The Leaflet GeoJSON layer does not retrieve the data automatically and therefore we need to obtain it ourselves. Once data is present, we can load it with the following code:

loadGeoJSON: function(map){ 
    Ext.Ajax.request({ 
        cors: true, 
        url: 'http://10.0.0.19:8080/geoserver/wfs?service=WFS&' +                
        'version=1.1.0&request=GetFeature&typename=
         mastering_postgis:earthquakes&' + 
         'outputFormat=application/json&srsname=EPSG:4326&' + 
         'bbox=-180,-90,180,90,EPSG:4326' 
    }).then(function(response){ 
        L.geoJson(Ext.JSON.decode(response.responseText).features, { 
            pointToLayer: function (feature, latlng) { 
                return L.circleMarker(latlng, { 
                    radius: feature.properties.mag * 5, 
                    fillColor: '#993366', 
                    weight: 1 
                }); 
            } 
        }).addTo(map); 
    }); 
}, 

As you can see, it is way simpler than consuming GeoJSON in ol3. We now need to navigate to the example's folder, run sencha app watch, and navigate to http://localhost:1841/apps/06_leaflet_geojson/. You should see a similar output to the following: