Android ImageView分析并展开

版本号：1.0 

日期：2014.6.11 2014.6.12

版权：© 2014 kince 转载注明出处

  ImageView是开发中经常使用到的一个控件，也能够说是不可缺少的。

对于它的使用。除了注意ScaleType的理解和设置外，还须要注意其它一些问题，比方设置一张大的背景图片内存占用和释放等。

还有它的拓展性方面，像圆角图片、圆形图片、图片边框等等。因此，假设想熟练使用这个控件，就须要对事实上现的机制有一个基本的了解。

  ImageView也是直接继承于View类。基本的结构图例如以下：

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 鉴于篇幅大小，就不copy ImageView的总体代码，选择结构图中的部分作为重点。首先是构造方法，代码例如以下：

 public ImageView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        initImageView();
 
        TypedArray a = context.obtainStyledAttributes(attrs,
                com.android.internal.R.styleable.ImageView, defStyle, 0);
 
        Drawable d = a.getDrawable(com.android.internal.R.styleable.ImageView_src);
        if (d != null) {
            setImageDrawable(d);
        }
 
        mBaselineAlignBottom = a.getBoolean(
                com.android.internal.R.styleable.ImageView_baselineAlignBottom, false);
 
        mBaseline = a.getDimensionPixelSize(
                com.android.internal.R.styleable.ImageView_baseline, -1);
 
        setAdjustViewBounds(
            a.getBoolean(com.android.internal.R.styleable.ImageView_adjustViewBounds,
            false));
 
        setMaxWidth(a.getDimensionPixelSize(
                com.android.internal.R.styleable.ImageView_maxWidth, Integer.MAX_VALUE));
 
        setMaxHeight(a.getDimensionPixelSize(
                com.android.internal.R.styleable.ImageView_maxHeight, Integer.MAX_VALUE));
 
        int index = a.getInt(com.android.internal.R.styleable.ImageView_scaleType, -1);
        if (index >= 0) {
            setScaleType(sScaleTypeArray[index]);
        }
 
        int tint = a.getInt(com.android.internal.R.styleable.ImageView_tint, 0);
        if (tint != 0) {
            setColorFilter(tint);
        }
 
        int alpha = a.getInt(com.android.internal.R.styleable.ImageView_drawableAlpha, 255);
        if (alpha != 255) {
            setAlpha(alpha);
        }
 
        mCropToPadding = a.getBoolean(
                com.android.internal.R.styleable.ImageView_cropToPadding, false);
 
        a.recycle();
 
        //need inflate syntax/reader for matrix
    }
 
    private void initImageView() {
        mMatrix     = new Matrix();
        mScaleType  = ScaleType.FIT_CENTER;
        mAdjustViewBoundsCompat = mContext.getApplicationInfo().targetSdkVersion <=
                Build.VERSION_CODES.JELLY_BEAN_MR1;
    }

  在构造方法中也是非经常规的从attrs文件里读取属性值，并进行设置。也能够看到ImageView默认使用的ScaleType是FIT_CENTER。说到ScaleType。它是一个枚举类型，用于设置。寻常使用的ScaleType就是在这里定义的。

