三、初始化canvas
canvas繪制的圖片在hdpi顯示屏上會(huì)出現(xiàn)模糊，具體原因這里不作分析，可以參考下這里。我這里的做法是讓canvas的width與height為其css width/height的devicePixelRatio倍，以及調(diào)用canvas api時(shí)所傳入的參數(shù)都要乘以window.devicePixelRatio。最后還要記錄一下兩個(gè)canvas坐標(biāo)原點(diǎn)的x, y差值（originXDiff與originYDiff）。如下
_ratio(size) {
return parseInt(window.devicePixelRatio * size);
},
_initCanvas() {
let $canvas = this.$refs.canvas,
$pCanvas = this.$refs.pCanvas,
clipperClientRect = this.$refs.clipper.getBoundingClientRect(),
clipperWidth = parseInt(this.clipperImgWidth / window.devicePixelRatio),
clipperHeight = parseInt(this.clipperImgHeight / window.devicePixelRatio);
this.ctx = $canvas.getContext('2d');
this.pCtx = $pCanvas.getContext('2d');
//判斷clipperWidth與clipperHeight有沒有超過容器值
if (clipperWidth < 0 || clipperWidth > clipperClientRect.width) {
clipperWidth = 250
}
if (clipperHeight < 0 || clipperHeight > clipperClientRect.height) {
clipperHeight = 100
}
//因?yàn)閏anvas在手機(jī)上會(huì)被放大，因此里面的內(nèi)容會(huì)模糊，這里根據(jù)手機(jī)的devicePixelRatio來放大canvas，然后再通過設(shè)置css來收縮，因此關(guān)于canvas的所有值或坐標(biāo)都要乘以devicePixelRatio
$canvas.style.width = clipperClientRect.width + 'px';
$canvas.style.height = clipperClientRect.height + 'px';
$canvas.width = this._ratio(clipperClientRect.width);
$canvas.height = this._ratio(clipperClientRect.height);
$pCanvas.style.width = clipperWidth + 'px';
$pCanvas.style.height = clipperHeight + 'px';
$pCanvas.width = this._ratio(clipperWidth);
$pCanvas.height = this._ratio(clipperHeight);
//計(jì)算兩個(gè)canvas原點(diǎn)的x y差值
let cClientRect = $canvas.getBoundingClientRect(),
pClientRect = $pCanvas.getBoundingClientRect();
this.originXDiff = pClientRect.left - cClientRect.left;
this.originYDiff = pClientRect.top - cClientRect.top;
this.cWidth = cClientRect.width;
this.cHeight = cClientRect.height;
} 四、加載圖片
加載圖片比較簡單，首先是創(chuàng)建一個(gè)Image對(duì)象并監(jiān)聽器onload事件（因?yàn)榧虞d的圖片有可能是跨域的，因此要設(shè)置其crossOrigin屬性為Anonymous，然后服務(wù)器上要設(shè)置Access-Control-Allow-Origin響應(yīng)頭）。加載的圖片如果寬高大于容器的寬高，要對(duì)其進(jìn)行縮小處理。最后垂直水平居中顯示()（這里注意的是要保存圖片繪制前的寬高值，因?yàn)槿蘸罂s放圖片是以該值為基礎(chǔ)再乘以縮放倍率，這里取imgStartWidth，imgStartHeight）如下
_loadImg() {
if (this.imgLoading || this.loadImgQueue.length === 0) {
return;
}
let img = this.loadImgQueue.shift();
if (!img) {
return;
}
let $img = new Image(),
onLoad = e => {
$img.removeEventListener('load', onLoad, false);
this.$img = $img;
this.imgLoaded = true;
this.imgLoading = false;
this._initImg($img.width, $img.height);
this.$emit('loadSuccess', e);
this.$emit('loadComplete', e);
this._loadImg();
},
onError = e => {
$img.removeEventListener('error', onError, false);
this.$img = $img = null;
this.imgLoading = false;
this.$emit('loadError', e);
this.$emit('loadComplete', e);
this._loadImg();
};
this.$emit('beforeLoad');
this.imgLoading = true;
this.imgLoaded = false;
$img.src = this.img;
