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Thursday, January 14, 2016

Let's write a mobile game with React Native (Part 2)

This is the second part of my React Native tutorial that shows you how to write a cross-platform mobile game. In Part 1, we've rendered a resolution-independent letter grid with custom glyphs. We're going to add event handling and animations to enliven the app.

The following guide builds on top of the v0.1 code release; the end result can be found in v0.2.

7. Touch event handling

The conventional way to capture click events in React Native is to use four Touchable* components: TouchableHighlight, TouchableActiveFeedback, TouchableOpacity, and TouchableWithoutFeedback.

TouchableActiveFeedback is Android-only; TouchableHighlight often causes undesirable artifacts; TouchableWithoutFeedback should be used with caution as it provides no visual cues; in most cases such as creating a button, you should consider using TouchableOpacity.

Let's give it a try by importing the component from React in boardview.js.

var {
  ...
  TouchableOpacity, // <- New
  ...
} = React;

We then extract a renderTile method from renderTiles and wrap each tile <View> inside a <TouchableOpacity>:

...
  renderTiles() {
    var result = [];
    for (var row = 0; row < SIZE; row++) {
      for (var col = 0; col < SIZE; col++) {
        var key = row * SIZE + col;
        var letter = String.fromCharCode(65 + key);
        var position = {
          left: col * CELL_SIZE + CELL_PADDING,
          top: row * CELL_SIZE + CELL_PADDING
        };
        result.push(this.renderTile(key, position, letter)); // <- New
      }
    }
    return result;
  },

  // New
  renderTile(id, position, letter) {
    return <TouchableOpacity key={id}
                             onPress={() => console.log(id)}>
             <View style={[styles.tile, position]}>
               <Text style={styles.letter}>{letter}</Text>
             </View>
           </TouchableOpacity>;
  },
...

Notice that each <TouchableOpacity> has a unique key property because they all nest under the same parent node. As mentioned in Part 1 Section 5, this helps React Native to efficiently compare virtual DOM trees. The inner <View> and <Text> don't need keys because they have no sibling nodes at all — it doesn't hurt if you set keys, though.

The onPress property uses ES6's arrow syntax to declare an anonymous, parameterless event handler. For now, it logs the tile id for debugging; our real game logic will be filled in later.

Play with the app on a device and you'll observe two flaws:

  1. The onPress event handler isn't triggered until you lift your finger (see the screencast above). This behavior is not bad for regular apps because it allows users to easily cancel a click by sliding outside the click target. However, responsiveness is paramount in a game, so we'd like to fire the event handler as soon as a touch starts. You could solve this by binding an onPressIn handler rather than an onPress, yet it won't fix the next issue.
  2. If you press a tile and hold your finger, then press another tile, the second click won't register. Again, this behavior is perfectly fine for regular apps, but in our fast-paced game, it'll cause tremendous frustration when a player taps with multiple fingers at high speed.

A quick solution that kills two birds with one stone is to get rid of the TouchableOpacity wrapper and directly attach an onStartShouldSetResponder handler to each tile <View>. The handler should always return false or a false-y value like undefined, so that it never nominates itself as the event responder. Our new renderTile method looks like this:

...
  renderTile(id, position, letter) {
    return <View key={id} style={[styles.tile, position]}
                 onStartShouldSetResponder={() => this.clickTile(id)}>
             <Text style={styles.letter}>{letter}</Text>
           </View>;
  },

  clickTile(id) {
    console.log(id);
  },
...

As the following screencast demonstrates, each click now immediately triggers the clickTile event handler. You may also verify on a device that no clicks are lost when multi-touch is involved.

The only thing missing is some form of visual feedback to highlight the tile being pressed. We're going to fix that with animations.

8. Property animation

React Native provides a convenient Animated module for animating component properties. It's designed in such a way that our rendering logic can remain largely intact when we plug in the animation logic. Let's add a simple opacity animation to see how it works.

First, import Animated and Easing:

var {
  Animated, // <- New
  Easing, // <- New
  ...
} = React;

Add a getInitialState() method to the BoardView component, where we initialize 16 (4x4) Animated.Value instances, each controlling the opacity of a single tile. Pass 1 to the constructor of Animated.Value so that all letter tiles are fully opaque at the start.

var BoardView = React.createClass({
  getInitialState() { // New method
    var opacities = new Array(SIZE * SIZE);
    for (var i = 0; i < opacities.length; i++) {
      opacities[i] = new Animated.Value(1);
    }
    return {opacities}; // ES6 shorthand for {opacities: opacities}
  },
  ...

Once React Native mounts the component, individual Animated.Value instances can be accessed by this.state.opacities[id]. Hook them up with the style object of letter tiles as shown below:

  renderTiles() {
    var result = [];
    for (var row = 0; row < SIZE; row++) {
      for (var col = 0; col < SIZE; col++) {
        var id = row * SIZE + col;
        var letter = String.fromCharCode(65 + id);
        var style = {
          left: col * CELL_SIZE + CELL_PADDING,
          top: row * CELL_SIZE + CELL_PADDING,
          opacity: this.state.opacities[id], // <- New
        };
        result.push(this.renderTile(id, style, letter));
      }
    }
    return result;
  },

  renderTile(id, style, letter) {
    //      v- New
    return <Animated.View key={id} style={[styles.tile, style]}
               onStartShouldSetResponder={() => this.clickTile(id)}>
             <Text style={styles.letter}>{letter}</Text>
           </Animated.View>;
  },

