java.lang.ClassCastException:GridLayout.LayoutParams无法转换为Fra

Question

OS X Yosemite 10.10.5与Android Studio 2.0

出于教学目的，我想将尽可能多的View属性从XML移动到Java。

我在GridLayout中有一个android.support.v7.widget.RecyclerView，我想移动RecyclerView的android:layout_gravity。

从GridLayout.setGravity和R.attr文档中，我知道layout_gravity由子对象用于设置父对象中的重力属性。

以下代码是异常的根本原因：

GridLayout.LayoutParams parentParams = new GridLayout.LayoutParams();
parentParams.setGravity(Gravity.CENTER_VERTICAL);
container.setLayoutParams(parentParams);

我想要收缩的XML：

<GridLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/recycler_view_container"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <android.support.v7.widget.RecyclerView
        android:id="@+id/alphabet_recycler_view"
        android:layout_width="match_parent"
        android:layout_height="wrap_content">
    <!--android:layout_gravity="center_vertical">-->
    </android.support.v7.widget.RecyclerView>
</GridLayout>

这是异常链：

java.lang.ClassCastException: android.widget.GridLayout$LayoutParams cannot be cast to android.widget.FrameLayout$LayoutParams
at android.widget.FrameLayout.onMeasure(FrameLayout.java:431)
at android.support.v7.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:135)
at android.view.View.measure(View.java:17430)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463)
at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1436)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:722)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:613)
at android.view.View.measure(View.java:17430)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
at android.view.View.measure(View.java:17430)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463)
at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1436)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:722)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:613)
at android.view.View.measure(View.java:17430)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2560)
at android.view.View.measure(View.java:17430)
at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2001)
at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1166)
at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1372)
at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1054)
at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5779)
at android.view.Choreographer$CallbackRecord.run(Choreographer.java:767)
at android.view.Choreographer.doCallbacks(Choreographer.java:580)
at android.view.Choreographer.doFrame(Choreographer.java:550)
at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:753)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5221)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)

我查找了android.view.view.measure（view.java：17430），找到了这个：

if (cacheIndex < 0 || sIgnoreMeasureCache) {
    // measure ourselves, this should set the measured dimension flag back
    onMeasure(widthMeasureSpec, heightMeasureSpec);
    mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
} else {

我真的不明白它是如何调用android.support.v7.widget.ContentFrameLayout.onMeasure instead of android.view.View.onMeasure 的

我错过了什么？

Answer 1

在您提供的代码片段中：

GridLayout.LayoutParams parentParams = new GridLayout.LayoutParams();
parentParams.setGravity(Gravity.CENTER_VERTICAL);
container.setLayoutParams(parentParams);

它出现在变量名container中，您正试图将构造的LayoutParams应用于GridLayout实例，而不是RecyclerView实例。我希望最后一行看起来更像

recyclerView.setLayoutParams(parentParams);

如果我的假设是正确的，那么GridLayout的父级最终将尝试解析您构建的LayoutParams。这个父级不太可能（！）本身是GridLayout（我从堆栈跟踪中猜测它是FrameLayout），并且当它试图处理特定于GridLayout的LayoutParams时会阻塞。