用Canvas制作能够依据手势摆动的树

依据工作的须要。制作一个摆动的树做为页面的背景。为了添加页面的交互性，我又为背景中的树添加了鼠标（触控）事件，使他可以依据鼠标（触控）做出对应的动作，当手指做上下或者左右滑动的时候树会跟着摆动。先看看终于效果。

Step1.完毕HTML页面。新建一个Tree类

完毕HTML页面后新建一个Tree类用来记录树的各个属性。当中x,y为树根部的坐标值。branchLen,branchWidth各自是树枝的长度与宽度，depth为树枝的层数。canvas用来接页面中的canvas元素（默认是ID为canvas的元素）。

function Tree(x,y,branchLen,branchWidth,depth,canvas){this.canvas = canvas || document.getElementById('canvas');this.ctx = this.canvas.getContext('2d');this.x = x||0;this.y = y||0;this.branchLen = branchLen||0;this.branchWidth = branchWidth||0;var depth = depth || 5;
}

点击查看历史代码

Step2.加入drawRoot方法，用来绘制树干

首先在drawRoot中画第一个枝干。

drawRoot的參数意义同上。而且在Tree类的构造函数中执行drawRoot并把Tree接受到的參数传入。最后new一个Tree类，使树根位于屏幕的底部正中心。树枝长100px，树枝宽度为8px，树枝层数为8层（临时用不上）。

var atree = new Tree(canvas.width/2-4,canvas.height,100,8,8,canvas);

在drawRoot中我们须要用lineTo()画出树枝。树枝的起始的坐标值(x,y)已经给出，结束的坐标值(toX,toY)须要进行计算。第一个画的是树干，因为树干垂直于地面所以结束坐标toX等于初始坐标x,而结束坐标toY等于初始y减去树干长度branchLen(注意坐标的0,0点在canvas的左上角)。var toX = x;var toY = y-branchLen;

function Tree(x,y,branchLen,branchWidth,depth,canvas){this.canvas = canvas || document.getElementById('canvas');this.ctx = this.canvas.getContext('2d');this.x = x||0;this.y = y||0;this.branchLen = branchLen||0;this.branchWidth = branchWidth||0;var depth = depth || 5;this.drawRoot(this.x,this.y,this.branchLen,this.branchWidth);
}
Tree.prototype.drawRoot = function(x,y,branchLen,branchWidth){var toX = x;var toY = y-branchLen;this.ctx.save();this.ctx.strokeStyle="rgba(37, 141, 194, 0.93)";this.ctx.beginPath();this.ctx.lineCap = "butt";this.ctx.lineJoin="round";this.ctx.lineWidth = branchWidth;this.ctx.moveTo(x,y);this.ctx.lineTo(toX,toY);this.ctx.closePath();this.ctx.stroke();this.ctx.restore();
}
var atree = new Tree(canvas.width/2-4,canvas.height,100,8,8,canvas);

执行代码：

点击查看历史代码

Step3.加入drawBranch方法，用来绘制树枝

drawBranch相同是依据初始与结束坐标画出一条直线代表树枝。

与树干不同的是树枝不再是垂直与地面而是与树干保持一定的角度。并且树枝的初始值是树干的结束点(toX,toY)。所以在drawBranch中我们增加新參数angle用来表示树枝与树干的垂直夹角α，这样就能够依据α算出toX与toY。请看图。

