在何处/何时使用 description 内容() ？

有人知道在哪里/什么时候调用 可包装的的这个方法吗？

@Override
public int describeContents() {
return 0;
}

它必须被重写，但是我应该考虑用它做些有用的事情吗？

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最佳答案

There is a constant defined in Parcelable called CONTENTS_FILE_DESCRIPTOR which is meant to be used in describeContents() to create bitmask return value.

Description for CONTENTS_FILE_DESCRIPTOR in the API ref is:

Bit masks for use with describeContents(): each bit represents a kind of object considered to have potential special significance when marshalled.

Which really means: If you need to put FileDescriptor object into Parcelable you should/must specify CONTENTS_FILE_DESCRIPTOR as return value of describeContents(), i.e. by "special object" (in describeContents()'s description) they really mean: FileDescriptor.

This whole Parcelable functionality looks unfinished (read: has bad design). There is one other strange thing in the docs:

Classes implementing the Parcelable interface must also have a static field called CREATOR, which is an object implementing the Parcelable.Creator interface

Implementing multiple inheritance by rules defined in human readable form? :-)

It seems like C++ programmer designed Parceable and at some point he realized: Oh, damn, there is no multiple inheritance in Java... :-)

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There is only two possible value, 0 or CONTENTS_FILE_DESCRIPTOR

if you are serializing POLO, this value should always be 0, the CONTENTS_FILE_DESCRIPTOR is reserved for ParcelFileDescriptor, which could serialize a File Descriptor(FD) in *unix system.

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From android framework, the only usage occurs in ActivityManagerService.java:

//ActivityManagerService.java
public int startActivityIntentSender(IApplicationThread caller,
IntentSender intent, Intent fillInIntent, String resolvedType,
IBinder resultTo, String resultWho, int requestCode,
int flagsMask, int flagsValues) {
// Refuse possible leaked file descriptors
if (fillInIntent != null && fillInIntent.hasFileDescriptors()) {
throw new IllegalArgumentException("File descriptors passed in Intent");
}
//...
}

Intent.java hasFileDescriptors() comes from Bundle.java hasFileDescriptors(). And the bundle will iterator all data in mMap(hashMap) or mParcelledData(Parcel). We will figure out intent.hasFileDescriptors() just wraps Parcel/Parcelable describeContents().

While, maybe this is the only usage for describeContents() : it use to filter FileDescriptor from Intent pass...


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