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A while loop in Dart is a pre-test control flow statement that repeatedly executes a block of code as long as a specified boolean condition evaluates to true. Because the condition is evaluated before the loop body executes, the code block will not execute at all if the initial condition is false.

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Syntax

while (condition) {
  // Statements to execute
}

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Execution Mechanics

  1. Condition Evaluation: The loop begins by evaluating the condition expression. In Dart, this expression must evaluate strictly to a boolean (bool) type. Dart does not support implicit truthy/falsy type coercion (e.g., while (1) will result in a compile-time error).
  2. Execution: If the condition evaluates to true, the statements within the loop body are executed sequentially.
  3. Iteration: After the loop body completes, control flow returns to the top of the loop, and the condition is re-evaluated.
  4. Termination: The loop terminates immediately when the condition evaluates to false. Control flow then passes to the first statement following the loop block.

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Code Example

int counter = 0;

while (counter < 3) {
  print(counter);
  counter++; // State mutation to ensure eventual termination
}

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Control Transfer Statements

You can alter the standard execution flow of a while loop using control transfer statements:
  • break: Immediately terminates the loop entirely, bypassing the condition check and transferring execution to the code following the loop block.
  • continue: Halts the current iteration, skips any remaining statements in the loop body, and immediately jumps back to re-evaluate the loop condition.
int i = 0;

while (i < 5) {
  i++;
  if (i == 2) {
    continue; // Skips the rest of the block, re-evaluates (i < 5)
  }
  if (i == 4) {
    break; // Exits the loop entirely
  }
  print(i);
}

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Infinite Loops

If the loop’s condition never evaluates to false—typically due to a lack of state mutation within the loop body—it results in an infinite loop. This will block the current isolate’s thread of execution indefinitely. 
// Example of an infinite loop
while (true) {
  // This block will execute forever unless a 'break' statement is encountered
}
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