meteor/packages/launch-screen/default-behavior.js

// Hold launch screen on app load. This reflects the fact that Meteor
// mobile apps that use this package always start with a launch screen
// visible. (see XXX comment at the top of package.js for more
// details)
var handle = LaunchScreen.hold();

var Template = Package.templating && Package.templating.Template;

Meteor.startup(function () {
  if (! Template) {
    handle.release();
  } else if (Package['iron:router']) {
    // XXX Instead of doing this here, this code should be in
    // iron:router directly. Note that since we're in a
    // `Meteor.startup` block it's ok that we don't have a
    // weak dependency on iron:router in package.js.
    Package['iron:router'].Router.onAfterAction(function () {
      handle.release();
    });
  } else {
    // We intentionally don't use `Template.body.rendered = ...` here
    // since other packages, or your app, may set the same
    // callback. What we should really have is the ability to set
    // multiple rendered callbacks (eg
    // https://github.com/meteor/meteor/issues/2805), while ensuring
    // that if one adds a callback after rendered already fired, the
    // callback is called immediately.
    //
    // So, instead we poll every 50ms to detect whether
    // `Template.body` has already been rendered.
    var checkBody = setInterval(function () {
      if (Template.body.view && Template.body.view.isRendered) {
        handle.release();
        clearInterval(checkBody);
      }
    }, 50);
  }
});