javascript - 如何清除 JavaScript 中的绘图 Canvas ？

Question

我是 HTML 新手。只是为了好玩。我通过观看 YouTube 上的教程用 HTML 制作绘图 Canvas 。

这里模拟代码: https://jsfiddle.net/MasoodSalik/yr1ezp4x/

我面临两个问题:

1. 当我清除 Canvas 时，画笔无法正常工作，如图所示。

2. 当我绘制或超出 Canvas 边缘时，画笔仍会在 Canvas 中拖动。我希望它在触及边缘时停止绘制。我该怎么做？

   

   *{
     -moz-box-sizing: border-box;
      box-sizing: border-box;
     font-family:sans-serif;
   
     user-select: none;
     -webkit-user-select:none;
      user-select: none;
   
   }
   
   
   #toolbara{
    width :329px;
    height :40px;
     padding:10px;
     position: relative;
     top:0px;
     background-color:#2f2f2f;
     color: white;
     }
   
   
   .radcontrol{
       width : 30px;
      height : 20px;
      background-color:#4f4f4f;
      display:inline-block;
      text-align:center;
   }
   
   #rad{
     float:left;
   }
   #colour{
    //position: relative;
     float:center;
   }
   
   .swatch{
     width:20px;
     height:20px;
     border-radius:10px;
     box-shadow: inset 0px 2px 0px rgba(255,255,255,0.5), 0px 2px 2px rgba(0,0,0,0.5);
     display:inline-block;
     margin-left:5px;
     margin-bottom:50px;
     background-color:cyan;
   }
   .swatch.active{
     border:2px solid white;
      box-shadow: inset 0px 1px 2px rgba(0,0,0,0.5) ;
   
   }
   #save{
       background-color: #4f4f4f;
      // width: 50px;
      padding: 5px;
      position: relative;
     float :right;
     top:-45px;
     right: 60px;
     margin-right:0px;
   }
   #save:hover{
     background-color: #818181;
   
   }
   
   #clear{
       background-color: #4f4f4f;
      // width: 50px;
      padding: 5px;
      position: relative;
     float :right;
     top:-45px;
     right: -40px;
    // margin-right:0px;
   }
   #clear:hover{
     background-color: #818181;
   
   }
   
   </style>
   
   
   <canvas id="canvas" width="325" height="500" style="border:2px solid">
   <p>Your browser doesn't support canvas.</p>
   </canvas>
   
   <div id ="toolbara">
       <div id="rad">
           Radius <span id="radval">1</span>
         <div id="decrad" class="radcontrol">-</div>
          <div id="incrad" class="radcontrol">+</div>
        </div>
        <div id="colors">
        </div>
        <div id="save"> Save </div>
        <div id="clear"> Clear </div>
   </div>
   
   <script>
   
   var canvas=document.getElementById('canvas');
   var context=canvas.getContext('2d');
   var radius=5;
   var dragging=false;
   
   context.beginPath();
   context.rect(0, 0, canvas.width, canvas.height);
   context.fillStyle = "white";
   context.fill();
   context.beginPath();
   
   context.lineWidth=radius*2;
   
   var putPoint = function(e){
       if(dragging) {
       context.lineTo(e.offsetX, e.offsetY);
       context.stroke();
       context.beginPath();
       context.arc(e.offsetX, e.offsetY, radius, 0, Math.PI*2);
        context.fill();
       context.beginPath();
       context.moveTo(e.offsetX, e.offsetY);
   }
   }
   
   var engage=function(e)
   { dragging=true;
      putPoint(e);
   }
   
   var disengage=function()
   {   
    dragging=false;
   context.beginPath();
   }
   
       canvas.addEventListener('mousedown',engage);
       canvas.addEventListener('mouseup',disengage);
       canvas.addEventListener('mousemove',putPoint);
   
    var setRadius = function (newRadius) {
                   if (newRadius < minRad) newRadius = minRad;
                   else if (newRadius > maxRad) newRadius = maxRad;
                   radius = newRadius;
                   context.lineWidth = radius * 2;
   
                   radSpan.innerHTML = radius;
               }
   
     ////////////////////////////////////////
    var minRad = 1,
                   maxRad = 10,
                   defaultRad = 1,
                   interval = 1,
                   radSpan = document.getElementById('radval'),
                   decRad = document.getElementById('decrad'),
                   incRad = document.getElementById('incrad');
   
               decRad.addEventListener('click', function () {
                   setRadius(radius - interval);
               });
               incRad.addEventListener('click', function () {
                    setRadius(radius < interval ? interval : radius + interval);
               });
   
               setRadius(defaultRad);
     //////////////////////////////////////////////////////
   
   var colors = ['black', 'white', 'red', 'yellow', 'green', 'blue']; //Color array to select from
   /*Handles the creation of color*/
   for(var i=0, n=colors.length;i<n; i++){
   var swatch = document.createElement('nav');
   swatch.className = 'swatch';
   swatch.style.backgroundColor = colors[i];
   swatch.addEventListener('click',setSwatch);
   document.getElementById('colors').appendChild(swatch);
   }
   /*set color*/
   function setColor(color){
   context.fillStyle = color;
   context.strokeStyle = color;
   var active = document.getElementsByClassName('active')[0];
   if(active){
   active.className = 'swatch';
   }
   }
   function setSwatch(e){
   //identify swatch
   var swatch = e.target;
   //set color
   setColor(swatch.style.backgroundColor);
   //give active class
   swatch.className += ' active';
   }
   setSwatch({target: document.getElementsByClassName('swatch')[0]}); //set default swatch
   
   //////////////////////////////////////
   
    var button = document.getElementById('save');
               button.addEventListener('click', saveImage)
   
    function saveImage()          
    {     //   context.clearRect(0, 0, canvas.width, canvas.height);
                 var data = canvas.toDataURL();
   
                window.open(data,'_blank,','location=0,menubar=0')
                //  button.href = dataURL;
               };
   
    var butonclear = document.getElementById('clear');
               butonclear.addEventListener('click', clearImage)
   
    function clearImage()          
    {      
               context.clearRect(0, 0, canvas.width, canvas.height);
    context.beginPath();
   context.rect(0, 0, canvas.width, canvas.height);
   context.fillStyle = "white";
   context.fill();
   context.beginPath();
   context.moveTo(e.offsetX, e.offsetY);
   
   
         };          
   
   </script>
   
   
   <!link rel="stylesheet" type="text/css" href="https://googledrive.com/host/0B7ZbA61nROnAMkFzSDVoOWdCWkk/noselect.css"></!link></!link>`
   

Answer 1

1. 画笔无法正常工作，因为当您调用 clearImage() 时，您正在将 context.fillStyle 更改为白色，同时 context.moveTo(e.offsetX, e.offsetY ); 这是不必要的。看看重现 https://jsfiddle.net/yr1ezp4x/2/

2. 在那里你会有更多的工作。由鼠标事件触发的内部函数你必须检查鼠标是否在你的 Canvas 上。