在上一篇文章【ImageView】自定义ImageView系列（三）——一个功能强大的圆角图片库（上） 中，介绍了原作者对于开源项目 SelectableRoundedImageView 的需求出现背景和技术实现思路。这篇文章，通过对核心自定义View源码的分析，学习一下它的实现原理。
SelectableRoundedImageView.png
Java源码:
package com.joooonho;
import com.joooonho.R;
import android.content.Context;
import android.content.res.ColorStateList;
import android.content.res.Resources;
import android.content.res.Resources.NotFoundException;
import android.content.res.TypedArray;
import android.graphics.Bitmap;
import android.graphics.Bitmap.Config;
import android.graphics.BitmapShader;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.ColorFilter;
import android.graphics.Matrix;
import android.graphics.Paint;
import android.graphics.Path;
import android.graphics.PixelFormat;
import android.graphics.Rect;
import android.graphics.RectF;
import android.graphics.Shader;
import android.graphics.drawable.BitmapDrawable;
import android.graphics.drawable.Drawable;
import android.graphics.drawable.LayerDrawable;
import android.net.Uri;
import android.util.AttributeSet;
import android.util.Log;
import android.widget.ImageView;
public class SelectableRoundedImageView extends ImageView {
public static final String TAG = "SelectableRoundedImageView";
private int mResource = 0;
private static final ScaleType[] sScaleTypeArray = {
ScaleType.MATRIX,
ScaleType.FIT_XY,
ScaleType.FIT_START,
ScaleType.FIT_CENTER,
ScaleType.FIT_END,
ScaleType.CENTER,
ScaleType.CENTER_CROP,
ScaleType.CENTER_INSIDE
};
// Set default scale type to FIT_CENTER, which is default scale type of
// original ImageView.
private ScaleType mScaleType = ScaleType.FIT_CENTER;
private float mLeftTopCornerRadius = 0.0f;
private float mRightTopCornerRadius = 0.0f;
private float mLeftBottomCornerRadius = 0.0f;
private float mRightBottomCornerRadius = 0.0f;
private float mBorderWidth = 0.0f;
private static final int DEFAULT_BORDER_COLOR = Color.BLACK;
private ColorStateList mBorderColor = ColorStateList.valueOf(DEFAULT_BORDER_COLOR);
private boolean isOval = false;
private Drawable mDrawable;
private float[] mRadii = new float[] { 0, 0, 0, 0, 0, 0, 0, 0 };
public SelectableRoundedImageView(Context context) {
super(context);
}
public SelectableRoundedImageView(Context context, AttributeSet attrs) {
this(context, attrs, 0);
}
public SelectableRoundedImageView(Context context, AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
TypedArray a = context.obtainStyledAttributes(attrs,
R.styleable.SelectableRoundedImageView, defStyle, 0);
final int index = a.getInt(R.styleable.SelectableRoundedImageView_android_scaleType, -1);
if (index >= 0) {
setScaleType(sScaleTypeArray[index]);
}
mLeftTopCornerRadius = a.getDimensionPixelSize(
R.styleable.SelectableRoundedImageView_sriv_left_top_corner_radius, 0);
mRightTopCornerRadius = a.getDimensionPixelSize(
R.styleable.SelectableRoundedImageView_sriv_right_top_corner_radius, 0);
mLeftBottomCornerRadius = a.getDimensionPixelSize(
R.styleable.SelectableRoundedImageView_sriv_left_bottom_corner_radius, 0);
mRightBottomCornerRadius = a.getDimensionPixelSize(
R.styleable.SelectableRoundedImageView_sriv_right_bottom_corner_radius, 0);
if (mLeftTopCornerRadius < 0.0f || mRightTopCornerRadius < 0.0f
|| mLeftBottomCornerRadius < 0.0f || mRightBottomCornerRadius < 0.0f) {
throw new IllegalArgumentException("radius values cannot be negative.");
}
mRadii = new float[] {
mLeftTopCornerRadius, mLeftTopCornerRadius,
mRightTopCornerRadius, mRightTopCornerRadius,
mRightBottomCornerRadius, mRightBottomCornerRadius,
mLeftBottomCornerRadius, mLeftBottomCornerRadius };
mBorderWidth = a.getDimensionPixelSize(
R.styleable.SelectableRoundedImageView_sriv_border_width, 0);
if (mBorderWidth < 0) {
throw new IllegalArgumentException("border width cannot be negative.");
}
mBorderColor = a
.getColorStateList(R.styleable.SelectableRoundedImageView_sriv_border_color);
if (mBorderColor == null) {
mBorderColor = ColorStateList.valueOf(DEFAULT_BORDER_COLOR);
}
isOval = a.getBoolean(R.styleable.SelectableRoundedImageView_sriv_oval, false);
a.recycle();
updateDrawable();
}
@Override
protected void drawableStateChanged() {
super.drawableStateChanged();
invalidate();
}
@Override
public ScaleType getScaleType() {
return mScaleType;
}
@Override
public void setScaleType(ScaleType scaleType) {
super.setScaleType(scaleType);
mScaleType = scaleType;
updateDrawable();
}
/*@Override
public void setImageDrawable(Drawable drawable) {
mResource = 0;
mDrawable = SelectableRoundedCornerDrawable.fromDrawable(drawable, getResources());
super.setImageDrawable(mDrawable);
updateDrawable();
}*/
@Override
public void setImageBitmap(Bitmap bm) {
mResource = 0;
mDrawable = SelectableRoundedCornerDrawable.fromBitmap(bm, getResources());
super.setImageDrawable(mDrawable);
updateDrawable();
}
@Override
public void setImageResource(int resId) {
if (mResource != resId) {
mResource = resId;
mDrawable = resolveResource();
super.setImageDrawable(mDrawable);
updateDrawable();
}
}
@Override
public void setImageURI(Uri uri) {
super.setImageURI(uri);
setImageDrawable(getDrawable());
}
private Drawable resolveResource() {
Resources rsrc = getResources();
if (rsrc == null) {
return null;
}
Drawable d = null;
if (mResource != 0) {
try {
d = rsrc.getDrawable(mResource);
} catch (NotFoundException e) {
Log.w(TAG, "Unable to find resource: " + mResource, e);
// Don't try again.
mResource = 0;
}
}
return SelectableRoundedCornerDrawable.fromDrawable(d, getResources());
}
private void updateDrawable() {
if (mDrawable == null) {
return;
}
((SelectableRoundedCornerDrawable) mDrawable).setScaleType(mScaleType);
((SelectableRoundedCornerDrawable) mDrawable).setCornerRadii(mRadii);
((SelectableRoundedCornerDrawable) mDrawable).setBorderWidth(mBorderWidth);
((SelectableRoundedCornerDrawable) mDrawable).setBorderColor(mBorderColor);
((SelectableRoundedCornerDrawable) mDrawable).setOval(isOval);
}
public float getCornerRadius() {
return mLeftTopCornerRadius;
}
/**
* Set radii for each corner.
*
* @param leftTop The desired radius for left-top corner in dip.
* @param rightTop The desired desired radius for right-top corner in dip.
* @param leftBottom The desired radius for left-bottom corner in dip.
* @param rightBottom The desired radius for right-bottom corner in dip.
*
*/
public void setCornerRadiiDP(float leftTop, float rightTop, float leftBottom, float rightBottom) {
final float density = getResources().getDisplayMetrics().density;
final float lt = leftTop * density;
final float rt = rightTop * density;
final float lb = leftBottom * density;
final float rb = rightBottom * density;
mRadii = new float[] { lt, lt, rt, rt, rb, rb, lb, lb };
updateDrawable();
}
public float getBorderWidth() {
return mBorderWidth;
}
/**
* Set border width.
*
* @param width
* The desired width in dip.