/**
     * Options for scaling the bounds of an image to the bounds of this view.
     */
    public enum ScaleType {
        /**
         * Scale using the image matrix when drawing. The image matrix can be set using
         * {@link ImageView#setImageMatrix(Matrix)}. From XML, use this syntax:
         * <code>android:scaleType="matrix"</code>.
         */
        MATRIX      (0),
        /**
         * Scale the image using {@link Matrix.ScaleToFit#FILL}.
         * From XML, use this syntax: <code>android:scaleType="fitXY"</code>.
         */
        FIT_XY      (1),
        /**
         * Scale the image using {@link Matrix.ScaleToFit#START}.
         * From XML, use this syntax: <code>android:scaleType="fitStart"</code>.
         */
        FIT_START   (2),
        /**
         * Scale the image using {@link Matrix.ScaleToFit#CENTER}.
         * From XML, use this syntax:
         * <code>android:scaleType="fitCenter"</code>.
         */
        FIT_CENTER  (3),
        /**
         * Scale the image using {@link Matrix.ScaleToFit#END}.
         * From XML, use this syntax: <code>android:scaleType="fitEnd"</code>.
         */
        FIT_END     (4),
        /**
         * Center the image in the view, but perform no scaling.
         * From XML, use this syntax: <code>android:scaleType="center"</code>.
         */
        CENTER      (5),
        /**
         * Scale the image uniformly (maintain the image's aspect ratio) so
         * that both dimensions (width and height) of the image will be equal
         * to or larger than the corresponding dimension of the view
         * (minus padding). The image is then centered in the view.
         * From XML, use this syntax: <code>android:scaleType="centerCrop"</code>.
         */
        CENTER_CROP (6),
        /**
         * Scale the image uniformly (maintain the image's aspect ratio) so
         * that both dimensions (width and height) of the image will be equal
         * to or less than the corresponding dimension of the view
         * (minus padding). The image is then centered in the view.
         * From XML, use this syntax: <code>android:scaleType="centerInside"</code>.
         */
        CENTER_INSIDE (7);
 
        ScaleType(int ni) {
            nativeInt = ni;
        }
        final int nativeInt;
    }

 功能是设置图片的显示位置和大小等方面。

接着就是onMeasure()方法了，它用于设置ImageView的大小。我们在xml文件里设置ImageView的时候，假设指定了固定的宽高，那么onMeasur()方法中測量的大小就是固定的宽高大小；假设是包裹内容，那么就须要进一步的计算。

@Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        resolveUri();//获取图片Drawable
        int w;
        int h;
 
        // Desired aspect ratio of the view's contents (not including padding)
        float desiredAspect = 0.0f;
 
        // We are allowed to change the view's width
        boolean resizeWidth = false;
 
        // We are allowed to change the view's height
        boolean resizeHeight = false;
 
        final int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);
        final int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec);
 
        if (mDrawable == null) {
            // If no drawable, its intrinsic size is 0.
            mDrawableWidth = -1;
            mDrawableHeight = -1;
            w = h = 0;
        } else {
            w = mDrawableWidth;在updateDrawable(Drawable d)方法赋值的。
            h = mDrawableHeight;
            if (w <= 0) w = 1;
            if (h <= 0) h = 1;
 
            // We are supposed to adjust view bounds to match the aspect
            // ratio of our drawable. See if that is possible.
            if (mAdjustViewBounds) {
                resizeWidth = widthSpecMode != MeasureSpec.EXACTLY;
                resizeHeight = heightSpecMode != MeasureSpec.EXACTLY;
 
                desiredAspect = (float) w / (float) h;
            }
        }
 
        int pleft = mPaddingLeft;
        int pright = mPaddingRight;
        int ptop = mPaddingTop;
        int pbottom = mPaddingBottom;
 
        int widthSize;
        int heightSize;
 
        if (resizeWidth || resizeHeight) {
            /* If we get here, it means we want to resize to match the
                drawables aspect ratio, and we have the freedom to change at
                least one dimension.
            */
 
            // Get the max possible width given our constraints
            widthSize = resolveAdjustedSize(w + pleft + pright, mMaxWidth, widthMeasureSpec);
 
            // Get the max possible height given our constraints
            heightSize = resolveAdjustedSize(h + ptop + pbottom, mMaxHeight, heightMeasureSpec);
 
            if (desiredAspect != 0.0f) {
                // See what our actual aspect ratio is
                float actualAspect = (float)(widthSize - pleft - pright) /
                                        (heightSize - ptop - pbottom);
 
                if (Math.abs(actualAspect - desiredAspect) > 0.0000001) {
 
                    boolean done = false;
 
                    // Try adjusting width to be proportional to height
                    if (resizeWidth) {
                        int newWidth = (int)(desiredAspect * (heightSize - ptop - pbottom)) +
                                pleft + pright;
 
