Use cold start time effectively with a branded launch theme
Pro-tip by +Ian Lake

When your app isn’t in memory and is launched, that ‘cold start’ can take significantly longer than if your app is already in memory. Depending on the size of your app and what you’re doing in your Application’s onCreate() (as little as possible I hope!), there may be lag between when the user starts your app and your Activity’s onCreate() is actually called. During that time, the window manager makes its best effort to draw a placeholder UI using elements from your theme such as the background and status bar color.

But the background doesn’t have to be a solid color: it can be an opportunity to add a little more personality and branding to your app without slowing down the user through the use of a branded launch screen (http://goo.gl/gp6FDE), allowing your app UI to focus on content rather than additional branding. The key is creating a custom theme that overrides android:windowBackground, then replacing that custom theme with your standard theme before calling super.onCreate().

Assuming you have a theme called AppTheme, your launcher theme would be:
<style name="AppTheme.Launcher">
  <item name="android:windowBackground">@drawable/launch_screen</item>
</style>

This implies that everything about the launcher theme is inherited from your main theme - you’re just changing the windowBackground. One other attribute you may consider changing here is colorPrimaryDark: the status bar color on Android 5.0+ devices. Setting colorPrimaryDark to your main background color can put more emphasis on your branding at the expense of another element changing when transitioning to your final theme.

But drawable/launch_screen can’t be just a simple image, unfortunately - it’ll end up stretched to fill the entire screen. Instead, you can use an XML file such as:
<layer-list xmlns:android="http://schemas.android.com/apk/res/android" android:opacity="opaque">
  <!-- The background color, preferably the same as your normal theme -->
  <item android:drawable="@android:color/white"/>
  <!-- Your product logo - 144dp color version of your app icon -->
  <item>
    <bitmap
      android:src="@drawable/product_logo_144dp"
      android:gravity="center"/>
  </item>
</layer-list>

Make particular note of the android:opacity=”opaque” line - this is critical in preventing a flash of black as your theme transitions.

Then apply your theme to your activity in your AndroidManifest.xml using android:theme="@style/AppTheme.Launcher".

The easiest way to transition back to your normal theme is to call setTheme(R.style.AppTheme) before super.onCreate() and setContentView():
public class MyMainActivity extends AppCompatActivity {
 @Override
  protected void onCreate(Bundle savedInstanceState) {
    // Make sure this is before calling super.onCreate
    setTheme(R.style.Theme_MyApp);
    super.onCreate(savedInstanceState);
    // ...
  }
}


Things to note with this approach:
- No launchpad activity - there’s no delay such as there would be if you were launching a second activity from a dedicated splash screen style activity
- No artificial delays - you’re only using the time that you have, just taking advantage of theming
- No extra overdraw - resetting your theme removes a layer of overdraw compared to having an opaque view with your normal background above the custom windowBackground
- Only for your launcher activity - this isn’t appropriate for deep links into your app or handling a URI, but for launches done through the home screen - the point is to minimize dead time, not to annoy users.
- Fast is best - keeping your app lean and minimizing work done at startup is critical to a good experience, even if that means slightly less time for branding - remember: getting users to the content they care about should be your #1 priority.
- Watch your transition - keep both the number and complexity of your transitions to a minimum by sharing as many elements (colors, etc) as possible to make for a seamless transition straight to content.

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