AIDL 传递对象集合所遇到的问题

以你之姓@ 2023-10-18 07:03 100阅读 0赞

在项目使用AIDL中，发现自己以前对于AIDL的理解有些误区。  
以前使用AIDL增加方法，只要iadl文件里的方法顺序不变，在所有的方法之后，增加新方法，新的AIDL是可以兼容原先的AIDL的。即用新的AIDL也可以和老的AIDL进行通讯 (只要不调用新增加的方法就不会有问题)。  
但是，在传递`LIst<Bean>`的过程中，发现，如果`Bean`的字段有增减，则新Aidl和老Aidl通讯会有问题，无法兼容。此时，Client端和Server端的`Bean`必须是同一个，不能有差别。

#### 以下是做的相关的实验: ####

RemoteSong.java

public class RemoteSong implements Parcelable {
        private int id;
        private String title;
        private int duration;
        //get / set 省略
    
        @Override
        public int describeContents() {
            return 0;
        }
    
        @Override
        public void writeToParcel(Parcel dest, int flags) {
            dest.writeInt(this.id);
            dest.writeString(this.title);
            dest.writeInt(this.duration);
        }
    
        public RemoteSong() {
        }
    
        protected RemoteSong(Parcel in) {
            readFromParcel(in);
        }
    
        public void readFromParcel(Parcel in) {
            this.id = in.readInt();
            this.title = in.readString();
            this.duration = in.readInt();
        }
    
        public static final Creator<RemoteSong> CREATOR = new Creator<RemoteSong>() {
            @Override
            public RemoteSong createFromParcel(Parcel source) {
                return new RemoteSong(source);
            }
    
            @Override
            public RemoteSong[] newArray(int size) {
                return new RemoteSong[size];
            }
        };
    }

RemotePlayList.java

public class RemotePlayList implements Parcelable {
        private int id;
        private String name;
        private List<RemoteSong> songs = new ArrayList<>();
        //get / set 省略
    
        public List<RemoteSong> getSongs() {
            if (songs == null) {
                return new ArrayList<>();
            }
            return songs;
        }
    
        public void setSongs(List<RemoteSong> songs) {
            this.songs = songs;
        }
    
        @Override
        public int describeContents() {
            return 0;
        }
    
        @Override
        public void writeToParcel(Parcel dest, int flags) {
            dest.writeInt(this.id);
            dest.writeString(this.name);
            dest.writeTypedList(this.songs);
        }
    
        public RemotePlayList() {
        }
    
        protected RemotePlayList(Parcel in) {
            readFromParcel(in);
        }
    
        public void readFromParcel(Parcel in) {
            this.id = in.readInt();
            this.name = in.readString();
            this.songs = in.createTypedArrayList(RemoteSong.CREATOR);
        }
    
        public static final Creator<RemotePlayList> CREATOR = new Creator<RemotePlayList>() {
            @Override
            public RemotePlayList createFromParcel(Parcel source) {
                return new RemotePlayList(source);
            }
    
            @Override
            public RemotePlayList[] newArray(int size) {
                return new RemotePlayList[size];
            }
        };
    }

IMyAidlInterface.aidl

// IMyAidlInterface.aidl
    package com.heiko.aidltest;
    
    import com.heiko.aidltest.bean.Person;
    import com.heiko.aidltest.bean.RemotePlayList;
    
    interface IMyAidlInterface {
        RemotePlayList getLocalList();
    }

此时，在Aidl Client端程序中，调用getLocalList()获取的数据是正确的

[{
            "duration": 1000,
            "id": 0,
            "title": "标题0"
        }, {
            "duration": 1001,
            "id": 1,
            "title": "标题1"
        }, {
            "duration": 1002,
            "id": 2,
            "title": "标题2"
        }, {
            "duration": 1003,
            "id": 3,
            "title": "标题3"
        }, {
            "duration": 1004,
            "id": 4,
            "title": "标题4"
        }]

接着，我们把 Aidl Server端程序的RemoteSong中增加一个fileName字段

public class RemoteSong implements Parcelable {
        private int id;
        private String title;
        private int duration;
        private String fileName;
        //get / set 省略
    
        public RemoteSong() {
        }
    
        @Override
        public int describeContents() {
            return 0;
        }
    
