A lightweight library written for Android to make common data loading tasks easy and simple.
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README.md

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FetchUtils

A lightweight library written for Android to make common data loading tasks easy and simple.

Purpose

The library started out as a refactor, in to reusable components, of data fetching code in the My Bus Edinburgh project.

The problem was that HTTP fetching and file reading code was re-implemented wherever it was used, which of course is bad programming practice. The code was tightly coupled in to its implementation and was not unit tested.

Work was undertaken to refactor this code out in to re-usable components, and for the components to be unit tested as much as possible. This was done, but the code still lived within the project. This project is the extraction of this code in to its own library which will be maintained independently and can easily be imported in to future projects.

Forking and pull requests are welcome.

It is acknowledged that there are better developed libraries out there for doing this sort of thing. This library does not attempt to compete with these libraries. This libraries merely attempts to do a small amount of things well, reliably and is easy to use with only basic knowledge required for most things.

Licence

This project uses the Apache 2.0 licence, as defined below.

Copyright (C) 2014 - 2015 Niall Scott
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

     http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.

Author

This library is created by Niall Scott. Twitter

In forks, credit to original source is appreciated.

Concepts

Fetcher - Reader

A fetcher describes the mechanism for retrieving data. For example, this might mean connecting to a remote server or opening a file.

A reader describes the process that is performed on the data once it reaches the device. For example, the data may be interpreted as plain text, a bitmap image, JSON or even piped straight out to something else (such as downloading a file over HTTP and piped straight out to a file on the device). The reader retains the data or a reference to it throughout its lifecycle.

The idea is that fetchers and readers can be used in any combination with each other. Both conform to their respective interfaces and a reader is passed in to a fetcher. When the fetcher receives data, it passes this data in to a reader. Generally, fetchers and readers are immutable - new instances should be created for new fetches.

In code, a fetcher is defined as;

public interface Fetcher {

    public void executeFetcher(FetcherStreamReader reader) throws IOException;
}

...and a reader is defined as;

public interface FetcherStreamReader {

    public void readInputStream(InputStream stream) throws IOException;
}

When executeFetcher(FetcherStreamReader reader) is called on the Fetcher, it will deal with the set up of the stream. When the stream is ready, readInputStream(InputStream stream) will be called on the reader and will block until the reader no longer requires the data stream. Once this unblocks, the Fetcher will deal with tearing down the stream.

Caveat: the whole process is blocking. There is no attempt at dealing with threading in this part of the library. Android already has good ways at dealing with threading, such as using Loaders in UI code, and AsyncTask in non-UI code. You may wish to use SimpleAsyncTaskLoader included in this library to perform asynchronous operations from the UI.

Loaders

Read the Android documentation about Loaders here. Understanding this is critical before using the Loader helper classes in this library.

Loaders are an advanced Android topic. They were introduced in Android 3.0 (Honeycomb) to get around the problem of asynchronously loading data in to a user interface that can be destroyed and recreated. This is because of Android's UI lifecycle - Activities can be destroyed and recreated due to system configuration changes. For example, this may happen when the device is rotated or the system language is changed. This is default behaviour and can be overridden - but this is discouraged by the engineers at Google.

When the user interface is destroyed and recreated, the reference to the loaded data is lost in the process as new object instances are used. It could be possible to get the data again by initiating another load (typically AsyncTasks were used), persisting the data on disk (SQLite database, files) or some convoluted and complicated solution to keep it in memory during rotation.

Loaders makes this much easier for us. It deals with the persistence of the data in memory during the re-creation of the user interface. Once the user interface is ready again, it simply connects back to the Loader and it can get the previously loaded data.

There are other benefits to Loaders and documenting them extensively here is beyond the scope of this document.

Loaders are also available in the v4 compatibility library, making their functionality available from Android 1.6 (Donut).

Why are Loaders mentioned here?

Good question.

Many of Google's examples of usage of Loaders depends on using a CursorLoader. What if you want to load data from another source, such as a JSON document over the network, and not persist it in a database? Then you may use an AsyncTaskLoader (CursorLoader actually extends from AsyncTaskLoader). The default Loader class does not provide any background threading at all - this is provided by AsyncTaskLoader, which, as you might have guessed, is backed by an AsyncTask.

