前言
最近TV开发需要加载的图片很长，大小也很大，并且还不允许压缩。比如显示：世界地图、清明上河图、微博长图、海报、活动照片等。
那么对于这种需求，该如何做呢？
首先不压缩，按照原图尺寸加载，那么屏幕肯定是不够大的，并且考虑到内存的情况，不可能一次性整图加载到内存中，所以肯定是局部加载，那么就需要用到一个类：
BitmapRegionDecoder
其次，既然屏幕显示不完，那么最起码要添加一个上下左右拖动的手势，让用户可以拖动查看。
实现方式有很多：
1.BitmapRegionDecoder：
分片加载，使用系统BitmapRegionDecoder去加载本地的图片，调用bitmapRegionDecoder.decodeRegion解析图片的矩形区域，返回bitmap，最终显示在ImageView上。这种方案需要手动处理滑动、缩放手势，网络图片还要处理缓存策略等问题。实现方式比较繁琐也不是很推荐。
2.SubsamplingScaleImageView
一款封装 BitmapRegionDecoder的三方库，已经处理了滑动，缩放手势。我们可以考虑选择这个库来进行加载长图，但是官方上的Demo示例加载的长图均为本地图片。这可能并不符合我们的网络场景需求，所以对于网络图片，我们还要考虑不同的加载框架，
3.Glide+SubsamplingScaleImageView混合加载渲染
对于图片加载框架，Glide当然是首选，我们使用Glide进行网络图片的下载和缓存管理，FileTarget作为桥梁，SubsamplingScaleImageView进行本地资源图片的分片加载，看起来很靠谱，那么一起来实现吧。
4.自定义LongImageView，结合BitmapRegionDecoder使用
本文采取的第四种方式，代码如下：
public class LongImageView extends AppCompatImageView {
private String TAG = getClass().getSimpleName();
private int mTargetY = 0;
private int scrollDistance = 0;
private int imgWidth = 0, imgHeight = 0;
private Bitmap imgBitmap = null;
private Bitmap holderBitmap;
private BitmapRegionDecoder bitmapRegionDecoder;
private boolean isScrolling = false;
private float startY = -1;
private int mStartTargetY = -1;
private BitmapFactory.Options scaleOptions = new BitmapFactory.Options();
public Bitmap bitmap;
public static final String EVENT_PROP_URL = "url";
public static final String EVENT_PROP_BITMAP_WIDTH = "resourceWidth";
public static final String EVENT_PROP_BITMAP_HEIGHT = "resourceHeight";
public LongImageView(Context context) {
super(context);
init();
}
public LongImageView(Context context, @Nullable AttributeSet attrs) {
super(context, attrs);
init();
}
public LongImageView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
init();
}
private void init() {
//遥控器按键事件
setOnKeyListener(new OnKeyListener() {
@Override
public boolean onKey(View v, int keyCode, KeyEvent event) {
scrollDistance = scrollDistance <= 0 ? getHeight() : scrollDistance;
if (event.getAction() == KeyEvent.ACTION_DOWN && !isScrolling) {
switch (event.getKeyCode()) {
case KeyEvent.KEYCODE_DPAD_UP:
scrollBy(0 - viewHeight2ImageHeight(scrollDistance));
break;
case KeyEvent.KEYCODE_DPAD_DOWN:
scrollBy(viewHeight2ImageHeight(scrollDistance));
break;
}
}
return false;
}
});
//响应鼠标拖拽(手指也可以)
setOnTouchListener(new OnTouchListener() {
@Override
public boolean onTouch(View v, MotionEvent event) {
switch (event.getAction()) {
case MotionEvent.ACTION_DOWN:
Log.e(TAG, " touch down");
startY = event.getRawY();
mStartTargetY = mTargetY;
break;
case MotionEvent.ACTION_MOVE:
float currentY = event.getRawY();
mTargetY = mStartTargetY + (int) (viewHeight2ImageHeight((int) (startY - currentY)) * 1f);
mTargetY = Math.max(0, Math.min(mTargetY, imgHeight - viewHeight2ImageHeight(getHeight())));
Log.e(TAG, " touch move " + mTargetY);
invalidate();
break;
case MotionEvent.ACTION_UP:
Log.e(TAG, " touch up");
startY = -1;
break;
}
return true;
}
});
}
private void startScroll(int targetY) {
targetY = Math.max(0, Math.min(targetY, imgHeight - viewHeight2ImageHeight(getHeight())));
ValueAnimator valueAnimator = ValueAnimator.ofInt(mTargetY, targetY);