*/
public void setBorderWidthDP(float width) {
float scaledWidth = getResources().getDisplayMetrics().density * width;
if (mBorderWidth == scaledWidth) {
return;
}
mBorderWidth = scaledWidth;
updateDrawable();
invalidate();
}
public int getBorderColor() {
return mBorderColor.getDefaultColor();
}
public void setBorderColor(int color) {
setBorderColor(ColorStateList.valueOf(color));
}
public ColorStateList getBorderColors() {
return mBorderColor;
}
public void setBorderColor(ColorStateList colors) {
if (mBorderColor.equals(colors)) {
return;
}
mBorderColor = (colors != null) ? colors : ColorStateList
.valueOf(DEFAULT_BORDER_COLOR);
updateDrawable();
if (mBorderWidth > 0) {
invalidate();
}
}
public boolean isOval() {
return isOval;
}
public void setOval(boolean oval) {
isOval = oval;
updateDrawable();
invalidate();
}
static class SelectableRoundedCornerDrawable extends Drawable {
private static final String TAG = "SelectableRoundedCornerDrawable";
private static final int DEFAULT_BORDER_COLOR = Color.BLACK;
private RectF mBounds = new RectF();
private RectF mBorderBounds = new RectF();
private final RectF mBitmapRect = new RectF();
private final int mBitmapWidth;
private final int mBitmapHeight;
private final Paint mBitmapPaint;
private final Paint mBorderPaint;
private BitmapShader mBitmapShader;
private float[] mRadii = new float[] { 0, 0, 0, 0, 0, 0, 0, 0 };
private float[] mBorderRadii = new float[] { 0, 0, 0, 0, 0, 0, 0, 0 };
private boolean mOval = false;
private float mBorderWidth = 0;
private ColorStateList mBorderColor = ColorStateList.valueOf(DEFAULT_BORDER_COLOR);
// Set default scale type to FIT_CENTER, which is default scale type of
// original ImageView.
private ScaleType mScaleType = ScaleType.FIT_CENTER;
private Path mPath = new Path();
private Bitmap mBitmap;
private boolean mBoundsConfigured = false;
public SelectableRoundedCornerDrawable(Bitmap bitmap, Resources r) {
mBitmap = bitmap;
mBitmapShader = new BitmapShader(bitmap, Shader.TileMode.CLAMP, Shader.TileMode.CLAMP);
if (bitmap != null) {
mBitmapWidth = bitmap.getScaledWidth(r.getDisplayMetrics());
mBitmapHeight = bitmap.getScaledHeight(r.getDisplayMetrics());
} else {
mBitmapWidth = mBitmapHeight = -1;
}
mBitmapRect.set(0, 0, mBitmapWidth, mBitmapHeight);
mBitmapPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
mBitmapPaint.setStyle(Paint.Style.FILL);
mBitmapPaint.setShader(mBitmapShader);
mBorderPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
mBorderPaint.setStyle(Paint.Style.STROKE);
mBorderPaint.setColor(mBorderColor.getColorForState(getState(), DEFAULT_BORDER_COLOR));
mBorderPaint.setStrokeWidth(mBorderWidth);
}
public static SelectableRoundedCornerDrawable fromBitmap(Bitmap bitmap, Resources r) {
if (bitmap != null) {
return new SelectableRoundedCornerDrawable(bitmap, r);
} else {
return null;
}
}
public static Drawable fromDrawable(Drawable drawable, Resources r) {
if (drawable != null) {
if (drawable instanceof SelectableRoundedCornerDrawable) {
return drawable;
} else if (drawable instanceof LayerDrawable) {
LayerDrawable ld = (LayerDrawable) drawable;
final int num = ld.getNumberOfLayers();
for (int i = 0; i < num; i++) {
Drawable d = ld.getDrawable(i);
ld.setDrawableByLayerId(ld.getId(i), fromDrawable(d, r));
}
return ld;
}
Bitmap bm = drawableToBitmap(drawable);
if (bm != null) {
