React: Canvas drawImage is not updating the canvas

Question

I am using react konva to draw images on a canvas. I have two different Layers (canvas in konva) inside a Stage Element. One picture is predetermined and one gets updated by an input at runtime. Both Pictures get shown fine, but when I try to save them, the one coming from the input is just a black png and not the picture I selected.

Javascript:

imageUpload when input is clicked.

  const handleImageUpload = e => {
    setSelected(false);
    let canvas = backgroundCanvas.current;
    let ctx = canvas.getContext('2d');
    newImage.onload = function() {
      newImage.crossOrigin = 'Anonymus';
      ctx.drawImage(newImage, 0, 0);
    };

    const [file] = e.target.files;
    if (file) {
      const reader = new FileReader();
      reader.readAsDataURL(file);
      reader.onload = e => {
        ctx.clearRect(0, 0, canvas.canvas.width, canvas.canvas.height);
        newImage.src = e.target.result;
      };
    }
  };

It shows the image on the screen like i want to and looks like it works, however, if I want to save the canvas as a png file, it is not there anymore.

Save function:

  const saveImage = () => {
    let backgroundCanvasSave = backgroundCanvas.current;
    const backgroundCanvasData = backgroundCanvasSave.toDataURL({
      mimeType: 'image/png',
    });
    downloadURI(backgroundCanvasData, 'stage.png');
  };

JSX:

<div>
      <Stage
        ref={canvasStage}
        width={window.innerWidth}
        height={window.innerHeight}
      >
        <Layer
          ref={backgroundCanvas}
          onClick={() => {
            setSelected(false);
          }}
          onTap={() => {
            setSelected(false);
          }}
        />
        <Layer ref={lampCanvas}>
          <Lamp
            shapeProps={shape}
            isSelected={selected}
            onSelect={() => {
              setSelected(true);
            }}
            onChange={setShape}
          />
        </Layer>
      </Stage>
      <label
        for="files"
        class="btn"
        style={{
          border: '1px solid',
          display: 'inline block',
          padding: '6px 12px',
          cursor: 'pointer',
        }}
      >
        Bild hochladen
        <input
          id="files"
          visibility="hidden"
          type="file"
          accept="image/*"
          style={{ display: 'none' }}
          onChange={handleImageUpload}
          ref={imageUploader}
        />
      </label>
      <label
        class="btn"
        style={{
          border: '1px solid',
          display: 'inline block',
          padding: '6px 12px',
          cursor: 'pointer',
        }}
        onClick={saveImage}
      >
        Bild speichern
      </label>
    </div>
  );

The goal of the application would be to save the Stage with both pictures inside it, but as I said, the background is always black, while the lamp is showing.

Thanks in advance!

Answer 1

Do not use the context of a layer directly. Konva may lose your changes while exporting the canvas.

To fix the issue just draw your input as <Image /> component:

const newImage = new Image();
newImage.onload = () => {
  this.setState({ image: newImage })
};
newImage.crossOrigin = 'Anonymus';
newImage.src = e.target.result;


// in render:
<Layer
  ref={backgroundCanvas}
  onClick={() => {
    setSelected(false);
  }}
  onTap={() => {
    setSelected(false);
  }}
>
  <Image image={this.state.image} />
</Layer>