        @Override
        public void writeToParcel(Parcel dest, int flags) {
            dest.writeInt(this.id);
            dest.writeString(this.title);
            dest.writeInt(this.duration);
            dest.writeString(this.fileName);
        }
    
        protected RemoteSong(Parcel in) {
            readFromParcel(in);
        }
    
        private void readFromParcel(Parcel in) {
            this.id = in.readInt();
            this.title = in.readString();
            this.duration = in.readInt();
            this.fileName = in.readString();
        }
    
        public static final Creator<RemoteSong> CREATOR = new Creator<RemoteSong>() {
            @Override
            public RemoteSong createFromParcel(Parcel source) {
                return new RemoteSong(source);
            }
    
            @Override
            public RemoteSong[] newArray(int size) {
                return new RemoteSong[size];
            }
        };
    }

此时，可以发现，再运行Server端和Client端程序，会发现，获取的数据不正确。

[{
        "duration": 1000,
        "id": 0,
        "title": "标题0"
    }, {
        "duration": 0,
        "id": 1324770695
    }, {
        "duration": 1001,
        "id": 1,
        "title": "标题1"
    }, {
        "duration": 0,
        "id": 1324770695
    }, {
        "duration": 1002,
        "id": 2,
        "title": "标题2"
    }]

#### 为什么数据会出现偏差 ####

先附上IMyAidlInterface.java

@Override
    public com.heiko.aidltest.bean.RemotePlayList getLocalList() throws android.os.RemoteException {
        android.os.Parcel _data = android.os.Parcel.obtain();
        android.os.Parcel _reply = android.os.Parcel.obtain();
        com.heiko.aidltest.bean.RemotePlayList _result;
        try {
            _data.writeInterfaceToken(DESCRIPTOR);
            mRemote.transact(Stub.TRANSACTION_getLocalList, _data, _reply, 0);
            _reply.readException();
            if ((0 != _reply.readInt())) {
                _result = com.heiko.aidltest.bean.RemotePlayList.CREATOR.createFromParcel(_reply);
            } else {
                _result = null;
            }
        } finally {
            _reply.recycle();
            _data.recycle();
        }
        return _result;
    }

查看AIDL生成的IMyAidlInterface.java文件及RemotePlayList里的Parcelable 读写规则，可以发现，是通过Parcelable来进行读写的

public void writeToParcel(Parcel dest, int flags) {
        dest.writeInt(this.id);
        dest.writeString(this.name);
        dest.writeTypedList(this.songs);
    }
    
    public void readFromParcel(Parcel in) {
        this.id = in.readInt();
        this.name = in.readString();
        this.songs = in.createTypedArrayList(RemoteSong.CREATOR);
    }

进入writeTypedList方法，我们来看下

public final <T extends Parcelable> void writeTypedList(List<T> val) {
            if (val == null) {
                writeInt(-1);
                return;
            }
            int N = val.size();
            int i=0;
            writeInt(N);
            while (i < N) {
                T item = val.get(i);
                if (item != null) {
                    writeInt(1);
                    item.writeToParcel(this, 0);
                } else {
                    writeInt(0);
                }
                i++;
            }
        }

可以看到，List会遍历，逐个调用RemoteSong的writeToParcel进行存储，我们知道，上面Demo中，Client端的RemoteSong只写入了4个字段

private void readFromParcel(Parcel in) {
        this.id = in.readInt();
        this.title = in.readString();
        this.duration = in.readInt();
        this.fileName = in.readString();
    }

而Server端的RemoteSong却只读取了3个字段

public void readFromParcel(Parcel in) {
        this.id = in.readInt();
        this.title = in.readString();
        this.duration = in.readInt();
    }

所以，多个遍历读取的时候，就会出现混乱，数据读取会出现错误。

public final <T> ArrayList<T> createTypedArrayList(Parcelable.Creator<T> c) {
        int N = readInt();
        if (N < 0) {
            return null;
        }
        ArrayList<T> l = new ArrayList<T>(N);
        while (N > 0) {
            if (readInt() != 0) {
                l.add(c.createFromParcel(this));
            } else {
                l.add(null);
            }
            N--;
        }
        return l;
    }

#### 小结 ####

所以，我们在使用AIDL传递对象集合的时候，尽量不要修改Bean对象，如果要修改，要确保Server端和Client端的Bean对象保持统一。

[相关Demo程序下载][Demo]

[Demo]: https://github.com/EthanCo/PettyModules/tree/master/AIDLTest/AidlListBeanTest