这样我们在画完树干后再分别画两个不同角度的树枝。一个是30°一个-30°。并将传给树枝的宽度branchWidth减小一个像素。使其与树干粗细不同。

Tree.prototype.drawRoot = function(x,y,branchLen,branchWidth){var toX = x;var toY = y-branchLen;this.ctx.save();this.ctx.strokeStyle="rgba(37, 141, 194, 0.93)";this.ctx.beginPath();this.ctx.lineCap = "butt";this.ctx.lineJoin="round";this.ctx.lineWidth = branchWidth;this.ctx.moveTo(x,y);this.ctx.lineTo(toX,toY);this.ctx.closePath();this.ctx.stroke();this.ctx.restore();this.drawBranch(toX,toY,branchLen,branchWidth-1,30);this.drawBranch(toX,toY,branchLen,branchWidth-1,-30);
}
Tree.prototype.drawBranch = function(x,y,branchLen,branchWidth,angle){var angle = angle || 0;var radian = (90-angle)*(Math.PI/180);var toX = x+Math.cos(radian)*branchLen;var toY = y-Math.sin(radian)*branchLen;this.ctx.save();this.ctx.strokeStyle="rgba(37, 141, 194, 0.93)";this.ctx.beginPath();this.ctx.lineCap = "butt";this.ctx.lineJoin="round";this.ctx.lineWidth = branchWidth;this.ctx.moveTo(x,y);this.ctx.lineTo(toX,toY);this.ctx.closePath();this.ctx.stroke();this.ctx.restore();
}

执行代码：

点击查看历史代码

Step4.改动drawBranch函数，反复画树枝

在drawBranch函数的最后再次调用两次drawBranch

this.drawBranch(toX,toY,branchLen,branchWidth-1,angle+30);
this.drawBranch(toX,toY,branchLen,branchWidth-1,angle-30);

使其调用自己完毕递归,注意这里传入的角度是在之前的角度的基础上在添加或者降低30度。

为了使递归停下来我们须要一个停止条件，就是之前一直没实用到的depth參数。我们在每次画下一层之前使其减1表示已经完毕了一层树枝的绘制，直至depth减小到0表示绘制全然部的层数。

function Tree(x,y,branchLen,branchWidth,depth,canvas){this.canvas = canvas || document.getElementById('canvas');this.ctx = this.canvas.getContext('2d');this.x = x||0;this.y = y||0;this.branchLen = branchLen||0;this.branchWidth = branchWidth||0;var depth = depth || 5;this.drawRoot(this.x,this.y,this.branchLen,this.branchWidth,depth);
}
Tree.prototype.drawRoot = function(x,y,branchLen,branchWidth,depth){var toX = x;var toY = y-branchLen;var depth = depth||5;this.ctx.save();this.ctx.strokeStyle="rgba(37, 141, 194, 0.93)";this.ctx.beginPath();this.ctx.lineCap = "butt";this.ctx.lineJoin="round";this.ctx.lineWidth = branchWidth;this.ctx.moveTo(x,y);this.ctx.lineTo(toX,toY);this.ctx.closePath();this.ctx.stroke();this.ctx.restore();depth--;if(depth>0){this.drawBranch(toX,toY,branchLen,branchWidth-1,30,depth);this.drawBranch(toX,toY,branchLen,branchWidth-1,-30,depth);}
}
Tree.prototype.drawBranch = function(x,y,branchLen,branchWidth,angle,depth){var angle = angle || 0;var radian = (90-angle)*(Math.PI/180);var toX = x+Math.cos(radian)*branchLen;var toY = y-Math.sin(radian)*branchLen;this.ctx.save();this.ctx.strokeStyle="rgba(37, 141, 194, 0.93)";this.ctx.beginPath();this.ctx.lineCap = "butt";this.ctx.lineJoin="round";this.ctx.lineWidth = branchWidth;this.ctx.moveTo(x,y);this.ctx.lineTo(toX,toY);this.ctx.closePath();this.ctx.stroke();this.ctx.restore();depth--;if(depth>0){this.drawBranch(toX,toY,branchLen,branchWidth-1,angle+30,depth);this.drawBranch(toX,toY,branchLen,branchWidth-1,angle-30,depth);}
}