                        // Allow the width to outgrow its original estimate if height is fixed.
                        if (!resizeHeight && !mAdjustViewBoundsCompat) {
                            widthSize = resolveAdjustedSize(newWidth, mMaxWidth, widthMeasureSpec);
                        }
 
                        if (newWidth <= widthSize) {
                            widthSize = newWidth;
                            done = true;
                        }
                    }
 
                    // Try adjusting height to be proportional to width
                    if (!done && resizeHeight) {
                        int newHeight = (int)((widthSize - pleft - pright) / desiredAspect) +
                                ptop + pbottom;
 
                        // Allow the height to outgrow its original estimate if width is fixed.
                        if (!resizeWidth && !mAdjustViewBoundsCompat) {
                            heightSize = resolveAdjustedSize(newHeight, mMaxHeight,
                                    heightMeasureSpec);
                        }
 
                        if (newHeight <= heightSize) {
                            heightSize = newHeight;
                        }
                    }
                }
            }
        } else {
            /* We are either don't want to preserve the drawables aspect ratio,
               or we are not allowed to change view dimensions. Just measure in
               the normal way.
            */
            w += pleft + pright;
            h += ptop + pbottom;
 
            w = Math.max(w, getSuggestedMinimumWidth());
            h = Math.max(h, getSuggestedMinimumHeight());
 
            widthSize = resolveSizeAndState(w, widthMeasureSpec, 0);
            heightSize = resolveSizeAndState(h, heightMeasureSpec, 0);
        }
 
        setMeasuredDimension(widthSize, heightSize);
    }

在onMeasure方法中，首先调用了resolveUri()这种方法。目的就是为了确定Drawable。

假设设置了drawableResource。那么Drawable就是其值；假设没有。那么就从ContentResolver获取一个Drawable。

 private void resolveUri() {
        if (mDrawable != null) {
            return;
        }
 
        Resources rsrc = getResources();
        if (rsrc == null) {
            return;
        }
 
        Drawable d = null;
 
        if (mResource != 0) {
            try {
                d = rsrc.getDrawable(mResource);
            } catch (Exception e) {
                Log.w("ImageView", "Unable to find resource: " + mResource, e);
                // Don't try again.
                mUri = null;
            }
        } else if (mUri != null) {
            String scheme = mUri.getScheme();
            if (ContentResolver.SCHEME_ANDROID_RESOURCE.equals(scheme)) {
                try {
                    // Load drawable through Resources, to get the source density information
                    ContentResolver.OpenResourceIdResult r =
                            mContext.getContentResolver().getResourceId(mUri);
                    d = r.r.getDrawable(r.id);
                } catch (Exception e) {
                    Log.w("ImageView", "Unable to open content: " + mUri, e);
                }
            } else if (ContentResolver.SCHEME_CONTENT.equals(scheme)
                    || ContentResolver.SCHEME_FILE.equals(scheme)) {
                InputStream stream = null;
                try {
                    stream = mContext.getContentResolver().openInputStream(mUri);
                    d = Drawable.createFromStream(stream, null);
                } catch (Exception e) {
                    Log.w("ImageView", "Unable to open content: " + mUri, e);
                } finally {
                    if (stream != null) {
                        try {
                            stream.close();
                        } catch (IOException e) {
                            Log.w("ImageView", "Unable to close content: " + mUri, e);
                        }
                    }
                }
        } else {
                d = Drawable.createFromPath(mUri.toString());
            }
 
            if (d == null) {
                System.out.println("resolveUri failed on bad bitmap uri: " + mUri);
                // Don't try again.
                mUri = null;
            }
        } else {
            return;
        }
 
