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：

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

未完待续