Tone.js has a few callback-generating classes which simplify the scheduling of complexly-timed events along the Transport. These events can be set to start and stop at specific moments along the Transport, as well as loop and playback at different rates. Events will not fire unless the Transport is started.

Tone.Event is the base-class for musical events. It creates a callback with a value which will be passed in as the second argument to the callback.

//create a looped note event every half-note
var note = new Tone.Event(function(time, pitch){
	synth.triggerAttackRelease(pitch, "16n", time);
}, "C2");

//set the note to loop every half measure
note.set({
	"loop" : true,
	"loopEnd" : "2n"
});

//start the note at the beginning of the Transport timeline
note.start(0);

//stop the note on the 4th measure
note.stop("4m");

Events have a probability parameter which allows you to adjust the probability of the event firing each time it is scheduled to.

//fire 50% of the time
note.probability = 0.5;

"Humanization" let's you adjust how rigid the callback timing is. If humanize is set to true, the passed-in time parameter will drift back and forth slightly to make the part feel a little more "human". You can also set humanize to a Time value, which will make it drift by that amount.

//drift by +/- a 32nd-note
note.humanize = "32n";

You can also adjust the playback-rate of all Event classes.

//loop the event twice as fast. 
note.playbackRate = 2;

Tone.Loop is a simplified Tone.Event. It has many of the same attributes as Tone.Event except instead of a loopEnd property, the duration of the loop is defined by the interval and it is set to loop by default. The constructor takes an interval after the callback

//loop the callback every 8th note from the beginning of the Timeline
var loop = new Tone.Loop(callback, "8n").start(0);

Tone.Part aggregates any number of Tone.Events which can be started, stopped and looped as a combined unit. Parts have all of the same methods as Tone.Events.

They can be constructed with either an array of [Time, Value] pairs, or with an array of objects that contain a "time" property.

var part = new Tone.Part(function(time, pitch){
	synth.triggerAttackRelease(pitch, "8n", time);
}, [["0", "C#3"], ["4n", "G3"], [3 * Tone.Time("8n"), "G#3"], ["2n", "C3"]]);

part.start("4m");

at let's you set or change values of a part at a given time.

//get the value at the given time
part.at("4n"); //returns "G3"

//change the first note to a G#
part.at("0", "G#2");

Tone.Sequence extends Tone.Part and simplifies the notation of composing sequential events. Pass in an array of evenly-spaced events at a given subdivision like so:

//a series of 8th notes
var seq = new Tone.Sequence(callback, ["C3", "Eb3", "F4", "Bb4"], "8n");

Nested arrays will subdivide that index by the length of the subarray and null is a rest

//a dotted quarter-note followed by an 8th note triplet
var seq = new Tone.Sequence(function(time, note){
	//play the note
}, ["C3", [null, "Eb3"], ["F4", "Bb4", "C5"]], "4n");

Sequences are set to loop by default at whatever the length of the events array is.

at works similarly to Tone.Part, but takes an index as the time value instead of the time.

seq.at(0); //returns "C3"
seq.at(1); //returns a Tone.Sequence

Tone.Pattern facilitates the creation of various arpeggiated note patterns. Its constructor takes a callback, an array of note values, and a string (the name of a supported classic "arpeggiator" pattern: "up", "down", "upDown", etc.)...

//cycle up and then down the array of values
var arp = new Tone.Pattern(callback, ["C3", "E3", "G3"], "upDown");
//callback order: "C3", "E3", "G3", "E3", ...repeat

arp.pattern = "downUp";
//callback order: "G3", "E3", "C3", "E3", ...repeat