Android canvas fill background color (Canvas appli

问题:

By having the following codes, I have some questions.

public class MainActivity extends Activity {

   @Override
   public void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView( new View(this) {
         Paint mPaint = new Paint();

         @Override
         protected void onDraw(Canvas canvas) {
            // TODO Auto-generated method stub
            super.onDraw(canvas);


            int width = this.getWidth();
            int height = this.getHeight();
            int radius = width > height ? height/2 : width/2;
            int center_x = width/2;
            int center_y = height/2;

            // prepare a paint
            mPaint.setStyle(Paint.Style.STROKE);
            mPaint.setStrokeWidth(5);
            mPaint.setAntiAlias(true);

            // draw a rectangle
            mPaint.setColor(Color.BLUE);
                mPaint.setStyle(Paint.Style.FILL); //fill the background with blue color
            canvas.drawRect(center_x - radius, center_y - radius, center_x + radius, center_y + radius, mPaint);
            // draw some text and rotation
            mPaint.setTextSize(50);
            mPaint.setTextAlign(Paint.Align.CENTER);
            mPaint.setColor(Color.BLACK);
            canvas.drawText( "Hello World" , center_x , center_y, mPaint);
         }
      });
    }
}

Q1: How can I fill blue colour in the frame? (The words still appear)

Q2: How many views and surfaces in this app? How can I count these in the app?

Q3: How many windows in this app?

Q4: In the code, I dont see any bitmap object in it. However, I thought that bitmap is the object that I can really draw things on it. Is my understanding incorrect? One possibility is that Canvas constructor initializes bitmap when it is newed.

Q5: I knew that these graphic thing will finally go to surface and then pass to surfaceflinger for final composition. Where does it locate in my code?

Thanks for any reply.

回答1:

Five questions. Let's see where I can help.

Q1: Tell the Paint to fill the rectangle: paint.setStyle(Paint.Style.FILL);

Q2: I see only the one view you create programmatically. Why would you like to count the views?

Q3: Again: One

Q4: You draw an mutable bitmaps by wrapping them with a Canvas. The method to actually draw are part of Canvas

Q5: The code you show is part of an Activity. The Activity is called by Android. It's your entry point into your App.

回答2:

Thanks for the answer. I did the job of making the code for marked answer, and it works.

    Bitmap bg = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888);
    Canvas canvas = new Canvas(bg);
    // paint background with the trick
    Paint rect_paint = new Paint();
    rect_paint.setStyle(Paint.Style.FILL);
    rect_paint.setColor(Color.rgb(0, 0, 0));
    rect_paint.setAlpha(0x80); // optional
    canvas.drawRect(0, 0, width, height, rect_paint); // that's painting the whole canvas in the chosen color.

回答3:

Q2:Hierarchy Viewer is very useful when you want to count how many views in your app. Optimizing Your UI