1.系统ProgressBar样式实现原理分析

普通的一直圆形转的ProgressBar

<style name="Widget.ProgressBar">
    <item name="indeterminateOnly">true</item>
    <item name="indeterminateDrawable">@drawable/progress_medium_white</item>
    <item name="indeterminateBehavior">repeat</item>
    <item name="indeterminateDuration">3500</item>
    <item name="minWidth">48dip</item>
    <item name="maxWidth">48dip</item>
    <item name="minHeight">48dip</item>
    <item name="maxHeight">48dip</item>
    <item name="mirrorForRtl">false</item>
</style> 

上面的“@drawable/progress_medium_white” 是一个帧动画

水平的ProgressBar

<style name="Widget.ProgressBar.Horizontal">
    <item name="indeterminateOnly">false</item>
    <item name="progressDrawable">@drawable/progress_horizontal</item>
    <item name="indeterminateDrawable">@drawable/progress_indeterminate_horizontal</item>
    <item name="minHeight">20dip</item>
    <item name="maxHeight">20dip</item>
    <item name="mirrorForRtl">true</item>
</style>

@drawable/progress_indeterminate_horizontal：是一个帧动画

@drawable/progress_horizontal：是一个LayerDrawable，主要由一个表示底色的Drawable，和表示Progress的ClipDrawable组成

<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:id="@android:id/background">
        <shape>
            <corners android:radius="5dip" />
            <gradient
                .....................
            />
        </shape>
    </item>
....................
<item android:id="@android:id/progress">
        <clip>
            <shape>
                <corners android:radius="5dip" />
                <gradient
                    .....................
                />
            </shape>
        </clip>
    </item>
</layer-list>

小结：
一直旋转的系统ProgressBar，其实只是一直播放的帧动画
水平的ProgressBar，是由于ClipDrawable根据Progress，从而改变ClipDrawable的Level

2.方案选择

1. 参考系统水平ProgressBar的样式实现，自定义特殊的Drawable，传入ProgressBar（由于是系统实现，优先考虑）
2. 继承ProgressBar，重写里面的Draw方法

3.方案一

1. 自定义一个Drawable，重写里面的Draw方法，绘制一个圆角矩形作为底色
2. 再自定义一个Drawable，重写里面的Draw方法，根据Level值，绘制一个当前的进度
3. 通过LayerDrawable把这两个Drawable层叠起来，得到最后的LayerDrawable
4. 调用ProgressBar的setProgressDrawable，设置该LayerDrawable

结果：失败。通过代码调用ProgressBar的setProgressDrawable发现调用无效，不解。

4.方案二

1. 生成一个矩形：RectF rectF = new RectF(mPathWidth,mPathWidth,getWidth()-mPathWidth,getHeight()-mPathWidth);
   注意：因为是只要边框，所以生成的矩形要减去边框的宽度
2. 生成圆角矩形的Path
3. 利用PathMeasure，获取Path的某一部分，并绘制到Canvas

具体代码：https://github.com/yinfork/Android-Dev-Demos/blob/master/app/src/main/java/com/inkenka/devdemos/processbar/RoundCornerProgressBar.java

public class RoundCornerProgressBar extends ProgressBar {
    private Paint mBackgroundPaint = new Paint();
    private Paint mProgressPaint = new Paint();
    private float mRadius;
    private int mPathWidth;
    private float mStartOffsetPercent;

构造函数（略）
 
    @Override
    public void onDraw(Canvas canvas) {
        drawBackground(canvas);
        drawProgress(canvas);
    }

    private void drawBackground(Canvas canvas){
        RectF rectF = new RectF(mPathWidth,mPathWidth,getWidth()-mPathWidth,getHeight()-mPathWidth);
        canvas.drawRoundRect(rectF, mRadius, mRadius, mBackgroundPaint);
    }

    private void drawProgress(Canvas canvas){
        RectF rectF = new RectF(mPathWidth,mPathWidth,getWidth()-mPathWidth,getHeight()-mPathWidth);
        Path path = new Path();

        path.addRoundRect(rectF, mRadius, mRadius, Path.Direction.CW);

        Path mRenderPaths = new Path();
        PathMeasure mPathMeasure = new PathMeasure(path, false);

        float pathOffset = mPathMeasure.getLength() * (getProgress()/100f);
        float startOffset = mPathMeasure.getLength() * mStartOffsetPercent;

        if(pathOffset + startOffset < mPathMeasure.getLength()){
            if (mPathMeasure.getSegment(startOffset, startOffset + pathOffset, mRenderPaths, true)) {
                canvas.drawPath(mRenderPaths, mProgressPaint);
            }
        } else {
            if (mPathMeasure.getSegment(startOffset, mPathMeasure.getLength(), mRenderPaths, true)) {
                canvas.drawPath(mRenderPaths, mProgressPaint);
            }
            if (mPathMeasure.getSegment(0, pathOffset + startOffset - mPathMeasure.getLength(), mRenderPaths, true)) {
                canvas.drawPath(mRenderPaths, mProgressPaint);
            }
        }
    }
}