调整输入图片大小以上传 [英] Resize image from input to upload

问题描述

我已经为这个问题苦苦挣扎了两个小时.我想从输入标签调整图像大小，然后将其上传到服务器.这是我的尝试.

I've been struggled with this problem for couple hours. I want to resize an image from an input tag and then upload it to the server. Here is my attempt.

我的输入元素:

<Input type="file" name="file" onChange={this.handleLoadAvatar} />

我的handleLoadAvatar函数:

handleLoadAvatar(e) {
var file = e.target.files[0];
var img = document.createElement("img");
var reader = new FileReader();
reader.onload = (e) => {
  img.src = e.target.result;
}
reader.readAsDataURL(file);

var canvas = document.createElement('canvas');
var ctx = canvas.getContext("2d");
ctx.drawImage(img, 0, 0);

var MAX_WIDTH = 300;
var MAX_HEIGHT = 300;
var width = img.width; // GET STUCK HERE
var height = img.height;

if (width > height) {
  if (width > MAX_WIDTH) {
    height *= MAX_WIDTH / width;
    width = MAX_WIDTH;
  }
} else {
  if (height > MAX_HEIGHT) {
    width *= MAX_HEIGHT / height;
    height = MAX_HEIGHT;
  }
}
canvas.width = width;
canvas.height = height;
var ctx = canvas.getContext("2d");
ctx.drawImage(img, 0, 0, width, height);
var dataurl = canvas.toDataURL("image/png");
this.setState({previewSrc: dataurl});

}

我正在为我的项目使用ReactJS.我在上面的评论中停留在行中，在该行中我无法获得图像的宽度.我在在上传之前对HTML5预先调整大小的图像进行了尝试，但这似乎不可行为我工作.有人可以指出我的代码有什么问题以及如何解决它吗?谢谢一堆！

I'm using ReactJS for my project. I got stuck at the line with the comment above where I can't get the image's width. I tried this solution at HTML5 Pre-resize images before uploading but this seems not to work for me. Can anybody point out what's wrong with my code and how to fix it? Thanks a bunch!

推荐答案

问题是您在访问图像width和height之前没有等待图像加载.

The problem is that you aren't waiting for the image to load before accessing its width and height.

在等待reader时，应该对img做同样的事情:

As you are waiting for the reader, you should do the same for the img:

handleLoadAvatar(e) {
  var file = e.target.files[0];
  var reader = new FileReader();
  reader.onload = (e) => {
    var img = document.createElement("img");
    img.onload = () => {
      var canvas = document.createElement('canvas');
      var ctx = canvas.getContext("2d");
      ctx.drawImage(img, 0, 0);

      var MAX_WIDTH = 300;
      var MAX_HEIGHT = 300;
      var width = img.width;
      var height = img.height;

      if (width > height) {
        if (width > MAX_WIDTH) {
          height *= MAX_WIDTH / width;
          width = MAX_WIDTH;
        }
      } else {
        if (height > MAX_HEIGHT) {
          width *= MAX_HEIGHT / height;
          height = MAX_HEIGHT;
        }
      }
      canvas.width = width;
      canvas.height = height;
      var ctx = canvas.getContext("2d");
      ctx.drawImage(img, 0, 0, width, height);
      var dataurl = canvas.toDataURL("image/png");
      this.setState({previewSrc: dataurl});
    }
    img.src = e.target.result;
  }
  reader.readAsDataURL(file);
}

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