android {
    compileSdkVersion 25
    defaultConfig {
	applicationId "com.anthonyfdev.dropdownviewexample"
	minSdkVersion 19
	targetSdkVersion 25
    }
}

allprojects {
	repositories {
	...
        maven { url 'https://jitpack.io' }
    }
}

dependencies {
    compile 'com.github.AnthonyFermin:DropDownView:1.0.1'
}

<com.anthonyfdev.dropdownview.DropDownView
    android:id="@+id/drop_down_view"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    app:containerBackgroundColor="#b71c1c"
    app:overlayColor="#64000000"/>

dropDownView = (DropDownView) findViewById(R.id.drop_down_view);
collapsedView = LayoutInflater.from(this).inflate(R.layout.view_my_drop_down_header, null, false);
expandedView = LayoutInflater.from(this).inflate(R.layout.view_my_drop_down_expanded, null, false);

dropDownView.setHeaderView(collapsedView);
dropDownView.setExpandedView(expandedView);

collapsedView.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        if (dropDownView.isExpanded()) {
            dropDownView.collapseDropDown();
        } else {
            dropDownView.expandDropDown();
        }
    }
});

/**
* A listener that wraps functionality to be performed when the drop down is successfully expanded
* or collapsed.
*/
private final DropDownView.DropDownListener dropDownListener = new DropDownView.DropDownListener() {
	@Override
	public void onExpandDropDown() {
	    adapter.notifyDataSetChanged();
	    ObjectAnimator.ofFloat(headerChevronIV, View.ROTATION.getName(), 180).start();
	}
  
	@Override
	public void onCollapseDropDown() {
	    ObjectAnimator.ofFloat(headerChevronIV, View.ROTATION.getName(), -180, 0).start();
	}
    };
     
...
 
dropDownView.setDropDownListener(dropDownListener);

Copyright 2017 DropDownView

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.