执行代码：

因为树之间角度过大，并且全部树枝长度都相等，看起来并不像一棵树。所以我们须要在Tree的构造函数中增加几个參数用来调整树的姿态。

function Tree(x,y,branchLen,branchWidth,depth,canvas){......this.branchLenFactor = 0.8;this.rootLenFactor = 1.2;this.branchAngle = 20;......
}

branchLenFactor:画每一层树枝的时候乘在branchLen上面，用来控制树枝长度。

rootLenFactor:画树根的时候乘在branchLen上面，用来控制树根长度。branchAngle: 用来控制树枝之间的角度。

Tree.prototype.drawRoot = function(x,y,branchLen,branchWidth,depth){var toX = x;var toY = y-branchLen*this.rootLenFactor;var depth = depth||5;this.ctx.save();this.ctx.strokeStyle="rgba(37, 141, 194, 0.93)";this.ctx.beginPath();this.ctx.lineCap = "butt";this.ctx.lineJoin="round";this.ctx.lineWidth = branchWidth;this.ctx.moveTo(x,y);this.ctx.lineTo(toX,toY);this.ctx.closePath();this.ctx.stroke();this.ctx.restore();depth--;if(depth>0){this.drawBranch(toX,toY,branchLen*this.branchLenFactor,branchWidth-1,this.branchAngle,depth);this.drawBranch(toX,toY,branchLen*this.branchLenFactor,branchWidth-1,-this.branchAngle,depth);}}Tree.prototype.drawBranch = function(x,y,branchLen,branchWidth,angle,depth){var angle = angle || 0;var radian = (90-angle)*(Math.PI/180);var toX = x+Math.cos(radian)*branchLen;var toY = y-Math.sin(radian)*branchLen;this.ctx.save();this.ctx.strokeStyle="rgba(37, 141, 194, 0.93)";this.ctx.beginPath();this.ctx.lineCap = "butt";this.ctx.lineJoin="round";this.ctx.lineWidth = branchWidth;this.ctx.moveTo(x,y);this.ctx.lineTo(toX,toY);this.ctx.closePath();this.ctx.stroke();this.ctx.restore();depth--;if(depth>0){this.drawBranch(toX,toY,branchLen*this.branchLenFactor,branchWidth-1,angle+this.branchAngle,depth);this.drawBranch(toX,toY,branchLen*this.branchLenFactor,branchWidth-1,angle-this.branchAngle,depth);}}

执行代码：

点击查看历史代码

Step5.使树枝晃动起来

为了使树枝有摇晃的效果。我们仅仅须要改变树枝之间的角度branchAngle就能够了。我须要在Tree的构造函数中添加三个新属性：oBranchAngle用来记录初始角度。branchAngleFactor用来控制角度随时间变化的变化量；swingAngle:随时间添加用来记录摇动的角度。
同一时候改动下drawRoot函数使其不用接受參数。调用更加方便。

function Tree(x,y,branchLen,branchWidth,depth,canvas){......this.branchAngle = 20;this.oBranchAngle = this.branchAngle;this.branchAngleFactor = 5;this.swingAngle = 0;......this.drawRoot();
}Tree.prototype.drawRoot = function(){var x = this.x,y=this.y,branchLen = this.branchLen,depth = this.depth,branchWidth = this.branchWidth;var toX = x;var toY = y-branchLen*this.rootLenFactor;var depth = depth||5;this.ctx.save();this.ctx.strokeStyle="rgba(37, 141, 194, 0.93)";this.ctx.beginPath();this.ctx.lineCap = "butt";this.ctx.lineJoin="round";this.ctx.lineWidth = this.branchWidth;this.ctx.moveTo(x,y);this.ctx.lineTo(toX,toY);this.ctx.closePath();this.ctx.stroke();this.ctx.restore();depth--;if(depth>0){this.drawBranch(toX,toY,branchLen*this.branchLenFactor,branchWidth-1,this.branchAngle,depth);this.drawBranch(toX,toY,branchLen*this.branchLenFactor,branchWidth-1,-this.branchAngle,depth);}}

添加循环函数，在循环函数中重绘整个树。而且每次重绘都要改动branchAngle值，使大树摇动起来。atree.swingAngle++;使摇动角度随时间变化。这里使用Math.sin(atree.swingAngle*(Math.PI/180))能够获得一个-1至1之间的连续变化值。atree.branchAngle = Math.sin(atree.swingAngle*(Math.PI/180))*atree.branchAngleFactor+atree.oBranchAngle;乘以系数并加在原角度上。

function loop(time){ctx.clearRect(0,0,canvas.width,canvas.height);atree.branchAngle = Math.sin(atree.swingAngle*(Math.PI/180))*atree.branchAngleFactor+atree.oBranchAngle;atree.drawRoot()requestAnimFrame(loop);}loop(0);

