Wait for selectFolder()

Processing Coding Questions
1

I am writing a program to do mass image editing and would like to have the end user select an input and output folder when the program starts.

I tried using selectFolder("Select a input folder:", "inputFolder"); and it opened the folder selection thing, but my code continues to run before a folder is selected, leading to a null error where one of my commands tries acting on the not yet initiated String iPath

Minimized code example: 
import java.io.File;

String inputPath;

void setup() {
  selectFolder("Select a input folder:", "inputFolder");
  
  CommandIWantToRunAFTERFolderIsSelected(inputPath);
}

void inputFolder(File selection) {
  if (selection == null) {
    javax.swing.JOptionPane.showMessageDialog(null, "Input Window was closed or the user hit cancel.", "Error", javax.swing.JOptionPane.ERROR_MESSAGE);
    exit();
  } else {
    inputPath = selection.toString();
    println("User selected " + selection.getAbsolutePath());
  }
}

void CommandIWantToRunAFTERFolderIsSelected(String path){
  if (path == null){
    println("Process crash due to null interperetation");
  } else {
    println("no crash as command ran after string initiated");
  }
}

Running the above code without modification, you will see that the command that interprets the path runs before a file path is selected.

How would I fix this issue

2

Basically in setup () have only size and SelectFolder

In draw() do what you want to achieve but with an if clause around it like if(ipath!=null) {

3

Prior to PS3 the select???? methods were modal i.e. blocked the main thread (which how Java) works. In pS3 the Processing developers changed it so that they are executed on a separate thread it means the main thread no longer stops.
There is no option in Processing to make them behave modally so you have 3 options

  1. Simply test the reference and only use the selected folders when they are not null i.e.
    if(inFolder != null){
      // safe to use inFolder
    }
  1. Write your own code to access the Java Swing file dialog boxes
  2. Use the equivalent functions in G4P because they are modal. I added these when PS3 was released.
4 
String inputPath;

void setup() {
  selectFolder("Select an input folder:", "inputFolder");

  while (inputPath == null)  delay(1);

  CommandIWantToRunAFTERFolderIsSelected(inputPath);
}
5

I really like this solution to the problem, but I marked @quark’s reply to the solution as it gave multiple, general solutions.