How can I set an entire view's alpha value in

问题:

I have an arbitrary view that I want to fade in on top of another view. In api level 11 I see there is a setAlpha, but I'm stuck supporting api level 7. I haven't run across a simple way to do this. How can I set the alpha for the entire view without messing with each individual component?

回答1:

You should be able to achieve a reasonable effect using an AlphaAnimation at API level 7.

    View v = findViewById(R.id.view2);

    AlphaAnimation aa = new AlphaAnimation(0f,1f);
    aa.setDuration(5000);
    v.startAnimation(aa);

回答2:

Using AlphaAnimation would be an excellent solution for most transitions, and would certainly have worked for me if I couldn't find a way to do exactly what I was trying to do, which involves fading between two views in a graduated way based on the tilt angle of the device. Fortunately I have! Here is the strategy I took: I wrapped the view in a custom subclass of FrameLayout, and implemented onDraw. There, I captured the child view as a bitmap, and then redrew the bitmap with the intended alpha. Here's some code. I'll edit when I get cleaned up, this is just proof of concept, but it works like a charm:

public class AlphaView extends FrameLayout {
    private int alpha = 255;

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

    public AlphaView(Context context, AttributeSet attrs) {
        super(context, attrs);
        setWillNotDraw(false);
    }

    public AlphaView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        setWillNotDraw(false);
    }

    public void setCustomAlpha(int alpha) {
        if (this.alpha != alpha) {
            this.alpha = alpha;
            invalidate();
        }
    }

    public int getCustomAlpha() {
        return alpha;
    }

    @Override
    protected void onDraw(Canvas canvas) {
        for(int index = 0; index < getChildCount(); index++ ) {
            View child  = getChildAt(index);
            child.setVisibility(View.INVISIBLE);
            child.setDrawingCacheEnabled(true);
            Bitmap bitmap = child.getDrawingCache(true);
            bitmap = Bitmap.createBitmap(bitmap);
            child.setDrawingCacheEnabled(false);
            Paint paint = new Paint();
            paint.setAlpha(alpha);
            canvas.drawBitmap(bitmap, 0, 0, paint);
        }
    }
}

回答3:

It would depend on the view type.

For a TextView, in xml you could have these attributes:

android:background="#00000000"
android:textColor="#77FFFFFF"

The first two numbers are the alpha values from 00 to FF (in hex). The background would be fully transparent, while the text would be white an partially transparent. I have not tested this, but it should work.

If you have an background that is an image, then the easiest thing to do would to pre-create your png drawable with transparency.