测量switchCompat抛出异常

Question

下面是我的代码，用于测量列表视图中项目的高度，并根据其子元素的总高度设置列表视图的高度。

在使用任何其他视图时，我没有任何问题。

public static void setListViewHeightBasedOnChildren(TwoWayView listView) {
    ListAdapter listAdapter = listView.getAdapter();
    if (listAdapter == null) {
        return;
    }
    int desiredWidth = MeasureSpec.makeMeasureSpec(listView.getWidth(), MeasureSpec.AT_MOST);
    int totalHeight = 0;
    View listItem = null;
    for (int i = 0; i < listAdapter.getCount(); i++) {
        listItem = listAdapter.getView(i, listItem, listView);
        if (listItem instanceof ViewGroup) {
            listItem.setLayoutParams(new ViewGroup.LayoutParams(desiredWidth, ViewGroup.LayoutParams.WRAP_CONTENT));
        }
        listItem.measure(desiredWidth, MeasureSpec.UNSPECIFIED); // this is where the exception happens
        totalHeight += listItem.getMeasuredHeight();
    }
    ViewGroup.LayoutParams params = listView.getLayoutParams();
    params.height = totalHeight + (listView.getItemMargin() * (listAdapter.getCount() - 1)) + listView.getPaddingTop() + listView.getPaddingBottom();
    listView.setLayoutParams(params);
    listView.requestLayout();
}

下面是堆栈跟踪：

java.lang.NullPointerException
       at android.text.StaticLayout.<init>(StaticLayout.java:50)
       at android.support.v7.widget.SwitchCompat.makeLayout(SwitchCompat.java:606)
       at android.support.v7.widget.SwitchCompat.onMeasure(SwitchCompat.java:526)
       at android.view.View.measure(View.java:15395)
       at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4826)
       at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1396)
       at android.widget.LinearLayout.measureHorizontal(LinearLayout.java:1038)
       at android.widget.LinearLayout.onMeasure(LinearLayout.java:576)
       at android.view.View.measure(View.java:15395)
       at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4826)
       at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1396)
       at android.widget.LinearLayout.measureHorizontal(LinearLayout.java:1038)
       at android.widget.LinearLayout.onMeasure(LinearLayout.java:576)
       at android.view.View.measure(View.java:15395)

Answer 1

在SwtichCompat的方法onMeasue中，如果showText为true，则使用textOn和textOff进行布局：

 @Override
public void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    if (mShowText) {
        if (mOnLayout == null) {
            mOnLayout = makeLayout(mTextOn);
        }

        if (mOffLayout == null) {
            mOffLayout = makeLayout(mTextOff);
        }
    }
    ...
 }

看一看方法makeLayout：

private Layout makeLayout(CharSequence text) {
    final CharSequence transformed = (mSwitchTransformationMethod != null)
            ? mSwitchTransformationMethod.getTransformation(text, this)
            : text;

    return new StaticLayout(transformed, mTextPaint,
            transformed != null ?
                    (int) Math.ceil(Layout.getDesiredWidth(transformed, mTextPaint)) : 0,
            Layout.Alignment.ALIGN_NORMAL, 1.f, 0, true);
}

如果参数文本为空，则转换后的文本也为空。然后，它将被传递给StaticLayout的构造函数，从而导致NullPointException。

因此，您应该为SwitchCompat设置android:textOn和android:textOff属性，或者将app:showText=设置为“false”：

<android.support.v7.widget.SwitchCompat
  android:layout_width="wrap_content"
  android:layout_height="wrap_content"
  android:textOn="@string/text_on"
  android:textOff="@string/text_off" />

或

<android.support.v7.widget.SwitchCompat
  xmlns:app="http://schemas.android.com/apk/res-auto"   
  android:layout_width="wrap_content"
  android:layout_height="wrap_content"
  app:showText="false" />

Answer 2

SwitchCompat要求你指定android:textOn和android:textOff

例如，您可以像这样进行更改

<android.support.v7.widget.SwitchCompat
android:id="@+id/on_off_switch"
android:textOn="On"
android:textOff="Off"
android:layout_width="60dp"
android:layout_height="60dp"
android:layout_centerVertical="true"
android:layout_alignParentEnd="true"/>