案例研究 - Onslaught!表演场馆

<style>
canvas {
  width: 640px;
  height: 320px;
}
</style>
<canvas width="320" height="240">
  Sorry, your browser is not supported.
</canvas>

// Get the "buffer" canvas (that isn't visible to the user)
var bufferCanvas = document.getElementById("buffer");
var buffer = bufferCanvas.getContext("2d");

// Draw your image on the buffer
buffer.drawImage(image, 0, 0);

// Draw a rectangle over the image using a nice translucent overlay
buffer.save();
buffer.globalCompositeOperation = "source-in";
buffer.fillStyle = "rgba(186, 51, 35, 0.6)"; // red
buffer.fillRect(0, 0, image.width, image.height);
buffer.restore();

// Copy the buffer onto the visible canvas
document.getElementById("stage").getContext("2d").drawImage(bufferCanvas, x, y);

function main () {
  handleInput();
  update();
  render();
};

setInterval(main, 1);

window.addEventListener("mousedown", function(e) {
  // A mouse click means the players wants to attack.
  // We don't actually do that yet, but instead tell the rest
  // of the program about the request.
  buttonStates[e.button] = true;
}, false);

function handleInput() {
  // Here is where we respond to the click
  if (buttonStates[LEFT_BUTTON]) {
    player.attacking = true;
    delete buttonStates[LEFT_BUTTON];
  }
};

function update() {
  // Check for collisions, states, whatever else is needed

  // If after that the player can still attack, do it!
  if (player.attacking && player.canAttack()) {
    player.attack();
  }
};

function render() {
  // First erase everything, something like:
  context.clearRect(0, 0, SCREEN_WIDTH, SCREEN_HEIGHT);

  // Draw the player (and whatever else you need)
  context.drawImage(
    player.getImage(),
    player.x, player.y
  );
};

function update() {

  // NOTE: You'll need to initially seed this.lastUpdate
  // with the current time when your game loop starts
  // this.lastUpdate = Date.now();

  // Calculate elapsed time since last frame
  var now = Date.now();
  var elapsed = (now - this.lastUpdate);
  this.lastUpdate = now;

  // Do stuff with elapsed

};

var Sprite = function() {

  // The sprite's position relative to the top left of the game world
  this.position = {x: 0, y: 0};

  // The sprite's direction. A positive x value indicates moving to the right
  this.direction = {x: 1, y: 0};

  // How many pixels the sprite moves per second
  this.speed = 50;
};

// Determine how far this sprite will move this frame
var distance = (sprite.speed / 1000) * elapsed;

// Apply the movement distance to the sprite's current position
// taking into account its direction
sprite.position.x += (distance * sprite.direction.x);
sprite.position.y += (distance * sprite.direction.y);

// Find the nearest hostile target (if any) to the player
var player = this.getPlayerObject();
var hostile = this.getNearestHostile(player);
if (hostile !== null) {
  // Found one! Shoot in its direction
  var shoot = hostile.boundingBox().center().subtract(
    player.boundingBox().center()
  ).normalize();
}

// Move towards where the player clicked/touched
var move = this.targetReticle.position.clone().subtract(
  player.boundingBox().center()
).normalize();
var distance = this.targetReticle.position.clone().subtract(
  player.boundingBox().center()
).magnitude();

// Prevent jittering if the character is close enough
if (distance < 3) {
  move.zero();
}

// Move the player
if ((move.x !== 0) || (move.y !== 0)) {
  player.setDirection(move);
}

/*
This example uses the SoundManager 2 library by Scott Schiller:
http://www.schillmania.com/projects/soundmanager2/
*/

// Default to sm2 (Flash)
var api = "sm2";

function initAudio (callback) {
  switch (api) {
    case "sm2":
      soundManager.onerror = (function (init) {
        return function () {
          api = "html5";
          init(callback);
        };
      }(arguments.callee));
      break;
    case "html5":
      var audio = document.createElement("audio");

      if (
        audio
        && audio.canPlayType
        && audio.canPlayType("audio/mpeg;")
      ) {
        callback();
      } else {
        // No audio support :(
      }
      break;
  }
};

/*
Note: you could instead use "new Audio()" here,
but the client will throw an error if it doesn't support Audio,
which makes using "document.createElement" a safer approach.
*/

var audio = document.createElement("audio");

if (audio && audio.canPlayType) {
  if (!audio.canPlayType("audio/mpeg;")) {
    // Here you know you CANNOT use .mp3 files
    if (audio.canPlayType("audio/ogg; codecs=vorbis")) {
      // Here you know you CAN use .ogg files
    }
  }
}

if (typeof localStorage == "object") {
  localStorage.setItem("foo", "bar");
  localStorage.getItem("foo"); // Value is "bar"
  localStorage.removeItem("foo");
  localStorage.getItem("foo"); // Value is now null
}

localStorage.setItem("foo", false);
typeof localStorage.getItem("foo"); // Value is "false" (a string literal)
if (localStorage.getItem("foo")) {
  // It's true!
}

// Don't pass objects into setItem
localStorage.setItem("bar", {"key": "value"});
localStorage.getItem("bar"); // Value is "[object Object]" (a string literal)

// JSON stringify and parse when dealing with localStorage
localStorage.setItem("json", JSON.stringify({"key": "value"}));
typeof localStorage.getItem("json"); // string
JSON.parse(localStorage.getItem("json")); // {"key": "value"}