angular-cn/modules/@angular/compiler/src/compiler_util/expression_converter.ts

649 lines
24 KiB
TypeScript

/**
* @license
* Copyright Google Inc. All Rights Reserved.
*
* Use of this source code is governed by an MIT-style license that can be
* found in the LICENSE file at https://angular.io/license
*/
import * as cdAst from '../expression_parser/ast';
import {isBlank} from '../facade/lang';
import {Identifiers, createIdentifier} from '../identifiers';
import {ClassBuilder} from '../output/class_builder';
import * as o from '../output/output_ast';
import {createPureProxy} from './identifier_util';
const VAL_UNWRAPPER_VAR = o.variable(`valUnwrapper`);
export interface NameResolver {
callPipe(name: string, input: o.Expression, args: o.Expression[]): o.Expression;
getLocal(name: string): o.Expression;
}
export class EventHandlerVars { static event = o.variable('$event'); }
export class ConvertPropertyBindingResult {
constructor(
public stmts: o.Statement[], public currValExpr: o.Expression,
public forceUpdate: o.Expression) {}
}
/**
* Converts the given expression AST into an executable output AST, assuming the expression is
* used in a property binding.
*/
export function convertPropertyBinding(
builder: ClassBuilder, nameResolver: NameResolver, implicitReceiver: o.Expression,
expression: cdAst.AST, bindingId: string): ConvertPropertyBindingResult {
const currValExpr = createCurrValueExpr(bindingId);
const stmts: o.Statement[] = [];
if (!nameResolver) {
nameResolver = new DefaultNameResolver();
}
const visitor = new _AstToIrVisitor(
builder, nameResolver, implicitReceiver, VAL_UNWRAPPER_VAR, bindingId, false);
const outputExpr: o.Expression = expression.visit(visitor, _Mode.Expression);
if (!outputExpr) {
// e.g. an empty expression was given
return null;
}
if (visitor.temporaryCount) {
for (let i = 0; i < visitor.temporaryCount; i++) {
stmts.push(temporaryDeclaration(bindingId, i));
}
}
if (visitor.needsValueUnwrapper) {
const initValueUnwrapperStmt = VAL_UNWRAPPER_VAR.callMethod('reset', []).toStmt();
stmts.push(initValueUnwrapperStmt);
}
stmts.push(currValExpr.set(outputExpr).toDeclStmt(null, [o.StmtModifier.Final]));
if (visitor.needsValueUnwrapper) {
return new ConvertPropertyBindingResult(
stmts, currValExpr, VAL_UNWRAPPER_VAR.prop('hasWrappedValue'));
} else {
return new ConvertPropertyBindingResult(stmts, currValExpr, null);
}
}
export class ConvertActionBindingResult {
constructor(public stmts: o.Statement[], public preventDefault: o.ReadVarExpr) {}
}
/**
* Converts the given expression AST into an executable output AST, assuming the expression is
* used in an action binding (e.g. an event handler).
*/
export function convertActionBinding(
builder: ClassBuilder, nameResolver: NameResolver, implicitReceiver: o.Expression,
action: cdAst.AST, bindingId: string): ConvertActionBindingResult {
if (!nameResolver) {
nameResolver = new DefaultNameResolver();
}
const visitor =
new _AstToIrVisitor(builder, nameResolver, implicitReceiver, null, bindingId, true);
const actionStmts: o.Statement[] = [];
flattenStatements(action.visit(visitor, _Mode.Statement), actionStmts);
prependTemporaryDecls(visitor.temporaryCount, bindingId, actionStmts);
const lastIndex = actionStmts.length - 1;
let preventDefaultVar: o.ReadVarExpr = null;
if (lastIndex >= 0) {
const lastStatement = actionStmts[lastIndex];
const returnExpr = convertStmtIntoExpression(lastStatement);
if (returnExpr) {
// Note: We need to cast the result of the method call to dynamic,
// as it might be a void method!
