android - Drawable 的 mutate 方法 Android 1.6 中的 NullPointerException

Question

在 Android 1.6 上使用带有列表状态 Drawable 的 mutate 时，我总是遇到空指针异常。它在更高的 Android 版本上完美运行。我正在使用默认的 android list_selector_background 作为要改变的 drawable，我需要改变它，否则按下的背景会变得非常困惑 -> 当按下一个时，全部或部分背景会突出显示。

项目在这里:

http://code.google.com/p/tree-view-list-android/

相关代码在这里:http://code.google.com/p/tree-view-list-android/source/browse/src/pl/polidea/treeview/AbstractTreeViewAdapter.java#205

return activity.getResources()
                    .getDrawable(android.R.drawable.list_selector_background)
                    .mutate();

下面的堆栈跟踪:

3-11 11:37:39.973: ERROR/AndroidRuntime(5304): java.lang.NullPointerException
03-11 11:37:39.973: ERROR/AndroidRuntime(5304):     at android.graphics.drawable.StateListDrawable.mutate(StateListDrawable.java:227)
03-11 11:37:39.973: ERROR/AndroidRuntime(5304):     at pl.polidea.treeview.AbstractTreeViewAdapter.getDrawableOrDefaultBackground(AbstractTreeViewAdapter.java:201)
03-11 11:37:39.973: ERROR/AndroidRuntime(5304):     at pl.polidea.treeview.AbstractTreeViewAdapter.populateTreeItem(AbstractTreeViewAdapter.java:210)
03-11 11:37:39.973: ERROR/AndroidRuntime(5304):     at pl.polidea.treeview.AbstractTreeViewAdapter.getView(AbstractTreeViewAdapter.java:153)
03-11 11:37:39.973: ERROR/AndroidRuntime(5304):     at android.widget.AbsListView.obtainView(AbsListView.java:1273)
03-11 11:37:39.973: ERROR/AndroidRuntime(5304):     at android.widget.ListView.makeAndAddView(ListView.java:1658)
03-11 11:37:39.973: ERROR/AndroidRuntime(5304):     at android.widget.ListView.fillDown(ListView.java:637)
03-11 11:37:39.973: ERROR/AndroidRuntime(5304):     at android.widget.ListView.fillFromTop(ListView.java:694)
03-11 11:37:39.973: ERROR/AndroidRuntime(5304):     at android.widget.ListView.layoutChildren(ListView.java:1516)
03-11 11:37:39.973: ERROR/AndroidRuntime(5304):     at android.widget.AbsListView.onLayout(AbsListView.java:1112)
03-11 11:37:39.973: ERROR/AndroidRuntime(5304):     at android.view.View.layout(View.java:6569)
03-11 11:37:39.973: ERROR/AndroidRuntime(5304):     at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1119)
03-11 11:37:39.973: ERROR/AndroidRuntime(5304):     at android.widget.LinearLayout.layoutVertical(LinearLayout.java:998)
03-11 11:37:39.973: ERROR/AndroidRuntime(5304):     at android.widget.LinearLayout.onLayout(LinearLayout.java:918)
03-11 11:37:39.973: ERROR/AndroidRuntime(5304):     at android.view.View.layout(View.java:6569)
03-11 11:37:39.973: ERROR/AndroidRuntime(5304):     at android.widget.FrameLayout.onLayout(FrameLayout.java:333)
03-11 11:37:39.973: ERROR/AndroidRuntime(5304):     at android.view.View.layout(View.java:6569)
03-11 11:37:39.973: ERROR/AndroidRuntime(5304):     at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1119)
03-11 11:37:39.973: ERROR/AndroidRuntime(5304):     at android.widget.LinearLayout.layoutVertical(LinearLayout.java:998)
03-11 11:37:39.973: ERROR/AndroidRuntime(5304):     at android.widget.LinearLayout.onLayout(LinearLayout.java:918)
03-11 11:37:39.973: ERROR/AndroidRuntime(5304):     at android.view.View.layout(View.java:6569)
03-11 11:37:39.973: ERROR/AndroidRuntime(5304):     at android.widget.FrameLayout.onLayout(FrameLayout.java:333)
03-11 11:37:39.973: ERROR/AndroidRuntime(5304):     at android.view.View.layout(View.java:6569)
03-11 11:37:39.973: ERROR/AndroidRuntime(5304):     at android.view.ViewRoot.performTraversals(ViewRoot.java:979)
03-11 11:37:39.973: ERROR/AndroidRuntime(5304):     at android.view.ViewRoot.handleMessage(ViewRoot.java:1613)
03-11 11:37:39.973: ERROR/AndroidRuntime(5304):     at android.os.Handler.dispatchMessage(Handler.java:99)
03-11 11:37:39.973: ERROR/AndroidRuntime(5304):     at android.os.Looper.loop(Looper.java:123)
03-11 11:37:39.973: ERROR/AndroidRuntime(5304):     at android.app.ActivityThread.main(ActivityThread.java:4203)
03-11 11:37:39.973: ERROR/AndroidRuntime(5304):     at java.lang.reflect.Method.invokeNative(Native Method)
03-11 11:37:39.973: ERROR/AndroidRuntime(5304):     at java.lang.reflect.Method.invoke(Method.java:521)
03-11 11:37:39.973: ERROR/AndroidRuntime(5304):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:791)
03-11 11:37:39.973: ERROR/AndroidRuntime(5304):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:549)
03-11 11:37:39.973: ERROR/AndroidRuntime(5304):     at dalvik.system.NativeStart.main(Native Method)

