Angular2 HTTP Providers, get a string from JSON for Amcharts

问题

This is a slightly messy questions. Although it appears I'm asking question about amCharts, I really just trying to figure how to extract an array from HTTP request and then turn it into a variable and place it in to 3-party javacript.

It all starts here, with this question, which was kindly answered by AmCharts support.

As one can see from the plnker. The chart is working. Data for the chart is hard coded:

`var chartData = [{date: new Date(2015,2,31,0,0,0,       0),value:372.10,volume:2506100},{date: new Date(2015,3,1,0, 0, 0, 0),value:370.26,volume:2458100},{date: new Date(2015,3,2,0, 0, 0, 0),value:372.25,volume:1875300},{date: new Date(2015,3,6,0, 0, 0, 0),value:377.04,volume:3050700}];`

So we know the amCharts part works. Know where the problem is changing hard coded data to a json request so it can be dynamic. I don't think this should be tremendously difficult, but for the life of me I can't seem figure it out.

The first issue is I can't find any documentation on .map, .subscribe, or .observable.

So here is a plunker that looks very similar to the first one, however it has an http providers and injectable. It's broken, because I can't figure out how to pull the data from the service an place it into the AmCharts function. I know how pull data from a http provider and display it in template using NgFor, but I don't need it in the template (view). As you can see, I'm successful in transferring the data from the service, with the getTitle() function.

    this.chart_data =_dataService.getEntries();
    console.log('Does this work? '+this.chart_data);

    this.title = _dataService.getTitle();
    console.log('This works '+this.title);

    // Transfer the http request to chartData to it can go into Amcharts
    // I think this should be string?
    var chartData = this.chart_data;

So the ultimate question is why can't I use a service to get data, turn that data into a variable and place it into a chart. I suspect a few clues might be in options.json as the json might not be formatted correctly? Am I declaring the correct variables? Finally, it might have something to do with observable / map?

回答1:

You have a few things here. First this is a class, keep it that way. By that I mean to move the functions you have inside your constructor out of it and make them methods of your class.

Second, you have this piece of code

this.chart_data =_dataService.getEntries().subscribe((data) => {
    this.chart_data = data; 
});

What happens inside subscribe runs asynchronously therefore this.chart_data won't exist out of it. What you're doing here is assigning the object itself, in this case what subscribe returns, not the http response. So you can simply put your library initialization inside of the subscribe and that'll work.

_dataService.getEntries().subscribe((data) => {

      if (AmCharts.isReady) {
        this.createStockChart(data);
      } else {
        AmCharts.ready(() => this.createStockChart(data));
      }
});

Now, finally you have an interesting thing. In your JSON you have your date properties contain a string with new Date inside, that's nothing but a string and your library requires (for what I tested) a Date object, so you need to parse it. The problem here is that you can't parse nor stringify by default a Date object. We need to convert that string to a Date object.

Look at this snippet code, I used eval (PLEASE DON'T DO IT YOURSELF, IS JUST FOR SHOWING PURPOSES!)

let chartData = [];
   for(let i = 0; i < data[0].chart_data.length; i++) {
       chartData.push({
          // FOR SHOWING PURPOSES ONLY, DON'T TRY IT AT HOME
          // This will parse the string to an actual Date object
          date : eval(data[0].chart_data[i].date);
          value : data[0].chart_data[i].value;
          volume : data[0].chart_data[i].volume;
   });
 }

Here what I'm doing is reconstructing the array so the values are as required.

For the latter case you'll have to construct your json using (new Date('YOUR DATE')).toJSON() and you can parse it to a Date object using new Date(yourJSON) (referece Date.prototype.toJSON() - MDN). This is something you should resolve in your server side. Assuming you already solved that, your code should look as follows

// The date property in your json file should be stringified using new Date(...).toJSON()
date : new Date(data[0].chart_data[i].date);

Here's a plnkr with the evil eval. Remember, you have to send the date as a JSON from the server to your client and in your client you have to parse it to a Date.

I hope this helps you a little bit.

回答2:

If the getEntries method of DataService returns an observable, you need to subscribe on it to get data:

_dataService.getEntries().subscribe(
  (data) => {
    this.chart_data = data;
  });

Don't forget that data are received asynchronously from an HTTP call. The http.get method returns an observable (something "similar" to promise) will receive the data in the future. But when the getEntries method returns the data aren't there yet...

The getTitle is a synchronous method so you can call it the way you did.

来源：https://stackoverflow.com/questions/35658236/angular2-http-providers-get-a-string-from-json-for-amcharts