使用javascript扩展typescript库

Question

I'm working with Ionic 3 to build a simple map application, in order to present the map I'm using leaflet (typed version) with the OpenStreet map tiles

I want to give the possibility to the user to download and cache the map, thus I found leaflet-offline here and wanted to use it since it allows me to use my localstorage to store the images.

Now my problem arise with the fact that I'm trying to mix typescript with javascript and I can not figure out how to make it work correctly.

What I have done and what is the problem:

I installed leaflet typed version and then leaflet-offline javascript version.

Now I imported them as follows inside my page:

import * as leaflet from 'leaflet';
import 'leaflet-offline';

Now if I try to use the library as explained in the example I get a typescript error which indicates me that the property offline is not in the object TileLayer.

leaflet.tileLayer.offline('https://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png', tilesDb, {
    attribution: '&copy; <a href="http://www.openstreetmap.org/copyright">OpenStreetMap</a>',
    subdomains: 'abc',
    minZoom: 13,
    maxZoom: 19,
    crossOrigin: true
}).addTo(this.map);

In order to avoid this I added

export namespace tileLayer {
function wms(baseUrl: string, options?: WMSOptions, offline?: any): TileLayer.WMS;
function offline(url?: any, tilesDb?: any, options?: any): any;
}

the function offline is in the namespace of tileLayer itself.

Current problem At the moment when I try to run the application everything works fine except for the actual map, I can't see it and the error that I get when I look into it is the following:

ERROR TypeError: Cannot read property 'then' of null at NewClass.getTileUrl (vendor.js:163411) at NewClass.createTile (vendor.js:163389) at NewClass._addTile (vendor.js:79419) at NewClass._update (vendor.js:79310) at NewClass._setView (vendor.js:79171) at NewClass._resetView (vendor.js:79129) at NewClass.fire (vendor.js:68787) at NewClass._move (vendor.js:72343) at NewClass._onZoomTransitionEnd (vendor.js:72800) at NewClass._catchTransitionEnd (vendor.js:72732)

I'm sure I'm doing some stupid mistake, but I lost a lot of time on this issue.

Anyone have some clue on how to go solve this?

Answer 1

You can see in the source https://github.com/robertomlsoares/leaflet-offline/blob/master/src/TileLayer.Offline.js

that you fail at:

    var resultPromise = this._tilesDb.getItem(dbStorageKey).then(function (data) {

So the call to

_tilesDb.getItem(dbStorageKey)

is returning null, at which point the code tries to call 'then' on null.

Since the tilesDB is something that you are supposed to be providing yourself, you can put a breakpoint in your getItem function to see what is going on. At some point you are returning null from your getItem function.

What getItem is supposed to do:

// return Promise that has the image Blob/File/Stream.

The key supplied to getItem is the url you supplied with a regex substitution applied to it (see _getStorageKey in same source file)

Answer 2

Cannot read property 'then' of null

Typescript gives this error when then receives null from the function it's applied on. So check that you are not missing return statement if your function is custom.

Now instead of adding layer to map directly at the time of say initialization. for that try to replace your existing code with

const offlineLayer = leaflet.tileLayer.offline('https://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png', tilesDb, {
    attribution: '&copy; <a href="http://www.openstreetmap.org/copyright">OpenStreetMap</a>',
    subdomains: 'abc',
    minZoom: 13,
    maxZoom: 19,
    crossOrigin: true
});

offlineLayer.addTo(this.map)

check the reference of the map either this.map or whatever the refernce in your code.