Answer 1

Android 1.6 好像有bug。当您深入研究 Android 1.6 源代码时，您会发现以下代码:

android.graphics.drawable.StateListDrawable.java:
...
public Drawable mutate() {
    if (!mMutated && super.mutate() == this) {
        final int[][] sets = mStateListState.mStateSets;
        final int count = sets.length;
        mStateListState.mStateSets = new int[count][];
        for (int i = 0; i < count; i++) {
            mStateListState.mStateSets[i] = sets[i].clone(); //NPE causing line
        }
        mMutated = true;
    }
    return this;
}
...

排队

mStateListState.mStateSets[i] = sets[i].clone(); //NPE causing line

clone() method is sometimes called on null object and it causes NullPointerException

在 Android > 1.6它已被修复:

public Drawable mutate() {
        if (!mMutated && super.mutate() == this) {
            final int[][] sets = mStateListState.mStateSets;
            final int count = sets.length;
            mStateListState.mStateSets = new int[count][];
            for (int i = 0; i < count; i++) {
                final int[] set = sets[i];
                if (set != null) {
                    mStateListState.mStateSets[i] = set.clone();
                }
            }
            mMutated = true;
        }
        return this;
    }

但在 Android 1.6 中我们需要做一些变通。让我们看看为什么 mStateSets[i] 有时包含空值:

android.graphics.drawable.DrawableContainer.DrowableContainerState:
... 
public final int addChild(Drawable dr) {
        final int pos = mNumChildren;

        if (pos >= mDrawables.length) {
            growArray(pos, pos+10); //Interesting line
        }

        dr.setVisible(false, true);
        dr.setCallback(mOwner);

        mDrawables[pos] = dr;
        mNumChildren++;
        mChildrenChangingConfigurations |= dr.getChangingConfigurations();
        mHaveOpacity = false;
        mHaveStateful = false;

        mConstantPadding = null;
        mPaddingChecked = false;
        mComputedConstantSize = false;

        return pos;
    }
...

上面的方法在从 xml 膨胀对象的过程中被调用。所以mStateListState.mStateSets的大小为N*10。

现在让我们看看您正试图从 android.R.drawable.list_selector_background 引用的资源中膨胀的 list_selector_background.xml 的主体:

<?xml version="1.0" encoding="utf-8"?>
<!-- Copyright (C) 2008 The Android Open Source Project

     Licensed under the Apache License, Version 2.0 (the "License");
     you may not use this file except in compliance with the License.
     You may obtain a copy of the License at

          http://www.apache.org/licenses/LICENSE-2.0

     Unless required by applicable law or agreed to in writing, software
     distributed under the License is distributed on an "AS IS" BASIS,
     WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     See the License for the specific language governing permissions and
     limitations under the License.
-->

<selector xmlns:android="http://schemas.android.com/apk/res/android">

    <item android:state_window_focused="false"
        android:drawable="@color/transparent" />

    <!-- Even though these two point to the same resource, have two states so the drawable will invalidate itself when coming out of pressed state. -->
    <item android:state_focused="true" android:state_enabled="false"
        android:state_pressed="true"
        android:drawable="@drawable/list_selector_background_disabled" />
    <item android:state_focused="true" android:state_enabled="false"
        android:drawable="@drawable/list_selector_background_disabled" />

    <item android:state_focused="true" android:state_pressed="true"
        android:drawable="@drawable/list_selector_background_transition" />
    <item android:state_focused="false" android:state_pressed="true"
        android:drawable="@drawable/list_selector_background_transition" />

    <item android:state_focused="true"
        android:drawable="@drawable/list_selector_background_focus" />

</selector>

此文件仅包含 6 个项目!那么我们如何解决这个问题呢？1. 在{Project}/res/drowable/下创建list_selector_background.xml 你可以复制Android的一个只改一行:

<item android:state_window_focused="false"
    android:drawable="@color/transparent" />

到

<item android:state_window_focused="false" android:drawable="@android:color/transparent" />

1. 复制 Android 或创建自己的文件:

   list_selector_background_disabled.9.pnglist_selector_background_focus.9.pnglist_selector_background_longpress.9.pnglist_selector_background_pressed.9.pnglist_selector_background_transition.xml

2. 修改 list_selector_background.xml 添加虚拟项目以完全匹配(6+4=10)个项目条件:

   <item android:animationCache="true"
       android:drawable="@android:color/transparent" />
   
   <item android:animationCache="false"
       android:drawable="@android:color/transparent" />
   
   <item android:alwaysDrawnWithCache="false"
       android:drawable="@android:color/transparent" />
   
   <item android:alwaysDrawnWithCache="true"
       android:drawable="@android:color/transparent" />
   

瞧。编译运行。它现在应该可以工作了。