为什么即使我没有实现必要的功能，Parcelable 也能工作？

Question

I wanted to retain a complex java object during screen rotation, so I made the object Parcelable and implemented necessary methods:

1. in writeToParcel(Parcel dest, int flags) method, I some saved values to "dest".
2. in Parcelable.Creator's createFromParcel(Parcel source) method, I obtained the values from "source" in correct order and returned the appropriate object.

Then in Fragment's onSaveInstanceState I saved the Parcelable:

@Override
public void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
    outState.putParcelable("myObject", myObject);
}

and got my object in Fragment's onCreate:

@Override
public void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  MyObject myObject = savedInstanceState.getParcelable("myObject");
}

This worked Perfectly.

Then I made the following test:

1. Deleted all my code in writeToParcel method.
2. Returned null in createFramParcel method.

When I ran the app I got THE EXACT SAME RESULT. I got an object with all the appropriate values in it.

Why did this work? Does Parcelable create Parcelable objects "automatically"?

Answer 1

Yeah, so this has to do with the way Bundle handles caching and parceling internally. When you call putParcelable() , it runs the following code:

public void putParcelable(@Nullable String key, @Nullable Parcelable value) {
    unparcel();
    mMap.put(key, value);
    mFdsKnown = false;
}

So basically, data in a Bundle is not immediately written to a Parcel -- mMap is an ArrayMap<String, Object> and it contains a cache of all the objects in the Bundle as they're inserted or removed.

At some point, writeToParcel() will be called on the Bundle , at which point everything in mMap gets written into mParcelledData .

So basically, when you do a config change, the Bundle still hasn't been written to a Parcel , so the same instance of the object you passed in is still stored in the Bundle 's mMap (so your Object also has never had writeToParcel() invoked -- you can confirm this by asserting that the object before and after config change have the same System.identityHashCode() ).

You can see notes about this in BaseBundle :

// Invariant - exactly one of mMap / mParcelledData will be null
// (except inside a call to unparcel)

ArrayMap<String, Object> mMap = null;

/*
 * If mParcelledData is non-null, then mMap will be null and the
 * data are stored as a Parcel containing a Bundle.  When the data
 * are unparcelled, mParcelledData willbe set to null.
 */
Parcel mParcelledData = null;

So if you were to write your Parcelable object to the save state bundle, and put your app in the background until the process dies (or I believe you can force that by running adb shell am kill <application_id> ) and then resume, you'll then run into the problem where your data isn't parceled correctly.