Leaflet 等待 promise 解决

Question

I am extending Leaflet getTileUrl for some sort of caching tiles. I use Vue but it shouldn't affect anything. It currently looks like this:

   //step 1 
   L.TileLayer.MyLayer = L.TileLayer.extend({
        getTileUrl: function(coords) {
            
            var url = this.getLayerUrl(coords.z, coords.x, coords.y);

            return url;

        }.bind(this)
    });

    //step 2
    L.tileLayer.myLayer =  function() {
        return new L.TileLayer.MyLayer();
    }

    //step 3
    L.tileLayer.myLayer().addTo(this.getParent('main-map').map);

The problem is that the getLayerUrl function returns a promise. Even when I tried to make getTileUrl async and than await for this.getLayerUrl (and also make async await step 2 and 3) or put .then(function(result) {return result;} after this.getLayerUrl, Leaflet show error in browser console that it is trying to get tile from url: GET http://localhost/project/public/[object%20Promise]

Also I should mention that this.getLayerUrl returns different url for every tile, it is actualy a blob url like: blob:http://localhost/f7c4298f-9e9d-423f-9d0b-3f7a301e433f but if the url is correctly returned leaflet doest have problem with it and the tile is correctly shown.

Answer 1

The approach here would be not to provide a getLayerUrl method, but to replace the implementation of the createTile element.

A "noop" replacement that replaces the createTile implementation with the original createTile implementation looks like:

L.TileLayer.MyLayer = L.TileLayer.extend({
  getTileUrl: function (coords) {
    var tile = document.createElement("img");

    DomEvent.on(tile, "load", Util.bind(this._tileOnLoad, this, done, tile));
    DomEvent.on(tile, "error", Util.bind(this._tileOnError, this, done, tile));

    if (this.options.crossOrigin || this.options.crossOrigin === "") {
      tile.crossOrigin =
        this.options.crossOrigin === true ? "" : this.options.crossOrigin;
    }

    tile.alt = "";
    tile.setAttribute("role", "presentation");

    tile.src = this.getTileUrl(coords);

    return tile;
  }
});

With that implementation in mind, it's possible to set the src attribute of the HTMLImageElement in a asynchronous manner replacing the synchronous tile.src = this.getTileUrl(coords); with something like eg:

asyncGetLayerUrl(coords)
  .then(function (url) {
    tile.src = url;
  })

And, for good measure, handle promise rejections by calling this._tileOnError , eg:

asyncGetLayerUrl(coords)
  .then(function (url) {
    tile.src = url;
  })
  .catch(
    function (err) {
      this._tileOnError(done, tile, err);
    }.bind(this)
  );

Answer 2

Unfortunately what you are trying to achieve is not possible. The getTileUrl() method is expected to return the url directly, ie not wrapped in a promise. However, you get the url from getLayerUrl that returns a promise. You simply can't wait for the result of the promise synchronously inside getTileUrl() .

The await keyword suggests that this is possible but it isn't. await is just syntactic sugar, a shorthand for chaining promises. I give you a simple example.

function getUrl() {
    return getAsyncUrl().then(url => {
         return transformUrl(url);
    });
}

async / await allows you to write it like this:

async function getUrl() {
    const url = await getAsyncUrl();
    return transformUrl(url);
}

But behind the scenes it still uses promises. An async function always returns a promise.

Answer 3

Hope this helps, information based on this article

L.TileLayer.YourLayer = L.TileLayer.extend({
 
  createTile: function (coords, done) {
    var tile = document.createElement('img');
    
    hereMakeSomeRemoteRequest().then(response => {
      tile.src = response.imageSrc
      done(null, tile)
    }, function (e) {
        console.log('error: ', e.message);
        tile.src = "data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7"; // empty gif 1x1 px
        done(e, tile)
    });
    return tile;
  }
});