面试之——Android-Universal-Image-Loa

通过前面两篇的学习，总算对Android-Universal-Image-Loader有些熟悉了。还没有看前面两篇的小伙伴们赶快去看看吧:Android-Universal-Image-Loader源码分析、Android-Universal-Image-Loader缓存策略，咋们还是从Android-Universal-Image-Loader的demo看他的效果图:

demo中的效果图.png image.png

这里demo中通过builder中的displayer方法中传入了一个CircleBitmapDisplayer对象，最后设置给了DisplayImageOptions成员变量displayer。那最终该displayer用在了什么地方呢？那咱们还是顺着源码去看下ImageLoader中displayImage方法最后调用了这么一句:

options.getDisplayer().display(bmp, imageAware, LoadedFrom.MEMORY_CACHE);

那咱们还是直接去刚才传的CircleBitmapDisplayer中display方法吧:

@Override
public void display(Bitmap bitmap, ImageAware imageAware, LoadedFrom loadedFrom) {
    if (!(imageAware instanceof ImageViewAware)) {
        throw new IllegalArgumentException("ImageAware should wrap ImageView. ImageViewAware is expected.");
    }
    imageAware.setImageDrawable(new CircleDrawable(bitmap, strokeColor, strokeWidth));
}

上面是通过ImageAware的setImageDrawable方法传了一个CircleDrawable对象，看名字都知道，其实CircleDrawable是一个Drawable对象，咱们此处的imageAware是一个ImageViewAware类型的对象，那咱们看看它的setImageDrawable方法是怎么完成显示的，该方法在父类ViewAware中实现的方法:

@Override
public boolean setImageDrawable(Drawable drawable) {
    if (Looper.myLooper() == Looper.getMainLooper()) {
        View view = viewRef.get();
        if (view != null) {
            setImageDrawableInto(drawable, view);
            return true;
        }
    } else {
        L.w(WARN_CANT_SET_DRAWABLE);
    }
    return false;
}

紧接着又调用了setImageDrawableInto方法，将传进来的drawable和当前的view作为参数:

/**
 * Should set drawable into incoming view. Incoming view is guaranteed not null.<br />
 * This method is called on UI thread.
 */
protected abstract void setImageDrawableInto(Drawable drawable, View view);

好吧，转来转去还是要去子类看看如何设置这个drawable了，还是回到ImageViewAware去看看吧:

@Override
protected void setImageDrawableInto(Drawable drawable, View view) {
    //子类中的view一定是ImageView，所以直接强转了
    ((ImageView) view).setImageDrawable(drawable);
    //如果是有动画的drawable才走这里
    if (drawable instanceof AnimationDrawable) {
        ((AnimationDrawable)drawable).start();
    }
}

oh，my god!!!就这么简单的一句啊，说白了这里就是定义一个圆形，带外框的drawable了，下面着重就是看下CircleBitmapDisplayer中内部类CircleDrawable:

public static class CircleDrawable extends Drawable {
    protected float radius;
    protected final RectF mRect = new RectF();
    protected final RectF mBitmapRect;
    protected final BitmapShader bitmapShader;
    protected final Paint paint;
    protected final Paint strokePaint;
    protected final float strokeWidth;
    protected float strokeRadius;
    public CircleDrawable(Bitmap bitmap, Integer strokeColor, float strokeWidth) {
        radius = Math.min(bitmap.getWidth(), bitmap.getHeight()) / 2;
        //将当前的bitmap放到bitmapShader上，用于后面画笔画出来
        bitmapShader = new BitmapShader(bitmap, Shader.TileMode.CLAMP, Shader.TileMode.CLAMP);
        mBitmapRect = new RectF(0, 0, bitmap.getWidth(), bitmap.getHeight());
        paint = new Paint();
        paint.setAntiAlias(true);
        //设置了bitmapShader后，说明该画笔画的是bitmap对象了
        paint.setShader(bitmapShader);
        paint.setFilterBitmap(true);
        paint.setDither(true);
        if (strokeColor == null) {
            strokePaint = null;
        } else {
            strokePaint = new Paint();
            strokePaint.setStyle(Paint.Style.STROKE);
            strokePaint.setColor(strokeColor);
            strokePaint.setStrokeWidth(strokeWidth);
            strokePaint.setAntiAlias(true);
        }
        this.strokeWidth = strokeWidth;
        strokeRadius = radius - strokeWidth / 2;
    }
    @Override
    protected void onBoundsChange(Rect bounds) {
        super.onBoundsChange(bounds);
        //获取到view占据的位置
        mRect.set(0, 0, bounds.width(), bounds.height());
        radius = Math.min(bounds.width(), bounds.height()) / 2;
        //方框的半径要小点
        strokeRadius = radius - strokeWidth / 2;
        // Resize the original bitmap to fit the new bound
        Matrix shaderMatrix = new Matrix();
        //重新设置图片的matrix，让图片大小适应view的大小
        shaderMatrix.setRectToRect(mBitmapRect, mRect, Matrix.ScaleToFit.FILL);
        bitmapShader.setLocalMatrix(shaderMatrix);
    }
    @Override
    public void draw(Canvas canvas) {
        //先要画bitmap对象
        canvas.drawCircle(radius, radius, radius, paint);
        if (strokePaint != null) {
            //方框放在上面
            canvas.drawCircle(radius, radius, strokeRadius, strokePaint);
        }
    }
    @Override
    public int getOpacity() {
        return PixelFormat.TRANSLUCENT;
    }
    @Override
    public void setAlpha(int alpha) {
        paint.setAlpha(alpha);
    }
    @Override
    public void setColorFilter(ColorFilter cf) {
        paint.setColorFilter(cf);
    }
}

上面画圆形的bitmap用到了paint的BitmapShader方式，这里也介绍下其他的方式实现，用paint的setXfermode来过滤也可以实现，用canvas的裁剪也可以实现。但是实现之前需要将bitmap适应当前view(放大或缩小)。

好了，图片转换就介绍到这里。期待最后一篇自定义Android-Universal-Image-Loader缓存和图片转换