Make sure that you change <View> to Animated.View where Animated.Values can be applied to. Otherwise, you'll run into a scary, red screen of error that isn't particularly informative:

Similarly, you can animate Text with Animated.Text, Image with Animated.Image, or custom components created with createAnimatedComponent. At this stage, nothing is actually animated yet. We'll kick off the opacity animation in the clickTile event handler using the Animated.timing API:

  clickTile(id) {
    var opacity = this.state.opacities[id];
    opacity.setValue(.5); // half transparent, half opaque
    Animated.timing(opacity, {
      toValue: 1, // fully opaque
      duration: 250, // milliseconds
    }).start();
  },

With these simple changes, we have achieved the following effect:

Animated.timing gives us two additional options to fine-tune the animation: easing (custom easing function) and delay (delay in milliseconds). You may also want to play with Animated.spring (bouncing animation) or orchestrate multiple animations with Animated.sequence (sequential animation) and Animated.parallel (simultaneous animation).

Faded opacity is a tad boring. Let's replace it with a 3D tilting effect as if tiles revolve around the X-axis. That's right — we can do simple 3D transformations in React Native!

First rename the Animated.Values and initialize them with 0:

  getInitialState() {
    var tilt = new Array(SIZE * SIZE);
    for (var i = 0; i < tilt.length; i++) {
      tilt[i] = new Animated.Value(0);
    }
    return {tilt};
  },

Then replace opacity in style with a transform — an array of transformation objects:

  • {perspective: ...} declares the virtual distance from the viewing point to the z=0 plane. Use it to control the intensity of 3D effect: the greater the value is, the further away you are from the objects, the less intense the distortion appears (i.e. objects look more flat).
  • {rotateX: ...} sets the rotational degrees around the X-axis, creating a tilting effect. Due to an unfortunate API design (as of RN 0.18), all the rotational properties (rotate, rotateX, rotateY, and rotateZ) only take a string (e.g. '30deg'). We can't directly apply a numerical degree or radian value, so we resort to Animated.Value's interpolate function to map a floating point number to a string.

  renderTiles() {
    var result = [];
    for (var row = 0; row < SIZE; row++) {
      for (var col = 0; col < SIZE; col++) {
        var id = row * SIZE + col;
        var letter = String.fromCharCode(65 + id);
        var tilt = this.state.tilt[id].interpolate({
          inputRange: [0, 1],
          outputRange: ['0deg', '-30deg']
        });
        var style = {
          left: col * CELL_SIZE + CELL_PADDING,
          top: row * CELL_SIZE + CELL_PADDING,
          transform: [{perspective: CELL_SIZE * 8},
                      {rotateX: tilt}]
        };
        result.push(this.renderTile(id, style, letter));
      }
    }
    return result;
  },

The final step is to patch the clickTile method to set up the new animation.

  clickTile(id) {
    var tilt = this.state.tilt[id];
    tilt.setValue(1); // mapped to -30 degrees
    Animated.timing(tilt, {
      toValue: 0, // mapped to 0 degrees (no tilt)
      duration: 250, // milliseconds
      easing: Easing.quad // quadratic easing function: (t) => t * t
    }).start();
  },

Technically, this 3D effect is not accurate — it's a mix of orthogonal projection and perspective projection. If you aspire to render a photorealistic scene with fancy shaders, I'd suggest that you take a look at gl-react-native — wait, no, please please use a real 3D engine. For our little game, I'd say this faux-3D animation is acceptable:

Full source up to this point can be found at https://github.com/zmxv/alpha-reflex/releases/tag/v0.2. In the next article, we'll create a timer component using a different animation technique and implement some game-specific logic.

20 comments:

  1. This is a great tutorial for Animated. Thanks so much!

    ReplyDelete
  2. Thanks so much for this tutorial. There's not many tutorials for React Native around and especially for animation. Looking forward to the next part.

    ReplyDelete
  3. Thanks so much for this tutorial. There's not many tutorials for React Native around and especially for animation. Looking forward to the next part.

    ReplyDelete
  4. Great tutorial
    Waiting for part3

    ReplyDelete
  5. Superb article! Waiting to see part 3.

    ReplyDelete
  6. Superb article! Waiting to see part 3.

    ReplyDelete
  7. Great tutorial. Any idea how I could swipe to select consecutive tiles?

    ReplyDelete
  8. Part 2 is great too! So many good points in here, and you describe it so well. Really appreciate you taking the time to share your knowledge.

    ReplyDelete
  9. If you are working in ES6 classes, and are getting stuck, here is the converted source code: https://github.com/ericchen0121/alphaReflex

    ReplyDelete
  10. Looking forward to Part 3... is it ever going to come?

    ReplyDelete
  11. Just a head's up, but the following didn't work for me:

    return console.log(id)}>

    {letter}

    ;

    But the following did:
    return console.log(id)}>
    {letter}
    ;

    ReplyDelete
  12. Hi,

    Nice Tutorial, When we will get next one.

    ReplyDelete
  13. Thank you for your sharing, it's helpful. When will you publish part 3?

    ReplyDelete
  14. Waiting for the next part. Not only me, but also professional website design service team is waiting for it.

    ReplyDelete
  15. This comment has been removed by the author.

    ReplyDelete
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    ReplyDelete