JavaScript代码在回调中无效

Question

In the following JavaScript code, if warnBeforeNew is false, the file open code works. However, it doesn't if warnBeforeNew is true, instead giving an error "Uncaught TypeError: Cannot read property 'root' of undefined".

I don't know if this is to do with scoping, but how do I get the file loading code to work within the callback? Thanks.

Editor.prototype.open = function(path) {
  if (Editor.warnBeforeNew==true){
    this.showDialog({
        dialogLabel: 'You have unsaved changes. Are you sure you want to discard them and open a different file?',
        submitLabel: 'Discard',
        cancelLabel: 'Cancel',
        submitCallback: function() {
          Editor.warnBeforeNew=false;
          this.filesystem.root.getFile(path, {}, this.load.bind(this), error.bind(null, "getFile " + path));
        }
    });
  } else {
    this.filesystem.root.getFile(path, {}, this.load.bind(this), error.bind(null, "getFile " + path));
  }
};

Answer 1

You have to save the value of this because when the callback is called, it's with a different receiver than the external function :

if (Editor.warnBeforeNew==true){
    var thing = this; // choose a more meaningful name if possible...
    this.showDialog({
        dialogLabel: 'You have unsaved changes. Are you sure you want to discard them and open a different file?',
        submitLabel: 'Discard',
        cancelLabel: 'Cancel',
        submitCallback: function() {
          Editor.warnBeforeNew=false;
          thing.filesystem.root.getFile(path, {}, thing.load.bind(thing), error.bind(null, "getFile " + path));
        }
    });
  } else {
    this.filesystem.root.getFile(path, {}, this.load.bind(this), error.bind(null, "getFile " + path));
  }

Answer 2

Try to catch the scope out side the callback and use it.

Editor.prototype.open = function(path) {
var that=this;
  if (Editor.warnBeforeNew==true){
    this.showDialog({
        dialogLabel: 'You have unsaved changes. Are you sure you want to discard them and open a different file?',
        submitLabel: 'Discard',
        cancelLabel: 'Cancel',
        submitCallback: function() {
          Editor.warnBeforeNew=false;
          that.filesystem.root.getFile(path, {}, that.load.bind(that), error.bind(null, "getFile " + path));
        }
    });
  } else {
    this.filesystem.root.getFile(path, {}, this.load.bind(this), error.bind(null, "getFile " + path));
  }
};

Answer 3

The submitCallback to your showDialog method needs to be bound, too - it accesses this.filesystem.root.… which fails.

this.showDialog({
    …,
    submitCallback: function() {
      Editor.warnBeforeNew=false;
      this.filesystem.root.getFile(path, {}, this.load.bind(this), error.bind(null, "getFile " + path));
    }.bind(this)
//    ^^^^^^^^^^
});