eventif

Clean, conditional event handlers.

Overview

eventif simplifies event handling. Instead of doing this:

emitter.on('anevent',function(a,b){
    if (a === 1 && b !== 1) {
        // do something
    } else if (a !== 1 && b === 2) {
        // do something else
    } else if (a > 5) {
        // another thing
    } else if (b < 20) {
        // and another something
    } else {
        // default case
    }
});
 
emitter.emit(2,20);
emitter.emit(1,0);
emitter.emit(-1);

Do this:

emitter.on({a: 1, b: 2}, handler1);
emitter.on({a: 0}, handler2);
emitter.on({a: {from: 3, to: 10}, b: -1, _join: 'OR'}, handler3);

Clean, simple, short readable handlers.

Installation

Simple:

npm install eventif

Usage

Eventif follows the syntax and style of Nodejs Event Emitter.

You follow three simple steps:

1. Create or enable an eventif emitter
2. Create zero, one or more handlers
3. Emit events

Create An Event Emitter

You have 2 ways to create an eventif emitter:

• Create a new eventif emitter
• Enable an existing event emitter to be eventif-capable

Create a New Emitter

You can create a new eventif-compliant emitter by having it inheirt from eventif. As the example in the docs shows, you can use either classical nodejs prototypal inheritance:

const Eventif = require('eventif');
const util = require('util');
 
function MyEmitter() {
  EventEmitter.call(this);
}
util.inherits(MyEmitter, Eventif);
 
myEmitter = new MyEmitter();

Or, if you prefer, you can use ES6-style syntax:

const EventIf = require('eventif');
 
class MyEmitter extends EventIf {}
 
const myEmitter = new MyEmitter();

In both of the above examples, you are creating a new emitter by inheriting from eventif instead of from events.

Enable an Existing Emitter

If you already have a standard event emitter class which, somewhere down the line, inherits from events, you may not be able to change it to inherit from eventif instead. In that case, instead of creating a new event emitter, you enable the existing one to be eventif capable.

const Eventif = require('eventif');
const EventEmitter = require('events');
const util = require('util');
 
function MyEmitter() {
  EventEmitter.call(this);
}
util.inherits(MyEmitter, Eventif);
 
Em = Eventif.y(MyEmitter);
// or an alias
Em = Eventif.enable(MyEmitter);
 
myEmitter = new Em();

If es6 classes make you happier:

const EventIf = require('eventif');
const EventEmitter = require('events');
class MyEmitter extends EventEmitter {}
 
Em = Eventif.y(MyEmitter);
// or an alias
Em = Eventif.enable(MyEmitter);
 
myEmitter = new Em();

NOTE: You only can enable an existing event emitter class if it inherits from EventEmitter.

Now we have an eventif-capable emitter. Let's use it!

Create zero or more handlers

How would you create a normal event handler? With on!

myEmitter.on('anevent',function(a,b){
    // do something
});

How would you create an eventif-capable handler? In exactly the same way!

myEmitter.on('anevent',function(a,b){
    // do something
});

Handlers do not need to be changed to support eventif.

However, you now have the option to make those handlers conditional:

myEmitter.on('anevent',{a:1, b:0},function(a,b){
    // do something
});

As you can see, handlers really do not need to be changed. Of course, you may want to change them, now that all of that extraneous if-else if-else if-else or perhaps switch-case-case-case-case-case-default code can go away!

The signature for emitter functions is as follows:

• emitter.on(event,conditions,handler)
• emitter.once(event,conditions,handler)
• emitter.emit(event,[param1,param2,...,paramN])

See below for details on the correct syntax for conditions and how it matches parameters.

No Conditions

If you call on() or once() with no conditions at all, it is handled directly by the original event emitter.

myEmitter.on('anevent',function(a,b){
    // do something
});

The above means, "for every event of type 'anevent', always call the handler".

