Android加载大图片，解决OOM问题

同学们还没有加载过大图片，都是使用框架加载的，框架内部已经处理过了。所以没有经过过因为加载图片而导致的OOM问题。你姑且认为这是一个存在的问题，我表演给大家看看吧。

加载大图片

    public void loadBigImage(View view) {
        ImageView imageView = this.findViewById(R.id.image_container);
        Bitmap bitmap = BitmapFactory.decodeResource(getResources(),R.mipmap.test_pic);
        imageView.setImageBitmap(bitmap);
    }

我这张图片有多大呢？

6.06M

分辨率是5400x4000

看看我们直接载入到控件里会怎么样？

崩溃了，报错信息如下：

2019-12-01 22:37:23.387 6292-6292/com.sunofbeaches.javanetdemo E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.sunofbeaches.javanetdemo, PID: 6292
    java.lang.RuntimeException: Canvas: trying to draw too large(117608400bytes) bitmap.
        at android.view.DisplayListCanvas.throwIfCannotDraw(DisplayListCanvas.java:260)
        at android.graphics.Canvas.drawBitmap(Canvas.java:1415)
        at android.graphics.drawable.BitmapDrawable.draw(BitmapDrawable.java:545)
        at android.widget.ImageView.onDraw(ImageView.java:1286)
        at android.view.View.draw(View.java:17071)
        at android.view.View.updateDisplayListIfDirty(View.java:16053)
        at android.view.View.draw(View.java:16837)
        at android.view.ViewGroup.drawChild(ViewGroup.java:3764)
        at android.view.ViewGroup.dispatchDraw(ViewGroup.java:3550)
        at android.view.View.updateDisplayListIfDirty(View.java:16048)
        at android.view.View.draw(View.java:16837)
        at android.view.ViewGroup.drawChild(ViewGroup.java:3764)
        at android.view.ViewGroup.dispatchDraw(ViewGroup.java:3550)
        at android.view.View.updateDisplayListIfDirty(View.java:16048)
        at android.view.View.draw(View.java:16837)
        at android.view.ViewGroup.drawChild(ViewGroup.java:3764)
        at android.view.ViewGroup.dispatchDraw(ViewGroup.java:3550)
        at android.view.View.updateDisplayListIfDirty(View.java:16048)
        at android.view.View.draw(View.java:16837)
        at android.view.ViewGroup.drawChild(ViewGroup.java:3764)
        at android.view.ViewGroup.dispatchDraw(ViewGroup.java:3550)
        at android.view.View.updateDisplayListIfDirty(View.java:16048)
        at android.view.View.draw(View.java:16837)
        at android.view.ViewGroup.drawChild(ViewGroup.java:3764)
        at android.view.ViewGroup.dispatchDraw(ViewGroup.java:3550)
        at android.view.View.updateDisplayListIfDirty(View.java:16048)
        at android.view.View.draw(View.java:16837)
        at android.view.ViewGroup.drawChild(ViewGroup.java:3764)
        at android.view.ViewGroup.dispatchDraw(ViewGroup.java:3550)
        at android.view.View.draw(View.java:17074)
        at com.android.internal.policy.DecorView.draw(DecorView.java:751)
        at android.view.View.updateDisplayListIfDirty(View.java:16053)
        at android.view.ThreadedRenderer.updateViewTreeDisplayList(ThreadedRenderer.java:656)
        at android.view.ThreadedRenderer.updateRootDisplayList(ThreadedRenderer.java:662)
        at android.view.ThreadedRenderer.draw(ThreadedRenderer.java:770)
        at android.view.ViewRootImpl.draw(ViewRootImpl.java:2796)
        at android.view.ViewRootImpl.performDraw(ViewRootImpl.java:2604)
        at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2211)
        at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1246)
        at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6301)
        at android.view.Choreographer$CallbackRecord.run(Choreographer.java:871)
        at android.view.Choreographer.doCallbacks(Choreographer.java:683)
        at android.view.Choreographer.doFrame(Choreographer.java:619)
        at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:857)
        at android.os.Handler.handleCallback(Handler.java:751)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:154)
        at android.app.ActivityThread.main(ActivityThread.java:6077)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:866)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:756)
    
