一、問題在哪里？
問題來源于app開發中一個很常見的場景――用戶頭像要展示成圓的：
二、怎么搞？
機智的我，第一想法就是，切一張中間圓形透明、四周與底色相同、尺寸與頭像相同的蒙板圖片，蓋在頭像上不就完事了嘛，哈哈哈！
在背景純色的前提下，這的確能簡單解決問題，但是如果背景沒有這么簡單呢？
在這種不規則背景下，有兩個問題：
1)、背景圖常常是適應手機寬度縮放，而頭像的尺寸又是固定寬高DP的，所以固定的蒙板圖片是沒法保證在不同機型上都和背景圖案吻合的。
2)、在這種非純色背景下，哪天想調整一下頭像位置就得重新換圖片蒙板，實在是太難維護了……
所以呢，既然頭像圖片肯定是方的，那就就讓ImageView圓起來吧。
三、開始干活
基本思路是，自定義一個ImageView，通過重寫onDraw方法畫出一個圓形的圖片來：
public class ImageViewPlus extends ImageView{ private Paint mPaintBitmap = new Paint(Paint.ANTI_ALIAS_FLAG); private Bitmap mRawBitmap; private BitmapShader mShader; private Matrix mMatrix = new Matrix(); public ImageViewPlus(Context context, AttributeSet attrs) { super(context, attrs); } @Override protected void onDraw(Canvas canvas) { Bitmap rawBitmap = getBitmap(getDrawable()); if (rawBitmap != null){ int viewWidth = getWidth(); int viewHeight = getHeight(); int viewMinSize = Math.min(viewWidth, viewHeight); float dstWidth = viewMinSize; float dstHeight = viewMinSize; if (mShader == null || !rawBitmap.equals(mRawBitmap)){ mRawBitmap = rawBitmap; mShader = new BitmapShader(mRawBitmap, TileMode.CLAMP, TileMode.CLAMP); } if (mShader != null){ mMatrix.setScale(dstWidth / rawBitmap.getWidth(), dstHeight / rawBitmap.getHeight()); mShader.setLocalMatrix(mMatrix); } mPaintBitmap.setShader(mShader); float radius = viewMinSize / 2.0f; canvas.drawCircle(radius, radius, radius, mPaintBitmap); } else { super.onDraw(canvas); } } private Bitmap getBitmap(Drawable drawable){ if (drawable instanceof BitmapDrawable){ return ((BitmapDrawable)drawable).getBitmap(); } else if (drawable instanceof ColorDrawable){ Rect rect = drawable.getBounds(); int width = rect.right - rect.left; int height = rect.bottom - rect.top; int color = ((ColorDrawable)drawable).getColor(); Bitmap bitmap = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888); Canvas canvas = new Canvas(bitmap); canvas.drawARGB(Color.alpha(color), Color.red(color), Color.green(color), Color.blue(color)); return bitmap; } else { return null; } }}分析一下代碼：
canvas.drawCircle 決定了畫出來的形狀是圓形，而圓形的內容則是通過 mPaintBitmap.setShader 搞定的。
其中，BitmapShader需要設置Bitmap填充ImageView的方式(CLAMP：拉伸邊緣, MIRROR：鏡像, REPEAT：整圖重復)。
這里其實設成什么不重要，因為我們實際需要的是將Bitmap按比例縮放成跟ImageView一樣大，而不是預置的三種效果。
所以，別忘了 mMatrix.setScale 和 mShader.setLocalMatrix 一起用，將圖片縮放一下。
四、更多玩法 ―― 支持邊框
看下面的效果圖，如果想給圓形的頭像上加一個邊框，該怎么搞呢？
public class ImageViewPlus extends ImageView{ private Paint mPaintBitmap = new Paint(Paint.ANTI_ALIAS_FLAG); private Paint mPaintBorder = new Paint(Paint.ANTI_ALIAS_FLAG); private Bitmap mRawBitmap; private BitmapShader mShader; private Matrix mMatrix = new Matrix(); private float mBorderWidth = dip2px(15); private int mBorderColor = 0xFF0080FF; public ImageViewPlus(Context context, AttributeSet attrs) { super(context, attrs); } @Override protected void onDraw(Canvas canvas) { Bitmap rawBitmap = getBitmap(getDrawable()); if (rawBitmap != null){ int viewWidth = getWidth(); int viewHeight = getHeight(); int viewMinSize = Math.min(viewWidth, viewHeight); float dstWidth = viewMinSize; float dstHeight = viewMinSize; if (mShader == null || !rawBitmap.equals(mRawBitmap)){ mRawBitmap = rawBitmap; mShader = new BitmapShader(mRawBitmap, TileMode.CLAMP, TileMode.CLAMP); } if (mShader != null){ mMatrix.setScale((dstWidth - mBorderWidth * 2) / rawBitmap.getWidth(), (dstHeight - mBorderWidth * 2) / rawBitmap.getHeight()); mShader.setLocalMatrix(mMatrix); } mPaintBitmap.setShader(mShader); mPaintBorder.setStyle(Paint.Style.STROKE); mPaintBorder.setStrokeWidth(mBorderWidth); mPaintBorder.setColor(mBorderColor); float radius = viewMinSize / 2.0f; canvas.drawCircle(radius, radius, radius - mBorderWidth / 2.0f, mPaintBorder); canvas.translate(mBorderWidth, mBorderWidth); canvas.drawCircle(radius - mBorderWidth, radius - mBorderWidth, radius - mBorderWidth, mPaintBitmap); } else { super.onDraw(canvas); } } private Bitmap getBitmap(Drawable drawable){ if (drawable instanceof BitmapDrawable){ return ((BitmapDrawable)drawable).getBitmap(); } else if (drawable instanceof ColorDrawable){ Rect rect = drawable.getBounds(); int width = rect.right - rect.left; int height = rect.bottom - rect.top; int color = ((ColorDrawable)drawable).getColor(); Bitmap bitmap = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888); Canvas canvas = new Canvas(bitmap); canvas.drawARGB(Color.alpha(color), Color.red(color), Color.green(color), Color.blue(color)); return bitmap; } else { return null; } } private int dip2px(int dipVal) { float scale = getResources().getDisplayMetrics().density; return (int)(dipVal * scale + 0.5f); }}看代碼中，加邊框實際上就是用實心純色的 Paint 畫了一個圓邊，在此基礎上畫上原來的頭像即可。
需要的注意的地方有三個：
1)、圓框的半徑不是 radius ，而應該是 radius - mBorderWidth / 2.0f 。想象著拿著筆去畫線，線其實是畫在右圖中白色圈的位置，只不過它很粗。
2)、在ImageView大小不變的基礎上，頭像的實際大小要比沒有邊框的時候小了，所以 mMatrix.setScale 的時候要把邊框的寬度去掉。
3)、畫頭像Bitmap的時候不能直接 canvas.drawCircle(radius, radius, radius - mBorderWidth, mPaintBitmap) ，這樣你會發現頭像的右側和下方邊緣被拉伸了（右圖）
為什么呢？因為 Paint 默認是以左上角為基準開始繪制的，此時頭像的實際區域是右圖中的紅框，而超過紅框的部分（圓形的右側和下方），自然被 TileMode.CLAMP效果沿邊緣拉伸了。
所以，需要通過挪動坐標系的位置和調整圓心，才能把頭像畫在正確的區域（右圖綠框）中。
五、更多玩法 ―― 支持xml配置
既然有了邊框，那如果想配置邊框的寬度和顏色該如何是好呢？
基本上兩個思路：
1)給ImageViewPlus加上set接口，設置完成之后通過 invalidate(); 重繪一下即可；
2)在xml里就支持配置一些自定義屬性，這樣用起來會方便很多。
這里重點說一下支持xml配置自定義屬性。
自定義控件要支持xml配置自定義屬性的話，首先需要在 /res/values 里去定義屬性：
<?xml version="1.0" encoding="utf-8"?> <resources> <attr name="borderColor" format="color" /> <attr name="borderWidth" format="dimension" /> <declare-styleable name="ImageViewPlus"> <attr name="borderColor" /> <attr name="borderWidth" /> </declare-styleable> </resources>
View attrs_imageviewplus.xml
然后在ImageViewPlus的構造函數中去讀取這些自定義屬性：
private static final int DEFAULT_BORDER_COLOR = Color.TRANSPARENT; private static final int DEFAULT_BORDER_WIDTH = 0; public ImageViewPlus(Context context, AttributeSet attrs) { super(context, attrs); //取xml文件中設定的參數 TypedArray ta = context.obtainStyledAttributes(attrs, R.styleable.ImageViewPlus); mBorderColor = ta.getColor(R.styleable.ImageViewPlus_borderColor, DEFAULT_BORDER_COLOR); mBorderWidth = ta.getDimensionPixelSize(R.styleable.ImageViewPlus_borderWidth, dip2px(DEFAULT_BORDER_WIDTH)); ta.recycle(); }在xml布局中使用自定義屬性：
