Sam Sam - 1 month ago 11
Android Question

Android canvas fill background color (Canvas application)

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);
}
});
}
}


enter image description here

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.

Answer

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.

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