1 /*
2  * Copyright (C) 2021 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 
17 package com.android.wm.shell.dagger;
18 
19 import java.lang.annotation.Documented;
20 import java.lang.annotation.Inherited;
21 import java.lang.annotation.Retention;
22 import java.lang.annotation.RetentionPolicy;
23 
24 import javax.inject.Qualifier;
25 
26 /**
27  * This is a qualifier that Shell uses to workaround an issue with providing nullable optionals
28  * which are by default unbound.
29  *
30  * For example, ideally we would have this scenario:
31  * BaseModule:
32  *   @BindsOptionalOf
33  *   abstract Optional<Interface> optionalInterface();
34  *
35  * SpecializedModule:
36  *   @Provides
37  *   static Interface providesInterface() {
38  *       return new InterfaceImpl();
39  *   }
40  *
41  * However, if the interface is supposed to be provided dynamically, then Dagger is not able to bind
42  * the optional interface to a null instance, and @BindsOptionalOf does not support @Nullable
43  * instances of the interface provided by the specialized module.
44  *
45  * For example, this does not work:
46  * BaseModule:
47  *   @BindsOptionalOf
48  *   abstract Optional<Interface> optionalInterface();
49  *
50  * SpecializedModule:
51  *   @Provides
52  *   static Interface providesInterface() {
53  *       if (systemSupportsInterfaceFeature) {
54  *           return new InterfaceImpl();
55  *       } else {
56  *           return null;
57  *       }
58  *   }
59  *
60  * To workaround this, we can instead upstream the check (assuming it can be upstreamed into the
61  * base module), and then always provide a non-null instance in the specialized module.
62  *
63  * For example:
64  * BaseModule:
65  *   @BindsOptionalOf
66  *   @DynamicOverride
67  *   abstract Interface dynamicInterface();
68  *
69  *   @Provides
70  *   static Optional<Interface> providesOptionalInterface(
71  *           @DynamicOverride Optional<Interface> interface) {
72  *       if (systemSupportsInterfaceFeature) {
73  *           return interface;
74  *       }
75  *       return Optional.empty();
76  *   }
77  *
78  * SpecializedModule:
79  *   @Provides
80  *   @DynamicOverride
81  *   static Interface providesInterface() {
82  *       return new InterfaceImpl();
83  *   }
84  *
85  * This is also useful in cases where there needs to be a default implementation in the base module
86  * which is also overridable in the specialized module.  This isn't generally recommended, but
87  * due to the nature of Shell modules being referenced from a number of various projects, this
88  * can be useful for *required* components that
89  * 1) clearly identifies which are intended for overriding in the base module, and
90  * 2) allows us to declare a default implementation in the base module, without having to force
91  *    every SysUI impl to explicitly provide it (if a large number of them share the default impl)
92  *
93  * For example, this uses the same setup as above, but the interface provided (if bound) is used
94  * otherwise the default is created:
95  *
96  * BaseModule:
97  *   @BindsOptionalOf
98  *   @DynamicOverride
99  *   abstract Interface dynamicInterface();
100  *
101  *   @Provides
102  *   static Optional<Interface> providesOptionalInterface(
103  *           @DynamicOverride Optional<Interface> overrideInterfaceImpl) {
104  *       if (overrideInterfaceImpl.isPresent()) {
105  *           return overrideInterfaceImpl.get();
106  *       }
107  *       return new DefaultImpl();
108  *   }
109  *
110  * SpecializedModule:
111  *   @Provides
112  *   @DynamicOverride
113  *   static Interface providesInterface() {
114  *       return new SuperSpecialImpl();
115  *   }
116  */
117 @Documented
118 @Inherited
119 @Qualifier
120 @Retention(RetentionPolicy.RUNTIME)
121 public @interface DynamicOverride {}