callbackDispatcher

Define a background callback with callbackDispatcher.

What is callbackDispatcher?

callbackDispatcher is the top-level function called when a background task runs. It runs in a separate isolate, so it cannot access the app's main state. The task id arrives via the task parameter and the appropriate work is performed.

bool callbackDispatcher(String task) {
  if (task == 'sync') {
    // sync data
  }
  return true;
}

The task parameter

callbackDispatcher receives a task id that identifies which task is running. This id matches the name given when the task was scheduled. Inside the function you use if or switch to perform the appropriate work.

bool callbackDispatcher(String task) {
  switch (task) {
    case 'sync':
      break;
  }
  return true;
}

Exercise Write a callbackDispatcher

Write a top-level callbackDispatcher function that takes String task, returns true for the 'sync' id, otherwise false. In main, call it with the 'sync' id and print the result.

Show example solution

bool callbackDispatcher(String task) {
  if (task == 'sync') {
    return true;
  }
  return false;
}

void main() {
  print(callbackDispatcher('sync'));
}

Test yourself

Why must callbackDispatcher be a top-level function?
- Because it cannot be defined inside main
- Because only top-level functions can be async
- Because it runs in a separate isolate and cannot access class state
- Because the compiler only compiles top-level functions
Show answer

Because it runs in a separate isolate and cannot access class state — Because callbackDispatcher runs in a separate isolate, it cannot access app state; therefore it must be top-level and independent.

Check the task id

bool callbackDispatcher(String ____) {
  if (task == 'sync') {
    return true;
  }
  return false;
}

Show answer

bool callbackDispatcher(String task) {
  if (task == 'sync') {
    return true;
  }
  return false;
}

callbackDispatcher parts

callbackDispatcher

Background callback
task

Task identifier
isolate

Separate execution environment
top-level

Top-level function

Next lesson: OneOffTask

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