Starting from version 2.8.5, LiteFlow provides conditional expression combinations.

Conditional expression is a variant of switch expression, SWITCH selects one of multiple subitems based on logic. The conditional expression has only two sub-items, true and false, which is very useful in the process of dealing with certain businesses.

In fact, conditional expression is to become an if else in the language. It's just that there are some different usages in the LiteFlow EL syntax.

The first parameter of the following IF and ELIF requires the definition of a if component. For how to define it, please refer to the chapter If component.

IF with two arguments

<chain name="chain1">
    THEN(
        IF(x, a),
        b
    );
</chain>

Flow chart

x is a conditional node. If it is true, the execution chain is x->a->b, and if it is false, it is x->b.

IF with three arguments

<chain name="chain1">
    THEN(
        IF(x, a, b),
        c
    );
</chain>

Flow chart

x is a conditional node. If it is true, the execution chain is x->a->c, and if it is false, it is x->b->c.

ELSE expression

LiteFlow also provides the ELSE expression, the IF with two arguments + ELSE expression is equivalent to the IF with three arguments, for example:

<chain name="chain1">
    IF(x, a).ELSE(b);
</chain>

It is equivalent to:

<chain name="chain1">
    IF(x, a, b);
</chain>

ELIF expression

The usage of the ELIF keyword is actually similar to the else if in the java language. It can be used with multiple ELIF, It will be followed by an ELSE at the end, which is used to judge multiple conditions:

<chain name="chain1">
    IF(x1, a).ELIF(x2, b).ELIF(x3, c).ELIF(x4, d).ELSE(THEN(m, n));
</chain>

Flow chart

Those who have written code should have a good understanding of this expression.

notice one

It is worth noting that only the two arguments expression of IF can be followed by ELIF. If the IF with three arguments is followed by ELIF, the last expression will be overwritten by the expression of ELIF. for example:

<chain name="chain1">
    IF(x1, a, b).ELIF(x2, c).ELSE(d);
</chain>

In this way, even if x1 is false, it will not execute to b, and will judge x2. Although the framework does fault-tolerant processing, it is not recommended to write expressions in this way. It is easy to cause confusion in understanding.

notice two

In fact, the IF with three arguments can already express all possibilities, which can be done by nesting, for example:

<chain name="chain1">
    IF(
        x1,
        a, 
        IF(
            x2,
            b,
            IF(x3, c, d)
        )
    );
</chain>

But it is still not recommended that you write this way. Multiple nesting will be difficult to understand, so try to use ELIF instead.