Overriding onDraw for ViewPager sometimes gives black screen

问题

I extended ViewPager to have a moving background (seems to be a fairly popular request too)

It seems to be mostly working, the problem I have is that sometimes the whole background turns black.

This actually specifically occurs when I rotate the screen while on another page than the first one and start moving (it is still ok after the screen rotation) - when the activity restarts after the screen rotation, it restores the ViewPager to the same page as it was before.

The whole code with application is on GitHub, but here is the class of interest.

package com.matthieu;

import android.annotation.SuppressLint;
import android.content.Context;
import android.graphics.*;
import android.os.Build;
import android.support.v4.view.ViewPager;
import android.util.AttributeSet;
import android.util.Log;

import java.io.IOException;
import java.io.InputStream;

public class ViewPagerParallax extends ViewPager {
    int background_id =-1;
    int background_saved_id =-1;
    int saved_width=-1, saved_height=-1, saved_max_num_pages =-1;
    Bitmap saved_bitmap;

    int max_num_pages=0;
    int imageHeight;
    int imageWidth;
    float zoom_level;

    Rect r = new Rect();

    private final static String TAG="ViewPagerParallax";

    public ViewPagerParallax(Context context) {
        super(context);
    }

    public ViewPagerParallax(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    @SuppressLint({"NewApi"})
    private void set_new_background() {
        if (background_id == -1)
            return;

        if (max_num_pages == 0)
            return;

        if (getWidth()==0 || getHeight()==0)
            return;

        if ((saved_height == getHeight()) && (saved_width == getWidth()) &&
                (background_saved_id==background_id) &&
                (saved_max_num_pages == max_num_pages))
            return;

        InputStream is;

        try {
            is = getContext().getResources().openRawResource(background_id);

            BitmapFactory.Options options = new BitmapFactory.Options();
            options.inJustDecodeBounds = true;
            BitmapFactory.decodeStream(is, null, options);

            imageHeight = options.outHeight;
            imageWidth = options.outWidth;

            zoom_level = ((float) imageHeight) / getHeight();  // we are always in 'fitY' mode
            is.reset();

            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.GINGERBREAD_MR1) {
                BitmapRegionDecoder brd = BitmapRegionDecoder.newInstance(is, true);

                options.inJustDecodeBounds = false;
                r.set(0, 0, Math.min((int) (getWidth() * ((max_num_pages + 4.0) / 5) * zoom_level), imageWidth), imageHeight);
                saved_bitmap = brd.decodeRegion(r, options);
                brd.recycle();
            } else {
                saved_bitmap = Bitmap.createBitmap(BitmapFactory.decodeStream(is), 0, 0, Math.min((int) (getWidth() * ((max_num_pages + 4.0) / 5) * zoom_level), imageWidth), imageHeight);
            }

            is.close();
        } catch (IOException e) {
            Log.e(TAG, "Cannot decode: " + e.getMessage());
            background_id = -1;
            return;
        }

        saved_height = getHeight();
        saved_width = getWidth();
        background_saved_id = background_id;
        saved_max_num_pages = max_num_pages;
    }

    int current_position;
    float current_offset;

    @Override
    protected void onPageScrolled(int position, float offset, int offsetPixels) {
        super.onPageScrolled(position, offset, offsetPixels);
        current_position = position;
        current_offset = offset;
    }

    Rect src = new Rect(), dst = new Rect();

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        src.set((int) (((current_position + current_offset) * getWidth() * zoom_level) / 5 ), 0,
                (int) ((((current_position + current_offset) * getWidth() * zoom_level) / 5)  + (getWidth() * zoom_level)), imageHeight);

        dst.set((int) ((current_position + current_offset) * getWidth()), 0, (int) ((current_position + current_offset) * getWidth()) + canvas.getWidth(), canvas.getHeight());
        // still confused why we need to shift also in the destination canvas

        canvas.drawBitmap(saved_bitmap, src, dst, null);
    }

    public void set_max_pages(int num_max_pages) {
        max_num_pages = num_max_pages;
        set_new_background();
    }

    public void setBackgroundAsset(int res_id) {
        background_id = res_id;
        set_new_background();
    }

    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        super.onLayout(changed, l, t, r, b);
        set_new_background();
    }
}

回答1:

The offset in the Canvas is calculated based on the first page being drawn... fixed the code o GitHub, now just hoping this is not something that is completely implementation dependent that would change later...

来源：https://stackoverflow.com/questions/14715435/overriding-ondraw-for-viewpager-sometimes-gives-black-screen