But there's a problem with AsyncTaskLoader. It does not deliver its data as expected with its lifecycle - and is poorly documented. Provided by this library is an extended AsyncTaskLoader that does deliver the data correctly by doing what CursorLoader does (albeit without the Cursor stuff).

Please see the documentation for SimpleAyncTaskLoader to see how it works and for a code example. Two versions of the class has been provided: one that extends the platform's AsyncTaskLoader (can be used on API 11+) and one that extends AsyncTaskLoader from the compatibility library (can be used on API 4+). It goes without saying that the compatibility library needs to be included before the compatibility SimpleAsyncTaskLoader can be used.

Usage

To use this library in your project, add the following as a dependency inside your build.gradle for the module that this library will be used in;

dependencies {
    compile 'uk.org.rivernile.android.fetchutils:fetchutils-library:1.1'
}

Fetcher - Reader

Select a fetcher class;

  • AssetFileFetcher - used to fetch the contents of an Android asset file
  • FileFetcher - used to fetch the contents of a File on disk
  • HttpFetcher - used to fetch the contents of data at a given URL from a HTTP server
  • Or create your own Fetcher by creating a class and implementing Fetcher

...then select a reader class;

  • BitmapFetcherStreamReader - used to read a stream of data when it describes a bitmap image
  • FileWriterStreamReader - used to read a stream of data and instantly output it to a file on disk
  • JSONFetcherStreamReader - used to read a stream of data and create a JSONArray or a JSONObject out of it
  • StringFetcherStreamReader - used to read a stream of data and hold the data as a String in memory
  • Or create your own FetcherStreamReader by creating a class and implementing FetcherStreamReader

Here is a code sample to open a file within the application's sandbox and load the entire contents of the file in to memory as a String;

// Create a File object pointing to the file on disk to read from. someContext is an instance of
// Android's Context class.
final File fileToOpen = new File(someContext.getFilesDir(), "yourfile.txt");
// Create an instance of the FileFetcher to read from a file on disk.
final FileFetcher fetcher = new FileFetcher(fileToOpen);
// Create an instance of StringFetcherStreamReader to handle the data that is read in.
final StringFetcherStreamReader reader = new StringFetcherStreamReader();

// Fetcher.executeFetcher() mandates that IOException should be caught.
try {
    fetcher.executeFetcher(reader); // This blocks while executing. Don't run on UI thread.
    System.out.println(reader.getData()); // Access the data from reader.getData()
} catch (IOException e) {
    // Catch errors here. You could also catch errors such as FileNotFoundException here if you
    // wanted to.
}

Here is a code sample to copy a file from Android assets and output it straight to a file in the application's sandbox;

// The name of the file to read from the assets directory.
final String fileName = "assetfile.txt";
// Create a File object pointing to the file on disk that the data will be written to.
final File fileToWrite = new File(someContext.getFilesDir(), "writtenfile.txt");
// Create an instance of the AssetFileFetcher  to read from a file in the application assets.
final AssetFileFetcher fetcher = new AssetFileFetcher(someContext, fileName);
// Create an instance of FileWriterStreamReader to output the data to the application sandbox.
final FileWriterStreamReader reader = new FileWriterStreamReader(fileToWrite, false);

try {
    fetcher.executeFetcher(reader); // Remember, this blocks.

    // File is now copied from assets to application sandbox.
} catch (IOException e) {
    // Catch errors here.
}

Again, the fetchers and readers can be used in any combination that makes sense, so they are flexible.

SimpleAsyncTaskLoader

This class exists in two forms: one which extends from the platform's AsyncTaskLoader (API level 11+) and one which extends from the v4 compatibility library AsyncTaskLoader. Be sure to use the correct one for your needs when importing the class.

import uk.org.rivernile.android.fetchutils.loaders.SimpleAsyncTaskLoader;

or

import uk.org.rivernile.android.fetchutils.loaders.support.SimpleAsyncTaskLoader;

SimpleAsyncTaskLoader is an abstract class - it's up to you to provide the implementation for what you want to load asynchronously. It may be a call to the network or to read from a file. Tip: you may want to use the Fetcher-Reader mechanism in your implementation.

