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<section id="user-controllable-objects">
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<section id="makegames-5">
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<span id="id1"></span><h2>5. User-controllable objects<a class="headerlink" href="#makegames-5" title="Permalink to this heading">¶</a></h2>
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<p>So far you can create a Pygame window, and render a ball that will fly across the screen. The next step is to make some bats which
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the user can control. This is potentially far more simple than the ball, because it requires no physics (unless your user-controlled
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object will move in ways more complex than up and down, e.g. a platform character like Mario, in which case you'll need more physics).
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User-controllable objects are pretty easy to create, thanks to Pygame's event queue system, as you'll see.</p>
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<section id="a-simple-bat-class">
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<span id="makegames-5-1"></span><h3>5.1. A simple bat class<a class="headerlink" href="#a-simple-bat-class" title="Permalink to this heading">¶</a></h3>
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<p>The principle behind the bat class is similar to that of the ball class. You need an <code class="docutils literal notranslate"><span class="pre">__init__</span></code> function to initialise the
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ball (so you can create object instances for each bat), an <code class="docutils literal notranslate"><span class="pre">update</span></code> function to perform per-frame changes on the bat before
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it is blitted the bat to the screen, and the functions that will define what this class will actually do. Here's some sample code:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Bat</span><span class="p">(</span><span class="n">pygame</span><span class="o">.</span><span class="n">sprite</span><span class="o">.</span><span class="n">Sprite</span><span class="p">):</span>
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<span class="w"> </span><span class="sd">"""Movable tennis 'bat' with which one hits the ball</span>
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<span class="sd"> Returns: bat object</span>
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<span class="sd"> Functions: reinit, update, moveup, movedown</span>
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<span class="sd"> Attributes: which, speed"""</span>
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<span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">side</span><span class="p">):</span>
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<span class="n">pygame</span><span class="o">.</span><span class="n">sprite</span><span class="o">.</span><span class="n">Sprite</span><span class="o">.</span><span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">)</span>
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<span class="bp">self</span><span class="o">.</span><span class="n">image</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">rect</span> <span class="o">=</span> <span class="n">load_png</span><span class="p">(</span><span class="s2">"bat.png"</span><span class="p">)</span>
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<span class="n">screen</span> <span class="o">=</span> <span class="n">pygame</span><span class="o">.</span><span class="n">display</span><span class="o">.</span><span class="n">get_surface</span><span class="p">()</span>
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<span class="bp">self</span><span class="o">.</span><span class="n">area</span> <span class="o">=</span> <span class="n">screen</span><span class="o">.</span><span class="n">get_rect</span><span class="p">()</span>
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<span class="bp">self</span><span class="o">.</span><span class="n">side</span> <span class="o">=</span> <span class="n">side</span>
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<span class="bp">self</span><span class="o">.</span><span class="n">speed</span> <span class="o">=</span> <span class="mi">10</span>
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<span class="bp">self</span><span class="o">.</span><span class="n">state</span> <span class="o">=</span> <span class="s2">"still"</span>
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<span class="bp">self</span><span class="o">.</span><span class="n">reinit</span><span class="p">()</span>
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<span class="k">def</span> <span class="nf">reinit</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
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<span class="bp">self</span><span class="o">.</span><span class="n">state</span> <span class="o">=</span> <span class="s2">"still"</span>
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<span class="bp">self</span><span class="o">.</span><span class="n">movepos</span> <span class="o">=</span> <span class="p">[</span><span class="mi">0</span><span class="p">,</span><span class="mi">0</span><span class="p">]</span>
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<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">side</span> <span class="o">==</span> <span class="s2">"left"</span><span class="p">:</span>
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<span class="bp">self</span><span class="o">.</span><span class="n">rect</span><span class="o">.</span><span class="n">midleft</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">area</span><span class="o">.</span><span class="n">midleft</span>
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<span class="k">elif</span> <span class="bp">self</span><span class="o">.</span><span class="n">side</span> <span class="o">==</span> <span class="s2">"right"</span><span class="p">:</span>
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<span class="bp">self</span><span class="o">.</span><span class="n">rect</span><span class="o">.</span><span class="n">midright</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">area</span><span class="o">.</span><span class="n">midright</span>
