Android:setBackgroundTintList 在支持 fab 上工作，setSupportBackgroundTintList 导致 npe

Question

我有一个支持工厂定义如下:

<android.support.design.widget.FloatingActionButton
        android:id="@+id/somefab"
        android:layout_width="50dp"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="20dp"
        app:elevation="6dp"
        app:rippleColor="@android:color/white"
        />

如果我像这样以编程方式设置 backgroundtintlist 那么它就可以工作:

somefab.setBackgroundTintList(ColorStateList.valueOf(Color.GRAY));

然而，这是一个支持 fab，应该在较低的 api 上可见，所以我想使用支持方法:

somefab.setSupportBackgroundTintList(ColorStateList.valueOf(Color.GRAY));

这里我已经得到一个空指针异常。

    java.lang.RuntimeException: Unable to start activity ComponentInfo{myprojectpath}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.graphics.drawable.Drawable android.graphics.drawable.Drawable$ConstantState.newDrawable()' on a null object reference
  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
  at android.app.ActivityThread.handleRelaunchActivity(ActivityThread.java:4077)
  at android.app.ActivityThread.-wrap15(ActivityThread.java)
  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1350)
  at android.os.Handler.dispatchMessage(Handler.java:102)
  at android.os.Looper.loop(Looper.java:148)
  at android.app.ActivityThread.main(ActivityThread.java:5417)
  at java.lang.reflect.Method.invoke(Native Method)
  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
                                                                            Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.graphics.drawable.Drawable android.graphics.drawable.Drawable$ConstantState.newDrawable()' on a null object reference
  at android.graphics.drawable.LayerDrawable$ChildDrawable.<init>(LayerDrawable.java:1723)
  at android.graphics.drawable.LayerDrawable$LayerState.<init>(LayerDrawable.java:1792)
  at android.graphics.drawable.RippleDrawable$RippleState.<init>(RippleDrawable.java:998)
  at android.graphics.drawable.RippleDrawable.createConstantState(RippleDrawable.java:989)
  at android.graphics.drawable.RippleDrawable.createConstantState(RippleDrawable.java:988)
  at android.graphics.drawable.LayerDrawable.mutate(LayerDrawable.java:1652)
  at android.graphics.drawable.RippleDrawable.mutate(RippleDrawable.java:975)
  at android.support.v7.widget.AppCompatDrawableManager.tintDrawable(AppCompatDrawableManager.java:773)
  at android.support.v7.widget.AppCompatBackgroundHelper.applySupportBackgroundTint(AppCompatBackgroundHelper.java:115)
  at android.support.v7.widget.AppCompatBackgroundHelper.setSupportBackgroundTintList(AppCompatBackgroundHelper.java:90)
  at android.support.v7.widget.AppCompatImageButton.setSupportBackgroundTintList(AppCompatImageButton.java:99)
  at myproject.myActivity$override.onCreate(myActivity.java:126)
  at myproject.myActivity$override.access$dispatch(myActivity.java)
  at myproject.myActivity.onCreate(myActivity.java:0)
  at android.app.Activity.performCreate(Activity.java:6251)
  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
  at android.app.ActivityThread.handleRelaunchActivity(ActivityThread.java:4077) 
  at android.app.ActivityThread.-wrap15(ActivityThread.java) 
  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1350) 
  at android.os.Handler.dispatchMessage(Handler.java:102) 
  at android.os.Looper.loop(Looper.java:148) 
  at android.app.ActivityThread.main(ActivityThread.java:5417) 
  at java.lang.reflect.Method.invoke(Native Method) 
  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 

为什么第一种方法有效而第二种方法无效？

Answer 1

这应该可行，我在 API 15、21 和 23 上测试过它:

ViewCompat.setBackgroundTintList(somefab, ColorStateList.valueOf(Color.GRAY));

为什么？转到 setSupportBackgroundTintList() 的源代码显示:

/**
 * This should be accessed via
 * {@link android.support.v4.view.ViewCompat#setBackgroundTintList(android.view.View, ColorStateList)}
 *
 * @hide
 */
@Override
public void setSupportBackgroundTintList(@Nullable ColorStateList tint) {
    if (mBackgroundTintHelper != null) {
        mBackgroundTintHelper.setSupportBackgroundTintList(tint);
    }
}

似乎应该使用 ViewCompat.setBackgroundTintList() 间接调用 setSupportBackgroundTintList() 方法。我尝试直接调用 setSupportBackgroundTintList()，它适用于 API 15 和 21，但在 API 23 上崩溃，给出与你给出的相同的 NullPointerException。