Timing control for script-based animations

Timing control is used mainly in animations for cyclic script execution synchronized with screen changes.

Making script-based animations requires updating the properties of the animated objects in each frame. In computerized animations, the frame rate is normally 30 or 60 fps (frames per second), because the human eye perceives a sequence of images as an animation when they change at least 12-15 times per second. The requestAnimationFrame() (or webkitRequestAnimationFrame()) method enables scheduling the animation frame update requests. The frame update rate depends on implementation. In Tizen 2.2, it is about 60 fps.

NoteIn mobile applications, when using the emulator, you must include the webkit prefix in method names, such as window.performance.webkitNow(). However, when using a device, the prefix must not be used. In addition, when using the emulator, methods assigned to variables must be formatted as follows:

window.performance.windowNow = window.performance.now || window.performance.webkitNow || Date.now;
window.performance.windowNow();

In wearable applications, when you are using the emulator and want to support backward compatibility, methods assigned to variables must be formatted as follows:

window.performance.windowNow = window.performance.now || window.performance.webkitNow || Date.now;
window.performance.windowNow();

Creating an Application with a Controllable Animation

To optimize the performance of your application, you must learn to create a simple controllable DOM animation.

This example creates a screen with a Tizen pinwheel rotating on it, and buttons to increase or decrease the rotation speed and start or stop the animation.

Figure: Controllable animation (in mobile applications only)

Controllable animation (in mobile applications only)

  1. Create the HTML layout, including a <nav> element with 3 buttons and <div> elements for the pinwheel image and text:

    <nav>
       <a href="#" id="leftBtn" class="btn">Left</a>
       <a href="#" id="playstopBtn" class="btn">Stop</a>
       <a href="#" id="rightBtn" class="btn">Right</a>
    </nav>
    
    <div id="pinwheel">
       <div id="rotateBackground"></div>
       TIZEN OS
    </div>
    
  2. Set the CSS properties:
    1. Define properties for a flexible vertical layout for the content area, and the nav container:

      body {
         display: -webkit-flex;
         -webkit-flex-direction: column;
      }
      
      nav {
         display: -webkit-flex;
         -webkit-flex-direction: row;
         -webkit-align-content: stretch;
         width: 100%;
      }
      
    2. To enable the rotation of the background, define the pinwheel element in relation to its children, and set its size to change according to the display size. Define the rotateBackground element to have a fixed size depending on the display size:

      #pinwheel {
         width: 90vw;
         height: 80vh;
         position: relative;
         overflow: hidden;
         line-height: 80vh;
         text-align: center;
      }
      
      #rotateBackground {
         background: url(images/Tizen-Pinwheel-On-Dark-RGB.png) no-repeat center;
         background-size: 90vw;
         content: "";
         position: absolute;
         width: 90vw;
         height: 90vh;
         z-index: -1;
      }
      
  3. Declare the variables used in the application:

    <script type="text/javascript">
        var leftDiv, rightDiv, playstopDiv,
            handlerRequest, rotateBackgroundDiv, rotation = 0, speed = -1;
    </script>
    

    The leftDiv, rightDiv, playstopDiv, and rotateBackgroundDiv variables contain DOM elements from the application UI. The handlerRequest variable handles the requestAnimationFrame() requests. The rotation variable is the current angle of the pinwheel, and the speed variable is the rotation speed in degrees per frame.

  4. Create a DOM object for the Play or Stop button and define button events:

    playstopDiv = document.getElementById('playstopBtn');
    
    playstopDiv.onclick = function() {
        if (handlerRequest) {
            window.webkitCancelRequestAnimationFrame(handlerRequest);
            handlerRequest = 0;
            playstopDiv.innerHTML = 'Play';
        } else {
            handlerRequest = window.webkitRequestAnimationFrame(nextFrame);
            playstopDiv.innerHTML = 'Stop';
        }
    };
    

    When the handlerRequest variable returns true, the animation is played. Clicking the button calls the cancelRequestAnimationFrame() method to stop the animation. The handlerRequest is reset to value 0, and the button text changes from Stop to Play.

    If the handlerRequest value is undefined or 0, the animation is not played. Clicking the button calls the requestAnimationFrame() method to play the animation, and the button text changes from Play to Stop. The parameter of the requestAnimationFrame() method defines the callback.

  5. In the callback of the requestAnimationFrame() method, to avoid rotation value overflow, define the rotation value to be decreased when the absolute value is greater than the round angle (360 degrees). Set the background rotation using the rotate() method, and use the requestAnimationFrame() method to update the next frame of the animation:

    function nextFrame(arg) {
        rotation += speed;
    
        if (rotation > 360) {
            rotation -= 360;
        } else if (rotation < -360) {
            rotation += 360;
        }
        rotateBackgroundDiv.style.webkitTransform = 'rotate(' + rotation + 'deg)';
        handlerRequest = window.webkitRequestAnimationFrame(nextFrame);
    }
    

    The callback method is automatically called before the screen is updated. To keep the animation going, the requestAnimationFrame() method is called at the end of the callback method. Because the callback is synchronized with screen updates, when application is in the background and not visible on the screen, the updates are not called and the animation stops.

  6. Set events for the Left and Right buttons. Tapping the buttons increases or decreases the animation speed.

    leftDiv = document.getElementById('leftBtn');
    leftDiv.onclick = function() {
        speed -= 0.5;
    };
    rightDiv = document.getElementById('rightBtn');
    rightDiv.onclick = function() {
        speed += 0.5;
    };
    handlerRequest = window.webkitRequestAnimationFrame(nextFrame);
    

Source Code

For the complete source code related to this use case, see the following file: