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 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.Arrays;
import java.util.List;

import lombok.ast.ClassDeclaration;
import lombok.ast.ConstructorDeclaration;
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",

        "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<String> applicableSuperClasses() {
        return Arrays.asList(CLASS_FRAGMENT, CLASS_V4_FRAGMENT);
    }

    @Override
    public void checkClass(@NonNull JavaContext context, @Nullable ClassDeclaration node,
            @NonNull Node declarationOrAnonymous, @NonNull ResolvedClass cls) {
        if (node == null) {
            return;
        }

        int flags = node.astModifiers().getEffectiveModifierFlags();
        if ((flags & Modifier.ABSTRACT) != 0) {
            return;
        }

        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);
            return;
        }

        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);
            return;
        }

        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");
                            // 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);
                    }
                }
            }
        }

        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);
        }
    }
}