Android，画布: 我如何清除(删除)一个画布(= 位图) ，生活在一个表面视图？

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最佳答案

How do I clear (or redraw) the WHOLE canvas for a new layout (= try at the game) ?

Just call Canvas.drawColor(Color.BLACK), or whatever color you want to clear your Canvas with.

And: how can I update just a part of the screen ?

There is no such method that just update a "part of the screen" since Android OS is redrawing every pixel when updating the screen. But, when you're not clearing old drawings on your Canvas, the old drawings are still on the surface and that is probably one way to "update just a part" of the screen.

So, if you want to "update a part of the screen", just avoid calling Canvas.drawColor() method.

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Here is the code of a minimal example showing that you always have to redraw every pixel of the Canvas at each frame.

This activity draw a new Bitmap every second on the SurfaceView, without clearing the screen before. If you test it, you will see that the bitmap is not always written to the same buffer, and the screen will alternate between the two buffers.

I tested it on my phone (Nexus S, Android 2.3.3), and on the emulator (Android 2.2).

public class TestCanvas extends Activity {
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(new TestView(this));
}
}


class TestView extends SurfaceView implements SurfaceHolder.Callback {


private TestThread mThread;
private int mWidth;
private int mHeight;
private Bitmap mBitmap;
private SurfaceHolder mSurfaceHolder;


public TestView(Context context) {
super(context);
mThread = new TestThread();
mBitmap = BitmapFactory.decodeResource(context.getResources(), R.drawable.icon);
mSurfaceHolder = getHolder();
mSurfaceHolder.addCallback(this);
}


@Override
public void surfaceChanged(SurfaceHolder holder, int format, int width,
int height) {
mWidth = width;
mHeight = height;
mThread.start();
}


@Override
public void surfaceCreated(SurfaceHolder holder) {/* Do nothing */}


@Override
public void surfaceDestroyed(SurfaceHolder holder) {
if (mThread != null && mThread.isAlive())
mThread.interrupt();
}


class TestThread extends Thread {
@Override
public void run() {
while (!isInterrupted()) {
Canvas c = null;
try {
c = mSurfaceHolder.lockCanvas(null);
synchronized (mSurfaceHolder) {
c.drawBitmap(mBitmap, (int) (Math.random() * mWidth), (int) (Math.random() * mHeight), null);
}
} finally {
if (c != null)
mSurfaceHolder.unlockCanvasAndPost(c);
}


try {
sleep(1000);
} catch (InterruptedException e) {
interrupt();
}
}
}
}
}

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Just call

canvas.drawColor(Color.TRANSPARENT)

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Draw transparent color with PorterDuff clear mode does the trick for what I wanted.

Canvas.drawColor(Color.TRANSPARENT, PorterDuff.Mode.CLEAR)

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please paste below code on surfaceview extend class constructor.............

constructor coding

    SurfaceHolder holder = getHolder();
holder.addCallback(this);


SurfaceView sur = (SurfaceView)findViewById(R.id.surfaceview);
sur.setZOrderOnTop(true);    // necessary
holder = sur.getHolder();
holder.setFormat(PixelFormat.TRANSPARENT);

xml coding

    <com.welcome.panelview.PanelViewWelcomeScreen
android:id="@+id/one"
android:layout_width="600px"
android:layout_height="312px"
android:layout_gravity="center"
android:layout_marginTop="10px"
android:background="@drawable/welcome" />

try above code...

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Found this in google groups and this worked for me..

Paint clearPaint = new Paint();
clearPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
canvas.drawRect(0, 0, width, height, clearPaint);

This removes drawings rectangles etc. while keeping set bitmap..

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mBitmap.eraseColor(Color.TRANSPARENT);


canvas.drawBitmap(mBitmap, 0, 0, mBitmapPaint);

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Your first requirement, how to clear or redraw whole canvas - Answer - use canvas.drawColor(color.Black) method for clearing the screen with a color of black or whatever you specify .

Your second requirement, how to update part of the screen - Answer - for example if you want to keep all other things unchanged on the screen but in a small area of screen to show an integer(say counter) which increases after every five seconds. then use canvas.drawrect method to draw that small area by specifying left top right bottom and paint. then compute your counter value(using postdalayed for 5 seconds etc., llike Handler.postDelayed(Runnable_Object, 5000);) , convert it to text string, compute the x and y coordinate in this small rect and use text view to display the changing counter value.

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use the reset method of Path class

Path.reset();

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I tried the answer of @mobistry:

canvas.drawColor(Color.TRANSPARENT, Mode.CLEAR);

But it doesn't worked for me.

The solution, for me, was:

canvas.drawColor(Color.TRANSPARENT, Mode.MULTIPLY);

Maybe some one has the same problem.

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For me calling Canvas.drawColor(Color.TRANSPARENT, PorterDuff.Mode.CLEAR) or something similar would only work after I touch the screen. SO I would call the above line of code but the screen would only clear after I then touched the screen. So what worked for me was to call invalidate() followed by init() which is called at the time of creation to initialize the view.

private void init() {
setFocusable(true);
setFocusableInTouchMode(true);
setOnTouchListener(this);


mPaint = new Paint();
mPaint.setAntiAlias(true);
mPaint.setDither(true);
mPaint.setColor(Color.BLACK);
mPaint.setStyle(Paint.Style.STROKE);
mPaint.setStrokeJoin(Paint.Join.ROUND);
mPaint.setStrokeCap(Paint.Cap.ROUND);
mPaint.setStrokeWidth(6);


mCanvas = new Canvas();
mPaths = new LinkedList<>();


addNewPath();
}

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canvas.drawColor(Color.TRANSPARENT, Mode.MULTIPLY);

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I had to use a separate drawing pass to clear the canvas (lock, draw and unlock):

Canvas canvas = null;
try {
canvas = holder.lockCanvas();
if (canvas == null) {
// exit drawing thread
break;
}
canvas.drawColor(colorToClearFromCanvas, PorterDuff.Mode.CLEAR);
} finally {
if (canvas != null) {
holder.unlockCanvasAndPost(canvas);
}
}

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Don't forget to call invalidate();

canvas.drawColor(backgroundColor);
invalidate();
path.reset();

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With the following approach, you can clear the whole canvas or just a part of it.
Please do not forget to disable Hardware acceleration since PorterDuff.Mode.CLEAR doesn’t work with hardware acceleration and finally call setWillNotDraw(false) because we override the onDraw method.

//view's constructor
setWillNotDraw(false);
setLayerType(LAYER_TYPE_SOFTWARE, null);


//view's onDraw
Paint TransparentPaint = new Paint();
TransparentPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
canvas.drawRect(0, 0, width, height, TransparentPaint);

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Try to remove the view at onPause() of an activity and add onRestart()

LayoutYouAddedYourView.addView(YourCustomView); LayoutYouAddedYourView.removeView(YourCustomView);

The moment you add your view, onDraw() method would get called.

YourCustomView, is a class which extends the View class.

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Erasing on Canvas in java android is similar erasing HTML Canvas via javascript with globalCompositeOperation. The logic was similar.

U will choose DST_OUT (Destination Out) logic.

paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_OUT));

Note: DST_OUT is more useful because it can erase 50% if the paint color have 50% alpha. So, to clear completely to transparent, the alpha of color must be 100%. Apply paint.setColor(Color.WHITE) is recommended. And make sure that the canvas image format was RGBA_8888.

After erased, go back to normal drawing with SRC_OVER (Source Over).

paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC_OVER));

Update small area display literally will need to access graphic hardware, and it maybe not supported.

The most close for highest performance is using multi image layer.

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In my case, I draw my canvas into linearlayout. To clean and redraw again:

    LinearLayout linearLayout = findViewById(R.id.myCanvas);
linearLayout.removeAllViews();

and then, I call the class with the new values:

    Lienzo fondo = new Lienzo(this,items);
linearLayout.addView(fondo);

This is the class Lienzo:

class Lienzo extends View {
Paint paint;
RectF contenedor;
Path path;
ArrayList<Items>elementos;


public Lienzo(Context context,ArrayList<Items> elementos) {
super(context);
this.elementos=elementos;
init();
}


private void init() {
path=new Path();
paint = new Paint();
contenedor = new RectF();
paint.setStyle(Paint.Style.FILL);
}




@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);


contenedor.left = oneValue;
contenedor.top = anotherValue;
contenedor.right = anotherValue;
contenedor.bottom = anotherValue;


float angulo = -90; //starts drawing at 12 o'clock
//total= sum of all element values
for (int i=0;i<elementos.size();i++){
if (elementos.get(i).angulo!=0 && elementos.get(i).visible){
paint.setColor(elementos.get(i).backColor);
canvas.drawArc(contenedor,angulo,(float)(elementos.get(i).value*360)/total,true,paint);


angulo+=(float)(elementos.get(i).value*360)/total;
}
} //for example
}
}

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In my case, creating canvas every time worked for me, even though it's not memory-friendly

Bitmap bm = BitmapFactory.decodeResource(getResources(), R.drawable.image);
imageBitmap = Bitmap.createBitmap(bm.getWidth(), bm.getHeight(), bm.getConfig());
canvas = new Canvas(imageBitmap);
canvas.drawBitmap(bm, 0, 0, null);

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The following worked for me:

canvas.drawColor(Color.TRANSPARENT, PorterDuff.Mode.SCREEN);

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I found my solution.

PaintView class:

public void clear() {
mPath.reset();
mCanvas.drawColor(Color.TRANSPARENT, PorterDuff.Mode.CLEAR);
paths.clear();
}

And MainActivity:

  clear_canvas_.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
paintView.clear();
}
});