Draw方法中Bitmap的NullPointer

Question

我正在尝试在画布上绘制位图。位图返回 null，但我不明白为什么。我已经阅读了 Bitmap 和 decodeResources 的文档。由于 decode 找不到它，drawable 似乎返回 null 。我试图使用调试会话，但我没有走得太远。

请就空异常提出建议。

错误类：

public class SnapShot extends Activity {

private Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);

public void draw(Canvas canvas) {
    drawIcon(canvas);    
}

private void drawIcon(Canvas canvas) {
    Bitmap bitmap = BitmapFactory.decodeResource(getResources(), R.drawable.buttongreen); //this is line 36
    canvas.drawBitmap(bitmap, 150, 150, paint);
}

}

堆栈跟踪：

10-01 09:24:30.638: E/AndroidRuntime(27206): FATAL EXCEPTION: main
10-01 09:24:30.638: E/AndroidRuntime(27206): java.lang.NullPointerException
10-01 09:24:30.638: E/AndroidRuntime(27206):    at android.content.ContextWrapper.getResources(ContextWrapper.java:80)
10-01 09:24:30.638: E/AndroidRuntime(27206):    at com.game.SnapShot.drawIcon(SnapShot.java:36)
10-01 09:24:30.638: E/AndroidRuntime(27206):    at com.game.SnapShot.draw(SnapShot.java:29)
10-01 09:24:30.638: E/AndroidRuntime(27206):    at com.game.onDraw(AugmentedView.java:77)
10-01 09:24:30.638: E/AndroidRuntime(27206):    at android.view.View.draw(View.java:6880)
10-01 09:24:30.638: E/AndroidRuntime(27206):    at android.view.ViewGroup.drawChild(ViewGroup.java:1646)
10-01 09:24:30.638: E/AndroidRuntime(27206):    at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1373)
10-01 09:24:30.638: E/AndroidRuntime(27206):    at android.view.View.draw(View.java:6883)
10-01 09:24:30.638: E/AndroidRuntime(27206):    at android.widget.FrameLayout.draw(FrameLayout.java:357)
10-01 09:24:30.638: E/AndroidRuntime(27206):    at android.view.ViewGroup.drawChild(ViewGroup.java:1646)
10-01 09:24:30.638: E/AndroidRuntime(27206):    at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1373)
10-01 09:24:30.638: E/AndroidRuntime(27206):    at android.view.View.draw(View.java:6883)
10-01 09:24:30.638: E/AndroidRuntime(27206):    at android.widget.FrameLayout.draw(FrameLayout.java:357)
10-01 09:24:30.638: E/AndroidRuntime(27206):    at com.android.internal.policy.impl.PhoneWindow$DecorView.draw(PhoneWindow.java:1862)
10-01 09:24:30.638: E/AndroidRuntime(27206):    at android.view.ViewRoot.draw(ViewRoot.java:1522)
10-01 09:24:30.638: E/AndroidRuntime(27206):    at android.view.ViewRoot.performTraversals(ViewRoot.java:1258)
10-01 09:24:30.638: E/AndroidRuntime(27206):    at android.view.ViewRoot.handleMessage(ViewRoot.java:1859)
10-01 09:24:30.638: E/AndroidRuntime(27206):    at android.os.Handler.dispatchMessage(Handler.java:99)
10-01 09:24:30.638: E/AndroidRuntime(27206):    at android.os.Looper.loop(Looper.java:130)
10-01 09:24:30.638: E/AndroidRuntime(27206):    at android.app.ActivityThread.main(ActivityThread.java:3683)
10-01 09:24:30.638: E/AndroidRuntime(27206):    at java.lang.reflect.Method.invokeNative(Native Method)
10-01 09:24:30.638: E/AndroidRuntime(27206):    at java.lang.reflect.Method.invoke(Method.java:507)
10-01 09:24:30.638: E/AndroidRuntime(27206):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
10-01 09:24:30.638: E/AndroidRuntime(27206):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
10-01 09:24:30.638: E/AndroidRuntime(27206):    at dalvik.system.NativeStart.main(Native Method)

Answer 1

如果你在视图中编写 drawIcon(Canvas canvas) 方法，那么你应该有一个上下文来调用 getResource()。

例如：

Bitmap bitmap = BitmapFactory.decodeResource(context.getResources(), R.drawable.buttongreen);
    canvas.drawBitmap(bitmap, 150, 150, paint);