preventDefaultVar = createPreventDefaultVar(bindingId);
actionStmts[lastIndex] =
preventDefaultVar.set(returnExpr.cast(o.DYNAMIC_TYPE).notIdentical(o.literal(false)))
.toDeclStmt(null, [o.StmtModifier.Final]);
}
}
return new ConvertActionBindingResult(actionStmts, preventDefaultVar);
}
/**
* Creates variables that are shared by multiple calls to `convertActionBinding` /
* `convertPropertyBinding`
*/
export function createSharedBindingVariablesIfNeeded(stmts: o.Statement[]): o.Statement[] {
const unwrapperStmts: o.Statement[] = [];
const readVars = o.findReadVarNames(stmts);
if (readVars.has(VAL_UNWRAPPER_VAR.name)) {
unwrapperStmts.push(
VAL_UNWRAPPER_VAR
.set(o.importExpr(createIdentifier(Identifiers.ValueUnwrapper)).instantiate([]))
.toDeclStmt(null, [o.StmtModifier.Final]));
}
return unwrapperStmts;
}
function temporaryName(bindingId: string, temporaryNumber: number): string {
return `tmp_${bindingId}_${temporaryNumber}`;
}
export function temporaryDeclaration(bindingId: string, temporaryNumber: number): o.Statement {
return new o.DeclareVarStmt(temporaryName(bindingId, temporaryNumber), o.NULL_EXPR);
}
function prependTemporaryDecls(
temporaryCount: number, bindingId: string, statements: o.Statement[]) {
for (let i = temporaryCount - 1; i >= 0; i--) {
statements.unshift(temporaryDeclaration(bindingId, i));
}
}
enum _Mode {
Statement,
Expression
}
function ensureStatementMode(mode: _Mode, ast: cdAst.AST) {
if (mode !== _Mode.Statement) {
throw new Error(`Expected a statement, but saw ${ast}`);
}
}
function ensureExpressionMode(mode: _Mode, ast: cdAst.AST) {
if (mode !== _Mode.Expression) {
throw new Error(`Expected an expression, but saw ${ast}`);
}
}
function convertToStatementIfNeeded(mode: _Mode, expr: o.Expression): o.Expression|o.Statement {
if (mode === _Mode.Statement) {
return expr.toStmt();
} else {
return expr;
}
}
class _AstToIrVisitor implements cdAst.AstVisitor {
private _nodeMap = new Map<cdAst.AST, cdAst.AST>();
private _resultMap = new Map<cdAst.AST, o.Expression>();
private _currentTemporary: number = 0;
public needsValueUnwrapper: boolean = false;
public temporaryCount: number = 0;
constructor(
private _builder: ClassBuilder, private _nameResolver: NameResolver,
private _implicitReceiver: o.Expression, private _valueUnwrapper: o.ReadVarExpr,
private bindingId: string, private isAction: boolean) {}
visitBinary(ast: cdAst.Binary, mode: _Mode): any {
let op: o.BinaryOperator;
switch (ast.operation) {
case '+':
op = o.BinaryOperator.Plus;
break;
case '-':
op = o.BinaryOperator.Minus;
break;
case '*':
op = o.BinaryOperator.Multiply;
break;
case '/':
op = o.BinaryOperator.Divide;
break;
case '%':
op = o.BinaryOperator.Modulo;
break;
case '&&':
op = o.BinaryOperator.And;
break;
case '||':
op = o.BinaryOperator.Or;
break;
case '==':
op = o.BinaryOperator.Equals;
break;
case '!=':
op = o.BinaryOperator.NotEquals;
break;
case '===':
op = o.BinaryOperator.Identical;
break;
case '!==':
op = o.BinaryOperator.NotIdentical;
break;
case '<':
op = o.BinaryOperator.Lower;
break;
case '>':
op = o.BinaryOperator.Bigger;
break;
case '<=':
op = o.BinaryOperator.LowerEquals;
break;
case '>=':
op = o.BinaryOperator.BiggerEquals;
break;
default:
throw new Error(`Unsupported operation ${ast.operation}`);
}
return convertToStatementIfNeeded(
mode,
new o.BinaryOperatorExpr(
op, this.visit(ast.left, _Mode.Expression), this.visit(ast.right, _Mode.Expression)));
}
visitChain(ast: cdAst.Chain, mode: _Mode): any {
ensureStatementMode(mode, ast);
return this.visitAll(ast.expressions, mode);
}
visitConditional(ast: cdAst.Conditional, mode: _Mode): any {
const value: o.Expression = this.visit(ast.condition, _Mode.Expression);
