html - Dart ，为什么我的形状不露面？

我试图做到这一点，以便在每个位置都会产生一个鼠标到一个黑圈，但我无法显示它，我知道我正在接收输入，但是我不知道为什么我的东西不正确。出现吗？我缺少某种重绘方法吗？我已经尝试了很多我不太了解requestAnimation在这里如何工作的事情。

我究竟做错了什么？

import 'dart:html';
import 'dart:async';
import 'package:polymer/polymer.dart';

CanvasElement canvas = document.querySelector('#myPaintCanvas');
CanvasRenderingContext2D context = canvas.getContext('2d');

//circle deminsions
var centerX;
var centerY;
var radius = 15;
var PI = 3.14;

void requestRedraw(){
  window.requestAnimationFrame(scribble());
 }

void scribble(pointX, pointY){
 print("MouseX: " + pointX.toString() + " MouseY: " + pointY.toString());


context.arc(pointX, pointY, radius, 0, 2 * PI, false);
context.fillStyle = 'black';
context.fill();
context.strokeStyle = '#003300';
context.stroke();

}

void main(){
 canvas.onMouseDown.listen((onDown){

    StreamSubscription mouseMoveStream = canvas.onMouseMove.listen((onData){
     scribble(onData.client.x, onData.client.y);
    });

//to end my stream when the mouse is up
canvas.onMouseUp.listen((onData) => mouseMoveStream.cancel());
  });

}

最佳答案

由于 Canvas 的实际绘图尺寸与其作为DOM元素的尺寸(canvas.width = 500;和canvas.style.width="500px";)之间不匹配，您很可能会遇到缩放问题

固定比例+一些其他修正:

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

CanvasElement canvas = document.querySelector('#myPaintCanvas');
CanvasRenderingContext2D context;

//circle deminsions
var centerX;
var centerY;
var radius = 15;
var PI = 3.14;


void scribble(pointX, pointY) {
  print("MouseX: " + pointX.toString() + " MouseY: " + pointY.toString());
  context.clearRect(0, 0, canvas.width, canvas.height);
  context.beginPath();
  context.arc(pointX, pointY, radius, 0, 2 * PI, false);
  context.fillStyle = 'black';
  context.fill();
  context.strokeStyle = '#003300';
  context.stroke();
  context.closePath();
}

void main() {
  canvas.width = 500;
  canvas.style.width="500px";
  canvas.height = 500;
  canvas.style.height="500px";
  context = canvas.getContext('2d');
  canvas.onMouseDown.listen((onDown) {

    StreamSubscription mouseMoveStream = canvas.onMouseMove.listen((onData) {
      var rec = canvas.getBoundingClientRect();
      scribble(onData.client.x- rec.left, onData.client.y - rec.top);
    });

    //to end my stream when the mouse is up
    document.onMouseUp.listen((onData) => mouseMoveStream.cancel());
  });
}

我添加了context.clearRect，以使圆圈清晰可见。需要context.beginPath();和context.closePath();来防止圆之间的桥梁。当在 Canvas 外释放鼠标按钮时，用document.onMouseUp而不是canvas.onMouseUp解决了问题。

关于html - Dart ，为什么我的形状不露面？，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/24318945/