$img.crossOrigin = 'Anonymous'; //因?yàn)閏anvas toDataUrl不能操作未經(jīng)允許的跨域圖片，這需要服務(wù)器設(shè)置Access-Control-Allow-Origin頭
$img.addEventListener('load', onLoad, false);
$img.addEventListener('error', onError, false);
}
_initImg(w, h) {
let eW = null,
eH = null,
maxW = this.cWidth,
maxH = this.cHeight - this.actionBarHeight;
//如果圖片的寬高都少于容器的寬高，則不做處理
if (w <= maxW && h <= maxH) {
eW = w;
eH = h;
} else if (w > maxW && h <= maxH) {
eW = maxW;
eH = parseInt(h / w * maxW);
} else if (w <= maxW && h > maxH) {
eW = parseInt(w / h * maxH);
eH = maxH;
} else {
//判斷是橫圖還是豎圖
if (h > w) {
eW = parseInt(w / h * maxH);
eH = maxH;
} else {
eW = maxW;
eH = parseInt(h / w * maxW);
}
}
if (eW <= maxW && eH <= maxH) {
//記錄其初始化的寬高，日后的縮放功能以此值為基礎(chǔ)
this.imgStartWidth = eW;
this.imgStartHeight = eH;
this._drawImage((maxW - eW) / 2, (maxH - eH) / 2, eW, eH);
} else {
this._initImg(eW, eH);
}
} 五、繪制圖片
下面的_drawImage有四個(gè)參數(shù)，分別是圖片對(duì)應(yīng)cCanvas的x，y坐標(biāo)以及圖片目前的寬高w，h。函數(shù)首先會(huì)清空兩個(gè)canvas的內(nèi)容，方法是重新設(shè)置canvas的寬高。然后更新組件實(shí)例中對(duì)應(yīng)的值，最后再調(diào)用兩個(gè)canvas的drawImage去繪制圖片。對(duì)于pCanvas來說，其繪制的圖片坐標(biāo)值為x，y減去對(duì)應(yīng)的originXDiff與originYDiff（其實(shí)相當(dāng)于切換坐標(biāo)系顯示而已，因此只需要減去兩個(gè)坐標(biāo)系原點(diǎn)的x，y差值即可）。看看代碼
_drawImage(x, y, w, h) {
this._clearCanvas();
this.imgX = parseInt(x);
this.imgY = parseInt(y);
this.imgCurrentWidth = parseInt(w);
this.imgCurrentHeight = parseInt(h);
//更新canvas
this.ctx.drawImage(this.$img, this._ratio(x), this._ratio(y), this._ratio(w), this._ratio(h));
//更新pCanvas，只需要減去兩個(gè)canvas坐標(biāo)原點(diǎn)對(duì)應(yīng)的差值即可
this.pCtx.drawImage(this.$img, this._ratio(x - this.originXDiff), this._ratio(y - this.originYDiff), this._ratio(w), this._ratio(h));
},
_clearCanvas() {
let $canvas = this.$refs.canvas,
$pCanvas = this.$refs.pCanvas;
$canvas.width = $canvas.width;
$canvas.height = $canvas.height;
$pCanvas.width = $pCanvas.width;
$pCanvas.height = $pCanvas.height;
} 六、移動(dòng)圖片
移動(dòng)圖片實(shí)現(xiàn)非常簡單，首先給gesture-mask綁定touchstart，touchmove，touchend事件，下面分別介紹這三個(gè)事件的內(nèi)容
首先定義四個(gè)變量scx, scy（手指的起始坐標(biāo)），iX，iY（圖片目前的坐標(biāo)，相對(duì)于cCanvas）。
1、touchstart
方法很簡單，就是獲取touches[0]的pageX，pageY來更新scx與scy以及更新iX與iY
2、touchmove
獲取touches[0]的pageX，聲明變量f1x存放，移動(dòng)后的x坐標(biāo)等于iX + f1x - scx，y坐標(biāo)同理，最后調(diào)用_drawImage來更新圖片。
看看代碼吧
_initEvent() {
let $gesture = this.$refs.gesture,
scx = 0,
scy = 0;
let iX = this.imgX,
iY = this.imgY;
$gesture.addEventListener('touchstart', e => {
if (!this.imgLoaded) {
return;
}
let finger = e.touches[0];
scx = finger.pageX;
scy = finger.pageY;
iX = this.imgX;
iY = this.imgY;
}, false);
$gesture.addEventListener('touchmove', e => {
e.preventDefault();
if (!this.imgLoaded) {
return;
}
let f1x = e.touches[0].pageX,
f1y = e.touches[0].pageY;
this._drawImage(iX + f1x - scx, iY + f1y - scy, this.imgCurrentWidth, this.imgCurrentHeight);
}, false);
}七、縮放圖片（這里不作特別說明的坐標(biāo)都是相對(duì)于cCanvas坐標(biāo)系）
