BitmapShader初步认识

参考文章链接：
自定义控件其实很简单1/3
Android BitmapShader 实战 实现圆形、圆角图片

这两天在看android中android.graphics.Paint类的时候关注到一个方法：

/** * Set or clear the shader object. * <p /> * Pass null to clear any previous shader. * As a convenience, the parameter passed is also returned. * * @param shader May be null. the new shader to be installed in the paint * @return       shader */public Shader setShader(Shader shader) {    long shaderNative = 0;    if (shader != null)        shaderNative = shader.getNativeInstance();    native_setShader(mNativePaint, shaderNative);    mShader = shader;    return shader;}

它的作用呢，就是为画笔着色，现在我们先看一个着色器 BitmapShader，它的构造方法如下：

/** * Call this to create a new shader that will draw with a bitmap. * * @param bitmap            The bitmap to use inside the shader * @param tileX             The tiling mode for x to draw the bitmap in. * @param tileY             The tiling mode for y to draw the bitmap in. */public BitmapShader(Bitmap bitmap, TileMode tileX, TileMode tileY) {    mBitmap = bitmap;    mTileX = tileX;    mTileY = tileY;    final long b = bitmap.ni();    init(nativeCreate(b, tileX.nativeInt, tileY.nativeInt));}

构造方法中的三个参数，第一个是一个Bitmap对象，而另外两个是什么鬼？我翻了源码之后发现它是在BitmapShader类内定义的一个枚举类型，如下：

public enum TileMode {    /**     * 拉伸最后一排或一列像素     * replicate the edge color if the shader draws outside of its     * original bounds     */    CLAMP   (0),    /**     * repeat the shader's image horizontally and vertically     */    REPEAT  (1),    /**     * repeat the shader's image horizontally and vertically, alternating     * mirror images so that adjacent images always seam     */    MIRROR  (2);    TileMode(int nativeInt) {        this.nativeInt = nativeInt;    }    final int nativeInt;}

代码很简洁，意思也很明了，TileMode的取值有三种：

• CLAMP 拉伸 这个和电脑屏保的模式应该有些不同，这个拉伸的是图片边缘的那列或一行像素

• REPEAT 重复 就是横向、纵向不断重复这个bitmap

• MIRROR 镜像 横向不断翻转重复，纵向不断翻转重复

那我们通过下面这张图片来实际看一下BitmapShader的使用：

首先，我们先初始化Paint对象：

mPaint = new Paint(Paint.ANTI_ALIAS_FLAG | Paint.DITHER_FLAG);mPaintBitmap = BitmapFactory.decodeResource(getResources(), R.drawable.cat);mPaint.setShader(new BitmapShader(mPaintBitmap, TileMode.CLAMP, TileMode.CLAMP));

然后在onDraw方法中，用这个Paint对象画出来：

@Overrideprotected void onDraw(Canvas canvas) {    canvas.drawRect(0, 0, getWidth(), getHeight(), mPaint);}

未完待续