Android Jetpack 之 App Startup，这个回答让我错失offer

App Startup 的普通使用:

==================

假设当前有三个Sdk库需要进行初始化 SdkA、SdkB、SdkC：（SdkB、SdkC的code和SdkA的结构一模一样）

class SdkA {

private object Instance {

val instance = SdkA()

}

companion object{

fun getInstance(): SdkA {

return Instance.instance

}

}

}

创建对应的初始化对象，必须接口 Initializer，接口 Initializer 的代码如下：

/**

• {@link Initializer}s can be used to initialize libraries during app startup, without

• the need to use additional {@link android.content.ContentProvider}s.

• @param The instance type being initialized

*/

public interface Initializer {

/**

• Initializes and a component given the application {@link Context}

• @param context The application context.

*/

@NonNull

T create(@NonNull Context context);

/**

• @return A list of dependencies that this {@link Initializer} depends on. This is

• used to determine initialization order of {@link Initializer}s.

• 
  

• For e.g. if a {@link Initializer} B defines another

• {@link Initializer} A as its dependency, then A gets initialized before B.

*/

@NonNull

List<Class<? extends Initializer<?>>> dependencies();

}

create 方法用于进行对象的初始化；

dependencies 方法用于定义需要在当前对象初始化之前进行初始化的对象对应的 Initializer。

SdkA 的对应的 SdkAInitializer：（SdkBInitializer、SdkCInitializer的code和SdkAInitializer的结构 一模一样）

class SdkAInitializer :Initializer{

val TAG = “SdkInitializer”

override fun create(context: Context): SdkA {

Log.d(TAG, "SdkA create: ")

return SdkA.getInstance()

}

override fun dependencies(): MutableList<Class<out Initializer<*>>> {

Log.d(TAG, "SdkA dependencies: ")

//SdkA 的初始化不需要其他库的依赖，所以这里 dependencies 方法返回一个空列表。

return Collections.emptyList()

}

}

接着在 manifest 中定义 InitializationProvider：

<provider

android:name=“androidx.startup.InitializationProvider”

android:authorities="${applicationId}.androidx-startup"

android:exported=“false”

tools:node=“merge”>

<meta-data

android:name=“com.noway.startup.SdkAInitializer”

android:value=“androidx.startup” />

<meta-data

android:name=“com.noway.startup.SdkBInitializer”

android:value=“androidx.startup” />

<meta-data

android:name=“com.noway.startup.SdkCInitializer”

android:value=“androidx.startup” />

run App 查看log：在App create之前SdkA、SdkB、SdkC依次先dependencies后create

2020-10-10 10:04:07.894 20431-20431/com.noway.startup D/SdkInitializer: App attachBaseContext:

2020-10-10 10:04:07.901 20431-20431/com.noway.startup D/SdkInitializer: SdkA dependencies:

2020-10-10 10:04:07.901 20431-20431/com.noway.startup D/SdkInitializer: SdkA create:

2020-10-10 10:04:07.902 20431-20431/com.noway.startup D/SdkInitializer: SdkB dependencies:

2020-10-10 10:04:07.902 20431-20431/com.noway.startup D/SdkInitializer: SdkB create:

2020-10-10 10:04:07.902 20431-20431/com.noway.startup D/SdkInitializer: SdkC dependencies:

2020-10-10 10:04:07.902 20431-20431/com.noway.startup D/SdkInitializer: SdkC create:

2020-10-10 10:04:07.904 20431-20431/com.noway.startup D/SdkInitializer: App onCreate:

2020-10-10 10:04:08.117 20431-20431/com.noway.startup D/SdkInitializer: MainActivity onCreate:

从log可以看出，dependencies 方法的执行顺序是 A→B→C，create 方法的执行顺序是 A → B → C。这里的 create 方法的顺序正是对象创建的顺序。

App Startup 的互相依赖使用:

====================

假设SdkB初始化需要依赖SdkC,需要先让SdkC初始化，只需要修改SdkB的code，把返回的空集合改为SdkC

class SdkBInitializer :Initializer{

val TAG = “SdkInitializer”

override fun create(context: Context): SdkB {

Log.d(TAG, "SdkB create: ")

return SdkB.getInstance()

}

override fun dependencies(): MutableList<Class<out Initializer<*>>> {

Log.d(TAG, "SdkB dependencies: ")

//SdkB 依赖 SdkC

return mutableListOf(SdkCInitializer::class.java)

// return Collections.emptyList()

}

}

run App 查看log：

2020-10-10 10:12:21.233 20922-20922/com.noway.startup D/SdkInitializer: App attachBaseContext:

2020-10-10 10:12:21.239 20922-20922/com.noway.startup D/SdkInitializer: SdkA dependencies:

2020-10-10 10:12:21.239 20922-20922/com.noway.startup D/SdkInitializer: SdkA create:

2020-10-10 10:12:21.239 20922-20922/com.noway.startup D/SdkInitializer: SdkB dependencies:

2020-10-10 10:12:21.240 20922-20922/com.noway.startup D/SdkInitializer: SdkC dependencies:

2020-10-10 10:12:21.240 20922-20922/com.noway.startup D/SdkInitializer: SdkC c

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开源分享完整内容戳这里

reate:

2020-10-10 10:12:21.241 20922-20922/com.noway.startup D/SdkInitializer: SdkB create:

2020-10-10 10:12:21.242 20922-20922/com.noway.startup D/SdkInitializer: App onCreate:

2020-10-10 10:12:21.357 20922-20922/com.noway.startup D/SdkInitializer: MainActivity onCreate:

从log可以看出，dependencies 方法的执行顺序是 A → B → C，create 方法的执行顺序是 A → C → B。

App Startup 的手动初始化（延时初始化）使用:

============================

当应用运行后，所有的 SDK 都进行了初始化。可是所有的 SDK 都在启动阶段进行初始化，会导致启动速度的变慢，某些 SDK 如何在需要使用的时候才进行初始化？

再创建一个 SdkD（和SdkA一样）需要在 manifest 中通过  标签中只需要定义 SdkDInitializer，并且添加  tools:node=“remove”：