不要使用Drawable的setBounds方法来指定其在控件作为背景的显示位置

假设一个控件使用Drawable作为背景,那么如果你想通过setBounds方法来控制背景的显示区域,那么你达不到目的.

原因是 当drawable被设置成控件背景后,当这个控件被绘制时(也就是draw(canvas)被调用时),控件在绘制背景时会自动更改其bounds为控件大小.

所以无论你怎么设置,系统都会在绘制这个控件背景时将其Bounds改为控件大小.

附上源码

public void draw(Canvas canvas) {
        final int privateFlags = mPrivateFlags;
        final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE &&
                (mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
        mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;

        /*
         * Draw traversal performs several drawing steps which must be executed
         * in the appropriate order:
         *
         *      1. Draw the background
         *      2. If necessary, save the canvas' layers to prepare for fading
         *      3. Draw view's content
         *      4. Draw children
         *      5. If necessary, draw the fading edges and restore layers
         *      6. Draw decorations (scrollbars for instance)
         */

        // Step 1, draw the background, if needed
        int saveCount;

        if (!dirtyOpaque) {
            drawBackground(canvas);
        }....

private void drawBackground(Canvas canvas) {
        final Drawable background = mBackground;
        if (background == null) {
            return;
        }

        if (mBackgroundSizeChanged) {
            background.setBounds(0, 0,  mRight - mLeft, mBottom - mTop);
            mBackgroundSizeChanged = false;
            mPrivateFlags3 |= PFLAG3_OUTLINE_INVALID;
        }....