How to Develop IME or SoftKeyboard or Custom Keyboard in Android?

Introduction:

BaseInputConnection
Defines the communication channel from an InputMethod back to the application that is receiving its input.
You use it to read text around the cursor, commit text to the text box, and send raw key events to the application.
Applications should extend this class rather than implementing the base interface InputConnection.

KeyboardView
An extension of View that renders a keyboard and responds to user input events.
The keyboard layout is specified by an instance of Keyboard, which you can define in an XML file.

Menifest:
In the Android system, an IME is an Android application that contains a IME service. The application's manifest file must declare the service, request the necessary permissions, provide an intent filter that matches the action action.view.InputMethod, and provide metadata that defines characteristics of the IME.
eg:-
<service android:name=".SimpleIME"
    android:label="@string/simple_ime"
    android:permission="android.permission.BIND_INPUT_METHOD"
    >
    <meta-data android:name="android.view.im" android:resource="@xml/method"/>
    <intent-filter>
        <action android:name="android.view.InputMethod" />
    </intent-filter>          
</service>

How can Design Keyboard Layout:
The layout of our keyboard contains only a KeyboardView.
<?xml version="1.0" encoding="UTF-8"?>
<android.inputmethodservice.KeyboardView
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/keyboard"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_alignParentBottom="true"
    android:keyPreviewLayout ="@layout/preview"
/>

Create a SoftKeyboard Class:

Create a new Java class and call it SoftKeyboard.java. The class should extend InputMethodService class and implement the OnKeyboardActionListener interface. The OnKeyboardActionListener interface contains the methods that are called when keys of the soft keyboard are tapped or pressed.

The SoftKeyboard class should have three member variables:

1.a KeyboardView referencing the view defined in the layout
2.a Keyboard instance that is assigned to the KeyboardView
3.a boolean telling us if the caps lock is enabled

After declaring these variables and adding the methods of the OnKeyboardActionListener interface, the SoftKeyboard class should look like this:

Switching among IME Subtypes:

You can allow users to switch easily among multiple IME subtypes by providing a switching key, such as the globe-shaped language icon, as part of the keyboard. Doing so greatly improves the keyboard's usability, and can help avoid user frustration. To enable such switching, perform the following steps:

Declare supportsSwitchingToNextInputMethod = "true" in the input method's XML resource files. Your declaration should look similar to the following snippet:
<input-method xmlns:android="http://schemas.android.com/apk/res/android"
        android:settingsActivity="com.example.softkeyboard.Settings"
        android:icon="@drawable/ime_icon"
        android:supportsSwitchingToNextInputMethod="true">

Call the shouldOfferSwitchingToNextInputMethod() method.