Question

我将ViewPager扩展为具有移动背景（似乎也是fairly popular请求）

它似乎主要是工作，我的问题是有时整个背景变黑。

这实际上特别发生在我在另一页上旋转屏幕而不是第一页并开始移动时（屏幕旋转后仍然可以） - 当屏幕旋转后活动重新开始时，它将ViewPager恢复到相同的状态以前的页面。

应用程序的整个代码都在GitHub上，但这里是感兴趣的类。

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();
    }
}

Answer 1

Canvas中的偏移量是根据绘制的第一页计算的...修复了代码o GitHub，现在只是希望这不是完全依赖于实现的东西，以后会改变......

覆盖ViewDager的onDraw有时会显示黑屏

1 个答案: