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Creating Cross-Platform Java and JavaScript Games with Amino, Part 2

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James Williams, author of Learning HTML5 Game Programming, continues his examination of how to use Amino to create cross-platform applications in JavaScript and Java. Learn how to use Amino to start and end a 'War.'
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In an earlier InformIT article, "Creating Cross-Platform Java and JavaScript Games with Amino," I took you through the basics of Amino by creating a binary clock in both Java and JavaScript. In this article, we'll progress a little further in Amino by creating a card game.

How the Game Is Played

First let's talk a little about the rules of the card game we'll be creating. War is an accumulation game of two or more players, where the goal is to end the game with the largest number of cards in your hand. Each player has a complete shuffled deck of cards. In clockwise order, each player selects a card from the top of his or her deck, placing the card face up in the playing surface between all the players. This stack in front of each player is called the pile. The player whose pile has the highest card (regardless of suit) wins the round. Cards are ranked in order from ace-high down through the face cards (king, queen, jack) and then the ordinals. The winner of the round collects all the cards from all the piles and places them at the bottom of his or her deck.

If two or more of the high cards in a round have the same value, a war is played. Each player in the war places four cards in a stack on top of his or her pile, three face down and the last card face up. The values of the visible cards are compared, and the player with the highest card wins the war and collects all the cards. If the cards in the war are equal, another war begins. This process repeats until someone wins the round. If a player in a war has fewer than four cards remaining in the deck, he or she plays all remaining cards in the deck face down, other than the last card. Players are removed from the game when they no longer have any cards.

To keep things simple, the version of War we'll create will be played with a human player and a computer player. Luckily for us, no strategy is involved for the computer player; it just waits for its turn and flips cards.

Drawing the Cards

The ImageView class is Amino's method of drawing nodes that contain images. It has a pretty straightforward constructor that takes a URL that points to the desired image resource:

new ImageView(srcUrl)

Amino uses an asynchronous call to load the requested image, showing a red rectangle if the requested image is not found. After the image is fully loaded, we can adjust basic properties on the node, such as the x and y coordinates, or we can use a Transform, which we'll examine next for complex transformations.

Transformations

The HTML5 Canvas allows you to do transformations such as translation, scaling, and rotation on drawings. The transformations at the 2D Context level are universally applied to all drawing instructions that follow the request to rotate, scale, or translate. The API has two instructions, save and restore, that allow you to restrict transformations to a subset of instructions. Amino exposes this functionality with the Transform class. It takes as a parameter the node that will be transformed. It includes functions to setTranslate*/setScale*/setRotate where * is a placeholder for either X or Y. Given a node named card, the following snippet shows how we would create a Transform, rotate it 45 degrees, and translate it to 10 pixels on the x axis:

var transform = new Transform(card);
transform.setRotate(45)
rootGroup.add(transform)
transform.setTranslateX(10)

Managing for Player Interaction

As is the case with the DOM element, the HTML5 Canvas can listen for and respond to events such as mouse clicks, keyboard presses, and touch events. However, most of the time the events have to be wired by the developer. Amino's JavaScript API handles this work for you by firing notifications when the canvas receives mousedown, mousemove, and mouseup events. On the enclosing HTML page, the API listens for the keystroke-related events, keyup and keydown. The following table shows the event names Amino assigns to the aforementioned types.

Mapping HTML Events to Amino Events

HTML Event Name

Amino Event Name

mousedown

MOUSE_PRESS

mousemove

MOUSE_DRAG

mouseup

MOUSE_RELEASE

keyup

KEY_RELEASED

keydown

KEY_PRESSED

Now that we know the events to which we want to subscribe, we need to communicate that information to Amino. Runner has a listen function with the following signature:

listen(eventType, eventTarget, callback)

where eventType is the String name of the event, eventTarget is the node whose events we want to listen to (or null to listen to all nodes), and callback is the function to execute when the event fires. To flip a player card every time the user hits the spacebar, for example, we would use the following code:

self.runner.listen("KEY_PRESSED", null, function(evt) {
      if (evt.key === 32)
            // Flip player card
});

When processing the raw event, Amino gives us MEvent or KEvent for the mouse and keyboard, respectively. KEvents contain a single property key that holds the integral value of the key that was pressed. On the other hand, MEvent has properties for the node (if any) that we interacted with, and the x and y positions of the mouse action. The computer player subscribes to an event that tells it when to play. We can have as many or as few listener callbacks as we want.

Optimizing the Game

Our game runs fast enough, so why would we need to optimize it? Simple: It could run faster and do the same work with less overhead. The game does its work in one canvas. Each individual card image is loaded on demand when it's instantiated. For the card fronts, this is no big deal because there's a lot of variance. But the back of the cards don't change as often. Instead of individually loading the image for the back of the card 52 times (for the 52 cards in a standard deck), we can load the image once and reuse the data. The following code shows the loading of an image and creation of cached assets. When the image finishes loading, it instantiates a CachedImageView node with the image data, width, and height.

backImage = new Image("images/90dpi/back.png")
backImage.onload = function() {
      console.log("loaded");
      // create our assets
      self.playerOneDeck = new CachedImageView(backImage, 169, 245);
}
backImage.src = "images/90dpi/back.png"

Note that CachedImageView is a tweaked version of ImageView to accept raw image data, and it's not in the core Amino APIs. Another way we couple implement caching is to use the Buffer and BufferNode classes. Both classes allow you to create an invisible canvas to draw components that might be costly to redraw on every frame. Buffer deals with raw pixel data, whereas BufferNode takes as input a node to draw on its own canvas. Shadow and blur effects are implemented using an internal Buffer.

At this point, each card front image is loaded individually. We could possibly optimize further by loading all the card fronts as a tiled single image and then using the lower-level canvas features to draw only a region of the larger image. There's no hard-and-fast rule on the use of native versus encapsulated implementation of buffering, other than the rule that doing less work is better.

Wrapping Up

In this article, you've seen how easily you can create a War card game using the JavaScript API for Amino. You've learned how the Canvas2D context allows for transforming objects with translation, rotation, and scaling, as well as how to draw text. We've also considered some strategies for optimization. Ready to try it yourself?

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