    
    --------- beginning of system

too large(117608400bytes) bitmap

这个bitmap太大了

那么如何解决这个问题呢？

这里有篇文章，大家可以稍微去看一下！

Volley/Xutils对大图片处理算法源码分析

避免OOM

前面这篇文章有这么些请：

理论：第一次读取图片，不写入内存，直接获取到图片的宽高，再通过用户设置的宽高，和这個拉伸形式來计算期望的宽高，结合这四個参数來找出最适合的采样率，或者直接根据屏幕大小，控件大小來计算最佳采样率。

    public void loadBigImage(View view) {
        ImageView imageView = this.findViewById(R.id.image_container);
        BitmapFactory.Options options = new BitmapFactory.Options();
        options.inSampleSize = 10;
        Bitmap bitmap = BitmapFactory.decodeResource(getResources(),R.mipmap.test_pic,options);
        imageView.setImageBitmap(bitmap);
    }

我改成这样子，就可以加载出来啦！

options.inSampleSize = 10;

这个是什么意思呢？

看看文档说明吧：

     /**
         * If set to a value > 1, requests the decoder to subsample the original
         * image, returning a smaller image to save memory. The sample size is
         * the number of pixels in either dimension that correspond to a single
         * pixel in the decoded bitmap. For example, inSampleSize == 4 returns
         * an image that is 1/4 the width/height of the original, and 1/16 the
         * number of pixels. Any value <= 1 is treated the same as 1. Note: the
         * decoder uses a final value based on powers of 2, any other value will
         * be rounded down to the nearest power of 2.
         */
        public int inSampleSize;

简单来说就是采样率，当这个值为1的时候，跟原图一样。如果为4，那么采样率为原图的4分之1，也就是大小会是原来的1/4。

但是我们不能把这个10写死呀。如果写成100，那不就成了1/100了，占的内存不更小吗？

大家看看100的效果如何

糊了吧！

那咋整呢？接下来我们就看看动态计算采样率吧。

动态计算采样率

你去看看前面的源码分析，就会发现框架的做法也是动态计算的，根据控件的大小，来动态计算。那么我们稍微修改一下代码如下：

    public void loadBigImage(View view) {
        ImageView imageView = this.findViewById(R.id.image_container);
        BitmapFactory.Options options = new BitmapFactory.Options();
        //If set to true, the decoder will return null (no bitmap)
        //设置为true以后呢，不是真的载入到内存中，只是获取到图片的相关信息
        options.inJustDecodeBounds = true;
        BitmapFactory.decodeResource(getResources(),R.mipmap.test_pic,options);
        int width = options.outWidth;
        int height = options.outHeight;
        int measuredWidth = imageView.getMeasuredWidth();
        int measuredHeight = imageView.getMeasuredHeight();
        Log.d(TAG,"width -- > " + width + " measure width -- > " + measuredWidth);
        Log.d(TAG,"height -- > " + height + " measure height -- > " + measuredHeight);
        int sampleSize;
        if(width < measuredWidth || height < measuredHeight) {
            sampleSize = 1;
        } else {
            int scaleX = width / measuredWidth;
            int scaleY = height / measuredHeight;
            sampleSize = scaleX > scaleY ? scaleX : scaleY;
        }
        Log.d(TAG,"sampleSize -- > " + sampleSize);
        options.inSampleSize = sampleSize;
        //If set to true, the decoder will return null (no bitmap)
        //要变成false了，因为真的要载入到内存中了
        options.inJustDecodeBounds = false;
        Bitmap bitmap = BitmapFactory.decodeResource(getResources(),R.mipmap.test_pic,options);
        imageView.setImageBitmap(bitmap);
    }

感觉截图会好看点，哈哈！！！

okay啦！

跑一次看看！！

log输出

D/BigImageLoadActivity: width -- > 5400 measure width -- > 1440
D/BigImageLoadActivity: height -- > 4000 measure height -- > 1944
D/BigImageLoadActivity: sampleSize -- > 3

效果：