版本：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，也可以说是必不可少的。对于它的使用，除了注意ScaleType的理解和设置外，还需要注意其他一些问题，比如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，如果想熟练使用这个控件，就需要对其实现的机制有一个基本的了解。
ImageView也是直接继承于View类，主要的结构图如下：
鉴于篇幅大小，就不copy ImageView的整体代码，选择结构图中的部分作为重点。首先是构造方法，代码如下：
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;
}
在构造方法中也是很常规的从attrs文件中读取属性值，并进行设置。也可以看到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，平常使用的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文件中设置ImageView的时候，如果指定了固定的宽高，那么onMeasur()方法中测量的大小就是固定的宽高大小；如果是包裹内容，那么就需要进一步的计算。
@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()这个方法，目的就是为了确定Drawable。如果设置了drawableResource，那么Drawable就是其值；如果没有，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()这个方法的最后，调用了 updateDrawable(d)方法，这个方法代码如下：
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;
}
}
可以看到就是为了Drawable宽高赋值的。回过头来继续看，如果Drawable的宽高不为空的话就分别赋值给w和h；如果为空的话值为-1。然后是一个if判断，mAdjustViewBounds作为判断的变量，它是在setAdjustViewBounds方法中设置的，默认为false，所以必须设置为true，这个判断才会执行。当然这个变量的值也可以在xml文件中设置（android:adjustViewBounds）。那这个方法是做什么用的呢？设置View的最大高度，单独使用无效，需要与setAdjustViewBounds一起使用。如果想设置图片固定大小，又想保持图片宽高比，需要如下设置：
1） 设置setAdjustViewBounds为true；2） 设置maxWidth、MaxHeight；
3） 设置设置layout_width和layout_height为wrap_content。
再看一下这个判断，
if (mAdjustViewBounds) {
resizeWidth = widthSpecMode != MeasureSpec.EXACTLY;
resizeHeight = heightSpecMode != MeasureSpec.EXACTLY;
desiredAspect = (float) w / (float) h;
}
widthSpecMode如果不是指定大小的话，因为如果指定了固定大小就不需要重新设置大小了。然后接下来的判断也是基于 resizeWidth和resizeHeight 的值，如果不为true的情况下，会执行如下代码：
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，实现方式比较简单，如果mDrawMatrix为空，那么就直接绘制出图片；如果不为空，那么还需要绘制矩阵。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));
}
}
}
可以看到在if判断中，对各个ScaleType的类型都进行了判断，根据不同的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，还有就是如何设置图片资源了，有以下几个方法：setImageResource(int resId)、setImageURI(Uri uri)、setImageDrawable(Drawable drawable)、setImageBitmap(Bitmap bm)等，或者也可以在xml文件中设置。但是这样直接使用会有一个隐形的弊端，如果显示的图片过多或者单张显示的图片像素过大，就容易出现OOM问题。因此就应该根据需求对图片进行预处理，常用方法有以下几种：
1、缩放、边界压缩
在内存中加载图片时直接在内存中做处理。关于图片压缩有很多方法，这里只是列举一个简单的例子，实际使用价值不大，如有需求可以自行参考其他资料。
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 这样的方法来设置一张大图片的时候，这些函数在完成decode后，最终都是通过java层的createBitmap来完成的，需要消耗更多内存。
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()来完成decode，无需再使用java层的createBitmap，从而节省了java层的空间。如果在读取时加上图片的Config参数，可以跟有效减少加载的内存，从而跟有效阻止抛out of Memory异常。另外，需要特别注意：decodeStream是直接读取图片资料的字节码了， 不会根据机器的各种分辨率来自动适应，使用了decodeStream之后，需要在hdpi和mdpi，ldpi中配置相应的图片资源，否则在不同分辨率机器上都是同样大小（像素点数量），显示出来的大小就不对了。
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
虽然虚拟机会自动回收垃圾资源，但是有时候不是那么及时，这时候可以手动回收。
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方法可以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时就可以调用
VMRuntime.getRuntime().setTargetHeapUtilization(TARGET_HEAP_UTILIZATION);
即可。
除了 优化Dalvik虚拟机的堆内存分配 外，还可以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为每个程序分配的内存可以通过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。
下面讲解一下如何自定义一个类继承于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。如下：
实现思路是这样的，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，关于详细的分析可以看android-circlebutton介绍 这篇文章。