繪制縮放后的圖片無非需要4個(gè)參數(shù)，縮放后圖片左上角的坐標(biāo)以及寬高。求寬高相對(duì)好辦，寬高等于imgStartWidth * 縮放比率與imgstartHeight * 縮放倍率（imgStartWidth ，imgstartHeight 上文第四節(jié)有提到）。接下來就是求縮放倍率的問題了，首先在touchstart事件上求取兩手指間的距離d1；然后在touchmove事件上繼續(xù)求取兩手指間的距離d2，當(dāng)前縮放倍率= 初始縮放倍率 + (d2-d1) / 步長（例如每60px算0.1），touchend事件上讓初始縮放倍率=當(dāng)前縮放倍率。
至于如何求取縮放后圖片左上角的坐標(biāo)值，在草稿紙上畫來畫去，畫了很久......終于有點(diǎn)眉目。首先要找到一個(gè)縮放中心（這里做法是取雙指的中點(diǎn)坐標(biāo)，但是這個(gè)坐標(biāo)必須要位于圖片上，如果不在圖片上，則取圖片上離該中點(diǎn)坐標(biāo)最近的點(diǎn)），然后存在下面這個(gè)等式
（縮放中心x坐標(biāo) - 縮放后圖片左上角x坐標(biāo)）/ 縮放后圖片的寬度 = （縮放中心x坐標(biāo) - 縮放前圖片左上角x坐標(biāo)）/ 縮放前圖片的寬度；（y坐標(biāo)同理）
接下來看看下面這個(gè)例子（在visio找了很久都沒有畫坐標(biāo)系的功能，所以只能手工畫了）
綠色框是一張10*5的圖片，藍(lán)色框是寬高放大兩倍后的圖片20*10，根據(jù)上面的公式推算的x2 = sx - w2(sx - x1) / w1，y2 = sy - h3(sy - y1) / h2。
堅(jiān)持...繼續(xù)看看代碼吧
_initEvent() {
let $gesture = this.$refs.gesture,
cClientRect = this.$refs.canvas.getBoundingClientRect(),
scx = 0, //對(duì)于單手操作是移動(dòng)的起點(diǎn)坐標(biāo)，對(duì)于縮放是圖片距離兩手指的中點(diǎn)最近的圖標(biāo)。
scy = 0,
fingers = {}; //記錄當(dāng)前有多少只手指在觸控屏幕
//one finger
let iX = this.imgX,
iY = this.imgY;
//two finger
let figuredistance = 0,
pinchScale = this.imgScale;
$gesture.addEventListener('touchstart', e => {
if (!this.imgLoaded) {
return;
}
if (e.touches.length === 1) {
let finger = e.touches[0];
scx = finger.pageX;
scy = finger.pageY;
iX = this.imgX;
iY = this.imgY;
fingers[finger.identifier] = finger;
} else if (e.touches.length === 2) {
let finger1 = e.touches[0],
finger2 = e.touches[1],
f1x = finger1.pageX - cClientRect.left,
f1y = finger1.pageY - cClientRect.top,
f2x = finger2.pageX - cClientRect.left,
f2y = finger2.pageY - cClientRect.top;
scx = parseInt((f1x + f2x) / 2);
scy = parseInt((f1y + f2y) / 2);
figureDistance = this._pointDistance(f1x, f1y, f2x, f2y);
fingers[finger1.identifier] = finger1;
fingers[finger2.identifier] = finger2;
//判斷變換中點(diǎn)是否在圖片中，如果不是則去離圖片最近的點(diǎn)
if (scx < this.imgX) {
scx = this.imgX;
}
if (scx > this.imgX + this.imgCurrentWidth) {
scx = this.imgX + this.imgCurrentHeight;
}
if (scy < this.imgY) {
scy = this.imgY;
}
if (scy > this.imgY + this.imgCurrentHeight) {
scy = this.imgY + this.imgCurrentHeight;
}
}
}, false);
$gesture.addEventListener('touchmove', e => {
e.preventDefault();
if (!this.imgLoaded) {
return;
}
this.maskShowTimer && clearTimeout(this.maskShowTimer);
this.maskShow = false;
if (e.touches.length === 1) {
let f1x = e.touches[0].pageX,
f1y = e.touches[0].pageY;
this._drawImage(iX + f1x - scx, iY + f1y - scy, this.imgCurrentWidth, this.imgCurrentHeight);
} else if (e.touches.length === 2) {
let finger1 = e.touches[0],
finger2 = e.touches[1],
f1x = finger1.pageX - cClientRect.left,
f1y = finger1.pageY - cClientRect.top,
f2x = finger2.pageX - cClientRect.left,
f2y = finger2.pageY - cClientRect.top,
newFigureDistance = this._pointDistance(f1x, f1y, f2x, f2y),
