This Drawable implements the "Drawer-Indicator to Arrow"-Animation as seen in several Material-Design-Apps; NOTE: Mind the updated constructors in Revision 5!

DrawerIconDrawable.java

/*
 * Copyright 2014 Johannes Homeier
 *
 * 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.
 */

package de.twoid.drawable;

import android.graphics.Canvas;
import android.graphics.ColorFilter;
import android.graphics.Paint;
import android.graphics.Rect;
import android.graphics.RectF;
import android.graphics.drawable.Drawable;
import android.view.Gravity;

public class DrawerIconDrawable extends Drawable {
	public static final int STATE_DRAWER = 0;
	public static final int STATE_ARROW = 1;

	private static final float BASE_DRAWABLE_SIZE = 48f;
	private static final float BASE_ICON_SIZE = 18f;
	private static final float BASE_BAR_WIDTH = BASE_ICON_SIZE;
	private static final float BASE_BAR_HEIGHT = 2f;
	private static final float BASE_BAR_SPACING = 5f;
	private static final float BASE_BAR_SHRINKAGE = BASE_BAR_WIDTH/6f;
	
	private static final float FULL_ROTATION = 720f;
	private static final float TOPRECT_FIRST_ROTATION = 450f;
	private static final float TOPRECT_SECOND_ROTATION = FULL_ROTATION-TOPRECT_FIRST_ROTATION;
	private static final float MIDRECT_FIRST_ROTATION = 360f;
	private static final float MIDRECT_SECOND_ROTATION = FULL_ROTATION-MIDRECT_FIRST_ROTATION;
	private static final float BOTRECT_FIRST_ROTATION = 270f;
	private static final float BOTRECT_SECOND_ROTATION = FULL_ROTATION-BOTRECT_FIRST_ROTATION;

	private static final float LEVEL_BREAKPOINT = 0.5f;

	// level of the animation
	private float level;

	// Dimensions
	private int width;
	private int height;
	private int offsetX;
	private int offsetY;
	private float barWidth;
	private float barHeight;
	private float barSpacing;
	private float barShrinkage;

	// Drawing-Objects
	private Paint mPaint;
	private Rect iconRect;
	private RectF topRect;
	private RectF middleRect;
	private RectF bottomRect;
	
	private int gravity = Gravity.CENTER;

	private boolean breakpointReached = false;
	

	/**
	 * Create a new DrawerIconDrawableV1 with size in pixel
	 * 
	 * @param size the size (both width and height) this drawable should have in pixel
	 */
	public DrawerIconDrawable(int size) {
		this(size, size);
	}

	/**
	 * Create a new DrawerIconDrawableV1 with a specfied width  and height  in pixel
	 * @param width the width this drawable should have in pixel
	 * @param height the height this drawable should have in pixel
	 */
	public DrawerIconDrawable(int width, int height) {
		this(width, height, 0, 0);
	}
	
	/**
	 * Create a new DrawerIconDrawableV1 with specified width and height in pixel and also apply a {@link Gravity} to align the icon
	 * @param width the width this drawable should have in pixel 
	 * @param height the height this drawable should have in pixel
	 * @param gravity the gravity to align the icon in this drawable
	 */
	public DrawerIconDrawable(int width, int height, int gravity) {
		this(width, height, 0, 0, gravity);
	}
	
	/**
	 * Create a new DrawerIconDrawableV1 with specified width and height in pixel and also apply a offset to the icon
	 * @param width the width this drawable should have in pixel
	 * @param height the height this drawable should have in pixel
	 * @param offsetX the offset the icon should have from its center in x-direction
	 * @param offsetY the offset the icon should have from its center in y-direction
	 */
	public DrawerIconDrawable(int width, int height, int offsetX, int offsetY) {
		this(width, height, 0, 0, Gravity.CENTER);
	}
	
	/**
	 * Create a new DrawerIconDrawableV1 with specified width and height in pixel and also apply a offset to the icon plus a {@link Gravity} to align the icon
	 * @param width the width this drawable should have in pixel
	 * @param height the height this drawable should have in pixel
	 * @param offsetX the offset the icon should have from its center in x-direction
	 * @param offsetY the offset the icon should have from its center in y-direction
	 * @param gravity the gravity to align the icon in this drawable
	 */
	public DrawerIconDrawable(int width, int height, int offsetX, int offsetY, int gravity) {
		this.width = width;
		this.height = height;
		this.offsetX = offsetX;
		this.offsetY = offsetY;
		this.gravity = gravity;

		setBounds(new Rect(0, 0, width, height));

		mPaint = new Paint();
		mPaint.setAntiAlias(true);
		mPaint.setColor(0xffffffff);

		iconRect = new Rect();
		topRect = new RectF();
		middleRect = new RectF();
		bottomRect = new RectF();

		setDefaultIconSize();
		setLevel(0);
	}

	@Override
	public void draw(Canvas canvas) {
		canvas.translate(iconRect.left + offsetX, iconRect.top + offsetY);
		float scaleFactor = level < LEVEL_BREAKPOINT ? level * 2 : (level - 0.5f) * 2;

		drawTopRect(canvas, scaleFactor);
		drawMiddleRect(canvas, scaleFactor);
		drawBottomRect(canvas, scaleFactor);
	}

	private void drawTopRect(Canvas canvas, float scaleFactor) {
		canvas.save();
		offsetTopRect(barShrinkage * scaleFactor, 0, -barShrinkage * scaleFactor, 0);
		canvas.rotate(
				level < LEVEL_BREAKPOINT 
					? level * TOPRECT_FIRST_ROTATION 
					: LEVEL_BREAKPOINT*TOPRECT_FIRST_ROTATION + (1 - level) * TOPRECT_SECOND_ROTATION
				, iconRect.width()/2
				, iconRect.height()/2);
		canvas.drawRect(topRect, mPaint);
		canvas.restore();
	}

	private void drawMiddleRect(Canvas canvas, float scaleFactor) {
		canvas.save();
		offsetMiddleRect(0, 0, -barShrinkage*2f/3f * scaleFactor, 0);
		canvas.rotate(
				level < LEVEL_BREAKPOINT 
					? level * MIDRECT_FIRST_ROTATION 
					: LEVEL_BREAKPOINT*MIDRECT_FIRST_ROTATION + (1 - level) * MIDRECT_SECOND_ROTATION
				, iconRect.width()/2
				, iconRect.height()/2);
		canvas.drawRect(middleRect, mPaint);
		canvas.restore();
	}

	private void drawBottomRect(Canvas canvas, float scaleFactor) {	
		canvas.save();
		offsetBottomRect(barShrinkage * scaleFactor, 0, -barShrinkage * scaleFactor,
				0);
		canvas.rotate(
				level < LEVEL_BREAKPOINT 
					? level * BOTRECT_FIRST_ROTATION 
					: LEVEL_BREAKPOINT*BOTRECT_FIRST_ROTATION + (1 - level) * BOTRECT_SECOND_ROTATION
				, iconRect.width()/2
				, iconRect.height()/2);
		canvas.drawRect(bottomRect, mPaint);
		canvas.restore();
	}

	private void offsetTopRect(float offsetLeft, float offsetTop,
			float offsetRight, float offsetBottom) {
		topRect.set(
				iconRect.width()/2 - barWidth/2 + offsetLeft
				, iconRect.height()/2 - barSpacing - barHeight/2 + offsetTop
				, iconRect.width()/2 + barWidth/2 + offsetRight
				, iconRect.height()/2 - barSpacing + barHeight/2 + offsetBottom);
	}

	private void offsetMiddleRect(float offsetLeft, float offsetTop,
			float offsetRight, float offsetBottom) {
		middleRect.set(
				iconRect.width()/2 - barWidth/2 + offsetLeft
				, iconRect.height()/2 - barHeight/2 + offsetTop
				, iconRect.width()/2 + barWidth/2 + offsetRight
				, iconRect.height()/2 + barHeight/2 + offsetBottom);
	}

	private void offsetBottomRect(float offsetLeft, float offsetTop,
			float offsetRight, float offsetBottom) {
		bottomRect.set(
				iconRect.width()/2 - barWidth/2 + offsetLeft
				, iconRect.height()/2 + barSpacing - barHeight/2 + offsetTop
				, iconRect.width()/2 + barWidth/2 + offsetRight
				, iconRect.height()/2 + barSpacing + barHeight/2 + offsetBottom);
	}

	@Override
	public void setAlpha(int alpha) {
		mPaint.setAlpha(alpha);
		invalidateSelf();
	}

	@Override
	public void setColorFilter(ColorFilter cf) {
		mPaint.setColorFilter(cf);
		invalidateSelf();
	}

	@Override
	public int getOpacity() {
		return 0;
	}

	/**
	 * set the color of the Drawable;
	 * @param color
	 */
	public void setColor(int color) {
		mPaint.setColor(color);
		invalidateSelf();
	}
	