        updateDrawable(d);
    }

  之后在resolveUri()这种方法的最后，调用了 updateDrawable(d)方法。这种方法代码例如以下：

 private void updateDrawable(Drawable d) {
        if (mDrawable != null) {
            mDrawable.setCallback(null);
            unscheduleDrawable(mDrawable);
        }
        mDrawable = d;
        if (d != null) {
            d.setCallback(this);
            if (d.isStateful()) {
                d.setState(getDrawableState());
            }
            d.setLevel(mLevel);
            d.setLayoutDirection(getLayoutDirection());
            d.setVisible(getVisibility() == VISIBLE, true);
            mDrawableWidth = d.getIntrinsicWidth();
            mDrawableHeight = d.getIntrinsicHeight();
            applyColorMod();
            configureBounds();
        } else {
            mDrawableWidth = mDrawableHeight = -1;
        }
    }

  能够看到就是为了Drawable宽高赋值的。回过头来继续看。假设Drawable的宽高不为空的话就分别赋值给w和h。假设为空的话值为-1。然后是一个if推断，mAdjustViewBounds作为推断的变量，它是在setAdjustViewBounds方法中设置的，默觉得false，所以必须设置为true，这个推断才会运行。当然这个变量的值也能够在xml文件里设置（android:adjustViewBounds）。

那这种方法是做什么用的呢？设置View的最大高度，单独使用无效，须要与setAdjustViewBounds一起使用。假设想设置图片固定大小，又想保持图片宽高比，须要例如以下设置：

1） 设置setAdjustViewBounds为true；
2） 设置maxWidth、MaxHeight；
3） 设置设置layout_width和layout_height为wrap_content。

  再看一下这个推断。

if (mAdjustViewBounds) {
                resizeWidth = widthSpecMode != MeasureSpec.EXACTLY;
                resizeHeight = heightSpecMode != MeasureSpec.EXACTLY;
 
                desiredAspect = (float) w / (float) h;
            }

  widthSpecMode假设不是指定大小的话。由于假设指定了固定大小就不须要又一次设置大小了。

然后接下来的推断也是基于 resizeWidth和resizeHeight 的值，假设不为true的情况下，会运行例如以下代码：

 w += pleft + pright;
            h += ptop + pbottom;
            w = Math.max(w, getSuggestedMinimumWidth());
            h = Math.max(h, getSuggestedMinimumHeight());
            widthSize = resolveSizeAndState(w, widthMeasureSpec, 0);
            heightSize = resolveSizeAndState(h, heightMeasureSpec, 0);
        }
        setMeasuredDimension(widthSize, heightSize);

  考虑了填充，最后设置ImageView的大小。

  最后看一下onDraw()方法，

  @Override
 protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
 
        if (mDrawable == null) {
            return; // couldn't resolve the URI
        }
 
        if (mDrawableWidth == 0 || mDrawableHeight == 0) {
            return;     // nothing to draw (empty bounds)
        }
 
        if (mDrawMatrix == null && mPaddingTop == 0 && mPaddingLeft == 0) {
            mDrawable.draw(canvas);
        } else {
            int saveCount = canvas.getSaveCount();
            canvas.save();
 
            if (mCropToPadding) {
                final int scrollX = mScrollX;
                final int scrollY = mScrollY;
                canvas.clipRect(scrollX + mPaddingLeft, scrollY + mPaddingTop,
                        scrollX + mRight - mLeft - mPaddingRight,
                        scrollY + mBottom - mTop - mPaddingBottom);
            }
 
            canvas.translate(mPaddingLeft, mPaddingTop);
 
            if (mDrawMatrix != null) {
                canvas.concat(mDrawMatrix);
            }
            mDrawable.draw(canvas);
            canvas.restoreToCount(saveCount);
        }
    }

  在onDraw()方法中，实现方式比較简单，假设mDrawMatrix为空，那么就直接绘制出图片；假设不为空，那么还须要绘制矩阵。这就涉及到mDrawMatrix矩阵了。它是在哪赋值的呢，就是ScaleType。