scale = this.imgScale + parseFloat(((newFigureDistance - figureDistance) / this.imgScaleStep).toFixed(1));
fingers[finger1.identifier] = finger1;
fingers[finger2.identifier] = finger2;
if (scale !== pinchScale) {
//目前縮放的最小比例是1，最大是5
if (scale < this.imgMinScale) {
scale = this.imgMinScale;
} else if (scale > this.imgMaxScale) {
scale = this.imgMaxScale;
}
pinchScale = scale;
this._scale(scx, scy, scale);
}
}
}, false);
$gesture.addEventListener('touchend', e => {
if (!this.imgLoaded) {
return;
}
this.imgScale = pinchScale;
//從finger刪除已經(jīng)離開的手指
let touches = Array.prototype.slice.call(e.changedTouches, 0);
touches.forEach(item => {
delete fingers[item.identifier];
});
//迭代fingers，如果存在finger則更新scx,scy,iX,iY，因?yàn)榭赡芸s放后立即單指拖動(dòng)
let i,
fingerArr = [];
for(i in fingers) {
if (fingers.hasOwnProperty(i)) {
fingerArr.push(fingers[i]);
}
}
if (fingerArr.length > 0) {
scx = fingerArr[0].pageX;
scy = fingerArr[0].pageY;
iX = this.imgX;
iY = this.imgY;
} else {
this.maskShowTimer = setTimeout(() => {
this.maskShow = true;
}, 300);
}
//做邊界值檢測(cè)
let x = this.imgX,
y = this.imgY,
pClientRect = this.$refs.pCanvas.getBoundingClientRect();
if (x > pClientRect.left + pClientRect.width) {
x = pClientRect.left
} else if (x + this.imgCurrentWidth < pClientRect.left) {
x = pClientRect.left + pClientRect.width - this.imgCurrentWidth;
}
if (y > pClientRect.top + pClientRect.height) {
y = pClientRect.top;
} else if (y + this.imgCurrentHeight < pClientRect.top) {
y = pClientRect.top + pClientRect.height - this.imgCurrentHeight;
}
if (this.imgX !== x || this.imgY !== y) {
this._drawImage(x, y, this.imgCurrentWidth, this.imgCurrentHeight);
}
});
},
_scale(x, y, scale) {
let newPicWidth = parseInt(this.imgStartWidth * scale),
newPicHeight = parseInt(this.imgStartHeight * scale),
newIX = parseInt(x - newPicWidth * (x - this.imgX) / this.imgCurrentWidth),
newIY = parseInt(y - newPicHeight * (y - this.imgY) / this.imgCurrentHeight);
this._drawImage(newIX, newIY, newPicWidth, newPicHeight);
},
_pointDistance(x1, y1, x2, y2) {
return parseInt(Math.sqrt((x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2)));
}說明一下fingers是干嘛的，是用來記錄當(dāng)前有多少只手指在屏幕上觸摸?？赡軙?huì)出現(xiàn)這種情況，雙指縮放后，其中一只手指移出顯示屏，而另外一個(gè)手指在顯示屏上移動(dòng)。針對(duì)這種情況，要在touchend事件上根據(jù)e.changedTouches來移除fingers里已經(jīng)離開顯示屏的finger，如果此時(shí)fingers里只剩下一個(gè)finger，則更新scx，scy，iX，iY為移動(dòng)圖片做初始化準(zhǔn)備。
八、裁剪圖片
這里很簡單，就調(diào)用pCanvas的toDataURL方法就可以了
_clipper() {
let imgData = null;
try {
imgData = this.$refs.pCanvas.toDataURL();
} catch (e) {
console.error('請(qǐng)?jiān)趓esponse header加上Access-Control-Allow-Origin，否則canvas無法裁剪未經(jīng)許可的跨域圖片');
}
this.$emit('sure', imgData);
}看完了這篇文章，相信你對(duì)“基于Vue如何實(shí)現(xiàn)移動(dòng)端圖片裁剪組件功能”有了一定的了解，如果想了解更多相關(guān)知識(shí)，歡迎關(guān)注創(chuàng)新互聯(lián)行業(yè)資訊頻道，感謝各位的閱讀！
名稱欄目：基于Vue如何實(shí)現(xiàn)移動(dòng)端圖片裁剪組件功能
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