return convertToStatementIfNeeded(
mode,
value.conditional(
this.visit(ast.trueExp, _Mode.Expression), this.visit(ast.falseExp, _Mode.Expression)));
}
visitPipe(ast: cdAst.BindingPipe, mode: _Mode): any {
const input = this.visit(ast.exp, _Mode.Expression);
const args = this.visitAll(ast.args, _Mode.Expression);
const value = this._nameResolver.callPipe(ast.name, input, args);
if (!value) {
throw new Error(`Illegal state: Pipe ${ast.name} is not allowed here!`);
}
this.needsValueUnwrapper = true;
return convertToStatementIfNeeded(mode, this._valueUnwrapper.callMethod('unwrap', [value]));
}
visitFunctionCall(ast: cdAst.FunctionCall, mode: _Mode): any {
return convertToStatementIfNeeded(
mode,
this.visit(ast.target, _Mode.Expression).callFn(this.visitAll(ast.args, _Mode.Expression)));
}
visitImplicitReceiver(ast: cdAst.ImplicitReceiver, mode: _Mode): any {
ensureExpressionMode(mode, ast);
return this._implicitReceiver;
}
visitInterpolation(ast: cdAst.Interpolation, mode: _Mode): any {
ensureExpressionMode(mode, ast);
const args = [o.literal(ast.expressions.length)];
for (let i = 0; i < ast.strings.length - 1; i++) {
args.push(o.literal(ast.strings[i]));
args.push(this.visit(ast.expressions[i], _Mode.Expression));
}
args.push(o.literal(ast.strings[ast.strings.length - 1]));
return ast.expressions.length <= 9 ?
o.importExpr(createIdentifier(Identifiers.inlineInterpolate)).callFn(args) :
o.importExpr(createIdentifier(Identifiers.interpolate)).callFn([
args[0], o.literalArr(args.slice(1))
]);
}
visitKeyedRead(ast: cdAst.KeyedRead, mode: _Mode): any {
const leftMostSafe = this.leftMostSafeNode(ast);
if (leftMostSafe) {
return this.convertSafeAccess(ast, leftMostSafe, mode);
} else {
return convertToStatementIfNeeded(
mode, this.visit(ast.obj, _Mode.Expression).key(this.visit(ast.key, _Mode.Expression)));
}
}
visitKeyedWrite(ast: cdAst.KeyedWrite, mode: _Mode): any {
const obj: o.Expression = this.visit(ast.obj, _Mode.Expression);
const key: o.Expression = this.visit(ast.key, _Mode.Expression);
const value: o.Expression = this.visit(ast.value, _Mode.Expression);
return convertToStatementIfNeeded(mode, obj.key(key).set(value));
}
visitLiteralArray(ast: cdAst.LiteralArray, mode: _Mode): any {
const parts = this.visitAll(ast.expressions, mode);
const literalArr =
this.isAction ? o.literalArr(parts) : createCachedLiteralArray(this._builder, parts);
return convertToStatementIfNeeded(mode, literalArr);
}
visitLiteralMap(ast: cdAst.LiteralMap, mode: _Mode): any {
const parts: any[] = [];
for (let i = 0; i < ast.keys.length; i++) {
parts.push([ast.keys[i], this.visit(ast.values[i], _Mode.Expression)]);
}
const literalMap =
this.isAction ? o.literalMap(parts) : createCachedLiteralMap(this._builder, parts);
return convertToStatementIfNeeded(mode, literalMap);
}
visitLiteralPrimitive(ast: cdAst.LiteralPrimitive, mode: _Mode): any {
return convertToStatementIfNeeded(mode, o.literal(ast.value));
}
private _getLocal(name: string): o.Expression {
if (this.isAction && name == EventHandlerVars.event.name) {
return EventHandlerVars.event;
}
return this._nameResolver.getLocal(name);
}
visitMethodCall(ast: cdAst.MethodCall, mode: _Mode): any {
const leftMostSafe = this.leftMostSafeNode(ast);
if (leftMostSafe) {
return this.convertSafeAccess(ast, leftMostSafe, mode);
} else {
const args = this.visitAll(ast.args, _Mode.Expression);
let result: any = null;
const receiver = this.visit(ast.receiver, _Mode.Expression);
if (receiver === this._implicitReceiver) {
const varExpr = this._getLocal(ast.name);
if (varExpr) {
result = varExpr.callFn(args);
}
}
if (isBlank(result)) {
result = receiver.callMethod(ast.name, args);
}
return convertToStatementIfNeeded(mode, result);
}
}
visitPrefixNot(ast: cdAst.PrefixNot, mode: _Mode): any {
return convertToStatementIfNeeded(mode, o.not(this.visit(ast.expression, _Mode.Expression)));
}
visitPropertyRead(ast: cdAst.PropertyRead, mode: _Mode): any {
const leftMostSafe = this.leftMostSafeNode(ast);
if (leftMostSafe) {
return this.convertSafeAccess(ast, leftMostSafe, mode);
} else {
let result: any = null;
const receiver = this.visit(ast.receiver, _Mode.Expression);
if (receiver === this._implicitReceiver) {
result = this._getLocal(ast.name);
}
if (isBlank(result)) {
result = receiver.prop(ast.name);
}
return convertToStatementIfNeeded(mode, result);
}
}
visitPropertyWrite(ast: cdAst.PropertyWrite, mode: _Mode): any {
const receiver: o.Expression = this.visit(ast.receiver, _Mode.Expression);
if (receiver === this._implicitReceiver) {
const varExpr = this._getLocal(ast.name);
if (varExpr) {
throw new Error('Cannot assign to a reference or variable!');
}
}
return convertToStatementIfNeeded(
mode, receiver.prop(ast.name).set(this.visit(ast.value, _Mode.Expression)));
}
visitSafePropertyRead(ast: cdAst.SafePropertyRead, mode: _Mode): any {
return this.convertSafeAccess(ast, this.leftMostSafeNode(ast), mode);
}
visitSafeMethodCall(ast: cdAst.SafeMethodCall, mode: _Mode): any {
return this.convertSafeAccess(ast, this.leftMostSafeNode(ast), mode);
}
visitAll(asts: cdAst.AST[], mode: _Mode): any { return asts.map(ast => this.visit(ast, mode)); }
visitQuote(ast: cdAst.Quote, mode: _Mode): any {
throw new Error('Quotes are not supported for evaluation!');
}
private visit(ast: cdAst.AST, mode: _Mode): any {
const result = this._resultMap.get(ast);
if (result) return result;
return (this._nodeMap.get(ast) || ast).visit(this, mode);
}
private convertSafeAccess(
ast: cdAst.AST, leftMostSafe: cdAst.SafeMethodCall|cdAst.SafePropertyRead, mode: _Mode): any {
// If the expression contains a safe access node on the left it needs to be converted to
// an expression that guards the access to the member by checking the receiver for blank. As
// execution proceeds from left to right, the left most part of the expression must be guarded
// first but, because member access is left associative, the right side of the expression is at
// the top of the AST. The desired result requires lifting a copy of the the left part of the
// expression up to test it for blank before generating the unguarded version.
// Consider, for example the following expression: a?.b.c?.d.e
// This results in the ast:
// .
// / \
// ?. e
// / \
// . d
// / \
// ?. c
// / \
// a b
// The following tree should be generated:
//
// /---- ? ----\
// / | \
// a /--- ? ---\ null
// / | \
// . . null
// / \ / \
// . c . e
// / \ / \
// a b , d
// / \
// . c
// / \
// a b
//
// Notice that the first guard condition is the left hand of the left most safe access node
// which comes in as leftMostSafe to this routine.
let guardedExpression = this.visit(leftMostSafe.receiver, _Mode.Expression);
let temporary: o.ReadVarExpr;
if (this.needsTemporary(leftMostSafe.receiver)) {
// If the expression has method calls or pipes then we need to save the result into a
// temporary variable to avoid calling stateful or impure code more than once.
temporary = this.allocateTemporary();
// Preserve the result in the temporary variable
guardedExpression = temporary.set(guardedExpression);
// Ensure all further references to the guarded expression refer to the temporary instead.
this._resultMap.set(leftMostSafe.receiver, temporary);
}
const condition = guardedExpression.isBlank();
// Convert the ast to an unguarded access to the receiver's member. The map will substitute
// leftMostNode with its unguarded version in the call to `this.visit()`.
