在画布周围拖动圆形矩形

什么是形状？

你通常通过创建表示每个形状的 JavaScript形状对象来保存形状。

var myCircle = { x:30, y:20, radius:15 };

当然，你并没有真正保存形状。相反，你将保存如何绘制形状的定义。

然后将每个 shape 对象放入一个数组中以便于参考。

// save relevant information about shapes drawn on the canvas
var shapes=[];

// define one circle and save it in the shapes[] array
shapes.push( {x:10, y:20, radius:15, fillcolor:'blue'} );

// define one rectangle and save it in the shapes[] array
shapes.push( {x:10, y:100, width:50, height:35, fillcolor:'red'} );

使用鼠标事件进行拖动

拖动形状或图像需要响应这些鼠标事件：

在 mousedown 上：

测试鼠标下是否有任何形状。如果形状位于鼠标下方，则用户打算拖动该形状。因此，请保留对该形状的引用，并设置一个 true / false isDragging 标志，指示拖动正在进行中。

在 mousemove 上：

计算自上次 mousemove 事件以来拖动鼠标的距离，并将拖动的形状的位置更改该距离。要更改形状的位置，请更改该形状对象中的 x,y 位置属性。

在 mouseup 或 mouseout 上：

用户打算停止拖动操作，因此清除 isDragging 标志。拖动完成。

演示：在画布上拖动圆形和矩形

该演示通过响应鼠标事件并通过清除和重绘来呈现移动的幻觉，从而在画布上拖动圆形和矩形。

// canvas related vars
var canvas=document.createElement("canvas");
var ctx=canvas.getContext("2d");
var cw=canvas.width;
var ch=canvas.height;
document.body.appendChild(canvas);
canvas.style.border='1px solid red';

// used to calc canvas position relative to window
function reOffset(){
    var BB=canvas.getBoundingClientRect();
    offsetX=BB.left;
    offsetY=BB.top;        
}
var offsetX,offsetY;
reOffset();
window.onscroll=function(e){ reOffset(); }
window.onresize=function(e){ reOffset(); }
canvas.onresize=function(e){ reOffset(); }

// save relevant information about shapes drawn on the canvas
var shapes=[];
// define one circle and save it in the shapes[] array
shapes.push( {x:30, y:30, radius:15, color:'blue'} );
// define one rectangle and save it in the shapes[] array
shapes.push( {x:100, y:-1, width:75, height:35, color:'red'} );

// drag related vars
var isDragging=false;
var startX,startY;

// hold the index of the shape being dragged (if any)
var selectedShapeIndex;

// draw the shapes on the canvas
drawAll();

// listen for mouse events
canvas.onmousedown=handleMouseDown;
canvas.onmousemove=handleMouseMove;
canvas.onmouseup=handleMouseUp;
canvas.onmouseout=handleMouseOut;

// given mouse X & Y (mx & my) and shape object
// return true/false whether mouse is inside the shape
function isMouseInShape(mx,my,shape){
    if(shape.radius){
        // this is a circle
        var dx=mx-shape.x;
        var dy=my-shape.y;
        // math test to see if mouse is inside circle
        if(dx*dx+dy*dy<shape.radius*shape.radius){
            // yes, mouse is inside this circle
            return(true);
        }
    }else if(shape.width){
        // this is a rectangle
        var rLeft=shape.x;
        var rRight=shape.x+shape.width;
        var rTop=shape.y;
        var rBott=shape.y+shape.height;
        // math test to see if mouse is inside rectangle
        if( mx>rLeft && mx<rRight && my>rTop && my<rBott){
            return(true);
        }
    }
    // the mouse isn't in any of the shapes
    return(false);
}

function handleMouseDown(e){
    // tell the browser we're handling this event
    e.preventDefault();
    e.stopPropagation();
    // calculate the current mouse position
    startX=parseInt(e.clientX-offsetX);
    startY=parseInt(e.clientY-offsetY);
    // test mouse position against all shapes
    // post result if mouse is in a shape
    for(var i=0;i<shapes.length;i++){
        if(isMouseInShape(startX,startY,shapes[i])){
            // the mouse is inside this shape
            // select this shape
            selectedShapeIndex=i;
            // set the isDragging flag
            isDragging=true;
            // and return (==stop looking for 
            //     further shapes under the mouse)
            return;
        }
    }
}

function handleMouseUp(e){
    // return if we're not dragging
    if(!isDragging){return;}
    // tell the browser we're handling this event
    e.preventDefault();
    e.stopPropagation();
    // the drag is over -- clear the isDragging flag
    isDragging=false;
}

function handleMouseOut(e){
    // return if we're not dragging
    if(!isDragging){return;}
    // tell the browser we're handling this event
    e.preventDefault();
    e.stopPropagation();
    // the drag is over -- clear the isDragging flag
    isDragging=false;
}

function handleMouseMove(e){
    // return if we're not dragging
    if(!isDragging){return;}
    // tell the browser we're handling this event
    e.preventDefault();
    e.stopPropagation();
    // calculate the current mouse position         
    mouseX=parseInt(e.clientX-offsetX);
    mouseY=parseInt(e.clientY-offsetY);
    // how far has the mouse dragged from its previous mousemove position?
    var dx=mouseX-startX;
    var dy=mouseY-startY;
    // move the selected shape by the drag distance
    var selectedShape=shapes[selectedShapeIndex];
    selectedShape.x+=dx;
    selectedShape.y+=dy;
    // clear the canvas and redraw all shapes
    drawAll();
    // update the starting drag position (== the current mouse position)
    startX=mouseX;
    startY=mouseY;
}

// clear the canvas and 
// redraw all shapes in their current positions
function drawAll(){
    ctx.clearRect(0,0,cw,ch);
    for(var i=0;i<shapes.length;i++){
        var shape=shapes[i];
        if(shape.radius){
            // it's a circle
            ctx.beginPath();
            ctx.arc(shape.x,shape.y,shape.radius,0,Math.PI*2);
            ctx.fillStyle=shape.color;
            ctx.fill();
        }else if(shape.width){
            // it's a rectangle
            ctx.fillStyle=shape.color;
            ctx.fillRect(shape.x,shape.y,shape.width,shape.height);
        }
    }
}