valueAnimator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
@Override
public void onAnimationUpdate(ValueAnimator animation) {
mTargetY = (int) animation.getAnimatedValue();
invalidate();
}
});
valueAnimator.addListener(new Animator.AnimatorListener() {
@Override
public void onAnimationStart(Animator animation) {
isScrolling = true;
}
@Override
public void onAnimationEnd(Animator animation) {
isScrolling = false;
}
@Override
public void onAnimationCancel(Animator animation) {
isScrolling = false;
}
@Override
public void onAnimationRepeat(Animator animation) {
}
});
valueAnimator.setInterpolator(new LinearInterpolator());
//设置滑动速度
valueAnimator.setDuration(10);
valueAnimator.start();
}
public void setImageStream(InputStream imgStream) {
try {
BitmapFactory.Options options = new BitmapFactory.Options();
options.inJustDecodeBounds = true;
BitmapFactory.decodeStream(imgStream, new Rect(0, 0, 0, 0), options);
imgWidth = options.outWidth;
imgHeight = options.outHeight;
//寻找最佳的缩放比例
int viewHeight2ImageHeight = viewHeight2ImageHeight(getHeight());
//设置缩放比例
int scale = getScaleValue(imgWidth, viewHeight2ImageHeight, 1);
scaleOptions.inSampleSize = scale;
} catch (Exception e) {
e.printStackTrace();
}
try {
bitmapRegionDecoder = BitmapRegionDecoder.newInstance(imgStream, false);
} catch (Exception e) {
e.printStackTrace();
}
}
public void setImageFile(File imgFile) {
try {
BitmapFactory.Options options = new BitmapFactory.Options();
options.inJustDecodeBounds = true;
BitmapFactory.decodeFile(imgFile.getAbsolutePath(), options);
imgWidth = options.outWidth;
imgHeight = options.outHeight;
//寻找最佳的缩放比例
int viewHeight2ImageHeight = viewHeight2ImageHeight(getHeight());
int scale = getScaleValue(imgWidth, viewHeight2ImageHeight, 1);
scaleOptions.inSampleSize = scale;
} catch (Exception e) {
e.printStackTrace();
}
try {
bitmapRegionDecoder = BitmapRegionDecoder.newInstance(imgFile.getAbsolutePath(), false);
} catch (Exception e) {
e.printStackTrace();
}
}
private int getScaleValue(int imgWidth, int imgHeight, int scaleValue) {
long memory = Runtime.getRuntime().maxMemory() / 4;
if (memory > 0) {
if (imgWidth * imgHeight * 4 > memory) {
scaleValue += 1;
return getScaleValue(imgWidth, imgHeight, scaleValue);
}
}
return scaleValue;
}
@SuppressLint("ResourceType")
public void setImageResource(int resourceId) {
InputStream imgStream = getResources().openRawResource(resourceId);
setImageStream(imgStream);
}
public void setPlaceHolder(int holderId) {
holderBitmap = BitmapFactory.decodeResource(getResources(), holderId);
}
private void scrollTo(int targetY) {
startScroll(targetY);
}
private void scrollBy(int distance) {
startScroll(mTargetY + distance);
}
public void setScrollDistance(int scrollDistance) {
this.scrollDistance = scrollDistance;
}
@Override
protected void onDraw(Canvas canvas) {
Log.e(getClass().getSimpleName(), "draw start " + getWidth() + " " + getHeight());
canvas.save();
int sr = canvas.saveLayer(0, 0, getWidth(), getHeight(), null, Canvas.ALL_SAVE_FLAG);
Paint paint = new Paint();
paint.setAntiAlias(true);
