A Parcelable Tutorial for Android

PARCELABLE INTERFACE OVERVIEW

In one of my earlier posts, I mentioned writing an article about FOSOAuthBundle integration with an Android client. To keep that article to the point, I need to explain some concepts beforehand. One of the important concepts is the Android Parcelable interface that allows data to be transferred between different processes/threads. Certain network operations with Android such as authentication with OAuth2 and then fetching data from a REST endpoint should be performed in the background in order not to block the UI thread. This requires data to be fetched by a service (I have opted for Intent Services in my implementation) in the background and then passed back to the calling activity/fragment with a result callback. This is where the Parcelable interface comes into play.

Basically, theParcelableinterface allows your classes to be flattened inside a message container called aParcelto facilitate high performance inter process communication. The received parcel data can then be unflattened to generate object/entity instances.

A BASIC PARCELABLE EXAMPLE

A Parcelable implementation is pretty straight forward. Override the necessary methods called writeToParcel() and describeContents(), add a static field called CREATOR which generates instances of your Parcelable class from a Parcel, and overload the class constructor which expects a Parcel as a parameter and calls the readFromParcel() utility method. Here is a basic example:

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publicclassConversationimplementsParcelable{
//...
protectedStringlastComment;
protectedIntegermessageCount;
protectedDatecreatedAt;
//...
//...
publicConversation(Parcelin){
readFromParcel(in);
}
@Override
publicvoidwriteToParcel(Parcelout,intflags){
//...
out.writeString(lastComment);
out.writeInt(messageCount);
out.writeSerializable(createdAt);
//...
}
privatevoidreadFromParcel(Parcelin){
//...
lastComment=in.readString();
messageCount=in.readInt();
createdAt=(Date)in.readSerializable();
//...
}
publicstaticfinalParcelable.Creator<conversation>CREATOR=newParcelable.Creator<conversation>(){
publicConversationcreateFromParcel(Parcelin){
returnnewConversation(in);
}
publicConversation[]newArray(intsize){
returnnewConversation[size];
}
};
@Override
publicintdescribeContents(){
return0;
}
}
</conversation></conversation>

NESTED PARCELABLE CLASSES

Lets say we have a Conversation class instance with an embedded User class instance that holds some information about a user that has initiated the conversation. Basically, we are talking about an one-to-one embedded mapping. In this case, if thefromUserproperty holds a User instance, then writing the user data to parcel would be accomplished as shown below:

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@Override
publicvoidwriteToParcel(Parcelout,intflags){
//...
out.writeParcelable(fromUser,flags);
//...
}

Reading the user data from parcel:

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privatevoidreadFromParcel(Parcelin){
//...
fromUser=in.readParcelable(User.class.getClassLoader());
//...
}

If you have a one-to-many embedded relationship such as a list of messages in a conversation, then the syntax would change as follows:

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@Override
publicvoidwriteToParcel(Parcelout,intflags){
//...
out.writeList(messages);
//...
}

Reading from the parcel:

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privatevoidreadFromParcel(Parcelin){
//...
in.readList(messages,Message.class.getClassLoader());
//...
}

BOOLEAN TYPES

Android API does not have a method to write a single boolean value to a parcel. In this case, you can utilize the writeInt() method as shown below:

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@Override
publicvoidwriteToParcel(Parcelout,intflags){
//...
out.writeInt(booleanValue?1:0);
//...
}

Reading from the parcel:

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privatevoidreadFromParcel(Parcelin){
//...
booleanValue=in.readInt()==1;
//...
}

ENUM TYPES

To flatten an Enum type in a parcel, simply implement the Parcelable interface for the Enum type. Here is an example:

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publicenumStatusimplementsParcelable{
STARTED,PAUSED,FINISHED;
publicstaticfinalParcelable.Creator<status>CREATOR=newParcelable.Creator<status>(){
publicStatuscreateFromParcel(Parcelin){
returnStatus.values()[in.readInt()];
}
publicStatus[]newArray(intsize){
returnnewStatus[size];
}
};
@Override
publicintdescribeContents(){
return0;
}
@Override
publicvoidwriteToParcel(Parcelout,intflags){
out.writeInt(ordinal());
}
}
</status></status>

When writing to the parcel, treat it as a nested Parcelable class:

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@Override
publicvoidwriteToParcel(Parcelout,intflags){
//...
out.writeParcelable(status,flags);
//...
}

Reading from the parcel:

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privatevoidreadFromParcel(Parcelin){
//...
status=in.readParcelable(Status.class.getClassLoader());
//...
}

PARCELABLE INTERFACE OVERVIEW

A BASIC PARCELABLE EXAMPLE

NESTED PARCELABLE CLASSES

BOOLEAN TYPES

ENUM TYPES

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