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<span class="k">def</span> <span class="nf">update</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
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<span class="n">newpos</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">rect</span><span class="o">.</span><span class="n">move</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">movepos</span><span class="p">)</span>
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<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">area</span><span class="o">.</span><span class="n">contains</span><span class="p">(</span><span class="n">newpos</span><span class="p">):</span>
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<span class="bp">self</span><span class="o">.</span><span class="n">rect</span> <span class="o">=</span> <span class="n">newpos</span>
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<span class="n">pygame</span><span class="o">.</span><span class="n">event</span><span class="o">.</span><span class="n">pump</span><span class="p">()</span>
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<span class="k">def</span> <span class="nf">moveup</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
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<span class="bp">self</span><span class="o">.</span><span class="n">movepos</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">movepos</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="o">-</span> <span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">speed</span><span class="p">)</span>
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<span class="bp">self</span><span class="o">.</span><span class="n">state</span> <span class="o">=</span> <span class="s2">"moveup"</span>
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<span class="k">def</span> <span class="nf">movedown</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
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<span class="bp">self</span><span class="o">.</span><span class="n">movepos</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">movepos</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="o">+</span> <span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">speed</span><span class="p">)</span>
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<span class="bp">self</span><span class="o">.</span><span class="n">state</span> <span class="o">=</span> <span class="s2">"movedown"</span>
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</pre></div>
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</div>
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<p>As you can see, this class is very similar to the ball class in its structure. But there are differences in what each function does.
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First of all, there is a reinit function, which is used when a round ends, and the bat needs to be set back in its starting place,
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with any attributes set back to their necessary values. Next, the way in which the bat is moved is a little more complex than with the
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ball, because here its movement is simple (up/down), but it relies on the user telling it to move, unlike the ball which just keeps
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moving in every frame. To make sense of how the bat moves, it is helpful to look at a quick diagram to show the sequence of events:</p>
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<img alt="../_images/tom_event-flowchart.png" src="../_images/tom_event-flowchart.png" />
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<p>What happens here is that the person controlling the bat pushes down on the key that moves the bat up. For each iteration of the main
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game loop (for every frame), if the key is still held down, then the <code class="docutils literal notranslate"><span class="pre">state</span></code> attribute of that bat object will be set to
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"moving", and the <code class="docutils literal notranslate"><span class="pre">moveup</span></code> function will be called, causing the ball's y position to be reduced by the value of the
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<code class="docutils literal notranslate"><span class="pre">speed</span></code> attribute (in this example, 10). In other words, so long as the key is held down, the bat will move up the screen
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by 10 pixels per frame. The <code class="docutils literal notranslate"><span class="pre">state</span></code> attribute isn't used here yet, but it's useful to know if you're dealing with spin, or
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would like some useful debugging output.</p>
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<p>As soon as the player lets go of that key, the second set of boxes is invoked, and the <code class="docutils literal notranslate"><span class="pre">state</span></code> attribute of the bat object
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will be set back to "still", and the <code class="docutils literal notranslate"><span class="pre">movepos</span></code> attribute will be set back to [0,0], meaning that when the <code class="docutils literal notranslate"><span class="pre">update</span></code> function is called, it won't move the bat any more. So when the player lets go of the key, the bat stops moving. Simple!</p>
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<section id="diversion-3-pygame-events">
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<span id="makegames-5-1-1"></span><h4>5.1.1. Diversion 3: Pygame events<a class="headerlink" href="#diversion-3-pygame-events" title="Permalink to this heading">¶</a></h4>
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<p>So how do we know when the player is pushing keys down, and then releasing them? With the Pygame event queue system, dummy! It's a
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really easy system to use and understand, so this shouldn't take long :) You've already seen the event queue in action in the basic
