我使用android studio并创建了自定义视图：

<LinearLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        android:orientation="vertical"
        android:gravity="center_horizontal|center_vertical"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_margin="4dp">

    <Button
            android:id="@+id/dateTile"
            android:layout_height="@dimen/date_tile_height"
            android:layout_width="@dimen/date_tile_width"
            android:background="@drawable/bg_january"/>

    <CheckBox
            android:id="@+id/dateTileCheckbox"
            android:button="@drawable/check_green"
            android:layout_width="30dp"
            android:layout_height="30dp"
            android:layout_marginLeft="15dp"
            android:layout_marginTop="-20dp"
            android:focusable="false"/>

    <TextView
            android:id="@+id/dateTileLabel"
            android:layout_width="wrap_content"
            android:layout_marginTop="-10dp"
            android:layout_height="wrap_content"/>

</LinearLayout>

<resources>
    <declare-styleable name="DateTileView">
        <attr name="monthLabel" format="string" localization="suggested" />
        <attr name="tileBackground" format="reference" />
    </declare-styleable>
</resources>

public class DateTileView extends LinearLayout
{
//....
//....
public DateTileView(Context context, AttributeSet attrs)
    {
        super(context, attrs);
        TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.DateTileView, 0, 0);

        String monthLabel = a.getString(R.styleable.DateTileView_monthLabel);
        Drawable monthBG = a.getDrawable(R.styleable.DateTileView_tileBackground);

        LayoutInflater inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        inflater.inflate(R.layout.custom_view_date_tile, this, true);

        TextView dateTileLabel = (TextView) findViewById(R.id.dateTileLabel);
        dateTileLabel.setText(monthLabel);

        Button dateTileButton = (Button) findViewById(R.id.dateTile);
        dateTileButton.setBackgroundDrawable(monthBG); //TODO: remove deprecated use if JELLY BEAN

        a.recycle();
    }
//...
//...
}

The following classes could not be instantiated:
- com.endilo.widget.DateTileView (Open Class, Show Exception)
  Tip: Use View.isInEditMode() in your custom views to skip code
  or show sample data when shown in the IDE  

 Exception Details java.lang.NullPointerException   at
 com.endilo.widget.DateTileView.<init>(DateTileView.java:35)   at
 java.lang.reflect.Constructor.newInstance   at
 android.view.LayoutInflater.rInflate_Original   at
 android.view.LayoutInflater_Delegate.rInflate   at
 android.view.LayoutInflater.rInflate   at
 android.view.LayoutInflater.rInflate_Original   at
 android.view.LayoutInflater_Delegate.rInflate   at
 android.view.LayoutInflater.rInflate   at
 android.view.LayoutInflater.rInflate_Original   at
 android.view.LayoutInflater_Delegate.rInflate   at
 android.view.LayoutInflater.rInflate   at
 android.view.LayoutInflater.inflate   at android.view.LayoutInflater.inflate

最佳答案

当应用程序未运行时，您不能执行第35行中的任何操作，因为设计模式将为某些仅在运行时实例化的对象获取空引用。
必须避免使用isineditmode（）在设计模式下执行这些操作，如下所示：

if(!isInEditMode()) {
    whatever there is in the line 35 of your class
}

@Override
protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);

    TypedArray a = this.getContext().obtainStyledAttributes(attrs, R.styleable.DateTileView, 0, 0);

    String monthLabel = a.getString(R.styleable.DateTileView_monthLabel);
    Drawable monthBG = a.getDrawable(R.styleable.DateTileView_tileBackground);

    LayoutInflater inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    inflater.inflate(R.layout.custom_view_date_tile, this, true);

    TextView dateTileLabel = (TextView) findViewById(R.id.dateTileLabel);
    dateTileLabel.setText(monthLabel);

    Button dateTileButton = (Button) findViewById(R.id.dateTile);
    dateTileButton.setBackgroundDrawable(monthBG); //TODO: remove deprecated use if JELLY BEAN

    a.recycle();
}

关于android - 在预览布局中使用自定义 View 中的自定义属性，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/18258719/