Rx.Observable.if(condition, thenSource, [elseSource])

给出一个或两个流，判断Observable应该取哪个流的值。（相当于 if...else）

参数

1. condition (Function): 判决条件，为true则取 thenSource, 否则执行 elseSource.
2. thenSource (Observable): Observable, 当condition为true时执行。
3. [elseSource] (Observable|Scheduler): Observable, 当condition为false时执行。如果没有提供，则默认为 Rx.Observabe.Empty.

返回值

(Observable): Observable

例

This uses and only then source

// 只传入 then source
var shouldRun = true;

var source = Rx.Observable.if(
    function () { return shouldRun; },
    Rx.Observable.return(42)
);

var subscription = source.subscribe(
    function (x) {
        console.log('Next: ' + x);
    },
    function (err) {
        console.log('Error: ' + err);   
    },
    function () {
        console.log('Completed');   
    });

// => Next: 42
// => Completed

The next example uses an elseSource

// 设置 elseSource
var shouldRun = false;

var source = Rx.Observable.if(
    function () { return shouldRun; },
    Rx.Observable.return(42),
    Rx.Observable.return(56)
);

var subscription = source.subscribe(
    function (x) {
        console.log('Next: ' + x);
    },
    function (err) {
        console.log('Error: ' + err);   
    },
    function () {
        console.log('Completed');   
    });

// => Next: 56
// => Completed