Using once

eventif fully supports once(). The rules for once() state that "This listener is invoked only the next time event is triggered, after which it is removed."

eventif follows the same logic. The handler is removed after the handler actually is invoked. Thus, if the handler would have been invoked if not for eventif, but the conditions do not match, then the handler remains in place. It is removed only after the actual handler function has been called at least once.

myEmitter.once({a:1},'anevent',function(a,b){
    // do something
});
myEmitter.emit('anevent'); // handler is NOT run or removed
myEmitter.emit('anevent',2); // handler is NOT run or removed
myEmitter.emit('anevent',1); // handler IS run and removed
myEmitter.emit('anevent',1); // handler already WAS removed, so it is NOT run

On the other hand, if you used a traditional event emitter, or even eventif without any conditions:

myEmitter.once('anevent',function(a,b){
    // do something
});
myEmitter.emit('anevent'); // handler IS run and removed
myEmitter.emit('anevent',2); // handler already WAS removed, so it is NOT run

Emit Events

This is easy. Just emit events like you always do!

myEmitter.emit('anevent');
// or if you want to pass parameters
myEmitter.emit('anevent',1,0,"hello world!");

Conditions and Syntax

In order for your conditions to work correctly, you need to specify the correct conditions syntax.

The syntax is the JSQL syntax of searchjs. The condition is what you pass as the (optional) second argument to .on() or .once(). The data checked against the condition is constructed of the name of the event and the parameters to your handler.

Let's take that apart.

Here's a simple example:

emitter.on('anevent',/* some condition */,function(a,b){
    // do something
});
emitter.emit('anevent',1,"hello");

Your handler expects the first argument to the event to be saved as a and the second as b. Thus, the object tested against will be:

{a:1, b:"hello"}

On the other hand, if you created the handler as follows:

emitter.on('anevent',/* some condition */,function(q,f){
    // do something
});
emitter.emit('anevent',1,"hello");

Then the data to test the condition against would be:

{q:1, f:"hello"}

In addition, the parameters are saved in order:

{1:1, 2: "hello"}

All that really happens with each event emission is:

1. Create a new object {}
2. Populate the object with the parameters to the event call {1:1, 2:"hello"}
3. If the handler has named arguments, populate the object with the named parameters {1:1, 2:"hello", a:1, b:"hello"}
4. Call searchjs.matchObject(data,condition), e.g. searchjs.match({1:1, 2:"hello", a:1, b:"hello"}, {1:1}).
5. If matchObject() returns true, call the handler, else do not.

Examples

In each example, we will show if it matches, and thus the handler is called, and why or why not.

Example 1

emitter.on('anevent',function(){
});
emitter.emit('anevent',1,2,'hello','jim',25,{a:1});

YES: With no condition, the handler always is called.

Example 2

emitter.on('anevent',{order:1,name:"jim"},function(order,count,message,name){
});
emitter.emit('anevent',1,2,'hello','jim',25,{a:1});

YES:

• order is 1st argument to function, which means 1st parameter to emit(). In condition, order:1 means it must equal 1. It does, and it matches.
• name is 4th argument to function, which means 4th parameter to emit(). In condition, name:"jim" means it must equal "jim". It does, and it matches.

Example 3

emitter.on('anevent',{order:1,name:"jill"},function(order,count,message,name){
});
emitter.emit('anevent',1,2,'hello','jim',25,{a:1});

NO:

• order is 1st argument to function, which means 1st parameter to emit(). In condition, order:1 means it must equal 1. It does, and it matches.
• name is 4th argument to function, which means 4th parameter to emit(). In condition, name:"jill" means it must equal "jill", but it equals "jim", so it fails.

Example 4

emitter.on('anevent',{3:"jim"},function(){
});
emitter.emit('anevent',1,2,'hello','jim',25,{a:1});

YES:

• Condition says that the 4th argument (3 from index 0) must be "jim". The 4th parameter (after the event), or 3rd from index 0, is indeed "jim".

Example 5

emitter.on('anevent',{order:1,name:"jill",_join:"OR"},function(order,count,message,name){
});
emitter.emit('anevent',1,2,'hello','jim',25,{a:1});

YES:

Condition says that "order" must equal 1 OR "name" must equal "jill". Since one of them is correct ("order" === 1), it matches.

Example 6

emitter.on('anevent',{count:{from: 1, to: 50} },function(order,count,message,name){
});
emitter.emit('anevent',1,32,'hello','jim',25,{a:1});

YES:

Condition says that "order" must be from 1 to 50. Since "order" is equal to 32, it is in the range and matches.