android - 如何从android的默认资源中获取位图？

Question

你好我是用这种方式做的

Bitmap bm = BitmapFactory.decodeResource(getResources(), android.R.drawable.list_selector_background);

int c = bm.getPixel(bm.getWidth()/2, bm.getHeight()/2);

但是 bm 是 null，因为在第二行给出了 NullPointerException。

错误:

02-12 18:29:57.568: E/AndroidRuntime(5086):     at android.app.ActivityThread.access$600(ActivityThread.java:123)
02-12 18:29:57.568: E/AndroidRuntime(5086):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1147)
02-12 18:29:57.568: E/AndroidRuntime(5086):     at android.os.Handler.dispatchMessage(Handler.java:99)
02-12 18:29:57.568: E/AndroidRuntime(5086):     at android.os.Looper.loop(Looper.java:137)
02-12 18:29:57.568: E/AndroidRuntime(5086):     at android.app.ActivityThread.main(ActivityThread.java:4424)
02-12 18:29:57.568: E/AndroidRuntime(5086):     at java.lang.reflect.Method.invokeNative(Native Method)
02-12 18:29:57.568: E/AndroidRuntime(5086):     at java.lang.reflect.Method.invoke(Method.java:511)
02-12 18:29:57.568: E/AndroidRuntime(5086):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
02-12 18:29:57.568: E/AndroidRuntime(5086):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
02-12 18:29:57.568: E/AndroidRuntime(5086):     at dalvik.system.NativeStart.main(Native Method)
02-12 18:29:57.568: E/AndroidRuntime(5086): Caused by: java.lang.NullPointerException
02-12 18:29:57.568: E/AndroidRuntime(5086):     at com.example.showhide.MainActivity.onCreate(MainActivity.java:38)
02-12 18:29:57.568: E/AndroidRuntime(5086):     at android.app.Activity.performCreate(Activity.java:4465)
02-12 18:29:57.568: E/AndroidRuntime(5086):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1049)
02-12 18:29:57.568: E/AndroidRuntime(5086):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1920)

Answer 1

因为这个资源是StateListDrawable，不是Bitmap。你应该使用:

 Drawable d = getResources().getDrawable(android.R.drawable.list_selector_background);

如果您确定它是位图，将可绘制对象转换为 BitmapDrawable 并从那里获取位图:

Drawable currentState = d.getCurrent();
if(currentState instanceof BitmapDrawable)
    Bitmap b = ((BitmapDrawable)currentState).getBitmap();

位图也可能为空。