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" xmlns:snser="http://schemas.android.com/apk/res/cc.snser.imageviewplus" android:layout_width="match_parent" android:layout_height="match_parent" android:background="@drawable/wallpaper" android:orientation="vertical" tools:context="${relativePackage}.${activityClass}" > <cc.snser.imageviewplus.ImageViewPlus android:id="@+id/imgplus" android:layout_width="200dp" android:layout_height="300dp" android:layout_marginBottom="50dp" android:layout_centerHorizontal="true" android:layout_alignParentBottom="true" android:src="@drawable/img_square" snser:borderColor="#FF0080FF" snser:borderWidth="15dp" /> </RelativeLayout>六、更多玩法 ―― 圓角ImageView
搞定了圓形ImageView以及對應的邊框，那如何實現下面這種圓角的ImageView呢？
其實原理上一樣，把 canvas.drawCircle 對應改成 canvas.drawRoundRect 就OK了，直接貼代碼吧：
public class ImageViewPlus extends ImageView{ /** * android.widget.ImageView */ public static final int TYPE_NONE = 0; /** * 圓形 */ public static final int TYPE_CIRCLE = 1; /** * 圓角矩形 */ public static final int TYPE_ROUNDED_RECT = 2; private static final int DEFAULT_TYPE = TYPE_NONE; private static final int DEFAULT_BORDER_COLOR = Color.TRANSPARENT; private static final int DEFAULT_BORDER_WIDTH = 0; private static final int DEFAULT_RECT_ROUND_RADIUS = 0; private int mType; private int mBorderColor; private int mBorderWidth; private int mRectRoundRadius; private Paint mPaintBitmap = new Paint(Paint.ANTI_ALIAS_FLAG); private Paint mPaintBorder = new Paint(Paint.ANTI_ALIAS_FLAG); private RectF mRectBorder = new RectF(); private RectF mRectBitmap = new RectF(); private Bitmap mRawBitmap; private BitmapShader mShader; private Matrix mMatrix = new Matrix(); public ImageViewPlus(Context context, AttributeSet attrs) { super(context, attrs); //取xml文件中設定的參數 TypedArray ta = context.obtainStyledAttributes(attrs, R.styleable.ImageViewPlus); mType = ta.getInt(R.styleable.ImageViewPlus_type, DEFAULT_TYPE); mBorderColor = ta.getColor(R.styleable.ImageViewPlus_borderColor, DEFAULT_BORDER_COLOR); mBorderWidth = ta.getDimensionPixelSize(R.styleable.ImageViewPlus_borderWidth, dip2px(DEFAULT_BORDER_WIDTH)); mRectRoundRadius = ta.getDimensionPixelSize(R.styleable.ImageViewPlus_rectRoundRadius, dip2px(DEFAULT_RECT_ROUND_RADIUS)); ta.recycle(); } @Override protected void onDraw(Canvas canvas) { Bitmap rawBitmap = getBitmap(getDrawable()); if (rawBitmap != null && mType != TYPE_NONE){ int viewWidth = getWidth(); int viewHeight = getHeight(); int viewMinSize = Math.min(viewWidth, viewHeight); float dstWidth = mType == TYPE_CIRCLE ? viewMinSize : viewWidth; float dstHeight = mType == TYPE_CIRCLE ? viewMinSize : viewHeight; float halfBorderWidth = mBorderWidth / 2.0f; float doubleBorderWidth = mBorderWidth * 2; if (mShader == null || !rawBitmap.equals(mRawBitmap)){ mRawBitmap = rawBitmap; mShader = new BitmapShader(mRawBitmap, TileMode.CLAMP, TileMode.CLAMP); } if (mShader != null){ mMatrix.setScale((dstWidth - doubleBorderWidth) / rawBitmap.getWidth(), (dstHeight - doubleBorderWidth) / rawBitmap.getHeight()); mShader.setLocalMatrix(mMatrix); } mPaintBitmap.setShader(mShader); mPaintBorder.setStyle(Paint.Style.STROKE); mPaintBorder.setStrokeWidth(mBorderWidth); mPaintBorder.setColor(mBorderWidth > 0 ? mBorderColor : Color.TRANSPARENT); if (mType == TYPE_CIRCLE){ float radius = viewMinSize / 2.0f; canvas.drawCircle(radius, radius, radius - halfBorderWidth, mPaintBorder); canvas.translate(mBorderWidth, mBorderWidth); canvas.drawCircle(radius - mBorderWidth, radius - mBorderWidth, radius - mBorderWidth, mPaintBitmap); } else if (mType == TYPE_ROUNDED_RECT){ mRectBorder.set(halfBorderWidth, halfBorderWidth, dstWidth - halfBorderWidth, dstHeight - halfBorderWidth); mRectBitmap.set(0.0f, 0.0f, dstWidth - doubleBorderWidth, dstHeight - doubleBorderWidth); float borderRadius = mRectRoundRadius - halfBorderWidth > 0.0f ? mRectRoundRadius - halfBorderWidth : 0.0f; float bitmapRadius = mRectRoundRadius - mBorderWidth > 0.0f ? mRectRoundRadius - mBorderWidth : 0.0f; canvas.drawRoundRect(mRectBorder, borderRadius, borderRadius, mPaintBorder); canvas.translate(mBorderWidth, mBorderWidth); canvas.drawRoundRect(mRectBitmap, bitmapRadius, bitmapRadius, mPaintBitmap); } } else { super.onDraw(canvas); } } private int dip2px(int dipVal) { float scale = getResources().getDisplayMetrics().density; return (int)(dipVal * scale + 0.5f); } private Bitmap getBitmap(Drawable drawable){ if (drawable instanceof BitmapDrawable){ return ((BitmapDrawable)drawable).getBitmap(); } else if (drawable instanceof ColorDrawable){ Rect rect = drawable.getBounds(); int width = rect.right - rect.left; int height = rect.bottom - rect.top; int color = ((ColorDrawable)drawable).getColor(); Bitmap bitmap = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888); Canvas canvas = new Canvas(bitmap); canvas.drawARGB(Color.alpha(color), Color.red(color), Color.green(color), Color.blue(color)); return bitmap; } else { return null; } }}View ImageViewPlus.java
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" xmlns:snser="http://schemas.android.com/apk/res/cc.snser.imageviewplus" android:layout_width="match_parent" android:layout_height="match_parent" android:background="@drawable/wallpaper" android:orientation="vertical" tools:context="${relativePackage}.${activityClass}" > <cc.snser.imageviewplus.ImageViewPlus android:id="@+id/imgplus" android:layout_width="200dp" android:layout_height="300dp" android:layout_marginBottom="50dp" android:layout_centerHorizontal="true" android:layout_alignParentBottom="true" android:src="@drawable/img_rectangle" snser:type="rounded_rect" snser:borderColor="#FF0080FF" snser:borderWidth="10dp" snser:rectRoundRadius="30dp" /> </RelativeLayout>View layout <?xml version="1.0" encoding="utf-8"?> <resources> <attr name="type"> <enum name="none" value="0" /> <enum name="circle" value="1" /> <enum name="rounded_rect" value="2" /> </attr> <attr name="borderColor" format="color" /> <attr name="borderWidth" format="dimension" /> <attr name="rectRoundRadius" format="dimension" /> <declare-styleable name="ImageViewPlus"> <attr name="type" /> <attr name="borderColor" /> <attr name="borderWidth" /> <attr name="rectRoundRadius" /> </declare-styleable></resources> View attrs_imageviewplus.xml
以上就是本文的全部內容，希望對大家學習Android軟件編程有所幫助。
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