执行代码：

点击查看历史代码

Step6.加入手势

这里为了省事仅仅加入了touch事件，mouse事件与touch事件的处理方法大体一致。
首先为Tree新加一个属性swingSwitch = true用来表示大树是否摆动。当手指触控到屏幕的时候摆动停止，离开屏幕的时候摆动继续。
加入strengthX,strengthY两个属性;分别表示树在x轴与y轴因受到的力而移动的距离。

加入strengthXFactor,strengthYFactor；分别用来表示再一次滑动中x轴与y轴移动的最大距离。

function Tree(x,y,branchLen,branchWidth,depth,canvas){......this.swingSwitch = true;......this.strengthX = 0;this.strengthY = 0;......}

//记录触控開始时的信息
var touchStart = {x:0,y:0,strengthX:0,strengthY:0};
document.addEventListener('touchstart',function(e){//让树停止摆动atree.swingSwitch = false;touchStart.x = e.touches[0].clientX;touchStart.y = e.touches[0].clientY;//记录触控開始时,原strength的值touchStart.strengthX = atree.strengthX;touchStart.strengthY = atree.strengthY;
});
document.addEventListener('touchmove',function(e){//阻止浏览器默认动作e.preventDefault();//(touchStart.x-e.touches[0].clientX)/canvas.width能够依据滑动距离获得一个0-1的值atree.strengthX = touchStart.strengthX-(touchStart.x-e.touches[0].clientX)/canvas.width*atree.strengthXFactor;atree.strengthY = touchStart.strengthY-(touchStart.y-e.touches[0].clientY)/canvas.height*atree.strengthYFactor;
});
document.addEventListener('touchend',function(e){//恢复摆动atree.swingSwitch = true;
});

改动drawBranch将strength的变化加入到角度与toX，toY的计算中，详情见凝视。

Tree.prototype.drawBranch = function(x,y,branchLen,branchWidth,angle,depth){var angle = angle || 0;//用strengthX乘以(depth/this.depth)使得树枝末梢对角度的变化不敏感angle += this.strengthX*(depth/this.depth)/this.strengthXFactor*this.branchAngle;var radian = (90-angle)*(Math.PI/180);//用strengthX乘以(1-depth/this.depth)使得树枝末梢对角度的变化敏感var toX = x+Math.cos(radian)*branchLen+this.strengthX*(1-depth/this.depth);var toY = y-Math.sin(radian)*branchLen+this.strengthY*(1-depth/this.depth);......
}

在动画循环中加入恢复代码，使strengthX，strengthY恢复为0。并添加swingSwitch的推断。

function loop(time){......//当swingSwitch开启时開始摆动if(atree.swingSwitch){//将strength恢复到0if(atree.strengthX >0){atree.strengthX -= 1;}if(atree.strengthX <0){atree.strengthX += 1;}if(atree.strengthY >0){atree.strengthY -= 1;}if(atree.strengthY <0){atree.strengthY += 1;}atree.swingAngle++;atree.branchAngle = Math.sin(atree.swingAngle*(Math.PI/180))*atree.branchAngleFactor+atree.oBranchAngle;}......
}
loop(0);

执行代码：

点击查看历史代码

Step7.加入缓动效果

Step6中的恢复strengthX,strengthY的代码过于简单，动画匀速恢复到0，显得过于突兀。比較真实的情况应该是由快变慢的恢复，所以我们要为恢复代码加上缓动。首先在Tree中加入recoverStartTime = 0用来记录恢复開始的时间。在手指离开屏幕的时候(touchend)将其赋为0，同一时候用oStrengthX,oStrengthY记录下来strengthX与strengthY的目标值。

