Canvas update? invalid ()?

Question

Hi, I am trying to get my canvas update application from a custom view I made. This view creates a square with lines and a circle in the center. I want to click a button and draw random x and y coordinates on the canvas.

Heres My MainActivity:

@Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); // EditText numDart = (EditText) findViewById(R.id.numDarts); setContentView(R.layout.activity_main); } @Override public boolean onCreateOptionsMenu(Menu menu) { // Inflate the menu; this adds items to the action bar if it is present. getMenuInflater().inflate(R.menu.main, menu); return true; } public int convertToDpi(int px){ DisplayMetrics metrics = getResources().getDisplayMetrics(); int unit = metrics.widthPixels/20; return px * unit; } public void drawCanvas(View v){ View view = (View) findViewById(R.id.canView); Paint black = new Paint(); black.setColor(Color.BLACK); black.setStyle(Style.FILL); view.invalidate(); //dont know where to go from here } 

Heres My Custom View:

 public CanView(Context context, AttributeSet attrs) { super(context, attrs); // TODO Auto-generated constructor stub } @Override protected void onDraw(Canvas canvas) { // TODO Auto-generated method stub super.onDraw(canvas); Rect myrect = new Rect(); myrect.set(-10, 10, 10, -10); Paint blue = new Paint(); Paint white = new Paint(); white.setColor(Color.WHITE); white.setStyle(Paint.Style.FILL); Paint black = new Paint(); black.setColor(Color.BLACK); black.setStyle(Paint.Style.FILL); blue.setColor(Color.BLUE); blue.setStyle(Paint.Style.FILL); canvas.drawRect(myrect, blue); canvas.drawCircle(convertToDpi(10), convertToDpi(10),convertToDpi(3), white); canvas.drawLine(convertToDpi(10), 0, convertToDpi(10), convertToDpi(20), black); canvas.drawLine(0, convertToDpi(10), convertToDpi(20), convertToDpi(10), black); canvas.scale(5, 5, 0, 0); } @Override public void postInvalidate() { //Logic for redrawig goes here??? // TODO Auto-generated method stub super.postInvalidate(); } public int convertToDpi(int px){ DisplayMetrics metrics = getResources().getDisplayMetrics(); int unit = metrics.widthPixels/20; return px * unit; } 

I don’t understand how to refer to the canvas from my user view and change it or redraw it. Im assuming you are using invalidate (); but I am puzzled by how this method works, can someone help me?

Answer 1

When invalidate is called onDraw is called again, you should also use this.getHolder().addCallback(this); , and let your class implement Callback and add unrealized methods

Answer 2

I think you only need to draw a presentation. First set the content as the view in which you draw on the canvas. You set your subnet view only as the main layout. If you want to see the "View" on which you draw on canvas. set this view to your content.

setContentView (canView);

Now, if you want this button too, create a relative layout and add a button view, and this canView will relate to this relative layout and set this as your content.