	/**
	 * set the size of the icon; this size should be smaller than the size of this drawable itself to fully show the transformation!
	 * @param size the size of the icon in pixel
	 */
	public void setIconSize(int size){
		if(size > Math.min(width, height)) size = Math.min(width, height);
		
		iconRect.set(0,0,size,size);
		Gravity.apply(gravity, iconRect.width(), iconRect.height(), getBounds(), iconRect);
		
		float ratio = size / BASE_ICON_SIZE;
		barWidth = BASE_BAR_WIDTH * ratio;
		barHeight = BASE_BAR_HEIGHT * ratio;
		barSpacing = BASE_BAR_SPACING * ratio;
		barShrinkage = BASE_BAR_SHRINKAGE * ratio;
		
		invalidateSelf();
	}
	
	/**
	 * Apply a {@link Gravity} to align the icon in this drawable
	 * @param gravity the gravity to align the icon in this drawable
	 */
	public void setGravity(int gravity){
		Gravity.apply(gravity, iconRect.width(), iconRect.height(), getBounds(), iconRect);
		invalidateSelf();
	}
	
	/**
	 * resets the icon size to its default size (as specified in the Material-Design-guidelines
	 * this means, the icon will be 1/3 of the minimal size of this drawable
	 */
	public void setDefaultIconSize(){
		setIconSize((int) (Math.min(width, height) * BASE_ICON_SIZE/BASE_DRAWABLE_SIZE));
	}
	
	/**
	 * offset the icon from its center
	 * @param offsetX the offset the icon should have from its center in x-direction
	 * @param offsetY the offset the icon should have from its center in y-direction
	 */
	public void offsetIcon(int offsetX, int offsetY){
		this.offsetX = offsetX;
		this.offsetY = offsetY;
		invalidateSelf();
	}

	/**
	 * set the state of the Drawable;
	 * 
	 * @param level
	 */
	public void setState(int state) {
		switch (state) {
		case STATE_DRAWER:
			setLevel((float) STATE_DRAWER);
			break;
		case STATE_ARROW:
			setLevel((float) STATE_ARROW);
			break;
		}
	}

	/**
	 * set the level of the animation; drawer indicator is fully displayed at 0;
	 * arrow is fully displayed at 1
	 * 
	 * @param level
	 */
	public void setLevel(float level) {
		if (level == 1)
			breakpointReached = true;
		if (level == 0)
			breakpointReached = false;

		this.level = (breakpointReached ? LEVEL_BREAKPOINT : 0) + level / 2;
		invalidateSelf();
	}

	@Override
	public int getIntrinsicWidth() {
		return width;
	}

	@Override
	public int getIntrinsicHeight() {
		return height;
	}
}

MainActivity.java

package de.twoid.drawericondrawabletest;

import android.app.Activity;
import android.os.Bundle;
import android.view.Gravity;

public class MainActivity extends Activity {
	DrawerIconDrawable drawerIconDrawable;
	
	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		
		float DIPS = getResources().getDisplayMetrics().density;
		
		drawerIconDrawable = new DrawerIconDrawable(
				(int) (72*DIPS)		//width of the drawable
				, (int) (48*DIPS)	//height of the drawable
				, (int) (18*DIPS)	//x-offset of the icon
				, 0			//y-offset of the icon
				, Gravity.CENTER_VERTICAL | Gravity.LEFT	//let the icon center vertically and align to the left of the drawable
				);
		
		getActionBar().setDisplayShowHomeEnabled(false);
		getActionBar().setDisplayHomeAsUpEnabled(true);
		getActionBar().setHomeAsUpIndicator(drawerIconDrawable);
	}
}

Any chance of a example of how to get this into the actionbar as the drawer icon?

This should do it:

getActionBar().setHomeAsUpIndiactor(drawerIconDrawable);

Do I just copy and paste this into the code of my app?

Yes, just copy it ;) I'm going to post a updated version soon though.. so you might just wait for that

I must be missing something completely obvious to get this working .. :(

mDrawerIcon = new DrawerIconDrawable(50, 50, 16);
getSupportActionBar().setDisplayShowTitleEnabled(false);
getSupportActionBar().setHomeAsUpIndicator(mDrawerIcon);

Is that all that's needed? When doing this, I only see my application icon in the action bar, not the drawable...

UPDATE:
Ok, I got it working! In case anyone has similar issues, the fix for me was to use:

getSupportActionBar().setLogo(mDrawerIcon);

And, then to achieve the spinning effect, in the ActionBarDrawerToggle::onDrawerSlide() set:
mDrawerIcon.setLevel(slideOffset);

And yes, homj, it sounds like you're aware of the scaling issues. It appears ~2x too small, and the incoming size isn't used.

Looking forward to your next update! And, thank you for providing a wonderfully self-contained "library"!!

I just updated the code! You can now set the icon-size besides the drawable-size itself and apply a offset and a gravity to the icon to arrange it.

Feel free to test it and provide some feedback if you notice any bugs or issues!