大图查看器是许多app的常用功能，主要使用场景是用户点击图片，然后启动一个新界面来展示图片的完整尺寸，并能通过手势移动图片以及放大缩小。当然，上面说的是最基本的功能，实际使用中还要包括：如果是本地图片应该可以移除，如果是网络图片，应提供一个保存到本地的功能等。
本文为什么叫封装一个大图查看器，而不是叫做编写一个大图查看器呢？因为大图查看器的最核心功能，展示图片以及手势操控我们使用了一个开源库来完成，这个开源库叫做subsampling-scale-image-view，这个开源库非常靠谱，使用也非常简单，我记得知乎的Android app中也是使用了这个库。这个库的Github地址是：subsampling-scale-image-view。
我先把我做出来的效果图展示出来：
首先第一张是：
然后来看第二张：
两张界面截图展示的是同一张图片，但区别在于，第一张图片右下角有一个下载的图标，对应的是查看网络图片这种方式（大家都需要保存网络图片这个功能）；而第二张图片右下角没有了图标，而右上角有一个垃圾桶的图标，用来进行移除操作，想象一下，本来查看的就是本地图片，那用户也就不用再保存它一次了，而移除操作在很多实际使用场景中有效，比如说在微信中，你想发朋友圈，你选择了一几张本地照片以后进行大图浏览，而这时候你发现其中几张不太好，想移除它，于是为了用户方便，应该给用户一个在大图查看器中进行移除的方法。
<style name="Dialog_Fullscreen"> <item name="android:windowFullscreen">true</item> <item name="android:windowNoTitle">true</item> </style>
这就是全屏Dialog的实现方式。接下来，定义Dialog的布局：
<?xml version="1.0" encoding="utf-8"?><RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent" android:layout_height="match_parent" android:background="@android:color/black"> <android.support.v4.view.ViewPager android:id="@+id/scale_image_view_pager" android:layout_width="match_parent" android:layout_height="match_parent" /> <ImageView android:id="@+id/scale_image_close" android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_alignParentTop="true" android:layout_alignParentStart="true" android:padding="24dp" android:src="@drawable/ic_clear_white_24dp" tools:ignore="ContentDescription"/> <ImageView android:id="@+id/scale_image_delete" android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_alignParentTop="true" android:layout_alignParentEnd="true" android:padding="24dp" android:src="@drawable/ic_delete_white_24dp" tools:ignore="ContentDescription"/> <ImageView android:id="@+id/scale_image_save" android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_alignParentBottom="true" android:layout_alignParentEnd="true" android:padding="24dp" android:src="@drawable/ic_file_download_white_24dp" tools:ignore="ContentDescription"/> <TextView android:id="@+id/scale_image_count" android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_centerHorizontal="true" android:layout_alignParentBottom="true" android:layout_marginBottom="32dp" android:textSize="16sp" android:textColor="@android:color/white"/></RelativeLayout>
代码很简单，唯一值得说的地方就是，里面放置了三个ImageView，分别作为：关闭Dialog，移除图片，保存图片到本地三个功能的按钮。
public class ScaleImageView { private static final byte URLS = 0; private static final byte FILES = 1; private byte status; private Activity activity; private List<String> urls; private List<File> files; private List<File> downloadFiles; private int selectedPosition; private Dialog dialog; private ImageView delete; private ImageView download; private TextView imageCount; private ViewPager viewPager; private List<View> views; private MyPagerAdapter adapter; private OnDeleteItemListener listener; private int startPosition; public ScaleImageView(Activity activity) { this.activity = activity; init(); } public void setOnDeleteItemListener(OnDeleteItemListener listener) { this.listener = listener; } private void init() { RelativeLayout relativeLayout = (RelativeLayout) activity.getLayoutInflater().inflate(R.layout.dialog_scale_image, null); ImageView close = (ImageView) relativeLayout.findViewById(R.id.scale_image_close); delete = (ImageView) relativeLayout.findViewById(R.id.scale_image_delete); download = (ImageView) relativeLayout.findViewById(R.id.scale_image_save); imageCount = (TextView) relativeLayout.findViewById(R.id.scale_image_count); viewPager = (ViewPager) relativeLayout.findViewById(R.id.scale_image_view_pager); dialog = new Dialog(activity, R.style.Dialog_Fullscreen); dialog.setContentView(relativeLayout); close.setOnClickListener(v -> dialog.dismiss()); delete.setOnClickListener(v -> { int size = views.size(); files.remove(selectedPosition); if (listener != null) { listener.onDelete(selectedPosition); } viewPager.removeView(views.remove(selectedPosition)); if (selectedPosition != size) { int position = selectedPosition + 1; String text = position + "/" + views.size(); imageCount.setText(text); } adapter.notifyDataSetChanged(); }); download.setOnClickListener(v -> { try { MediaStore.Images.Media.insertImage(activity.getContentResolver(), downloadFiles.get(selectedPosition).getAbsolutePath(), downloadFiles.get(selectedPosition).getName(), null); } catch (FileNotFoundException e) { e.printStackTrace(); } Snackbar.make(viewPager, "图片保存成功", Snackbar.LENGTH_SHORT).show(); }); viewPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() { @Override public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) { } @Override public void onPageSelected(int position) { selectedPosition = position; String text = ++position + "/" + views.size(); imageCount.setText(text); } @Override public void onPageScrollStateChanged(int state) { } }); }}首先，我们关注一下最上面三行定义的几个变量，URLS和FILES两个常量分别表示网络查看模式和本地查看模式，而status则用来标示现在查看器处于哪个模式下。
init()方法主要用来加载Dialog的布局，并获取其中每一个View的Id，并给几个可点击项设置了点击事件监听。例如close代表最上面界面截图中最左上角的“叉”，在它的点击事件监听中我们设置了关闭Dialog，即dialog.dissmiss()。delete代表本地查看模式下移除图片，download代表网络查看模式下的保存图片到本地，我们先说download，因为它比较简单，就是调用了一行代码，将下载的文件保存到相册并通知系统。其中值得说的是downFiles的类型是List<File>，它用来储存从网络下载下来的图片。而seletePosition则用来表示用户当前浏览的图片在ViewPager中的下标，说白了它就等于用户当前查看的第n张图片减一。
public interface OnDeleteItemListener { void onDelete(int position); }这个接口的作用在于，当调用者在移除了某张图片后，还需要做的事情就应该写在接口的onDelete方法中。举个例子，假如你用了一个GridView来展示9张图片，当用户在大图查看器中将第3张图片移除的时候，这个时候你的GridView应该和大图查看器保持一致，也把第三张图片移除掉，而GridView的移除逻辑，就应该写在onDelete方法中。
public void setUrls(List<String> urls, int startPosition) { if (this.urls == null) { this.urls = new ArrayList<>(); } else { this.urls.clear(); } this.urls.addAll(urls); status = URLS; delete.setVisibility(View.GONE); if (downloadFiles == null) { downloadFiles = new ArrayList<>(); } else { downloadFiles.clear(); } this.startPosition = startPosition++; String text = startPosition + "/" + urls.size(); imageCount.setText(text); } public void setFiles(List<File> files, int startPosition) { if (this.files == null) { this.files = new LinkedList<>(); } else { this.files.clear(); } this.files.addAll(files); status = FILES; download.setVisibility(View.GONE); this.startPosition = startPosition++; String text = startPosition + "/" + files.size(); imageCount.setText(text); }当调用者想使用网络查看模式的时候，应该使用第一个方法——setUrls。调用者需要传入想要查看的这组图片的url并传入起始查看位置，举例来说，当调用者使用GridView来为用户展示图片的时候，用户可能点击了GridView的第三张图片，他想从第三张开始看起，而这第二个参数就是指的这个。后面的逻辑包括给类设置状态，即给status的值设置为URLS，然后清空或者初始化下载的图片，并给imageCount设置正确的显示内容。而setFiles也是类似的，我就不展开讲了，大致就是，要查看本地图片，调用者应该拿到本地图片的File对象并装入一个List然后传入。
public void create() { dialog.show(); views = new ArrayList<>(); adapter = new MyPagerAdapter(views, dialog); if (status == URLS) { for (String url : urls) { FrameLayout frameLayout = (FrameLayout) activity.getLayoutInflater().inflate(R.layout.view_scale_image, null); SubsamplingScaleImageView imageView = (SubsamplingScaleImageView) frameLayout.findViewById(R.id.scale_image_view); views.add(frameLayout); IOThread.getSingleThread().execute(() -> { File downLoadFile; try { downLoadFile = Glide.with(activity).load(url).downloadOnly(Target.SIZE_ORIGINAL, Target.SIZE_ORIGINAL).get(); downloadFiles.add(downLoadFile); activity.runOnUiThread(() -> imageView.setImage(ImageSource.uri(Uri.fromFile(downLoadFile)))); } catch (Exception e) { e.printStackTrace(); } }); } viewPager.setAdapter(adapter); } else if (status == FILES) { for (File file : files) { FrameLayout frameLayout = (FrameLayout) activity.getLayoutInflater().inflate(R.layout.view_scale_image, null); SubsamplingScaleImageView imageView = (SubsamplingScaleImageView) frameLayout.findViewById(R.id.scale_image_view); views.add(frameLayout); imageView.setImage(ImageSource.uri(Uri.fromFile(file))); } viewPager.setAdapter(adapter); } viewPager.setCurrentItem(startPosition); }