return new SelectableRoundedCornerDrawable(bm, r);
} else {
Log.w(TAG, "Failed to create bitmap from drawable!");
}
}
return drawable;
}
public static Bitmap drawableToBitmap(Drawable drawable) {
if (drawable == null) {
return null;
}
if (drawable instanceof BitmapDrawable) {
return ((BitmapDrawable) drawable).getBitmap();
}
Bitmap bitmap;
int width = Math.max(drawable.getIntrinsicWidth(), 2);
int height = Math.max(drawable.getIntrinsicHeight(), 2);
try {
bitmap = Bitmap.createBitmap(width, height, Config.ARGB_8888);
Canvas canvas = new Canvas(bitmap);
drawable.setBounds(0, 0, canvas.getWidth(), canvas.getHeight());
drawable.draw(canvas);
} catch (IllegalArgumentException e) {
e.printStackTrace();
bitmap = null;
}
return bitmap;
}
@Override
public boolean isStateful() {
return mBorderColor.isStateful();
}
@Override
protected boolean onStateChange(int[] state) {
int newColor = mBorderColor.getColorForState(state, 0);
if (mBorderPaint.getColor() != newColor) {
mBorderPaint.setColor(newColor);
return true;
} else {
return super.onStateChange(state);
}
}
private void configureBounds(Canvas canvas) {
// I have discovered a truly marvelous explanation of this,
// which this comment space is too narrow to contain. :)
// If you want to understand what's going on here,
// See http://www.joooooooooonhokim.com/?p=289
Rect clipBounds = canvas.getClipBounds();
Matrix canvasMatrix = canvas.getMatrix();
if (ScaleType.CENTER == mScaleType) {
mBounds.set(clipBounds);
} else if (ScaleType.CENTER_CROP == mScaleType) {
applyScaleToRadii(canvasMatrix);
mBounds.set(clipBounds);
} else if (ScaleType.FIT_XY == mScaleType) {
Matrix m = new Matrix();
m.setRectToRect(mBitmapRect, new RectF(clipBounds), Matrix.ScaleToFit.FILL);
mBitmapShader.setLocalMatrix(m);
mBounds.set(clipBounds);
} else if (ScaleType.FIT_START == mScaleType || ScaleType.FIT_END == mScaleType
|| ScaleType.FIT_CENTER == mScaleType || ScaleType.CENTER_INSIDE == mScaleType) {
applyScaleToRadii(canvasMatrix);
mBounds.set(mBitmapRect);
} else if (ScaleType.MATRIX == mScaleType) {
applyScaleToRadii(canvasMatrix);
mBounds.set(mBitmapRect);
}
}
private void applyScaleToRadii(Matrix m) {
float[] values = new float[9];
m.getValues(values);
for (int i = 0; i < mRadii.length; i++) {
mRadii[i] = mRadii[i] / values[0];
}
}
private void adjustCanvasForBorder(Canvas canvas) {
Matrix canvasMatrix = canvas.getMatrix();
final float[] values = new float[9];
canvasMatrix.getValues(values);
final float scaleFactorX = values[0];
final float scaleFactorY = values[4];
final float translateX = values[2];
final float translateY = values[5];
final float newScaleX = mBounds.width()
/ (mBounds.width() + mBorderWidth + mBorderWidth);
final float newScaleY = mBounds.height()
/ (mBounds.height() + mBorderWidth + mBorderWidth);
canvas.scale(newScaleX, newScaleY);
if (ScaleType.FIT_START == mScaleType || ScaleType.FIT_END == mScaleType
|| ScaleType.FIT_XY == mScaleType || ScaleType.FIT_CENTER == mScaleType
|| ScaleType.CENTER_INSIDE == mScaleType || ScaleType.MATRIX == mScaleType) {
canvas.translate(mBorderWidth, mBorderWidth);
} else if (ScaleType.CENTER == mScaleType || ScaleType.CENTER_CROP == mScaleType) {
// First, make translate values to 0
canvas.translate(
-translateX / (newScaleX * scaleFactorX),
-translateY / (newScaleY * scaleFactorY));
// Then, set the final translate values.
