dart - Dart Canvas 运动

Question

用 Dart 做这样的 Canvas 运动的最佳方法是什么？
http://jsfiddle.net/loktar/dMYvG/

我正在尝试使 Canvas 运动平稳，并想看看Dart可以做什么。

还有Dart的jsfiddle替代项吗？

var canvas = document.getElementById("canvas"),
ctx = canvas.getContext("2d");

canvas.width = canvas.height = 300;

var x = 150,
y = 150,
velY = 0,
velX = 0,
speed = 2,
friction = 0.98,
keys = [];

function update() {

if (keys[38]) {
    if (velY > -speed) {
        velY--;
    }
}

if (keys[40]) {
    if (velY < speed) {
        velY++;
    }
}
if (keys[39]) {
    if (velX < speed) {
        velX++;
    }
}
if (keys[37]) {
    if (velX > -speed) {
        velX--;
    }
}

velY *= friction;
y += velY;
velX *= friction;
x += velX;

if (x >= 295) {
    x = 295;
} else if (x <= 5) {
    x = 5;
}

if (y > 295) {
    y = 295;
} else if (y <= 5) {
    y = 5;
}

ctx.clearRect(0, 0, 300, 300);
ctx.beginPath();
ctx.arc(x, y, 5, 0, Math.PI * 2);
ctx.fill();

setTimeout(update, 10);
}

update();

document.body.addEventListener("keydown", function (e) {
keys[e.keyCode] = true;
});
document.body.addEventListener("keyup", function (e) {
keys[e.keyCode] = false;
});

Answer 1

我知道有TryDart!，但它似乎不如JSFiddle强大。

对于您的示例，这是等效的Dart代码:

import 'dart:html';
import 'dart:math';
import 'dart:async';

void main() {
    CanvasElement canvas = querySelector('canvas');
    CanvasRenderingContext2D ctx = canvas.context2D;
    canvas.width = canvas.height = 300;

    int x = 150;
    int y = 150;
    double velY = 0.0;
    double velX = 0.0;
    int speed = 2;
    double friction = 0.98;
    Map<int, bool> keys = new Map<int, bool>();

    void update() {
        if (keys[38]) {
            if (velY > -speed)
            {
                velY--;
            }
        }
        if (keys[40]) {
            if (velY < speed) {
                velY++;
            }
        }
        if (keys[39]) {
            if (velX < speed) {
                velX++;
            }
        }
        if (keys[37]) {
            if (velX > -speed) {
                velX--;
            }
        }

        velY *= friction;
        y += velY;
        velX *= friction;
        x += velX;

        if (x >= 295) {
            x = 295;
        } else if (x <= 5) {
            x = 5;
        }

        if (y > 295) {
            y = 295;
        } else if (y <= 5) {
            y = 5;
        }

        ctx.clearRect(0, 0, 300, 300);
        ctx.beginPath();
        ctx.arc(x, y, 5, 0, PI * 2);
        ctx.fill();
        var timer = new Timer(new Duration(milliseconds:10), update);
     }

    update();

    document.body.onKeyDown.listen((e) => keys[e.keyCode] = true);
    document.body.onKeyUp.listen((e) => keys[e.keyCode] = false);
  }

您还可以利用Dart API中的诸如num.clamp之类的东西来简化代码。