DelegateManager.java revision 9fe7fca9bcdceade9c654c6a8dcf0c48be16d78d 
1/*
2 * Copyright (C) 2010 The Android Open Source Project
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 *      http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17package com.android.layoutlib.bridge.impl;
18
19import com.android.annotations.Nullable;
20import com.android.layoutlib.bridge.util.Debug;
21import com.android.layoutlib.bridge.util.SparseWeakArray;
22
23import android.util.SparseArray;
24
25import java.lang.ref.WeakReference;
26import java.util.ArrayList;
27import java.util.List;
28
29/**
30 * Manages native delegates.
31 *
32 * This is used in conjunction with layoublib_create: certain Android java classes are mere
33 * wrappers around a heavily native based implementation, and we need a way to run these classes
34 * in our Eclipse rendering framework without bringing all the native code from the Android
35 * platform.
36 *
37 * Thus we instruct layoutlib_create to modify the bytecode of these classes to replace their
38 * native methods by "delegate calls".
39 *
40 * For example, a native method android.graphics.Matrix.init(...) will actually become
41 * a call to android.graphics.Matrix_Delegate.init(...).
42 *
43 * The Android java classes that use native code uses an int (Java side) to reference native
44 * objects. This int is generally directly the pointer to the C structure counterpart.
45 * Typically a creation method will return such an int, and then this int will be passed later
46 * to a Java method to identify the C object to manipulate.
47 *
48 * Since we cannot use the Java object reference as the int directly, DelegateManager manages the
49 * int -> Delegate class link.
50 *
51 * Native methods usually always have the int as parameters. The first thing the delegate method
52 * will do is call {@link #getDelegate(long)} to get the Java object matching the int.
53 *
54 * Typical native init methods are returning a new int back to the Java class, so
55 * {@link #addNewDelegate(Object)} does the same.
56 *
57 * The JNI references are counted, so we do the same through a {@link WeakReference}. Because
58 * the Java object needs to count as a reference (even though it only holds an int), we use the
59 * following mechanism:
60 *
61 * - {@link #addNewDelegate(Object)} and {@link #removeJavaReferenceFor(long)} adds and removes
62 *   the delegate to/from a list. This list hold the reference and prevents the GC from reclaiming
63 *   the delegate.
64 *
65 * - {@link #addNewDelegate(Object)} also adds the delegate to a {@link SparseArray} that holds a
66 *   {@link WeakReference} to the delegate. This allows the delegate to be deleted automatically
67 *   when nothing references it. This means that any class that holds a delegate (except for the
68 *   Java main class) must not use the int but the Delegate class instead. The integers must
69 *   only be used in the API between the main Java class and the Delegate.
70 *
71 * @param <T> the delegate class to manage
72 */
73public final class DelegateManager<T> {
74    @SuppressWarnings("FieldCanBeLocal")
75    private final Class<T> mClass;
76    private final SparseWeakArray<T> mDelegates = new SparseWeakArray<T>();
77    /** list used to store delegates when their main object holds a reference to them.
78     * This is to ensure that the WeakReference in the SparseWeakArray doesn't get GC'ed
79     * @see #addNewDelegate(Object)
80     * @see #removeJavaReferenceFor(long)
81     */
82    private final List<T> mJavaReferences = new ArrayList<T>();
83    private int mDelegateCounter = 0;
84
85    public DelegateManager(Class<T> theClass) {
86        mClass = theClass;
87    }
88
89    /**
90     * Returns the delegate from the given native int.
91     * <p>
92     * If the int is zero, then this will always return null.
93     * <p>
94     * If the int is non zero and the delegate is not found, this will throw an assert.
95     *
96     * @param native_object the native int.
97     * @return the delegate or null if not found.
98     */
99    @Nullable
100    public T getDelegate(long native_object) {
101        if (native_object > 0) {
102            T delegate =  mDelegates.get(native_object);
103
104            if (Debug.DEBUG) {
105                if (delegate == null) {
106                    System.out.println("Unknown " + mClass.getSimpleName() + " with int " +
107                            native_object);
108                }
109            }
110
111            assert delegate != null;
112            return delegate;
113        }
114        return null;
115    }
116
117    /**
118     * Adds a delegate to the manager and returns the native int used to identify it.
119     * @param newDelegate the delegate to add
120     * @return a unique native int to identify the delegate
121     */
122    public long addNewDelegate(T newDelegate) {
123        long native_object = ++mDelegateCounter;
124        mDelegates.put(native_object, newDelegate);
125        assert !mJavaReferences.contains(newDelegate);
126        mJavaReferences.add(newDelegate);
127
128        if (Debug.DEBUG) {
129            System.out.println("New " + mClass.getSimpleName() + " with int " + native_object);
130        }
131
132        return native_object;
133    }
134
135    /**
136     * Removes the main reference on the given delegate.
137     * @param native_object the native integer representing the delegate.
138     */
139    public void removeJavaReferenceFor(long native_object) {
140        T delegate = getDelegate(native_object);
141
142        if (Debug.DEBUG) {
143            System.out.println("Removing main Java ref on " + mClass.getSimpleName() +
144                    " with int " + native_object);
145        }
146
147        mJavaReferences.remove(delegate);
148    }
149}
150