使用Canvas和ForEach处理异步呼叫

Question

I am trying to understand how I can utilize angular's $q library to display images which are based on a canvas drawing and then converted using .toDataURL() ;

Basically I want to:

1. Loop over a images ( $scope.targetImages )
2. Draw them on a canvas
3. Convert them to images using .toDataURL() and store it in ( $scope.outputImages );
4. Display the images using ng-repeat

The problem is, that the function .toDataURL() can take some time before executed, thus resulting in a delayed call of step 4, and thus nothing being displayed.

I have tried the following, but it still resolves before all the images are converted.

As I have it now, when I call drawCanvas () for the second time, then the images are shown.

    // 1
    $scope.targetImages= {}

    drawCanvas().then(function(data){
      console.log("done: " + new Date())
      console.log(data)
      $scope.outputImages = data;
      $scope.previewMode = false; // switch views, display canvas, remove preview
    });


    function drawCanvas() {

    var defer = $q.defer();

    var targetImages = {} 
    angular.forEach($scope.targetImages , function(imageObj, key) {

      var canvas = document.getElementById("canvas");
      var ctx = canvas.getContext("2d");
      var img = new Image();
      img.src = imageObj.nativeURL;
      img.onload = start

      // 2
      function start() {

        ctx.drawImage(img, 0, 0, img.width, img.height);

        outputImages[key] = {
          IID: key,
          dataURL: canvas.toDataURL()
        }

      } // start

    }); // for loop target images

    defer.resolve(outputImages);
    return defer.promise;

    } // draw canvas

And displayed as:

<img ng-show="!previewMode" ng-src="{{image.dataURL || ''}}" style="width: 100vw; max-width: 600px;">

Answer 1

First, define a function that draws an image to the canvas and returns a promise for the result:

function drawToCanvas(nativeURL) {
    return $q(function (resolve) {
        var canvas = document.getElementById("canvas");
        var ctx = canvas.getContext("2d");
        var img = new Image();

        img.src = nativeURL;
        img.onload = function () {
            ctx.drawImage(img, 0, 0, img.width, img.height);

            resolve(canvas.toDataURL());
        };
    });
}

The solution then becomes:

$scope.targetImages = [];

function drawCanvas() {
    // pResults is an array of promises
    var pResults = $scope.targetImages.map(function (imageObj) {
        return drawToCanvas(imageObj.nativeURL);
    });

    return $q.all(pResults);
}

drawCanvas().then(function(data) {
    // data is an array of imageUrls
    console.log("done: " + new Date())
    console.log(data)
    $scope.outputImages = data;
    $scope.previewMode = false; 
    // switch views, display canvas, remove preview
});

To simplify, I have changed $scope.targetImages and $scope.outputImages to be arrays instead of objects, but it shouldn't be too hard to go back to using objects for them if that's what you need.