HTML5之canvas简单射箭小游戏

最近折腾一个自己个人主页，无奈履历太渣，能放在首页的东西不多，于是想给自己的个人主页上添加一个小游戏。遂参考了各种教程，使用HTML5的canvas元素做了一个相当原始的东西出来，效果如图~

如图所示，右边是一个目标靶子，当我们点击start按钮后它会以一个特定的速度上下来回移动。靶子动起来之后我们点击shoot按钮就会从左边射出一支“箭”（其实就是一条红线啦(⊙﹏⊙)）。这支箭射中靶子后根据你射中的位置有0到10十个级别的分数。Ok，接下来是源码。主要是在两个循环里面移动箭和靶子的位置，不断调用制绘制它们的方法。然后用一个主循环不断判定游戏是否终止。

<div>
    <canvas id="canvas" width="360" height="500">您的浏览器不支持canvas，请使用chrome或者firefox体验~</canvas>
    <button onclick="startGame()">Start</button>
    <button onclick="shoot()">shoot</button>
    <script language="JavaScript">
        var canvas = document.getElementById("canvas");
        var cans = canvas.getContext("2d");
        cans.strokeStyle = "grey";
        cans.strokeRect(0, 0, 360, 500);
        cans.moveTo(0, 250);
        cans.arc(0, 250, 28, 0.5 * Math.PI, 90 + Math.PI);
        cans.stroke();

        var targetInterval, arrowInterval;
        var started = false;
        var targetUp = 0;

        var target = {x: 356, y: 10, w: 2, h: 160};
        var arrow = {x: 1, y: 250, len: 50};

        drawArrow = function () {
            cans.fillStyle = 'red';
            cans.fillRect(arrow.x, arrow.y, arrow.len, 1);
        };
        clearArrow = function () {
            cans.clearRect(arrow.x, arrow.y - 1, arrow.len, 2);
        };

        drawTarget = function () {
            cans.fillStyle = 'green';
            cans.fillRect(target.x, target.y, target.w, target.h);
            for (var yi = target.y, xi = target.x, hi = target.h; yi < target.y + target.h / 2; yi += 8, xi -= 2, hi -= 16) {
                cans.fillRect(xi, yi, 2, hi);
            }
        };
        clearTarget = function () {
            cans.clearRect(target.x - 20, target.y, 22, target.h);
        };
        drawArrow();
        drawTarget();

        moveArrow = function () {
            clearArrow();
            arrow.x += 2;
            drawArrow();
        };

        moveTarget = function () {
            clearTarget();
            if (targetUp == 0) {
                target.y += 2;
                if (target.y + target.h > 490) {
                    targetUp = 1;
                }
            } else {
                target.y -= 2;
                if (target.y < 10) {
                    targetUp = 0;
                }
            }
            drawTarget();
        };


        function getScore() {
            if (arrow.x + arrow.len < target.x - 2 * 9) {
                return -1;
            }
            var yDiff = Math.abs(target.y + target.h / 2 - arrow.y);
            var nCircle = Math.floor(yDiff / 8);
            var tarX = target.x - 9 * 2 + 2 * nCircle;
            if (arrow.x + arrow.len < tarX) {
                return -1;
            }
            if (10 - nCircle < 0) {
                return 0;
            }
            return 10 - nCircle;
        }

        startGame = function () {
            clearArrow();
            clearTarget();

            started = true;
            target.y = 10;
            arrow.x = 1;
            drawArrow();
            drawTarget();
            targetUp = 0;
            targetInterval = setInterval(moveTarget, 50);
            var mainInterval = setInterval(function () {
                if (getScore() != -1) {
                    alert("you get score " + getScore());
                    clearInterval(arrowInterval);
                    clearInterval(targetInterval);
                    clearInterval(mainInterval);
                    started = false;
                }
            }, 10);
        };

        shoot = function () {
            if (started) {
                arrowInterval = setInterval(moveArrow, 10);
            } else {
                alert("you should click start first");
            }
        };
    </script>
</div>