javascript - 如何让用户使用纯 Javascript 将剪贴板中的图像数据粘贴到 Firefox 中的 Canvas 元素中？

Question

我已尽最大努力找到适用于最新版本的 Firefox 的简单、相关且最新的示例，但我真的很努力。

标题真的说明了一切。我希望用户能够从 Windows Paint 等编辑器复制图像的一部分或使用“打印屏幕”按钮，然后将其粘贴到 Canvas 元素中。如果 Canvas 调整大小以完全适合粘贴的图像(字面意思)，则加分。

如果合理，希望避免使用基于 Flash 或 Java 的解决方案。

我在 Javascript 方面还算不错，但对最新的 HTML5 功能相对缺乏经验，而且对 Canvas 元素完全陌生。请帮忙!

Answer 1

2.0 版:更小、更简洁的代码适用于 Chrome、Firefox、Edge、Opera。没有更多的黑客。但如果您需要支持 IE 和 Safari，请检查 v1 版本。

http://jsfiddle.net/viliusl/xq2aLj4b/5/

---

版本 1.0Chrome 实现很简单。 Firefox(和 IE)有一些限制，用户必须发出命令才能像键盘事件一样进行粘贴，并且可编辑的输入必须被聚焦，所以我们在这里做了一些技巧 - 在按下 ctrl 时我们聚焦那个输入字段，在释放时取消聚焦。

浏览器支持(图像数据):

• 火狐
• Chrome
• 边缘
• IE-11
• 歌剧

var CLIPBOARD = new CLIPBOARD_CLASS("my_canvas", true);

/**
 * image pasting into canvas
 * 
 * @param {string} canvas_id - canvas id
 * @param {boolean} autoresize - if canvas will be resized
 */
function CLIPBOARD_CLASS(canvas_id, autoresize) {
	var _self = this;
	var canvas = document.getElementById(canvas_id);
	var ctx = document.getElementById(canvas_id).getContext("2d");
	var ctrl_pressed = false;
	var command_pressed = false;
	var paste_event_support;
	var pasteCatcher;

	//handlers
	document.addEventListener('keydown', function (e) {
		_self.on_keyboard_action(e);
	}, false); //firefox fix
	document.addEventListener('keyup', function (e) {
		_self.on_keyboardup_action(e);
	}, false); //firefox fix
	document.addEventListener('paste', function (e) {
		_self.paste_auto(e);
	}, false); //official paste handler

	//constructor - we ignore security checks here
	this.init = function () {
		pasteCatcher = document.createElement("div");
		pasteCatcher.setAttribute("id", "paste_ff");
		pasteCatcher.setAttribute("contenteditable", "");
		pasteCatcher.style.cssText = 'opacity:0;position:fixed;top:0px;left:0px;width:10px;margin-left:-20px;';
		document.body.appendChild(pasteCatcher);

		// create an observer instance
		var observer = new MutationObserver(function(mutations) {
			mutations.forEach(function(mutation) {
				if (paste_event_support === true || ctrl_pressed == false || mutation.type != 'childList'){
					//we already got data in paste_auto()
					return true;
				}

				//if paste handle failed - capture pasted object manually
				if(mutation.addedNodes.length == 1) {
					if (mutation.addedNodes[0].src != undefined) {
						//image
						_self.paste_createImage(mutation.addedNodes[0].src);
					}
					//register cleanup after some time.
					setTimeout(function () {
						pasteCatcher.innerHTML = '';
					}, 20);
				}
			});
		});
		var target = document.getElementById('paste_ff');
		var config = { attributes: true, childList: true, characterData: true };
		observer.observe(target, config);
	}();
	//default paste action
	this.paste_auto = function (e) {
		paste_event_support = false;
		if(pasteCatcher != undefined){
			pasteCatcher.innerHTML = '';
		}
		if (e.clipboardData) {
			var items = e.clipboardData.items;
			if (items) {
				paste_event_support = true;
				//access data directly
				for (var i = 0; i < items.length; i++) {
					if (items[i].type.indexOf("image") !== -1) {
						//image
						var blob = items[i].getAsFile();
						var URLObj = window.URL || window.webkitURL;
						var source = URLObj.createObjectURL(blob);
						this.paste_createImage(source);
					}
				}
				e.preventDefault();
			}
			else {
				//wait for DOMSubtreeModified event
				//https://bugzilla.mozilla.org/show_bug.cgi?id=891247
			}
		}
	};
	//on keyboard press
	this.on_keyboard_action = function (event) {
		k = event.keyCode;
		//ctrl
		if (k == 17 || event.metaKey || event.ctrlKey) {
			if (ctrl_pressed == false)
				ctrl_pressed = true;
		}
		//v
		if (k == 86) {
			if (document.activeElement != undefined && document.activeElement.type == 'text') {
				//let user paste into some input
				return false;
			}

			if (ctrl_pressed == true && pasteCatcher != undefined){
				pasteCatcher.focus();
			}
		}
	};
	//on kaybord release
	this.on_keyboardup_action = function (event) {
		//ctrl
		if (event.ctrlKey == false && ctrl_pressed == true) {
			ctrl_pressed = false;
		}
		//command
		else if(event.metaKey == false && command_pressed == true){
			command_pressed = false;
			ctrl_pressed = false;
		}
	};
	//draw pasted image to canvas
	this.paste_createImage = function (source) {
		var pastedImage = new Image();
		pastedImage.onload = function () {
			if(autoresize == true){
				//resize
				canvas.width = pastedImage.width;
				canvas.height = pastedImage.height;
			}
			else{
				//clear canvas
				ctx.clearRect(0, 0, canvas.width, canvas.height);
			}
			ctx.drawImage(pastedImage, 0, 0);
		};
		pastedImage.src = source;
	};
}

1. Copy image data into clipboard or press Print Screen <br>
2. Press Ctrl+V (page/iframe must be focused):
<br /><br />
<canvas style="border:1px solid grey;" id="my_canvas" width="300" height="300"></canvas>