GWT read mime type client side - asynchronous

Question

I'm trying to read the mime type in GWT client side in order to validate a file before upload it. To do this I use JSNI to read the file header using HTML5 filereader API. However my problem is that GWT does not wait for the result of the reading and continue the code execution. The side effect is that my boolean is not set yet and my condition goes wrong. Is there any mechanism like promise implemented in GWT?
Any help on this would be much appreciated!
UploadImageButtonWidget.java
private boolean isMimeTypeValid = false;
private String mimeType = null;
public native boolean isValid(Element element)/*-{
var widget = this;
var files = element.files;
var reader = new FileReader();
var CountdownLatch = function (limit){
this.limit = limit;
this.count = 0;
this.waitBlock = function (){};
};
CountdownLatch.prototype.countDown = function (){
this.count = this.count + 1;
if(this.limit <= this.count){
return this.waitBlock();
}
};
CountdownLatch.prototype.await = function(callback){
this.waitBlock = callback;
};
var barrier = new CountdownLatch(1);
reader.readAsArrayBuffer(files[0]);
reader.onloadend = function(e) {
var arr = (new Uint8Array(e.target.result)).subarray(0, 4);
var header = "";
for (var i = 0; i < arr.length; i++) {
header += arr[i].toString(16);
}
widget.#com.portal.client.widgets.base.UploadImageButtonWidget::setMimeType(Ljava/lang/String;)(header);
barrier.countDown();
}
return barrier.await(function(){
return widget.#com.portal.client.widgets.base.UploadImageButtonWidget::isMimeTypeValid();
});
}-*/
public void setMimeType(String headerString) {
boolean mimeValid = true;
if (headerString.equalsIgnoreCase(PNG_HEADER)) {
mimeType = PNG_MIMETYPE;
} else if (headerString.equalsIgnoreCase(GIF_HEADER)) {
mimeType = GIF_MIMETYPE;
} else if (headerString.equalsIgnoreCase(JPG_HEADER1) || headerString.equalsIgnoreCase(JPG_HEADER2) || headerString.equalsIgnoreCase(JPG_HEADER3)) {
mimeType = JPG_MIMETYPE;
} else {
mimeValid = false;
setValidationError(i18n.uploadErrorNotImageBasedOnMimeType());
fileChooser.getElement().setPropertyJSO("files", null);
setErrorStatus();
}
setMimeTypeValid(mimeValid);
}
public boolean isMimeTypeValid() {
GWT.log("mimeType" + mimeType);
GWT.log("isMimetypeValid" + String.valueOf(isMimeTypeValid));
return mimeType != null;
}
in the activity:
public void validateAndUpload() {
UploadImageButtonWidget uploadImageButtonWidget = view.getUpload();
if (uploadImageButtonWidget.isValid()) {
GWT.log("mime ok: will be uploaded");
uploadImage();
} else {
GWT.log("mime not ok: will not be uploaded");
}
}