Tasker: Flow Control

Flow Control

Overview

Task flow control is based on the following Tasker elements:

On the Wiki there is a detailed example of processing a file's content [External Link].

Tip: if you accidentally create a task that never ends when experimenting with loops, use the Kill button in the Task Edit screen to end it manually.

Conditions
Every action can have a condition associated with it (specify this in the Action Edit screen). If the condition does not match, the action will be skipped.

A condition consists of an operator ('equals' etc) and two parameters. The possible operators are:

Expressions which are not mathematically valid e.g. I Am The Walrus > 5 give a warning and evaluate to false when used with a mathematical operator.

Foreach Loop

Goal: perform a set of actions for each of apple, pear and banana.

1. For
%item
apple,pear,banana		 Loop once for each of apple, pear and banana
2.   Action One
 Example: Flash %item
3.   Action Two
 ...
4. End For
 Return to action 1 if we havn't done all the items yet

Result: Action One and Action Two are performed three times. The first time, the variable %item is set to apple, the second time pear and the last time banana.

You can insert a Goto action in the loop with either Top of Loop (meaning continue, skip to the next item straight away) or End of Loop (meaning break, stop without doing any more items) specified.

In adition to simple text, the For action accepts any comma-separated combination of these Items:

A common case is to use %arr(), which performs a loop for each element in the array %arr.

Warning: the Values parameter in the loop is reevaluated with each iteration of the loop, meaning that modifying array variables which appear there from within the loop can have unexpected effects. To workaround that, it can be best to use the following sequence: 

   Variables Set, %values, %arrayWhichWillChange()
   Variable Split, %values
   For, %value, %values()
      ...

For Loop

Goal: perform a set of actions for each of a set of elements in turn.

Use the Foreach Loop as described above, with the Items parameter being a range specification e.g. 4:0, 100, 0:8:2 (= 4,3,2,1,0,100,0,2,4,6,8).

Until Loop

Goal: perform a Task X until some condition is met (at least once)

1. Action One
 ...
2. Action Two
 ...
3. Goto
1
If %qtime < 20		 Return to action 1 if runtime < 20

Result: Action One and Action Two are performed until %QTIME contains the value 20 or more i.e. until the task has been running for 20 seconds.

Note: %QTIME is a builtin local variable available in all tasks.

While Loop

Goal: perform a Task X while some condition is met.

1. Stop

If %fruit Not Matches Apple		 Stop task if it's not crunchy, otherwise go to next action
2. Action One
 ...
3. Action Two
 ...
4. Goto
1		 Go back and see if we're still crunchy

Result: Action One and Action Two are performed while %fruit contains the value Apple.

Counter Loop

Goal: perform a Task X a set number of times.

1. Variable Set
%count, 0		 Initialize the counter
2. Action One
Label: LoopStart		 ...
3. Action Two
 ...
4. Variable Add
%count, 1		 Add one to %count
5. Goto
LoopStart
If %count < 10		 Return to action 2 if count < 10

Result: after initialization of %count to 0, the task loops around the actions from 2-5 until %count reaches 10, at which point the condition on the Goto fails and the end of the task is reached.

Note that we used a Goto to a labelled action this time. In all but the very simplest tasks it's better to use a label rather than a number. It's easier to work out what's happening and if you insert or delete actions before the loop starts, the Goto will still jump to the right place.

An alternative way to do this loop is to use a For action specified as 0:10.

If / Then / Else Condition

Goal: perform certain Tasks if conditions are met, otherwise perform a different task.

1. If
%fruit ~ Apple		 ~ is short for 'matches'
2.   Action One ...
3.   Action Two
 ...
4. Else If
%fruit ~ Pear		 an Else action with a condition
5.   Action Three ...
6. Else
7.   Action Four
 ...

Result: actions One and Two are executed if %fruit matches Apple, Action Three is executed if %fruit matches Pear, otherwise Action Four is executed.

Notes:

Subroutines

To call another task, use the Perform Task action. To use it as a subroutine, you just need to ensure that the priority of the calling task is less than the priority of the called task (more info: scheduling).

The parent can optionally pass values to the child and receive a result back:

Parent Task

1.   Perform Task
Child,
Priority, 10
%par1, 5,
Result Value Variable, %result 		pass 5 to the child, expect a result in %result
2.   Variable Flash
Result: %result 		what did we get back ?

Child Task

1.   Variable Set
%newval, %par1 + 1, Do Maths 		add one to the value that was passed
1.   Return
%newval
set %result in the parent to the value of %newval in the child

Result: the parent flashes 6

Notes: