lint/libs/lint-checks/src/main/java/com/android/tools/lint/checks/FragmentDetector.java

/*
 * Copyright (C) 2012 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.android.tools.lint.checks;

import static com.android.SdkConstants.CLASS_FRAGMENT;
import static com.android.SdkConstants.CLASS_V4_FRAGMENT;
import static com.android.tools.lint.client.api.JavaParser.ResolvedClass;
import static com.android.tools.lint.client.api.JavaParser.ResolvedNode;

import com.android.annotations.NonNull;
import com.android.annotations.Nullable;
import com.android.tools.lint.detector.api.Category;
import com.android.tools.lint.detector.api.Detector;
import com.android.tools.lint.detector.api.Detector.JavaScanner;
import com.android.tools.lint.detector.api.Implementation;
import com.android.tools.lint.detector.api.Issue;
import com.android.tools.lint.detector.api.JavaContext;
import com.android.tools.lint.detector.api.Location;
import com.android.tools.lint.detector.api.Scope;
import com.android.tools.lint.detector.api.Severity;
import com.android.tools.lint.detector.api.Speed;

import java.lang.reflect.Modifier;
import java.util.Collections;
import java.util.List;

import lombok.ast.AstVisitor;
import lombok.ast.ClassDeclaration;
import lombok.ast.ConstructorDeclaration;
import lombok.ast.ForwardingAstVisitor;
import lombok.ast.Node;
import lombok.ast.NormalTypeBody;
import lombok.ast.TypeMember;

/**
 * Checks that Fragment subclasses can be instantiated via
 * {link {@link Class#newInstance()}}: the class is public, static, and has
 * a public null constructor.
 * <p>
 * This helps track down issues like
 *   http://stackoverflow.com/questions/8058809/fragment-activity-crashes-on-screen-rotate
 * (and countless duplicates)
 */
public class FragmentDetector extends Detector implements JavaScanner {
    /** Are fragment subclasses instantiatable? */
    public static final Issue ISSUE = Issue.create(
        "ValidFragment", //$NON-NLS-1$
        "Fragment not instantiatable",
        "Ensures that `Fragment` subclasses can be instantiated",

        "From the Fragment documentation:\n" +
        "*Every* fragment must have an empty constructor, so it can be instantiated when " +
        "restoring its activity's state. It is strongly recommended that subclasses do not " +
        "have other constructors with parameters, since these constructors will not be " +
        "called when the fragment is re-instantiated; instead, arguments can be supplied " +
        "by the caller with `setArguments(Bundle)` and later retrieved by the Fragment " +
        "with `getArguments()`.",

        Category.CORRECTNESS,
        6,
        Severity.FATAL,
        new Implementation(
                FragmentDetector.class,
                Scope.JAVA_FILE_SCOPE)
        ).addMoreInfo(
            "http://developer.android.com/reference/android/app/Fragment.html#Fragment()"); //$NON-NLS-1$


    /** Constructs a new {@link FragmentDetector} */
    public FragmentDetector() {
    }

    @NonNull
    @Override
    public Speed getSpeed() {
        return Speed.FAST;
    }

    // ---- Implements JavaScanner ----

    @Nullable
    @Override
    public List<Class<? extends Node>> getApplicableNodeTypes() {
        return Collections.<Class<? extends Node>>singletonList(ClassDeclaration.class);
    }

    @Nullable
    @Override
    public AstVisitor createJavaVisitor(@NonNull final JavaContext context) {
        return new ForwardingAstVisitor() {
            @Override
            public boolean visitClassDeclaration(ClassDeclaration node) {
                int flags = node.astModifiers().getEffectiveModifierFlags();
                if ((flags & Modifier.ABSTRACT) != 0) {
                    return true;
                }

                ResolvedNode resolved = context.resolve(node);
                if (!(resolved instanceof ResolvedClass)) {
                    return true;
                }

                ResolvedClass cls = (ResolvedClass) resolved;

                if (!cls.isSubclassOf(CLASS_FRAGMENT, false)
                        && !cls.isSubclassOf(CLASS_V4_FRAGMENT, false)) {
                    return true;
                }

                if ((flags & Modifier.PUBLIC) == 0) {
                    String message = String.format("This fragment class should be public (%1$s)",
                            cls.getName());
                    context.report(ISSUE, node, context.getLocation(node.astName()), message,
                            null);
                    return true;
                }

                if (cls.getContainingClass() != null && (flags & Modifier.STATIC) == 0) {
                    String message = String.format(
                            "This fragment inner class should be static (%1$s)", cls.getName());
                    context.report(ISSUE, node, context.getLocation(node.astName()), message,
                            null);
                    return true;
                }

                boolean hasDefaultConstructor = false;
                boolean hasConstructor = false;
                NormalTypeBody body = node.astBody();
                if (body != null) {
                    for (TypeMember member : body.astMembers()) {
                        if (member instanceof ConstructorDeclaration) {
                            hasConstructor = true;
                            ConstructorDeclaration constructor = (ConstructorDeclaration) member;
                            if (constructor.astParameters().isEmpty()) {
                                // The constructor must be public
                                if (constructor.astModifiers().isPublic()) {
                                    hasDefaultConstructor = true;
                                } else {
                                    Location location = context.getLocation(
                                            constructor.astTypeName());
                                    context.report(ISSUE, constructor, location,
                                            "The default constructor must be public",
                                            null);
                                    // Also mark that we have a constructor so we don't complain again
                                    // below since we've already emitted a more specific error related
                                    // to the default constructor
                                    hasDefaultConstructor = true;
                                }
                            } else {
                                Location location = context.getLocation(constructor.astTypeName());
                                // TODO: Use separate issue for this which isn't an error
                                String message = "Avoid non-default constructors in fragments: "
                                        + "use a default constructor plus "
                                        + "Fragment#setArguments(Bundle) instead";
                                context.report(ISSUE, constructor, location, message, null);
                            }
                        }
                    }
                }

                if (!hasDefaultConstructor && hasConstructor) {
                    String message = String.format(
                            "This fragment should provide a default constructor (a public " +
                            "constructor with no arguments) (%1$s)",
                            cls.getName()
                    );
                    context.report(ISSUE, node, context.getLocation(node.astName()), message,
                            null);
                }

                return true;
            }
        };
    }
}