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Pygame program, where it was used to check if the user was quitting the application. The code for moving the bat is about as simple
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as that:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">for</span> <span class="n">event</span> <span class="ow">in</span> <span class="n">pygame</span><span class="o">.</span><span class="n">event</span><span class="o">.</span><span class="n">get</span><span class="p">():</span>
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<span class="k">if</span> <span class="n">event</span><span class="o">.</span><span class="n">type</span> <span class="o">==</span> <span class="n">QUIT</span><span class="p">:</span>
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<span class="k">return</span>
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<span class="k">elif</span> <span class="n">event</span><span class="o">.</span><span class="n">type</span> <span class="o">==</span> <span class="n">KEYDOWN</span><span class="p">:</span>
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<span class="k">if</span> <span class="n">event</span><span class="o">.</span><span class="n">key</span> <span class="o">==</span> <span class="n">K_UP</span><span class="p">:</span>
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<span class="n">player</span><span class="o">.</span><span class="n">moveup</span><span class="p">()</span>
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<span class="k">if</span> <span class="n">event</span><span class="o">.</span><span class="n">key</span> <span class="o">==</span> <span class="n">K_DOWN</span><span class="p">:</span>
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<span class="n">player</span><span class="o">.</span><span class="n">movedown</span><span class="p">()</span>
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<span class="k">elif</span> <span class="n">event</span><span class="o">.</span><span class="n">type</span> <span class="o">==</span> <span class="n">KEYUP</span><span class="p">:</span>
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<span class="k">if</span> <span class="n">event</span><span class="o">.</span><span class="n">key</span> <span class="o">==</span> <span class="n">K_UP</span> <span class="ow">or</span> <span class="n">event</span><span class="o">.</span><span class="n">key</span> <span class="o">==</span> <span class="n">K_DOWN</span><span class="p">:</span>
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<span class="n">player</span><span class="o">.</span><span class="n">movepos</span> <span class="o">=</span> <span class="p">[</span><span class="mi">0</span><span class="p">,</span><span class="mi">0</span><span class="p">]</span>
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<span class="n">player</span><span class="o">.</span><span class="n">state</span> <span class="o">=</span> <span class="s2">"still"</span>
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</pre></div>
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</div>
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<p>Here assume that you've already created an instance of a bat, and called the object <code class="docutils literal notranslate"><span class="pre">player</span></code>. You can see the familiar
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layout of the <code class="docutils literal notranslate"><span class="pre">for</span></code> structure, which iterates through each event found in the Pygame event queue, which is retrieved with
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the <a class="reference internal" href="../ref/event.html#pygame.event.get" title="pygame.event.get"><code class="xref py py-mod docutils literal notranslate"><span class="pre">event.get()</span></code></a> function. As the user hits keys, pushes mouse buttons and moves the joystick about, those actions are
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pumped into the Pygame event queue, and left there until dealt with. So in each iteration of the main game loop, you go through
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these events, checking if they're ones you want to deal with, and then dealing with them appropriately. The <a class="reference internal" href="../ref/event.html#pygame.event.pump" title="pygame.event.pump"><code class="xref py py-func docutils literal notranslate"><span class="pre">event.pump()</span></code></a>
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function that was in the <code class="docutils literal notranslate"><span class="pre">Bat.update</span></code> function is then called in every iteration to pump out old events, and keep the queue
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current.</p>
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<p>First we check if the user is quitting the program, and quit it if they are. Then we check if any keys are being pushed down, and if
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they are, we check if they're the designated keys for moving the bat up and down. If they are, then we call the appropriate moving
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function, and set the player state appropriately (though the states moveup and movedown and changed in the <code class="docutils literal notranslate"><span class="pre">moveup()</span></code> and
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<code class="docutils literal notranslate"><span class="pre">movedown()</span></code> functions, which makes for neater code, and doesn't break <em>encapsulation</em>, which means that you
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assign attributes to the object itself, without referring to the name of the instance of that object). Notice here we have three
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states: still, moveup, and movedown. Again, these come in handy if you want to debug or calculate spin. We also check if any keys have
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been "let go" (i.e. are no longer being held down), and again if they're the right keys, we stop the bat from moving.</p>
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</section>
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