canvas.translate(-(mBounds.left - mBorderWidth), -(mBounds.top - mBorderWidth));
}
}
private void adjustBorderWidthAndBorderBounds(Canvas canvas) {
Matrix canvasMatrix = canvas.getMatrix();
final float[] values = new float[9];
canvasMatrix.getValues(values);
final float scaleFactor = values[0];
float viewWidth = mBounds.width() * scaleFactor;
mBorderWidth = (mBorderWidth * mBounds.width()) / (viewWidth - (2 * mBorderWidth));
mBorderPaint.setStrokeWidth(mBorderWidth);
mBorderBounds.set(mBounds);
mBorderBounds.inset(- mBorderWidth / 2, - mBorderWidth / 2);
}
private void setBorderRadii() {
for (int i = 0; i < mRadii.length; i++) {
if (mRadii[i] > 0) {
mBorderRadii[i] = mRadii[i];
mRadii[i] = mRadii[i] - mBorderWidth;
}
}
}
@Override
public void draw(Canvas canvas) {
canvas.save();
if (!mBoundsConfigured) {
configureBounds(canvas);
if (mBorderWidth > 0) {
adjustBorderWidthAndBorderBounds(canvas);
setBorderRadii();
}
mBoundsConfigured = true;
}
if (mOval) {
if (mBorderWidth > 0) {
adjustCanvasForBorder(canvas);
mPath.addOval(mBounds, Path.Direction.CW);
canvas.drawPath(mPath, mBitmapPaint);
mPath.reset();
mPath.addOval(mBorderBounds, Path.Direction.CW);
canvas.drawPath(mPath, mBorderPaint);
} else {
mPath.addOval(mBounds, Path.Direction.CW);
canvas.drawPath(mPath, mBitmapPaint);
}
} else {
if (mBorderWidth > 0) {
adjustCanvasForBorder(canvas);
mPath.addRoundRect(mBounds, mRadii, Path.Direction.CW);
canvas.drawPath(mPath, mBitmapPaint);
mPath.reset();
mPath.addRoundRect(mBorderBounds, mBorderRadii, Path.Direction.CW);
canvas.drawPath(mPath, mBorderPaint);
} else {
mPath.addRoundRect(mBounds, mRadii, Path.Direction.CW);
canvas.drawPath(mPath, mBitmapPaint);
}
}
canvas.restore();
}
public void setCornerRadii(float[] radii) {
if (radii == null)
return;
if (radii.length != 8) {
throw new ArrayIndexOutOfBoundsException("radii[] needs 8 values");
}
for (int i = 0; i < radii.length; i++) {
mRadii[i] = radii[i];
}
}
@Override
public int getOpacity() {
return (mBitmap == null || mBitmap.hasAlpha() || mBitmapPaint.getAlpha() < 255) ? PixelFormat.TRANSLUCENT
: PixelFormat.OPAQUE;
}
@Override
public void setAlpha(int alpha) {
mBitmapPaint.setAlpha(alpha);
invalidateSelf();
}
@Override
public void setColorFilter(ColorFilter cf) {
mBitmapPaint.setColorFilter(cf);
invalidateSelf();
}
@Override
public void setDither(boolean dither) {
mBitmapPaint.setDither(dither);
invalidateSelf();
}
@Override
public void setFilterBitmap(boolean filter) {
mBitmapPaint.setFilterBitmap(filter);
invalidateSelf();
}
@Override
public int getIntrinsicWidth() {
return mBitmapWidth;
}
@Override
public int getIntrinsicHeight() {
return mBitmapHeight;
}
public float getBorderWidth() {
return mBorderWidth;
}
public void setBorderWidth(float width) {
mBorderWidth = width;
mBorderPaint.setStrokeWidth(width);
}
public int getBorderColor() {
return mBorderColor.getDefaultColor();
}
public void setBorderColor(int color) {
setBorderColor(ColorStateList.valueOf(color));
}
public ColorStateList getBorderColors() {
return mBorderColor;
}
/**
* Controls border color of this ImageView.