function Tree(x,y,branchLen,branchWidth,depth,canvas){......this.recoverStartTime = 0;......
}
function loop(time){......if(atree.swingSwitch){if(atree.strengthX > 0){if(atree.recoverStartTime == 0){atree.recoverStartTime = time;}var t = time-atree.recoverStartTime;//五次方的缓动atree.strengthX =  Math.max(atree.oStrengthX-atree.oStrengthX*((t=t/2000-1)*t*t*t*t + 1)+0,0);}if(atree.strengthX < 0){if(atree.recoverStartTime == 0){atree.recoverStartTime = time;}var t = time-atree.recoverStartTime;//五次方的缓动atree.strengthX =  Math.min(atree.oStrengthX-atree.oStrengthX*((t=t/2000-1)*t*t*t*t + 1)+0,0);}if(atree.strengthY > 0){if(atree.recoverStartTime == 0){atree.recoverStartTime = time;}var t = time-atree.recoverStartTime;//五次方的缓动atree.strengthY =  Math.max(atree.oStrengthY-atree.oStrengthY*((t=t/2000-1)*t*t*t*t + 1)+0,0);}if(atree.strengthY < 0){if(atree.recoverStartTime == 0){atree.recoverStartTime = time;}var t = time-atree.recoverStartTime;//五次方的缓动atree.strengthY =  Math.min(atree.oStrengthY-atree.oStrengthY*((t=t/2000-1)*t*t*t*t + 1)+0,0);}}......
}
document.addEventListener('touchend',function(e){atree.recoverStartTime = 0;atree.oStrengthX = atree.strengthX;atree.oStrengthY = atree.strengthY;......
});

执行代码：

点击查看历史代码

Step7.使树干摇动并移至屏幕左边

改动drawRoot使树干也能够晃动，并改动var atree = new Tree(10,canvas.height,100,8,8,canvas);使其移至左边。

Tree.prototype.drawRoot = function(){......//添加strengthvar angle = 0;angle += this.strengthX/this.strengthXFactor*this.branchAngle;var radian = (90-angle)*(Math.PI/180);var toX = x+Math.cos(radian)*branchLen*this.rootLenFactor;var toY = y-Math.sin(radian)*branchLen*this.rootLenFactor;......
}
var atree = new Tree(10,canvas.height,100,8,8,canvas);

执行代码：

点击查看历史代码

Step8.

将动画循环中处理角度的部分加入到Tree的swing()中。

Tree.prototype.swing = function(time){this.ctx.clearRect(0,0,this.canvas.width,this.canvas.height);if(this.swingSwitch){if(this.strengthX > 0){if(this.recoverStartTime == 0){this.recoverStartTime = time;}var t = time-this.recoverStartTime;this.strengthX =  Math.max(this.oStrengthX-this.oStrengthX*((t=t/2000-1)*t*t*t*t + 1)+0,0);}if(this.strengthX < 0){if(this.recoverStartTime == 0){this.recoverStartTime = time;}var t = time-this.recoverStartTime;this.strengthX =  Math.min(this.oStrengthX-this.oStrengthX*((t=t/2000-1)*t*t*t*t + 1)+0,0);}if(this.strengthY > 0){if(this.recoverStartTime == 0){this.recoverStartTime = time;}var t = time-this.recoverStartTime;this.strengthY =  Math.max(this.oStrengthY-this.oStrengthY*((t=t/2000-1)*t*t*t*t + 1)+0,0);}if(this.strengthY < 0){if(this.recoverStartTime == 0){this.recoverStartTime = time;}var t = time-this.recoverStartTime;this.strengthY =  Math.min(this.oStrengthY-this.oStrengthY*((t=t/2000-1)*t*t*t*t + 1)+0,0);}this.swingAngle++;this.branchAngle = Math.sin(this.swingAngle*(Math.PI/180))*this.branchAngleFactor+this.oBranchAngle;}this.drawRoot();
}
var atree = new Tree(10,canvas.height,100,8,8,canvas);
function loop(time){atree.swing(time);requestAnimFrame(loop);
}
loop(0);

执行代码：

查看全部代码请去Github

如有问题或者建议请微博@UED天机。

我会及时回复

也能够收藏天机的官网，http://ued.sexy/  经常更新了最新的教程。