Composition over inheritance for Android components like Activity or Fragment
Composition over inheritance
Allows to add functionality into an Android Activity
. Just because we all have a BaseActivity
in our projects containing too much unused stuff. When it grows, it get unmaintainable.
setContentView()
MvpActivity
) or Flow to your Activities when you need itGiven you have an Activity
showing a list of tweets (TweetStreamActivity
) and you want add view tracking.
You could do it with inheritance and use TrackedTweetStreamActivity
from now on:
public class TrackedTweetStreamActivity extends TweetStreamActivity {
@Override
protected void onResume() {
super.onResume();
Analytics.trackView("stream");
}
}
more likely you would create a TrackedActivity
and extend the TweetStreamActivity
from it:
public abstract class TrackedActivity extends AppCompatActivity {
public abstract String getTrackingName();
@Override
protected void onResume() {
super.onResume();
Analytics.trackView(getTrackingName());
}
}
public class TrackedTweetStreamActivity extends TrackedActivity {
@Override
public String getTrackingName() {
return "stream";
}
}
Both solutions work but don't scale well. You'll most likely end up with big inheritance structures:
class MvpActivity extends AppCompatActivity { ... }
class BaseActivity extends AppCompatActivity { ... }
class BaseMvpActivity extends MvpActivity { ... }
class WizardUiActivity extends BaseActivity { ... }
class TrackedWizardUiActivity extends WizardUiActivity { ... }
class TrackedBaseActivity extends BaseActivity { ... }
class TrackedMvpBaseActivity extends BaseMvpActivity { ... }
Some libraries out there provide both, a specialized Activity
extending AppCompatActivity
and a delegate with a documentation when to call which function of the delegate in your Activity
.
public class TrackingDelegate {
/**
* usage:
* <pre>{@code
*
* @Override
* protected void onResume() {
* super.onResume();
* mTrackingDelegate.onResume();
* }
* } </pre>
*/
public void onResume() {
Analytics.trackView("<viewName>");
}
}
public class TweetStreamActivity extends AppCompatActivity {
private final TrackingDelegate mTrackingDelegate = new TrackingDelegate();
@Override
protected void onResume() {
super.onResume();
mTrackingDelegate.onResume();
}
}
This is an elegant solution but breaks when updating such a library and the delegate call position has changed. Or when the delegate added new callbacks which don't get automatically implemented by increasing the version number in the build.gradle
.
CompositeAndroid let's you add delegates to your Activity without adding calls to the correct location. Such delegates are called Plugins
. A Plugin is able to inject code at every position in the Activity lifecycle. It is able to override every method.
CompositeAndroid is available via jcenter
dependencies {
// it's very important to use the same version as the support library
def supportLibraryVersion = "25.0.0"
// contains CompositeActivity
implementation "com.pascalwelsch.compositeandroid:activity:$supportLibraryVersion"
// contains CompositeFragment and CompositeDialogFragment
implementation "com.pascalwelsch.compositeandroid:fragment:$supportLibraryVersion"
// core module (not required, only abstract classes and utils)
implementation "com.pascalwelsch.compositeandroid:core:$supportLibraryVersion"
}
Extend from one of the composite implementations when you want to add plugins. This is the only inheritance you have to make.
- public class MyActivity extends AppCompatActivity {
+ public class MyActivity extends CompositeActivity {
- public class MyFragment extends Fragment { // v4 support library
+ public class MyFragment extends CompositeFragment {
Use the constructor to add plugins. Do not add plugins in #onCreate()
. That's too late. Many Activity
methods are called before #onCreate()
which could be important for a plugin to work.
public class MainActivity extends CompositeActivity {
final LoadingIndicatorPlugin loadingPlugin = new LoadingIndicatorPlugin();
public MainActivity() {
addPlugin(new ViewTrackingPlugin("Main"));
addPlugin(loadingPlugin);
}
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// ...
// example usage of the LoadingIndicatorPlugin
loadingPlugin.showLoadingIndicator();
}
}
Read more about the ordering of the Plugins here
This is the strength of CompositeAndroid. You don't really have to learn something new. It works like you'd extend you Activity
to add functionality. Let's change the TrackedActivity
from above and create a ViewTrackingPlugin
.
Here the original
public abstract class TrackedActivity extends AppCompatActivity {
public abstract String getTrackingName();
@Override
protected void onResume() {
super.onResume();
Analytics.trackView(getTrackingName());
}
}
As plugin:
public class ViewTrackingPlugin extends ActivityPlugin {
private final String mViewName;
protected TrackedPlugin(final String viewName) {
mViewName = viewName;
}
@Override
public void onResume() {
super.onResume();
Analytics.trackView(mViewName);
}
}
The implementation inside of onResume()
hasn't changed!
Here some information about plugins. The Activity example is used but it works the same for other classes, too.
Plugin
super
executes code before super
of Activitysuper
is allowed and results in not calling super of the Activity
. (The activity will tell if the super
call was required)super
executes code after super
of ActivityNot everything works exactly like you'd use inheritance. Here is a small list of minor things you have to know:
Activity
method of a Plugin
such as onResume()
or getResources()
. Otherwise the call order of the added plugins is not guaranteed. Instead call those methods on the real Activity
with getActivity.onResume()
or getActivity.getResources()
.CompositeActivity#onRetainCustomNonConfigurationInstance()
is final and required for internal usage, use CompositeActivity#onRetainCompositeCustomNonConfigurationInstance()
insteadCompositeActivity#getLastCustomNonConfigurationInstance()
is final and required for internal usage, use CompositeActivity#getLastCompositeCustomNonConfigurationInstance()
insteadPlugin
works by overriding onRetainNonConfigurationInstance()
and returning an instance of CompositeNonConfigurationInstance(key, object)
. Get the data again with getLastNonConfigurationInstance(key)
and make sure you use the correct key
.CompositeAndroid
gets used in productions without major problems. There could be more performance related improvements but it works reliably right now.
Minor problems are:
CompositeAndroid
. I didn't expect this because the API should be really stable, but it happened in the past (upgrading from 24.1.0
to 24.2.0
). That's why CompositeAndroid
has the same version name as the support library. Yes, the support library can be used with and older CompositeAndroid
version. But it can break, as it happened already. Then again all upgrades from 24.2.1
where 100% backwards compatible. We'll see what the future brings.getLastNonConfigurationInstance()
and onRetainCustomNonConfigurationInstance()
did require manual written code.It was a proof of conecpt and it turned out to work great. So great I haven't touched it a lot after the initial draft. Things like the documentation are still missing. I'm still keeping this project up to date but I don't invest much time in performance improvements. I don't need it, it works at it is for me.
super
, not very flexibleCopyright 2016 Pascal Welsch
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
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