title: Best way to draw an image dynamically
body: <p>I'm creating an android app that has to draw a map of an ambient that i have explored with laserscans. 
  I have e text file with all my data like:</p>
  
  <pre><code>
  index x y
  path   0 0 0
  path   1 1 0
  path   2 2 0
  
  ...etc
  
  obstacle 0 10 10
  obstacle 1 10 22
  
  ..etc
  </code></pre>
  
  <p>so I have <em>xy</em> coordinates of where I've been and <em>xy</em> of obstacles I've seen.
  I have a <code>thread</code> that reads the data from the text file and stores that data in a <code>list</code>.</p>
  
  <p>Another <code>thread</code> reads that <code>list</code> and draws all the points that are put in the <code>list</code> until that moment by the reading <code>thread</code>.</p>
  
  <p>My problem is that I don't want to re-read everything every time the reading <code>thread</code> has put new data into the data <code>list</code>. There is a way to draw something like a bitmap and modify this dynamically? I mean that every time I have read some new data I "open" the bitmap, I add to that the new points, "close" that bitmap and show on the screen?</p>
  
  <p>what I am doing now is to read all the list in my <code>onDraw()</code> function and draw point by point, but I have <strong>170 000</strong> points and that is a useful work because every time the points are in the old position, I only have some new points...</p>