android形状xml旋转可绘制以编程方式改变颜色

这是三角形的 xml：

<layer-list xmlns:android="http://schemas.android.com/apk/res/android" >
    <item android:id="@+id/shape_id">
        <rotate
            android:fromDegrees="45"
            android:toDegrees="45"
            android:pivotX="-40%"
            android:pivotY="87%" >
            <shape android:shape="rectangle" >
                <stroke android:width="10dp"/>
            </shape>
        </rotate>
    </item>
</layer-list>

这是文本视图的背景

<TextView
    android:id="@+id/headlineSelect_TXT2"
    android:layout_width="50dp"
    android:layout_height="50dp"
    android:layout_weight="1"
    android:background="@drawable/category_triangle_shape1"
    android:visibility="invisible" />

我想以编程方式改变形状的颜色。我尝试了这个，但出现了空指针异常

LayerDrawable bgDrawable = (LayerDrawable) getActivity()
    .getResources()
    .getDrawable(R.drawable.category_triangle_shape1);
final GradientDrawable shape = (GradientDrawable) bgDrawable
    .findDrawableByLayerId(R.id.shape_id);
shape.setStroke(10,Color.GREEN);

我怎样才能做到这一点？感谢帮助。

这有点棘手，涉及很多演员：

TextView view = (TextView) findViewById( R.id.my_text_view );

// Get the drawable from the view, if you're using an imageview src
// element you'll go with view.getDrawable()
LayerDrawable layers = (LayerDrawable) view.getBackground();

// Now get your shape by selecting the id
RotateDrawable rotate = (RotateDrawable) layers.findDrawableByLayerId( R.id.shape_id );

// Finally you can access the underlying shape
GradientDrawable drawable = (GradientDrawable) rotate.getDrawable();

// ... and do you fancy things
drawable.setColor( ... );