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JavaScript 使 getContext() 公开?

转载 作者:行者123 更新时间:2023-12-02 19:42:16 28 4
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我正在尝试获取 Canvas 的上下文,显然我收到错误Uncaught TypeError: Cannot call method 'getContext' of null

显然我在它初始化之前就得到了它?我该怎么办呢。如何公开我的 Canvas ,以便可以在其他函数中访问它并消除错误?

    var spawnX = 5;
var spawnY = 7;
var realSpawnX = spawnX*32;
var realSpawnY = spawnY*32;
var playerImg = new Image();
var playerImgX = new Image();
var currX = 5;
var currY = 7;
var canvas = document.getElementById("canvas");
var context = canvas.getContext("2d");
var imageObj = new Image();

window.onLoad = function() {

loadGame();

};

// The map
function loadMap(map) {
if (map == 1) {
return [
[ 11, 1, 1, 1, 1, 1, 1, 190, 115, 1, 1, 1, 1, 1, 1, 2],
[ 190, 190, 190, 190, 190, 190, 190, 190, 13, 148, 148, 148, 148, 148, 121, 2],
[ 1, 520, 127, 127, 127, 127, 127, 13, 13, 148, 167, 167, 167, 148, 343, 1],
[ 1, 520, 127, 166, 166, 166, 127, 13, 13, 148, 167, 167, 167, 148, 343, 1],
[ 1, 520, 127, 166, 166, 166, 127, 13, 13, 148, 148, 148, 183, 148, 343, 1],
[ 1, 520, 364, 174, 127, 361, 127, 13, 13, 13, 13, 13, 13, 13, 13, 1],
[ 115, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 115],
[ 1, 514, 13, 13, 394, 343, 145, 220, 145, 145, 145, 13, 13, 13, 13, 1],
[ 1, 514, 13, 13, 343, 118, 145, 166, 166, 166, 145, 13, 13, 13, 13, 1],
[ 1, 514, 514, 13, 118, 118, 145, 166, 166, 166, 145, 13, 13, 13, 13, 1],
[ 1, 1, 1, 115, 1, 1, 145, 145, 145, 145, 145, 1, 1, 1, 1, 1]
];
}
}

function loadGame(){
// Load Game
canvas = document.getElementById("canvas");
context = canvas.getContext("2d");
imageObj = new Image();
var tiles = [];
var board = loadMap(1);

canvas.width = 512;
canvas.height = 352;

// Set up the tiles
for (x = 0; x <= 520; x++) {
imageObj = new Image(); // new instance for each image
imageObj.src = "line_tile/t"+x+".png";
tiles.push(imageObj);
}
var theX;
var theY;
// Draw the map by rows and cols
for (x = 0; x <= 10; x++) {
for (y = 0; y <= 15; y++) {

theX = x*32;
theY = y*32;
//context.drawImage(tiles[board[x][y]], theY, theX,32,32);
//console.log("Tile X: " + x + " | Tile Y: " + y + " - X Pos: " + theX + " | Y Pos: " + theY);
}
}

// DRAW THE PLAYER
spawnX = 5;
spawnY = 7;
realSpawnX = spawnX*32;
realSpawnY = spawnY*32;
currX = 5;
currY = 7;
playerImg.src = "me.gif";
context.drawImage(playerImg, realSpawnY, realSpawnX,32,32);
console.log("Player drawn at ("+spawnX+","+spawnY+") Real spawn: X: " +realSpawnX + " Y: " + realSpawnY);

}

// Pressing arrow keys 'moves' the player
$(document).keydown(function(e){
if (e.keyCode == 37) { // LEFT ARROW
currX = currX-1;
console.log("New X:" + currX);
return false;
}
if (e.keyCode == 39) { // RIGHT ARROW
currX = currX+1;
console.log("New X:" + currX);
return false;
}

spawnX = 1;
spawnY = 1;
realSpawnX = spawnX*32;
realSpawnY = spawnY*32;
playerImgX.src = "me.gif";
context.drawImage(playerImgX, realSpawnY, realSpawnX,32,32);
});

这是 HTML 页面

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">

<head>
<meta http-equiv="content-type" content="text/html; charset=iso-8859-1" />

<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.18/jquery-ui.min.js"></script>


<title>Untitled 1</title>

<script type="text/javascript" src="theGame.js"></script>
<style type="text/css">
<!--
#canvas {
background:red;
}
-->
</style>
</head>

<body>
<canvas id="canvas"></canvas>
</body>
</html>

最佳答案

您应该等待初始化变量,直到页面加载完毕:

var canvas = null;
var context = null;

window.onload = function() {
canvas = document.getElementById("canvas");
var context = canvas.getContext("2d");

loadGame();
};

关于JavaScript 使 getContext() 公开?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/10256601/

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