这个是在configureBounds()方法中设置的，

private void configureBounds() {
        if (mDrawable == null || !mHaveFrame) {
            return;
        }
 
        int dwidth = mDrawableWidth;
        int dheight = mDrawableHeight;
 
        int vwidth = getWidth() - mPaddingLeft - mPaddingRight;
        int vheight = getHeight() - mPaddingTop - mPaddingBottom;
 
        boolean fits = (dwidth < 0 || vwidth == dwidth) &&
                       (dheight < 0 || vheight == dheight);
 
        if (dwidth <= 0 || dheight <= 0 || ScaleType.FIT_XY == mScaleType) {
            /* If the drawable has no intrinsic size, or we're told to
                scaletofit, then we just fill our entire view.
            */
            mDrawable.setBounds(0, 0, vwidth, vheight);
            mDrawMatrix = null;
        } else {
            // We need to do the scaling ourself, so have the drawable
            // use its native size.
            mDrawable.setBounds(0, 0, dwidth, dheight);
 
            if (ScaleType.MATRIX == mScaleType) {
                // Use the specified matrix as-is.
                if (mMatrix.isIdentity()) {
                    mDrawMatrix = null;
                } else {
                    mDrawMatrix = mMatrix;
                }
            } else if (fits) {
                // The bitmap fits exactly, no transform needed.
                mDrawMatrix = null;
            } else if (ScaleType.CENTER == mScaleType) {
                // Center bitmap in view, no scaling.
                mDrawMatrix = mMatrix;
                mDrawMatrix.setTranslate((int) ((vwidth - dwidth) * 0.5f + 0.5f),
                                         (int) ((vheight - dheight) * 0.5f + 0.5f));
            } else if (ScaleType.CENTER_CROP == mScaleType) {
                mDrawMatrix = mMatrix;
 
                float scale;
                float dx = 0, dy = 0;
 
                if (dwidth * vheight > vwidth * dheight) {
                    scale = (float) vheight / (float) dheight;
                    dx = (vwidth - dwidth * scale) * 0.5f;
                } else {
                    scale = (float) vwidth / (float) dwidth;
                    dy = (vheight - dheight * scale) * 0.5f;
                }
 
                mDrawMatrix.setScale(scale, scale);
                mDrawMatrix.postTranslate((int) (dx + 0.5f), (int) (dy + 0.5f));
            } else if (ScaleType.CENTER_INSIDE == mScaleType) {
                mDrawMatrix = mMatrix;
                float scale;
                float dx;
                float dy;
 
                if (dwidth <= vwidth && dheight <= vheight) {
                    scale = 1.0f;
                } else {
                    scale = Math.min((float) vwidth / (float) dwidth,
                            (float) vheight / (float) dheight);
                }
 
                dx = (int) ((vwidth - dwidth * scale) * 0.5f + 0.5f);
                dy = (int) ((vheight - dheight * scale) * 0.5f + 0.5f);
 
                mDrawMatrix.setScale(scale, scale);
                mDrawMatrix.postTranslate(dx, dy);
            } else {
                // Generate the required transform.
                mTempSrc.set(0, 0, dwidth, dheight);
                mTempDst.set(0, 0, vwidth, vheight);
 
                mDrawMatrix = mMatrix;
                mDrawMatrix.setRectToRect(mTempSrc, mTempDst, scaleTypeToScaleToFit(mScaleType));
            }
        }
    }

  能够看到在if推断中，对各个ScaleType的类型都进行了推断。依据不同的ScaleType设置不同的矩阵mDrawMatrix。然后通过矩阵对图像进行变换，从而显示出不同的效果。

  除了这一点经常使用到之外，还有就是怎样设置图片资源了，有下面几个方法：setImageResource(int resId)、setImageURI(Uri uri)、setImageDrawable(Drawable drawable)、setImageBitmap(Bitmap bm)等。或者也能够在xml文件里设置。

可是这样直接使用会有一个隐形的弊端，假设显示的图片过多或者单张显示的图片像素过大，就easy出现OOM问题。因此就应该依据需求对图片进行预处理，经常用法有下面几种：

