Picasso 2.5.2的fit、resize在android5.1无法加载图片的解决方案

使用Transformation代替fit、resize

一开始使用下面代码加载图片，发现图片显示不出来

Picasso.with(context)
                    .load(new File(data.content))
                    .placeholder(R.drawable.default_publishlist)
                    .fit()
                    .centerInside()
                    .into(binding.ivContent);

后来去掉fit、centerInside方法，图片可以加载出来，不过占用内存非常大，显然还是存在问题。经过探索发现Picasso有一个Transformation接口，可以自定义转换图像，于是有了的处理方式：

Picasso.with(context)
                    .load(new File(data.content))
                    .placeholder(R.drawable.default_publishlist)
                    .transform(getTransformation(binding.ivContent))
                    .into(binding.ivContent);

private static Transformation getTransformation(final ImageView view) {
        return new Transformation() {
            @Override
            public Bitmap transform(Bitmap source) {
                int targetWidth = view.getWidth();

                //返回原图
                if (source.getWidth() == 0 || source.getWidth() < targetWidth) {
                    return source;
                }

                //如果图片大小大于等于设置的宽度，则按照设置的宽度比例来缩放
                double aspectRatio = (double) source.getHeight() / (double) source.getWidth();
                int targetHeight = (int) (targetWidth * aspectRatio);
                if (targetHeight == 0 || targetWidth == 0) {
                    return source;
                }
                Bitmap result = Bitmap.createScaledBitmap(source, targetWidth, targetHeight, false);
                if (result != source) {
                    // Same bitmap is returned if sizes are the same
                    source.recycle();
                }
                return result;
            }

            @Override
            public String key() {
                return "transformation" + " desiredWidth";
            }
        };
    }

根据ImageView宽高来计算加载图片的大小，可以大大节省占用的内存，避免发生OOM