xml 资源中 Bitmap.decodeResources() 中的空指针异常

Question

在我当前的 android 项目中，我从 xml 资源创建了两个位图。每次我使用 xml 位图作为 Bitmap.decodeResources() 的资源来构建我的项目时，当我尝试访问位图时都会出现空指针异常。但是当我切换到使用直接的 .png 时，错误就消失了。

为什么decodeResources() 返回 null？

创建位图的代码：

public OvertButton(float x, float y, float width, float height, int pressedId, int  unPressedId, Resources res, boolean visible) {
    \\these are null if xml bitmap is passed
    pressed = BitmapFactory.decodeResource(res, pressedId); 
    unPressed = BitmapFactory.decodeResource(res, unPressedId);

    this.visible = visible;
    relocate(x, y);
    resize(width, height); //throws NPE when I createScaledBitmap()

    ClickableManager.registerClickable(this);
}

资源文件

<bitmap xmlns:android="http://schemas.android.com/apk/res/android"
    android:src="@drawable/play_button_pressed"
    android:filter="false"
    android:antialias="false"
    android:dither="false"/>

和resize方法

@Override
    public void resize(float width, float height) {
        box.set(box.left, box.top, box.left + width, box.top + height);
        Log.d("resize",String.valueOf(height));
        pressed = Bitmap.createScaledBitmap(pressed, (int)width, (int)height, true);
        unPressed = Bitmap.createScaledBitmap(unPressed, (int)width, (int)height, true);
    }

还有日志猫

04-16 09:45:06.341  11824-11824/com.handmade.eed D/dalvikvm﹕ Late-enabling CheckJNI
04-16 09:45:06.450  11824-11824/com.handmade.eed D/MenuActivity﹕ 2130837594$$$2130837596
04-16 09:45:06.451  11824-11824/com.handmade.eed D/skia﹕ --- SkImageDecoder::Factory returned null
04-16 09:45:06.451  11824-11824/com.handmade.eed D/OvertButton﹕ true2130837591
04-16 09:45:06.457  11824-11824/com.handmade.eed D/OvertButton﹕ false2130837595
04-16 09:45:06.457  11824-11824/com.handmade.eed D/resize﹕ 252.0
04-16 09:45:06.458  11824-11824/com.handmade.eed D/AndroidRuntime﹕ Shutting down VM
04-16 09:45:06.459  11824-11824/com.handmade.eed W/dalvikvm﹕ threadid=1: thread exiting with uncaught exception (group=0x41610d40)
04-16 09:45:06.461  11824-11824/com.handmade.eed E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.handmade.eed, PID: 11824
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.handmade.eed/com.handmade.eed.MenuActivity}: java.lang.NullPointerException
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2209)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2269)
            at android.app.ActivityThread.access$800(ActivityThread.java:139)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1210)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5102)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
            at android.graphics.Bitmap.createScaledBitmap(Bitmap.java:615)
            at com.handmade.overt.visible.OvertButton.resize(OvertButton.java:61)
            at com.handmade.overt.visible.OvertButton.<init>(OvertButton.java:29)
            at com.handmade.overt.visible.OvertButton.<init>(OvertButton.java:34)
            at com.handmade.eed.MenuActivity.onCreate(MenuActivity.java:72)
            at android.app.Activity.performCreate(Activity.java:5248)
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1110)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2173)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2269)
            at android.app.ActivityThread.access$800(ActivityThread.java:139)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1210)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5102)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
            at dalvik.system.NativeStart.main(Native Method)
04-16 09:45:09.872  11824-11824/com.handmade.eed I/Process﹕ Sending signal. PID: 11824 SIG: 9

Answer 1

这是我愚蠢的情况，但这是后代的答案。 Bitmap与BitmapDrawable不一样

从上面的 Drawable Resources 链接：

编译的资源数据类型：
指向 BitmapDrawable 的资源指针。

因此我不得不改变这个：

Bitmap pressed = BitmapFactory.decodeResource(res, R.drawable.play_button_pressed_res); //this is the id for the xml referring to the png

到此：

Drawable pressed = res.getDrawable(R.drawable.play_button_pressed_res);

或者这个：

Bitmap pressed = BitmapFactory.decodeResource(res, R.drawable.play_button_pressed); //this is the id for the png 

我决定使用bitmap，因为据我所知，它更快。