2
我伸出ViewPager有一個移動背景（似乎是一個fairly popular要求太）重寫的onDraw爲ViewPager有時會黑屏
這似乎是主要的工作，我的問題是，有時整個背景匝黑色。
實際上，當我在另一個頁面上旋轉屏幕而不是第一個屏幕時開始移動（在屏幕旋轉後它仍然可以） - 當屏幕旋轉後重新開始活動時，它將ViewPager恢復爲與之前相同的頁面。
整個代碼與應用程序在GitHub，但這裏是感興趣的類。
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();
}
}