为TextSwitcher使用setCurrentText或setText时,为什么会出现NullPointerException？

Question

TextSwitcher是ListView行的一部分(在xml中定义).我在setViewValue中定义TextSwitcher的文本:

switch(viewId) {
case R.id.tw_text:
    TextSwitcher twText = (TextSwitcher) view;
    twText.setCurrentText("test text");
    return true;

此代码的工作完美,如果twText是TextView.但它返回NullPointerException TextSwitcher:

05-22 10:24:08.781: ERROR/AndroidRuntime(3348): FATAL EXCEPTION: main
05-22 10:24:08.781: ERROR/AndroidRuntime(3348): java.lang.NullPointerException
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.widget.TextSwitcher.setCurrentText(TextSwitcher.java:89)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at my.package.MainActivity$1.setViewValue(MainActivity.java:87)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.widget.SimpleCursorAdapter.bindView(SimpleCursorAdapter.java:126)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.widget.CursorAdapter.getView(CursorAdapter.java:186)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.widget.AbsListView.obtainView(AbsListView.java:1315)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.widget.ListView.measureHeightOfChildren(ListView.java:1198)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.widget.ListView.onMeasure(ListView.java:1109)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.view.View.measure(View.java:8171)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:3132)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1012)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.widget.LinearLayout.measureHorizontal(LinearLayout.java:696)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.widget.LinearLayout.onMeasure(LinearLayout.java:306)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.view.View.measure(View.java:8171)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:3132)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.widget.FrameLayout.onMeasure(FrameLayout.java:245)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.view.View.measure(View.java:8171)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:3132)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.widget.FrameLayout.onMeasure(FrameLayout.java:245)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.view.View.measure(View.java:8171)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:3132)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.widget.FrameLayout.onMeasure(FrameLayout.java:245)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.view.View.measure(View.java:8171)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:3132)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1012)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.widget.LinearLayout.measureVertical(LinearLayout.java:381)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.widget.LinearLayout.onMeasure(LinearLayout.java:304)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.view.View.measure(View.java:8171)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:3132)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.widget.FrameLayout.onMeasure(FrameLayout.java:245)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.view.View.measure(View.java:8171)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:3132)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.widget.FrameLayout.onMeasure(FrameLayout.java:245)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.view.View.measure(View.java:8171)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.widget.LinearLayout.measureVertical(LinearLayout.java:526)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.widget.LinearLayout.onMeasure(LinearLayout.java:304)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.view.View.measure(View.java:8171)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:3132)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.widget.FrameLayout.onMeasure(FrameLayout.java:245)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.view.View.measure(View.java:8171)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.view.ViewRoot.performTraversals(ViewRoot.java:801)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.view.ViewRoot.handleMessage(ViewRoot.java:1727)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.os.Handler.dispatchMessage(Handler.java:99)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.os.Looper.loop(Looper.java:123)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at android.app.ActivityThread.main(ActivityThread.java:4627)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at java.lang.reflect.Method.invokeNative(Native Method)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at java.lang.reflect.Method.invoke(Method.java:521)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
05-22 10:24:08.781: ERROR/AndroidRuntime(3348):     at dalvik.system.NativeStart.main(Native Method)

更新1:看起来我需要将setFactory()添加到我的代码中.但如果我做以下事情:

  TextSwitcher twText = (TextSwitcher) view;
  twText.setFactory(new ViewSwitcher.ViewFactory() {
      @Override
      public View makeView() {
      TextView t = new TextView(MainActivity.this);
      return t;
      }
      });

我收到另一个错误:

05-22 10:54:42.852: ERROR/AndroidRuntime(3907): java.lang.IllegalStateException: Can't add more than 2 views to a ViewSwitcher
05-22 10:54:42.852: ERROR/AndroidRuntime(3907):     at android.widget.ViewSwitcher.addView(ViewSwitcher.java:64)
05-22 10:54:42.852: ERROR/AndroidRuntime(3907):     at android.widget.TextSwitcher.addView(TextSwitcher.java:67)
05-22 10:54:42.852: ERROR/AndroidRuntime(3907):     at android.view.ViewGroup.addView(ViewGroup.java:1845)

更新2:我之前添加twText.removeAllViews()过twText.setFactory().它可以工作,但看起来不是再次删除和添加视图的最佳方式.如何查看是否已有视图？

另外,我想在xml中保留TextSwitcher的格式/样式.setFactory的使用不允许我这样做.我能做什么？

Answer 1

看起来在setCurrentText调用时没有向文本切换器添加任何视图.此方法基本上只获取当前文本视图并为其设置文本,因此如果NullPointerException出现,则可能仅由于文本切换器中缺少视图而发生.

关于工厂的错误:您必须小心设置工厂或自己添加两个视图.您有以下任一选项:

• 调用setFactory将创建两个视图.
• 通过调用手动添加两个视图addView.

在这两种情况下,只能有两个视图,不多也不少.如果您设置了工厂并且已添加了一些视图,则会出现错误.您可以通过检查getChildCount()返回的方法来检查是否需要设置工厂:

TextSwitcher switcher = (TextSwitcher) view;
if (switcher.getChildCount() != 2) {
    switcher.removeAllViews();
    switcher.setFactory(new ViewSwitcher.ViewFactory() {
        @Override
        public View makeView() {
            return new TextView(MainActivity.this);
        }
    });
}