add rule for creating node child within a loop

README.md

@@ -255,6 +255,8 @@ Default rules:
 
 -   `no-array-component-field-type`: Using 'array' type in component markup can result in inefficient copying of data during transfer to the render thread. Use 'node' type if possible for more efficient transfer of data from the task thread to the render thread (`error | warn | info | off`)
 
+- `no-createchild-in-loop`: CreateChild should be used when you want to append a single instance of a type of node to a parent, but avoid modifiing that child after it's created. Do not use in a loop to append multiple children. Use CreateChildren() instead (`error | warn | info | off`)
+
 ### Strictness rules
 
 - `type-annotations`: validation of presence of `as` type annotations, for function

src/index.ts

@@ -51,6 +51,7 @@ export type BsLintConfig = Pick<BsConfig, 'project' | 'rootDir' | 'files' | 'cwd
         'color-cert'?: RuleColorCertCompliant;
         'no-assocarray-component-field-type'?: RuleSeverity;
         'no-array-component-field-type'?: RuleSeverity;
+        'no-createchild-in-loop'?: RuleSeverity;
     };
     globals?: string[];
     ignores?: string[];
@@ -86,6 +87,7 @@ export interface BsLintRules {
     colorCertCompliant: RuleColorCertCompliant;
     noAssocarrayComponentFieldType: BsLintSeverity;
     noArrayComponentFieldType: BsLintSeverity;
+    noCreateObjectInLoop: BsLintSeverity;
 }
 
 export { Linter };

src/plugins/codeStyle/diagnosticMessages.ts

@@ -25,7 +25,8 @@ export enum CodeStyleError {
     ColorAlphaDefaults = 'LINT3022',
     ColorCertCompliant = 'LINT3023',
     NoAssocarrayFieldType = 'LINT3024',
-    NoArrayFieldType = 'LINT3025'
+    NoArrayFieldType = 'LINT3025',
+    NoCreateChildInLoop = 'LINT3027'
 }
 
 const CS = 'Code style:';
@@ -215,5 +216,12 @@ export const messages = {
         severity: severity,
         source: 'bslint',
         range
+    }),
+    NoCreateChildInLoop: (range: Range, severity: DiagnosticSeverity) => ({
+        severity,
+        code: CodeStyleError.NoCreateChildInLoop,
+        source: 'bslint',
+        message: 'Avoid setting a value to node field after its creation within the same loop',
+        range
     })
 };

src/plugins/codeStyle/index.ts

@@ -4,6 +4,12 @@ import {
     FunctionExpression,
     isBrsFile,
     isXmlFile,
+    isFunctionExpression,
+    isForEachStatement,
+    isForStatement,
+    isWhileStatement,
+    isDottedGetExpression,
+    isComponentType,
     isGroupingExpression,
     TokenKind,
     WalkMode,
@@ -18,7 +24,12 @@ import {
     isVoidType,
     Statement,
     SymbolTypeFlag,
-    XmlFile
+    XmlFile,
+    isAssignmentStatement,
+    isDottedSetStatement,
+    isVariableExpression,
+    CallExpression,
+    AstNode
 } from 'brighterscript';
 import { RuleAAComma } from '../..';
 import { addFixesToEvent } from '../../textEdit';
@@ -76,7 +87,7 @@ export default class CodeStyle implements CompilerPlugin {
     validateBrsFile(file: BrsFile) {
         const diagnostics: (Omit<BsDiagnostic, 'file'>)[] = [];
         const { severity } = this.lintContext;
-        const { inlineIfStyle, blockIfStyle, conditionStyle, noPrint, noTodo, noStop, aaCommaStyle, eolLast, colorFormat } = severity;
+        const { inlineIfStyle, blockIfStyle, conditionStyle, noPrint, noTodo, noStop, aaCommaStyle, eolLast, colorFormat, noCreateObjectInLoop } = severity;
         const validatePrint = noPrint !== DiagnosticSeverity.Hint;
         const validateTodo = noTodo !== DiagnosticSeverity.Hint;
         const validateNoStop = noStop !== DiagnosticSeverity.Hint;
@@ -92,6 +103,7 @@ export default class CodeStyle implements CompilerPlugin {
         const validateEolLast = eolLast !== 'off';
         const disallowEolLast = eolLast === 'never';
         const validateColorStyle = validateColorFormat ? createColorValidator(severity) : undefined;
+        const validateNoCreateChildInLoop = noCreateObjectInLoop !== DiagnosticSeverity.Hint;
 