*
* @param colors
* The desired border color. If it's null, no border will be
* drawn.
*
*/
public void setBorderColor(ColorStateList colors) {
if (colors == null) {
mBorderWidth = 0;
mBorderColor = ColorStateList.valueOf(Color.TRANSPARENT);
mBorderPaint.setColor(Color.TRANSPARENT);
} else {
mBorderColor = colors;
mBorderPaint.setColor(mBorderColor.getColorForState(getState(),
DEFAULT_BORDER_COLOR));
}
}
public boolean isOval() {
return mOval;
}
public void setOval(boolean oval) {
mOval = oval;
}
public ScaleType getScaleType() {
return mScaleType;
}
public void setScaleType(ScaleType scaleType) {
if (scaleType == null) {
return;
}
mScaleType = scaleType;
}
}
}
attrs.xml资源：
为了便于扩展，实现通过代码和xml属性自由控制各个属性，所以源码中的getter和setter方法较多，导致源码较长。通过阅读源码，鸟哥总结了以下几个要点部分，帮助大家理解源码。
消失的onDraw方法
看完整个源码，会发现，没有自定义View经典的onDraw绘制方法（将导致在布局中无法预览自定义界面效果，这也是这个开源库的一个缺陷吧，大家感兴趣的话可以在源码的基础上进行修改实现）。而是通过自定义Drawable类——SelectableRoundedCornerDrawable，对ImageView的drawable进行圆角和圆形处理，再通过ImageView的setImageDrawable等系统API传递给ImageView。BitmapShader给画笔着色
自定义View实现圆形或圆角的方式有多种，常见的比如Paint.setXfermode、Paint.setShader（BitmapShader），鸟哥前面介绍几篇文章如【ImageView】自定义ImageView系列（一）——简单圆形图片中用到的是前者，本文中介绍的SelectableRoundedImageView用到的是后者。使用着色器BitmapShader给绘制图片的画笔着色，通过画笔展示图片。闭合的圆形或圆角Path
在自定义Drawable内部类中，重写draw(Canvas canvas)方法，通过canvas.drawPath方法使用被BitmapShader着色器着色过的Paint绘制闭合路径。源码中使用一个布尔型变量mOval控制是否绘制圆形图片，使用Path.addOval(RectF oval, Direction dir)添加圆形路径或者Path.addRoundRect(RectF rect, float[] radii, Direction dir)方法添加圆角矩形路径。
了解以上几点，基本上能掌握 SelectableRoundedImageView 核心的实现原理。通过缩放方式和一些几何运算，便可以计算出边框的宽度、圆角的半径、图片的大小等，这里不细作分析，查看源码即可。
在布局中使用：
在代码中使用：
SelectableRoundedImageView sriv = new SelectableRoundedImageView(context);
sriv.setScaleType(ScaleType.CENTER_CROP);
sriv.setCornerRadiiDP(4, 4, 0, 0);
sriv.setBorderWidthDP(4);
sriv.setBorderColor(Color.BLUE);
sriv.setImageDrawable(drawable);
sriv.setOval(true);
需要注意的一点是，如果使用Android图片加载开源框架 Android-Universal-Image-Loader ，确保在配置 DisplayImageOptions 参数时使用 SimpleBitmapDisplayer 或者 FadeInBitmapDisplayer，而不是 RoundedBitmapDisplayer 或者 RoundedVignetteBitmapDisplayer，如：
options = new DisplayImageOptions.Builder()
.showImageOnLoading(R.drawable.ic_stub)
.showImageForEmptyUri(R.drawable.ic_empty)
.showImageOnFail(R.drawable.ic_error)
.cacheInMemory(true)
.cacheOnDisk(true)
.considerExifParams(true)
// .displayer(new RoundedBitmapDisplayer(20))
// DO NOT USE RoundedBitmapDisplayer. Use SimpleBitmapDisplayer!
.displayer(new SimpleBitmapDisplayer())
.build();
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