if (leftMostSafe instanceof cdAst.SafeMethodCall) {
this._nodeMap.set(
leftMostSafe,
new cdAst.MethodCall(
leftMostSafe.span, leftMostSafe.receiver, leftMostSafe.name, leftMostSafe.args));
} else {
this._nodeMap.set(
leftMostSafe,
new cdAst.PropertyRead(leftMostSafe.span, leftMostSafe.receiver, leftMostSafe.name));
}
// Recursively convert the node now without the guarded member access.
const access = this.visit(ast, _Mode.Expression);
// Remove the mapping. This is not strictly required as the converter only traverses each node
// once but is safer if the conversion is changed to traverse the nodes more than once.
this._nodeMap.delete(leftMostSafe);
// If we allcoated a temporary, release it.
if (temporary) {
this.releaseTemporary(temporary);
}
// Produce the conditional
return convertToStatementIfNeeded(mode, condition.conditional(o.literal(null), access));
}
// Given a expression of the form a?.b.c?.d.e the the left most safe node is
// the (a?.b). The . and ?. are left associative thus can be rewritten as:
// ((((a?.c).b).c)?.d).e. This returns the most deeply nested safe read or
// safe method call as this needs be transform initially to:
// a == null ? null : a.c.b.c?.d.e
// then to:
// a == null ? null : a.b.c == null ? null : a.b.c.d.e
private leftMostSafeNode(ast: cdAst.AST): cdAst.SafePropertyRead|cdAst.SafeMethodCall {
const visit = (visitor: cdAst.AstVisitor, ast: cdAst.AST): any => {
return (this._nodeMap.get(ast) || ast).visit(visitor);
};
return ast.visit({
visitBinary(ast: cdAst.Binary) { return null; },
visitChain(ast: cdAst.Chain) { return null; },
visitConditional(ast: cdAst.Conditional) { return null; },
visitFunctionCall(ast: cdAst.FunctionCall) { return null; },
visitImplicitReceiver(ast: cdAst.ImplicitReceiver) { return null; },
visitInterpolation(ast: cdAst.Interpolation) { return null; },
visitKeyedRead(ast: cdAst.KeyedRead) { return visit(this, ast.obj); },
visitKeyedWrite(ast: cdAst.KeyedWrite) { return null; },
visitLiteralArray(ast: cdAst.LiteralArray) { return null; },
visitLiteralMap(ast: cdAst.LiteralMap) { return null; },
visitLiteralPrimitive(ast: cdAst.LiteralPrimitive) { return null; },
visitMethodCall(ast: cdAst.MethodCall) { return visit(this, ast.receiver); },
visitPipe(ast: cdAst.BindingPipe) { return null; },
visitPrefixNot(ast: cdAst.PrefixNot) { return null; },
visitPropertyRead(ast: cdAst.PropertyRead) { return visit(this, ast.receiver); },
visitPropertyWrite(ast: cdAst.PropertyWrite) { return null; },
visitQuote(ast: cdAst.Quote) { return null; },
visitSafeMethodCall(ast: cdAst.SafeMethodCall) { return visit(this, ast.receiver) || ast; },
visitSafePropertyRead(ast: cdAst.SafePropertyRead) {
return visit(this, ast.receiver) || ast;
}
});
}
// Returns true of the AST includes a method or a pipe indicating that, if the
// expression is used as the target of a safe property or method access then
// the expression should be stored into a temporary variable.
private needsTemporary(ast: cdAst.AST): boolean {
const visit = (visitor: cdAst.AstVisitor, ast: cdAst.AST): boolean => {
return ast && (this._nodeMap.get(ast) || ast).visit(visitor);
};
const visitSome = (visitor: cdAst.AstVisitor, ast: cdAst.AST[]): boolean => {
return ast.some(ast => visit(visitor, ast));
};
return ast.visit({
visitBinary(ast: cdAst.Binary):
boolean{return visit(this, ast.left) || visit(this, ast.right);},
visitChain(ast: cdAst.Chain) { return false; },
visitConditional(ast: cdAst.Conditional):
boolean{return visit(this, ast.condition) || visit(this, ast.trueExp) ||
visit(this, ast.falseExp);},
visitFunctionCall(ast: cdAst.FunctionCall) { return true; },
visitImplicitReceiver(ast: cdAst.ImplicitReceiver) { return false; },
visitInterpolation(ast: cdAst.Interpolation) { return visitSome(this, ast.expressions); },
visitKeyedRead(ast: cdAst.KeyedRead) { return false; },
visitKeyedWrite(ast: cdAst.KeyedWrite) { return false; },
visitLiteralArray(ast: cdAst.LiteralArray) { return true; },
visitLiteralMap(ast: cdAst.LiteralMap) { return true; },
visitLiteralPrimitive(ast: cdAst.LiteralPrimitive) { return false; },
visitMethodCall(ast: cdAst.MethodCall) { return true; },
visitPipe(ast: cdAst.BindingPipe) { return true; },
visitPrefixNot(ast: cdAst.PrefixNot) { return visit(this, ast.expression); },
visitPropertyRead(ast: cdAst.PropertyRead) { return false; },
visitPropertyWrite(ast: cdAst.PropertyWrite) { return false; },
visitQuote(ast: cdAst.Quote) { return false; },
visitSafeMethodCall(ast: cdAst.SafeMethodCall) { return true; },
visitSafePropertyRead(ast: cdAst.SafePropertyRead) { return false; }
});
}
private allocateTemporary(): o.ReadVarExpr {
const tempNumber = this._currentTemporary++;
this.temporaryCount = Math.max(this._currentTemporary, this.temporaryCount);
return new o.ReadVarExpr(temporaryName(this.bindingId, tempNumber));
}
private releaseTemporary(temporary: o.ReadVarExpr) {
this._currentTemporary--;
if (temporary.name != temporaryName(this.bindingId, this._currentTemporary)) {
throw new Error(`Temporary ${temporary.name} released out of order`);
}
}
}
function flattenStatements(arg: any, output: o.Statement[]) {
if (Array.isArray(arg)) {
(<any[]>arg).forEach((entry) => flattenStatements(entry, output));
} else {
output.push(arg);
}
}
function createCachedLiteralArray(builder: ClassBuilder, values: o.Expression[]): o.Expression {
if (values.length === 0) {
return o.importExpr(createIdentifier(Identifiers.EMPTY_ARRAY));
}
const proxyExpr = o.THIS_EXPR.prop(`_arr_${builder.fields.length}`);
const proxyParams: o.FnParam[] = [];
const proxyReturnEntries: o.Expression[] = [];
for (let i = 0; i < values.length; i++) {
const paramName = `p${i}`;
proxyParams.push(new o.FnParam(paramName));
proxyReturnEntries.push(o.variable(paramName));
}
createPureProxy(
o.fn(
proxyParams, [new o.ReturnStatement(o.literalArr(proxyReturnEntries))],
new o.ArrayType(o.DYNAMIC_TYPE)),
values.length, proxyExpr, builder);
return proxyExpr.callFn(values);
}
function createCachedLiteralMap(
builder: ClassBuilder, entries: [string, o.Expression][]): o.Expression {
if (entries.length === 0) {
return o.importExpr(createIdentifier(Identifiers.EMPTY_MAP));
}
const proxyExpr = o.THIS_EXPR.prop(`_map_${builder.fields.length}`);
const proxyParams: o.FnParam[] = [];
const proxyReturnEntries: [string, o.Expression][] = [];
const values: o.Expression[] = [];
for (let i = 0; i < entries.length; i++) {
const paramName = `p${i}`;
proxyParams.push(new o.FnParam(paramName));
proxyReturnEntries.push([entries[i][0], o.variable(paramName)]);
values.push(<o.Expression>entries[i][1]);
}
createPureProxy(
o.fn(
proxyParams, [new o.ReturnStatement(o.literalMap(proxyReturnEntries))],
new o.MapType(o.DYNAMIC_TYPE)),
entries.length, proxyExpr, builder);
return proxyExpr.callFn(values);
}
class DefaultNameResolver implements NameResolver {
callPipe(name: string, input: o.Expression, args: o.Expression[]): o.Expression { return null; }
getLocal(name: string): o.Expression { return null; }
}
function createCurrValueExpr(bindingId: string): o.ReadVarExpr {
return o.variable(`currVal_${bindingId}`); // fix syntax highlighting: `
}
function createPreventDefaultVar(bindingId: string): o.ReadVarExpr {
return o.variable(`pd_${bindingId}`);
}
function convertStmtIntoExpression(stmt: o.Statement): o.Expression {
if (stmt instanceof o.ExpressionStatement) {
return stmt.expr;
} else if (stmt instanceof o.ReturnStatement) {
return stmt.value;
}
return null;
}