Tool of Thought

APL for the Practical Man

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Modal Dialog Boxes 5: More on ProgressBar

May 18, 2024

The ProgressBar.Run operator, so far, handles iterative processes; we effectively have a loop, whether implemented using the each operator or recursion. This gives us the opportunitiy to check for a Pause click on each iteration, to cleanly pause the process, and clean up if the operation is canceled.

Sometimes we have a potentially long running process that is not itererative, or it is iterative but we don't want to bother with refactoring it to fit the needs of the Progress.Run operator. We want to put up a spinner or loader - and provide a cancel button to kill the process.

The HTML progress element provides an oscillating bar when no max attribute is set, giving us a JavaScript-free and CSS-free spinner.

For this case we can wrap our long running code with the Start and Stop functions:

_←d Progress.Start ''
... your code here
_←d Progress.Stop ''

The Start function puts up a modal dialog, but does not wait:

     h1←e A.New'h1'c
     p←e A.New'progress'←'progress'
     b←e A.New'button' 'Cancel'←'Cancel'
     _←⍺ A.ShowModal e

And Stop just closes the dialog:

     d←⍺ A.GetElementById'progress-bar'
     ⍺ A.DeleteElement d

In the Start function the thread id of the current process is noted in the component, so when Cancel is pressed, we can kill it:

     d←⍵.CurrentTarget A.GetNearest'dialog'
     _←⎕TKILL d.TID
     ⍵.Document A.DeleteElement d

This is a fairly crude technique, but useful in some circumstances.

There is another progress bar case that we have not handled. Consider a process of N discrete but non-iterative steps. As the code is running through these N steps, we want to provide feedback to the user about where we are in the process, and allow the process to be cancelled. For this we will need something like Begin, Update and End functions to sprinkle throughout our code.

In addition we may want to consider what happens if and when these various processes are scripted and have no user interaction.