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Android – get DIP with Java

Here's a little class to get density-independent pixels, and pixels from DIP:

package org.upshots.utils;

import android.content.Context;

public class DensityManager {

	private static DensityManager instance = null;
	public static DensityManager getInstance(Context context){
		if(instance == null){
			instance = new DensityManager(context);
		}
		return instance;
	}

	private float scale = 1f;
	private DensityManager(Context context){
		scale = context.getResources().getDisplayMetrics().density;
	}

	public int DIPToPixels(float dip){
		return (int) (0.5 + (dip * scale));
	}

	public int DIPToPixels(int dip){
		return DIPToPixels((float) dip);
	}

	public int DIPToPixels(double dip){
		return DIPToPixels((float) dip);
	}

	public int pixelsToDIP(float pixels){
		return (int) (0.5 + (pixels / scale));
	}

	public int pixelsToDIP(int pixels){
		return pixelsToDIP((float) pixels);
	}

	public int pixelsToDIP(double pixels){
		return pixelsToDIP((float) pixels);
	}
}

usage...

// argument 'context' would be 'this' from an Activity
// or getContext() from a view
DensityManager.getInstance(context).pixelsFromDIP(100);
// or...
DensityManager.getInstance(context).DIPFromPixels(100);

Android – set alpha (opacity) for a custom view

While API 3.0 has a setAlpha method, most of us are still targeting 2.x since penetration for anything over 2.2 is pretty slim.

ImageViews and Drawables already have setAlpha methods, and TextViews can be manipulated using various ARGB colors. Some people just use zero-duration AlphaAnimations, so generally you can get the effect you need, but sometimes you'll need a little more complex behavior (e.g., setting opacity based on drag position) or convenient (e.g., setting the alpha of the contents an entire layout).

Fortunately, it's pretty simple - despite the lack of documentation.

Just extend your Layout and override onDraw, and use the canvas's setLayerAlpha method:

@Override
public void onDraw(Canvas canvas){
	canvas.saveLayerAlpha(0, 0, canvas.getWidth(), canvas.getHeight(), 0x66, Canvas.HAS_ALPHA_LAYER_SAVE_FLAG);
	super.onDraw(canvas);
}

Extended to support method calls:

package org.upshots.widgets;

import android.content.Context;
import android.graphics.Canvas;
import android.widget.RelativeLayout;

public class FadingLayout extends RelativeLayout {

	private int alpha = 255;

	public FadingLayout(Context context) {
		super(context);
		setWillNotDraw(false);
	}

	public int getAlpha(){
		return alpha;
	}

	public void setAlpha(int opacity){
		alpha = opacity;
		invalidate();
	}

	@Override
	public void onDraw(Canvas canvas){
		canvas.saveLayerAlpha(0, 0, canvas.getWidth(), canvas.getHeight(), alpha, Canvas.HAS_ALPHA_LAYER_SAVE_FLAG);
		super.onDraw(canvas);
	}

}

Android – get dimensions of image resource

Context context = // "this" if from an Activity, or "getContext()" if from a View
Resources resources = context.getResources();
BitmapFactory.Options bounds = new BitmapFactory.Options();
bounds.inJustDecodeBounds = true;
// resId below might be something like R.drawable.my_image
BitmapFactory.decodeResource(resources, resId, bounds);
int imageHeight = bounds.outHeight;
int imageWidth = bounds.outWidth;

Android – WindowManager for overlays

Several core classes (including Dialog, PopupWindow and some Menu implementations) create an "overlay layer" that exists above, and separate from, existing UI elements - this is why we don't need to use special layoutmanagers on underlying Activity contentViews in order to position them. While the documentation is sparse, and cloning those classes is challenging (due to the inaccessible com.android.internal classes like PolicyManager), it's actually fairly simple to implement the functionality.

Use WindowManager.addView(view, windowManagerLayoutParams) to add a new layer. E.g., to add a view glued to the bottom of the screen, that'll overlay existing content:

WindowManager.LayoutParams wlp = new WindowManager.LayoutParams();
wlp.gravity = Gravity.BOTTOM;
wlp.height = WindowManager.LayoutParams.WRAP_CONTENT;
wlp.width = WindowManager.LayoutParams.FILL_PARENT;
wlp.flags = WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON | WindowManager.LayoutParams.FLAG_LAYOUT_IN_SCREEN;
wlp.format = PixelFormat.RGBA_8888;

WindowManager wm = (WindowManager) getSystemService("window");
wm.addView(someView, wlp);

You can supply animation resources to the WindowManager.LayoutParams instance, but AFAICT there is no way to add or use programmatic animation to either the window or any of it's content children.