Result is a class used to encapsulate the result of a load. Loaders can only return one object to their caller upon completion, so a Result object is used to hold a success object and a failure object. It's up to you what data is returned from the Loader - Result merely provides an easy way to return a success object and a failure object. In this object, the failure state can only accept an object that is of the type Exception or its descendants. If this is not suitable, then you may create your own class that can be returned by the Loader.

Here is a code example of a SimpleAsyncTaskLoader returning a String;

public class HttpAsyncTaskLoader extends SimpleAsyncTaskLoader<String> {

    // Context is required.
    private final Context context;
    // The URL to connect to.
    private final String url;

    public HttpAsyncTaskLoader(final Context context, final String url) {
        // Super needs to be called with a valid Context object.
        super(context);

        // Arguments are sent in to the constructor.
        this.context = context;
        this.url = url;
    }

    @Override
    public String loadInBackground() {
        // The Fetcher using the URL passed in to the constructor.
        final HttpFetcher.Builder builder = new HttpFetcher.Builder(context);
        builder.setUrl(url)
                .setAllowHostRedirects(false);
        final HttpFetcher fetcher = builder.build();
        final StringFetcherStreamReader reader = new StringFetcherStreamReader();

        try {
            fetcher.executeFetcher(reader);
            // If fetching was successful, return the data in the reader.
            return reader.getData();
        } catch (IOException e) {
            // If it failed, return null.
            return null;
        }
    }
}

Here is a code example, similar to above, that uses a Result instead;

public class HttpResultAsyncTaskLoader extends SimpleAsyncTaskLoader<Result<String, IOException>> {

    // Context is required.
    private final Context context;
    // The URL to connect to.
    private final String url;

    public HttpResultAsyncTaskLoader(final Context context, final String url) {
        // Super needs to be called with a valid Context object.
        super(context);

        // Arguments are sent in to the constructor.
        this.context = context;
        this.url = url;
    }

    @Override
    public Result<String, IOException> loadInBackground() {
        // The Fetcher using the URL passed in to the constructor.
        final HttpFetcher.Builder builder = new HttpFetcher.Builder(context);
        builder.setUrl(url)
                .setAllowHostRedirects(false);
        final HttpFetcher fetcher = builder.build();
        final StringFetcherStreamReader reader = new StringFetcherStreamReader();

        try {
            fetcher.executeFetcher(reader);
            // If fetching was successful, return the data in the reader via the Result object.
            return new Result<String, IOException>(reader.getData());
        } catch (IOException e) {
            // If it failed, pass the Exception in to the Result object.
            return new Result<String, IOException>(e);
        }
    }
}

...and where you handle your Loader callbacks, most probably in your Activity or Fragment;

...
public void onLoadFinished(Loader<Result<String, IOException>> loader,
                           Result<String, IOException> data) {
    if (data.isError()) {
        doSuccessThing(data.getSuccess());
    } else {
        doErrorThing(data.getError());
    }
}
...

Javadoc

See http://www.rivernile.org.uk/projects/fetchutils/1.1/docs/

To do

  • Add ability to make HttpFetcher also accept data to be posted up to the server
  • Improve unit tests
  • Add an example project

Versions

1.1.1

  • Fixed a crash that could occur in HttpFetcher when there is a connectivity problem
  • Updated library dependency versions
  • There have been no API changes in this release

1.1

  • More comprehensive API for HttpFetcher to match HttpURLConnection as much as possible
  • If the calling application has the permission ACCESS_NETWORK_STATE, then HttpFetcher will check connectivity with ConnectivityManager prior to attempting the connection
  • Target latest Android version (API level 22)
  • Update to latest Android Gradle version and latest build tools
  • The project is now distributed via Maven Central

1.0.1 (development - never released)

  • Added SimpleAsyncTaskLoader
  • Added Javadoc generation task to build.gradle
  • Added integration with Travis CI server
  • Added Maven Central push support

1.0

Initial release.