首先，这里要将Dialog显示出来，然后初始化ViewPager的Adapter，Adapter的代码如下所示：
private static class MyPagerAdapter extends PagerAdapter { private List<View> views; private Dialog dialog; MyPagerAdapter(List<View> views, Dialog dialog) { this.views = views; this.dialog = dialog; } @Override public Object instantiateItem(ViewGroup container, int position) { container.addView(views.get(position)); return views.get(position); } @Override public void destroyItem(ViewGroup container, int position, Object object) { if (position == 0 && views.size() == 0) { dialog.dismiss(); return; } if (position == views.size()) { container.removeView(views.get(--position)); } else { container.removeView(views.get(position)); } } @Override public int getCount() { return views.size(); } @Override public boolean isViewFromObject(View view, Object object) { return view == object; } @Override public int getItemPosition(Object object) { return POSITION_NONE; } }这是一个很简单的ViewPager适配器，唯一要说明的就是复写的destroyItem这个方法，它里面对应了多种情况，例如，当图片只有一张的时候，当移除的图片是一组图片的最后一张的时候，这些都要做相映的判断，否则使用的时候都会发生异常。
<?xml version="1.0" encoding="utf-8"?><FrameLayout xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="match_parent" android:layout_height="match_parent"> <com.davemorrissey.labs.subscaleview.SubsamplingScaleImageView android:id="@+id/scale_image_view" android:layout_width="match_parent" android:layout_height="match_parent"/></FrameLayout>
ScaleImageView scaleImageView = new ScaleImageView(activity);scaleImageView.setUrls(urls, position);scaleImageView.create();
如果查看本地图片：
ScaleImageView scaleImageView = new ScaleImageView(activity);scaleImageView.setFiles(files, position);scaleImageView.setOnDeleteItemListener(deletePosition -> adapter.removeItem(deletePosition));scaleImageView.create();
最后附上完整的java代码，一共也就两百多行，很简单：
public class ScaleImageView { private static final byte URLS = 0; private static final byte FILES = 1; private byte status; private Activity activity; private List<String> urls; private List<File> files; private List<File> downloadFiles; private int selectedPosition; private Dialog dialog; private ImageView delete; private ImageView download; private TextView imageCount; private ViewPager viewPager; private List<View> views; private MyPagerAdapter adapter; private OnDeleteItemListener listener; private int startPosition; public ScaleImageView(Activity activity) { this.activity = activity; init(); } public void setUrls(List<String> urls, int startPosition) { if (this.urls == null) { this.urls = new ArrayList<>(); } else { this.urls.clear(); } this.urls.addAll(urls); status = URLS; delete.setVisibility(View.GONE); if (downloadFiles == null) { downloadFiles = new ArrayList<>(); } else { downloadFiles.clear(); } this.startPosition = startPosition++; String text = startPosition + "/" + urls.size(); imageCount.setText(text); } public void setFiles(List<File> files, int startPosition) { if (this.files == null) { this.files = new LinkedList<>(); } else { this.files.clear(); } this.files.addAll(files); status = FILES; download.setVisibility(View.GONE); this.startPosition = startPosition++; String text = startPosition + "/" + files.size(); imageCount.setText(text); } public void setOnDeleteItemListener(OnDeleteItemListener listener) { this.listener = listener; } private void init() { RelativeLayout relativeLayout = (RelativeLayout) activity.getLayoutInflater().inflate(R.layout.dialog_scale_image, null); ImageView close = (ImageView) relativeLayout.findViewById(R.id.scale_image_close); delete = (ImageView) relativeLayout.findViewById(R.id.scale_image_delete); download = (ImageView) relativeLayout.findViewById(R.id.scale_image_save); imageCount = (TextView) relativeLayout.findViewById(R.id.scale_image_count); viewPager = (ViewPager) relativeLayout.findViewById(R.id.scale_image_view_pager); dialog = new Dialog(activity, R.style.Dialog_Fullscreen); dialog.setContentView(relativeLayout); close.setOnClickListener(v -> dialog.dismiss()); delete.setOnClickListener(v -> { int size = views.size(); files.remove(selectedPosition); if (listener != null) { listener.onDelete(selectedPosition); } viewPager.removeView(views.remove(selectedPosition)); if (selectedPosition != size) { int position = selectedPosition + 1; String text = position + "/" + views.size(); imageCount.setText(text); } adapter.notifyDataSetChanged(); }); download.setOnClickListener(v -> { try { MediaStore.Images.Media.insertImage(activity.getContentResolver(), downloadFiles.get(selectedPosition).getAbsolutePath(), downloadFiles.get(selectedPosition).getName(), null); } catch (FileNotFoundException e) { e.printStackTrace(); } Snackbar.make(viewPager, "图片保存成功", Snackbar.LENGTH_SHORT).show(); }); viewPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() { @Override public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) { } @Override public void onPageSelected(int position) { selectedPosition = position; String text = ++position + "/" + views.size(); imageCount.setText(text); } @Override public void onPageScrollStateChanged(int state) { } }); } public void create() { dialog.show(); views = new ArrayList<>(); adapter = new MyPagerAdapter(views, dialog); if (status == URLS) { for (String url : urls) { FrameLayout frameLayout = (FrameLayout) activity.getLayoutInflater().inflate(R.layout.view_scale_image, null); SubsamplingScaleImageView imageView = (SubsamplingScaleImageView) frameLayout.findViewById(R.id.scale_image_view); views.add(frameLayout); IOThread.getSingleThread().execute(() -> { File downLoadFile; try { downLoadFile = Glide.with(activity).load(url).downloadOnly(Target.SIZE_ORIGINAL, Target.SIZE_ORIGINAL).get(); downloadFiles.add(downLoadFile); activity.runOnUiThread(() -> imageView.setImage(ImageSource.uri(Uri.fromFile(downLoadFile)))); } catch (Exception e) { e.printStackTrace(); } }); } viewPager.setAdapter(adapter); } else if (status == FILES) { for (File file : files) { FrameLayout frameLayout = (FrameLayout) activity.getLayoutInflater().inflate(R.layout.view_scale_image, null); SubsamplingScaleImageView imageView = (SubsamplingScaleImageView) frameLayout.findViewById(R.id.scale_image_view); views.add(frameLayout); imageView.setImage(ImageSource.uri(Uri.fromFile(file))); } viewPager.setAdapter(adapter); } viewPager.setCurrentItem(startPosition); } private static class MyPagerAdapter extends PagerAdapter { private List<View> views; private Dialog dialog; MyPagerAdapter(List<View> views, Dialog dialog) { this.views = views; this.dialog = dialog; } @Override public Object instantiateItem(ViewGroup container, int position) { container.addView(views.get(position)); return views.get(position); } @Override public void destroyItem(ViewGroup container, int position, Object object) { if (position == 0 && views.size() == 0) { dialog.dismiss(); return; } if (position == views.size()) { container.removeView(views.get(--position)); } else { container.removeView(views.get(position)); } } @Override public int getCount() { return views.size(); } @Override public boolean isViewFromObject(View view, Object object) { return view == object; } @Override public int getItemPosition(Object object) { return POSITION_NONE; } } public interface OnDeleteItemListener { void onDelete(int position); }}
