Android：为什么旋转屏幕后不能在画布位图上绘画？

Question

I am using Android's Fingerpaint demo[1] to experiment with Canvas and Bitmaps. I want to draw an object on a screen, and continue drawing the object after the screen has rotated. The Fingerpaint demo erases the screen after a screen rotation - I want to preserve the contents of the screen and just rotate it along with the screen.

With my code, I can rotate the screen and the image I've drawn. But I'm no longer able to add any additional path markings to the bitmap. It becomes like a read-only Bitmap. Does anyone know what I am I doing wrong?

Here's the code where I save the image and restore it after a rotation. Notice that I am saving it as a PNG in a byte array (in onSaveInstanceState()) and then creating a new Bitmap from that byte array in onCreate() (I think this is okay?):

@Override
public void onSaveInstanceState(Bundle savedInstanceState) {
    Log.d("TAG", "Saving state...");
    ByteArrayOutputStream stream = new ByteArrayOutputStream();
    canvasView.mBitmap.compress(Bitmap.CompressFormat.PNG, 100, stream);
    imageByteArray = stream.toByteArray();
    savedInstanceState.putSerializable("ByteArray", imageByteArray);
    super.onSaveInstanceState(savedInstanceState);
}

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    canvasView = new MyView(this);
    setContentView(canvasView);

    mPaint = new Paint();
    mPaint.setAntiAlias(true);
    mPaint.setDither(true);
    mPaint.setColor(0xFF00FF00);
    mPaint.setStyle(Paint.Style.STROKE);
    mPaint.setStrokeJoin(Paint.Join.MITER);
    mPaint.setStrokeCap(Paint.Cap.ROUND);
    mPaint.setStrokeWidth(12);

    if (savedInstanceState != null) {
        Log.d("TAG", "Restoring any bitmaps...");
        byte[] imageByteArray = (byte[]) savedInstanceState.getSerializable("ByteArray");
        BitmapFactory.Options opt = new BitmapFactory.Options();
        opt.inMutable = true;
        Bitmap savedImage = BitmapFactory.decodeByteArray(imageByteArray, 0, imageByteArray.length, opt);
        canvasView.mBitmap = savedImage;
    }
}

In my custom view, MyView, here's the code where I rotate the bitmap when the screen changes:

@Override
protected void onSizeChanged(int w, int h, int oldw, int oldh) {
    super.onSizeChanged(w, h, oldw, oldh);
    Display display = ((WindowManager) getSystemService(Context.WINDOW_SERVICE)).getDefaultDisplay();
    int orientation = display.getRotation();
    Log.d("CANVAS", "Rotation: " + orientation);
    if (mBitmap == null) {
        mBitmap = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
        mCanvas = new Canvas(mBitmap);
    } else {
        Matrix rotator = new Matrix();
        rotator.postRotate(orientation * 3);
        Bitmap rotatedBitmap = Bitmap.createBitmap(mBitmap, 0, 0, mBitmap.getWidth(), mBitmap.getHeight(), rotator, true);
        mCanvas = new Canvas(rotatedBitmap);
        mCanvas.drawBitmap(rotatedBitmap, 0, 0, mPaint);
    }
}

Just about everything else is the same as in the Fingerpaint demo. I can push down to make markings on the screen, but when I lift my finger the path I've created is not applied to the bitmap.

Here's a duplication of the onTouchEvent() to illustrate (I did not modify it):

   public boolean onTouchEvent(MotionEvent event) {
            float x = event.getX();
            float y = event.getY();
            switch (event.getAction()) {
                case MotionEvent.ACTION_DOWN:
                    touch_start(x, y);
                    invalidate();
                    break;
                case MotionEvent.ACTION_MOVE:
                    touch_move(x, y);
                    invalidate();
                    break;
                case MotionEvent.ACTION_UP:
                    touch_up();
                    invalidate();
                    break;
            }
            return true;
        }

Thanks in advance for any insight. I suspect my understanding of how Canvas should work is not correct, and hence my confusion!

[1] The full Fingerpaint demo is here: https://android.googlesource.com/platform/development/+/master/samples/ApiDemos/src/com/example/android/apis/graphics/FingerPaint.java

Answer 1

BitmapFactory.decodeByteArray() ignores the option where you asked for a mutable bitmap and gives you one that isn't mutable because that's how it rolls.

https://developer.android.com/reference/android/graphics/BitmapFactory.html#decodeByteArray(byte[], int, int, android.graphics.BitmapFactory.Options)

decodeByteArray
    Bitmap decodeByteArray (byte[] data, 
                    int offset, 
                    int length, 
                    BitmapFactory.Options opts)
    Decode an immutable bitmap from the specified byte array.

You need to create a new bitmap object, using one of the ways to create a mutable bitmap. Then paint the immutable bitmap into the mutable one. Then toss the one you loaded with that function.

The reason for the immutability is that the bitmap was created from those bytes you gave it. So for speed they are backed by those bytes. They are the same memory. And the way actual bitmap objects work is different than that, and you can't just magically make a mutable bitmap by forcing those bytes from normal memory into the GPU. You can just make a second bitmap object and paint the data into that second bitmap.

Answer 2

You must control your configuration change for the activity.

<activity android:name=".MyActivity"
      android:configChanges="orientation|keyboardHidden"
      android:label="@string/app_name">

For more details just go for https://developer.android.com/guide/topics/resources/runtime-changes.html

Hope it helps..