Why won't my MithrilJS UI render unless I call redraw after a programmatic redirect? - mithril.js

Question

Here is a snippet of view code. Why won't it work without the m.redraw()? If I don't call that, the route changes and the login controller loads, but nothing is rendered into the DOM.
home.view = function(ctrl) {
console.log('in home view');
if (!mo_portal.logged_in) {
console.log('redirecting to login');
m.route("/login");
m.redraw();
return;
}
return m("div","HOME");
}

Answer 1

Changing route will always trigger a redraw. If you're not seeing the login page view without manually calling m.redraw it's probably down to bugs in the login controller or view that happen during the route change redraw - bugs whose failure conditions are reset when you call m.redraw again.
Here's an extension of your code with a login view and controller. mo_portal.logged_in is set to true or false depending on whether the user is one in the usersList or not, so we can test success and failure.
I took out the m.redraw (I also put the redirect logic in the home controller) and everything works fine.
var usersList = [
'john',
'alexia'
];
var mo_portal = {
username : '',
logged_in: false
};
var login = {};
login.controller = function(){
this.username = function( input ){
if( arguments.length ){
mo_portal.username = input;
mo_portal.logged_in = !!~usersList.indexOf( input );
}
return mo_portal.username;
};
};
login.view = function(ctrl){
console.log('in login view');
return [
m( 'input', { oninput : m.withAttr( 'value', ctrl.username ), value : ctrl.username() } ),
m( 'a[href=/home]', { config : m.route }, 'Login' )
];
};
var home = {};
home.controller = function(){
if (!mo_portal.logged_in) {
console.log('redirecting to login');
m.route("/login");
}
};
home.view = function(ctrl) {
console.log('in home view');
return m("div","HOME");
};
m.route( document.body, '/login', {
'/login' : login,
'/home' : home
} );
<script src="https://rawgit.com/lhorie/mithril.js/next/mithril.js"></script>

Answer 2

I think that's not how mithril is going to be used. Mithril does not expect a view update (aka redraw) during ongoing view construction.
I suppose you should do the route-change in the related controller.
Keep in mind, that a view is rendered evertime the page changes somehow. You probably don't want to check the login state evert time.