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To share binary data across app boundaries Android's programming model expects you to use content providers. And while I have covered how to access content providers and how to code one yourself , I haven't covered how to deal with binary data. I am going to correct that in this post. The ContentResolver class offers two methods to access binary data of content providers: Obvioulsly you use the openInputStream method to read data and the openOutputStream method to write data.
Both methods expect an id-based URI as parameter. It's up to you to close the stream later on. The contract of content providers requires you to implement the openFile method. The method of the base class will always throw a FileNotFoundException. The message is a tad confusing though the URI might differ, of course:. I think an IllegalStateException with a message text of "method not supported" would be better.
Android provides the helper method openFileHelper that makes implementing the openFile method very easy. As you can see in the next snippet, implementing the openFile method get's pretty simple:. Three messages are possible:. If you want to use another column for your filename or if you can compute it in some way, you cannot use openFileHelper. In this case you have to implement the openFile method yourself. Have a look at the source code of ContentProvider's openFileHelper method to get you started.
What I haven't covered so far is when and how to create the file. And there is a reason for that. The problem with binary data and content providers is, that you cannot create them whenever you feel like it. Instead for all this to function, you have to create a filename at the same time as the record is created. But the file will be added later on using the openOutputStream method. It get's complicated when you have a model in which binary data are optional. Because now you cannot create a filename when the record is created since you do not know whether the client intends to add binary data at all.
The solution to this dilemma is to use indirection. The sample project deals with keeping track of lent items - and the user can add a snapshot of the item to the record but is not required to do so. Thus I have added a table solely for those snapshots. The big downside of adding binary data with content providers is that you rely on a well-behaving client.
Of course normally it should be the other way round. The client could just insert a record into your content provider but never actually add the file. Should another client try to use it, your provider would throw an exception. This probably is the reason why the MediaStore content provider offers methods to insert images in its nested MediaStore. Media class and why for other media the media scanner is the preferred solution. You really should think about using the same approach.
Write a class that defines the contract of your provider, exports the URIs as constants, defines the columns and that offers helper methods to create whatever binary data you need. Within these methods you do what I have described above - but now you are no longer at the mercy of your clients. Sometimes you might have multiple binary data entries that belong to one object. Say you are developing a birdwatcher app. Now you might want to show videos of birds, as well as photos and maybe even the sound they are making.
At least three items, maybe even multiple items per media type. I think there are two approaches you could use in this case. For example separate URIs for bird videos, bird twitterings and bird photos. But if the meta data are the same for all types e. He has been interested in Android for quite a while and has been blogging about all kind of topics around Android. Great article, very helpful. However one aspect was neglected. Lets say the user deletes the record, How do I then delete file.
I have one concern that I have come across as I am trying to implement something like this. If I use openFileHelper , then a call comes to my query method. This returns a Cursor interface. But because Cursor is an interface, I have no idea what concrete class to actually create and return. Do you have any suggestions? Erik, one of the simplest implementations of Cursor interface is MatrixCursor. It is based on java collections to back the data.
Or you can see the other implementors of Cursor interface. There is also an abstract base cursor and wrappers. Wolfram, very nice article! Helped me so much! I think you need to call cr. Your email address will not be published. How to access binary data of existing content providers The ContentResolver class offers two methods to access binary data of content providers: No files supported by provider at content: Use an id-based URI only. Wolfram Rittmeyer lives in Germany and has been developing with Java for many years.
Writing your own Content Provider Android Tutorial: Using Content Providers Android Tutorial: October 24, at October 9, at November 28, at Leave a Reply Cancel reply Your email address will not be published.