1、缩放、边界压缩

     在内存中载入图片时直接在内存中做处理。

关于图片压缩有非常多方法，这里仅仅是列举一个简单的样例，实际使用价值不大。如有需求能够自行參考其它资料。

  InputStream is = this.getResources().openRawResource(R.drawable.xx);
     BitmapFactory.Options options=new BitmapFactory.Options();
     options.inJustDecodeBounds = false;
     options.inSampleSize = 10;   //width。hight设为原来的十分一
     Bitmap btp =BitmapFactory.decodeStream(is,null,options); 

2、直接调用JNI

     当使用像 imageView.setBackgroundResource，imageView.setImageResource, 或者 BitmapFactory.decodeResource 这种方法来设置一张大图片的时候，这些函数在完毕decode后，终于都是通过java层的createBitmap来完毕的，须要消耗很多其它内存。

因此，改用先通过BitmapFactory.decodeStream方法，创建出一个bitmap。再将其设为ImageView的 source，decodeStream最大的秘密在于其直接调用JNI>>nativeDecodeAsset()来完毕decode。无需再使用java层的createBitmap，从而节省了java层的空间。

假设在读取时加上图片的Config參数，能够跟有效降低载入的内存。从而跟有效阻止抛out of Memory异常。
  另外，须要特别注意：decodeStream是直接读取图片资料的字节码了， 不会依据机器的各种分辨率来自己主动适应。使用了decodeStream之后。须要在hdpi和mdpi，ldpi中配置对应的图片资源，否则在不同分辨率机器上都是相同大小（像素点数量）。显示出来的大小就不正确了。

  public static Bitmap readBitMap(Context context, int resId){
     BitmapFactory.Options opt = new BitmapFactory.Options();
         opt.inPreferredConfig = Bitmap.Config.RGB_565;
       opt.inPurgeable = true;
       opt.inInputShareable = true;
      InputStream is = context.getResources().openRawResource(resId);
        return BitmapFactory.decodeStream(is,null,opt);
      }

3、手动收回占用资源

     尽管虚拟机会自己主动回收垃圾资源，可是有时候不是那么及时，这时候能够手动回收。

   if(!bmp.isRecycle() ){
         bmp.recycle()   //回收图片所占的内存
         system.gc()  //提醒系统及时回收
     } 

4、优化Dalvik虚拟机的堆内存分配

     使用 dalvik.system.VMRuntime类提供的setTargetHeapUtilization方法能够增强程序堆内存的处理效率。

  private final static float TARGET_HEAP_UTILIZATION = 0.75f; 

在程序onCreate时就能够调用

 VMRuntime.getRuntime().setTargetHeapUtilization(TARGET_HEAP_UTILIZATION);

 就可以。

     除了 优化Dalvik虚拟机的堆内存分配 外，还能够强制定义自己软件的对内存大小。使用Dalvik提供的 dalvik.system.VMRuntime类来设置最小堆内存为例:Dalvik.VMRuntime类，提供对虚拟机全局，Dalvik的特定功能的接口。

Android为每一个程序分配的内存能够通过Runtime类的 totalMemory() 、freeMemory() 两个方法获取VM的一些内存信息。

private final static int CWJ_HEAP_SIZE = 6* 1024* 1024 ;
VMRuntime.getRuntime().setMinimumHeapSize(CWJ_HEAP_SIZE); //设置最小heap内存为6MB大小。

  

  下面解说一下怎样自己定义一个类继承于ImageView。首先以CircleButton为例，这是github上一个项目，实现一个圆形有点击效果的按钮。例如以下：

   实现思路是这种。先画两个圆形图案，一个是实心的圆。一个是圆环。圆环半径小于实心圆半径。这样默认就看不到圆环，然后再画出设置的图片，覆盖在二者之上。

最后在按下的时候启动一个属性动画，将圆环放大显示，关于具体的分析能够看android-circlebutton介绍 这篇文章。

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