if (bitmapRegionDecoder != null) {
int targetHeight = viewHeight2ImageHeight(getHeight());//根据控件的高度获取需要在原始图片上截取的高度
Log.e(getClass().getSimpleName(), "targetHeight " + targetHeight);
Log.e(getClass().getSimpleName(), "draw resource "
+ " " + imgWidth + " " + imgHeight
+ " " + mTargetY + " " + targetHeight);
imgBitmap = null;
if (imgHeight - mTargetY >= targetHeight) {//剩余区域大于 当前控件高度
imgBitmap = bitmapRegionDecoder.decodeRegion(new Rect(0, mTargetY, imgWidth, mTargetY + targetHeight)
, scaleOptions);
} else {//剩余区域小于 当前控件高度
imgBitmap = bitmapRegionDecoder.decodeRegion(new Rect(0, imgHeight - targetHeight, imgWidth, imgHeight)
, scaleOptions);
}
if (imgBitmap != null) {
//绘制需要展示的图片
canvas.drawBitmap(imgBitmap
, new Rect(0, 0, imgBitmap.getWidth(), imgBitmap.getHeight())
, new Rect(0, 0, getWidth(), getHeight())
, paint);
}
imgBitmap = null;
holderBitmap = null;
} else {
if (holderBitmap != null) {//绘制占位图
canvas.drawBitmap(holderBitmap
, new Rect(0, 0, holderBitmap.getWidth(), holderBitmap.getHeight())
, new Rect(0, 0, getWidth(), getHeight())
, paint);
}
}
canvas.restoreToCount(sr);
canvas.restore();
Log.e(getClass().getSimpleName(), "draw end");
}
private int imageHeight2ViewHeight(int imgHeight) {
if (this.imgHeight <= 0) {
return 0;
}
return (int) (imgHeight / ((float) getWidth() / imgWidth * imgHeight) * getHeight());
}
private int viewHeight2ImageHeight(int viewHeight) {
if (getHeight() <= 0) {
return 0;
}
return (int) (viewHeight / ((float) getWidth() / imgWidth * imgHeight) * imgHeight);
}
@Override
protected void onDetachedFromWindow() {
super.onDetachedFromWindow();
//回收资源和释放内存
release();
}
public void release() {
if (imgBitmap != null && !imgBitmap.isRecycled()) {
imgBitmap.recycle();
imgBitmap = null;
}
if (holderBitmap != null && !holderBitmap.isRecycled()) {
holderBitmap.recycle();
holderBitmap = null;
}
System.gc();
}
}5.在MainActivity中的使用:
public class MainActivity extends AppCompatActivity {
public String url = "https://qcloudimg-moss.cp47.ott.cibntv.net/data_center/files/2022/10/26/67a66d35-3f7c-4de8-9dfe-c706e42f44f2.jpg";
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
initView();
}
private void initView() {
LongImageView mImageView = findViewById(R.id.imageView);
mImageView.setScrollDistance((int) ((float) ScreenUtils.getScreenHeight(this) / 3 * 2));
mImageView.setFocusable(true);
try {
Glide.with(this)
.load(url)
.downloadOnly(new SimpleTarget<File>() {
@Override
public void onResourceReady(@NonNull File resource, @Nullable Transition<? super File> transition) {
mImageView.setImageFile(resource);
}
});
} catch (Exception e) {
e.printStackTrace();
}
}
}6.布局文件代码：
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
xmlns:app="http://schemas.android.com/apk/res-auto">
<com.example.longimageView.view.LongImageView
android:id="@+id/imageView"
android:layout_width="0dp"
android:layout_height="0dp"
android:focusable="true"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintBottom_toBottomOf="parent"/>
</androidx.constraintlayout.widget.ConstraintLayout>7.实现的效果如下：
总结
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