference"/>
        <attr name="errorImageResId" format="reference"/>
    </declare-styleable>
</resources>

将新类文件添加到项目中：

package my.namespace;

import android.content.Context;
import android.content.res.TypedArray;
import android.support.annotation.NonNull;
import android.util.AttributeSet;

import com.android.volley.toolbox.NetworkImageView;

public class MoreNetworkImageView extends NetworkImageView {
    public MoreNetworkImageView(@NonNull final Context context) {
        super(context);
    }

    public MoreNetworkImageView(@NonNull final Context context, @NonNull final AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public MoreNetworkImageView(@NonNull final Context context, @NonNull final AttributeSet attrs, final int defStyle) {
        super(context, attrs, defStyle);

        final TypedArray attributes = context.obtainStyledAttributes(attrs, R.styleable.MoreNetworkImageView, defStyle, 0);

        // load defaultImageResId from XML
        int defaultImageResId = attributes.getResourceId(R.styleable.MoreNetworkImageView_defaultImageResId, 0);
        if (defaultImageResId > 0) {
            setDefaultImageResId(defaultImageResId);
        }

        // load errorImageResId from XML
        int errorImageResId = attributes.getResourceId(R.styleable.MoreNetworkImageView_errorImageResId, 0);
        if (errorImageResId > 0) {
            setErrorImageResId(errorImageResId);
        }
    }
}

显示自定义属性使用的示例布局文件：

<?xml version="1.0" encoding="utf-8"?>
<android.support.v7.widget.CardView
  xmlns:android="http://schemas.android.com/apk/res/android"
  xmlns:app="http://schemas.android.com/apk/res-auto"
  xmlns:tools="http://schemas.android.com/tools"
  android:layout_width="wrap_content"
  android:layout_height="fill_parent">

  <my.namespace.MoreNetworkImageView
    android:layout_width="64dp"
    android:layout_height="64dp"
    app:errorImageResId="@drawable/error_img"
    app:defaultImageResId="@drawable/default_img"
    tools:defaultImageResId="@drawable/editor_only_default_img"/>
    <!-- 
      Note: The "tools:" prefix does NOT work for custom attributes in Android Studio 2.1 and 
      older at least, so in this example the defaultImageResId would show "default_img" in the 
      editor, not the "editor_only_default_img" drawable even though it should if it was 
      supported as an editor-only override correctly like standard Android properties.
    -->

</android.support.v7.widget.CardView>