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

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我是靠谱客的博主温暖小蝴蝶，这篇文章主要介绍不要使用Drawable的setBounds方法来指定其在控件作为背景的显示位置，现在分享给大家，希望可以做个参考。

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

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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) {
<strong>drawBackground(canvas);</strong>
}....

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private void drawBackground(Canvas canvas) {
final Drawable background = mBackground;
if (background == null) {
return;
}
if (mBackgroundSizeChanged) {
<strong>background.setBounds(0, 0,
mRight - mLeft, mBottom - mTop);</strong>
mBackgroundSizeChanged = false;
mPrivateFlags3 |= PFLAG3_OUTLINE_INVALID;
}....