c# - Why is a loop of async HttpClient calls in Xamarin blocking untill all are finished? -

i've written loop in xamarin android load 10 images asynchronously using system.net.http.httpclient. initiating 10 requests executes instantly, i'm not getting any continuation before 10 responses completed - 5 7 seconds later. why not responses coming individually, asynchronously?

i don't know how threads handled in xamarin might doing wrong. should not call ui thread? there way can specify scheduler or threading policy httpclient?

the code load function:

// method called 10 times in foreach loop, initated ui thread, not awaited private async void loadasync(string uri) {     bitmap bitmap = await imgsvc.loadanddecodebitmap(uri);     image.setimagebitmap(bitmap); }  public async task<bitmap> loadanddecodebitmap(string uri) {      var client = new httpclient();     byte[] data = await client.getbytearrayasync(uri);     bitmap img = bitmapfactory.decodebytearray(data, 0, data.length);     return img; } 

link full solution

load log 10 images
load log if load 1

you should consider loading them @ same time using task.whenall. creating multiple instance of httpclient can have negative effects on performance.

first updating call avoid using async void.

articleteaserview.cs

public async task setmodel(articleteaser model) {     title.settext(model.promotioncontent.title.value, textview.buffertype.normal);     description.settext(model.promotioncontent.description.value, textview.buffertype.normal);     try {         var uri = android.net.uri.parse(model.promotioncontent.imageasset.urls[0].url);         log.debug(tag, "image " + (++ctr) + " load starting...");         await loadasync(model.promotioncontent.imageasset.geturlwithminheight(240), image);         log.debug(tag, "image " + ctr + " load completed");     } catch (exception ex) {         log.debug(tag, ex.message);     } }  static imagesvc imgsvc = new imagesvc(); //should consider injecting service  private async task<image> loadasync(string uri, imageview image) {     bitmap bitmap = await imgsvc.loadanddecodebitmap(uri);     image.setimagebitmap(bitmap); }  //use 1 shared instance of `httpclient` life of application private static httpclient client = new httpclient();  public async task<bitmap> loadanddecodebitmap(string uri) {             byte[] data = await client.getbytearrayasync(uri);     bitmap img = bitmapfactory.decodebytearray(data, 0, data.length);     return img; } 

finally assuming have collection of urls. create tasks , invoke of them @ same time.

mainactivity.cs

private event eventhandler loaddata = delegate { };  protected override void oncreate(bundle bundle) {     base.oncreate(bundle);      // set our view "main" layout resource     setcontentview (resource.layout.main);      initviews();     loaddata += ondataloading; // subscribe event     loaddata(this, eventargs.empty); // raise event }  //async void allowed on event handlers (actual event handler) //*oncreate* not event handler. based method. private async void ondataloading(object sender, eventargs e) {     loaddata -= ondataloading;     await loaddataasync(); }  private void initviews() {     //... }  private async task loaddataasync() {     var svc = new newsservice();     var promcontent = svc.syncloadstrong();     await bindview(promcontent); }  private async task bindview(articlelist list) {     log.debug(tag, "binding mainactivity");     viewgroup scroller = findviewbyid<viewgroup>(resource.id.scroller);             if (list != null) {         var tasks = new list<task>();         foreach (articleteaser teaser in list) {             var atv = new articleteaserview(this, null);             tasks.add(atv.setmodel(teaser));             scroller.addview(atv);         }         await task.whenall(tasks);     } } 

make sure not mixing blocking calls .result, .wait() async/await calls , avoid using async void unless event handler.