         // Check if the file is empty by going backwards from the last token,
         // meaning there are tokens other than `Eof` and `Newline`.
@@ -133,6 +145,8 @@ export default class CodeStyle implements CompilerPlugin {
             }
         }
 
+        const createChildinLoop = new Map<AstNode, string[]>();
+
         file.ast.walk(createVisitor({
             // validate function style (`function` or `sub`)
             FunctionExpression: (func) => {
@@ -212,12 +226,85 @@ export default class CodeStyle implements CompilerPlugin {
                         }
                     }
                 }
+            },
+            CallExpression: (node) => {
+                if (validateNoCreateChildInLoop) {
+                    this.findCreateChildinLoop(node, createChildinLoop);
+                }
             }
         }), { walkMode: WalkMode.visitAllRecursive });
 
+        for (const [loop, candidates] of createChildinLoop) {
+            loop.walk(createVisitor({
+                DottedSetStatement: (s) => {
+                    const leftExpression = s.obj;
+                    let varStr = '';
+                    if (isVariableExpression(leftExpression)) {
+                        varStr = leftExpression.tokens.name.text;
+                    } else if (isDottedGetExpression(leftExpression)) {
+                        varStr = this.getDottedStr(leftExpression.obj);
+                    }
+
+                    if (candidates.includes(varStr)) {
+                        diagnostics.push(messages.NoCreateChildInLoop(s.tokens.name.location.range, noCreateObjectInLoop));
+                    }
+                }
+            }), { walkMode: WalkMode.visitAllRecursive });
+        }
+
         return diagnostics;
     }
 
+    isLoop(statement: AstNode) {
+        return isForStatement(statement) || isForEachStatement(statement) || isWhileStatement(statement);
+    }
+
+    findCreateChildinLoop(node: CallExpression, createChildinLoop: Map<AstNode, string[]>) {
+        if (!isDottedGetExpression(node.callee) || node.callee?.tokens?.name.text.toLowerCase() !== 'createchild') {
+            return;
+        }
+        const objType = node.callee.obj.getType({ flags: SymbolTypeFlag.runtime });
+        if (isComponentType(objType)) {
+            const parentLoop = node.findAncestor((node, cancel) => {
+                if (isFunctionExpression(node)) {
+                    cancel.cancel();
+                } else if (this.isLoop(node)) {
+                    return true;
+                }
+            });
+            if (this.isLoop(parentLoop)) {
+                let candidatesArray = createChildinLoop.get(parentLoop);
+
+                if (!candidatesArray) {
+                    candidatesArray = [];
+                    createChildinLoop.set(parentLoop, []);
+                }
+
+                let varStr = '';
+                if (isAssignmentStatement(node.parent)) {
+                    varStr = node.parent.tokens.name.text;
+                } else if (isDottedSetStatement(node.parent)) {
+                    varStr = this.getDottedStr(node.parent.obj);
+                }
+                createChildinLoop.get(parentLoop).push(varStr);
+            }
+        }
+    }
+
+    getDottedStr(expression: Expression) {
+        let currentExpression = expression;
+        let varStr = '';
+        while (isDottedGetExpression(currentExpression)) {
+            varStr = `.${currentExpression.tokens.name.text}${varStr}`;
+            currentExpression = currentExpression.obj;
+        }
+        if (isVariableExpression(currentExpression)) {
+            varStr = currentExpression.tokens.name.text + varStr;
+        }
+
+        return varStr;
+    }
+
     afterFileValidate(event: AfterFileValidateEvent) {
         const { file } = event;
         if (this.lintContext.ignores(file)) {

src/util.ts

@@ -28,7 +28,8 @@ export function getDefaultRules(): BsLintConfig['rules'] {
         'type-annotations': 'off',
         'no-print': 'off',
         'no-assocarray-component-field-type': 'off',
-        'no-array-component-field-type': 'off'
+        'no-array-component-field-type': 'off',
+        'no-createchild-in-loop': 'off'
     };
 }
 
@@ -166,7 +167,8 @@ function rulesToSeverity(rules: BsLintConfig['rules']) {
         colorAlphaDefaults: rules['color-alpha-defaults'],
         colorCertCompliant: rules['color-cert'],
         noAssocarrayComponentFieldType: ruleToSeverity(rules['no-assocarray-component-field-type']),
-        noArrayComponentFieldType: ruleToSeverity(rules['no-array-component-field-type'])
+        noArrayComponentFieldType: ruleToSeverity(rules['no-array-component-field-type']),
+        noCreateObjectInLoop: ruleToSeverity(rules['no-createchild-in-loop'])
     };
 }