html - 达特，为什么我的形状不显示？

Question

我试图做到这一点，以便在我的鼠标到达的每个位置都会生成一个黑色圆圈，但我无法让它显示出来，我知道我正在接收输入，但我不知道为什么我的东西没有显示？我是否缺少某种重绘方法？我已经尝试了很多东西，但我不太明白 requestAnimation 在这里是如何工作的。

我做错了什么？

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());
  });

}

Answer 1

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

修复了缩放+一些额外的修复:

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 来明确圆圈的位置。需要 context.beginPath(); 和 context.closePath(); 来防止圆之间的桥梁。 document.onMouseUp 而不是 canvas.onMouseUp 修复了在 Canvas 外释放鼠标按钮时的问题。