DrawableContainer

和 DrawableWrapper 一样，DrawableContainer 也是一个容器。

它内部保存了 mDrawables ，然后在适当的时候切换它们。

@Override
public void draw(Canvas canvas) {
if (mCurrDrawable != null) {
mCurrDrawable.draw(canvas);
}
if (mLastDrawable != null) {
mLastDrawable.draw(canvas);
}
}实际展示的是 mCurrDrawable ，而 mLastDrawable 是为了切换时的动画效果。

StateListDrawable 是它的实现类，因为版本的限制，目前实现按钮点击效果，主要是靠它。

StateListDrawable 内部保存了 int[][] mStateSets，结合父类的中的 mDrawables，实现根据 state 切换背景。

@Override
protected boolean onStateChange(int[] stateSet) {
final boolean changed = super.onStateChange(stateSet);

int idx = mStateListState.indexOfStateSet(stateSet);
if (DEBUG) android.util.Log.i(TAG, "onStateChange " + this + " states "
+ Arrays.toString(stateSet) + " found " + idx);
if (idx < 0) {
idx = mStateListState.indexOfStateSet(StateSet.WILD_CARD);
}

return selectDrawable(idx) || changed;
}

LevelListDrawable 则是根据 level 进行切换。

@Override
protected boolean onLevelChange(int level) {
int idx = mLevelListState.indexOfLevel(level);
if (selectDrawable(idx)) {
return true;
}
return super.onLevelChange(level);
}

AnimationDrawable，它按一定的 duration 不断切换 drawable。

public void start() {
mAnimating = true;

if (!isRunning()) {
// Start from 0th frame.
setFrame(0, false, mAnimationState.getChildCount() > 1
|| !mAnimationState.mOneShot);
}
}

private void setFrame(int frame, boolean unschedule, boolean animate) {
if (frame >= mAnimationState.getChildCount()) {
return;
}
mAnimating = animate;
mCurFrame = frame;
selectDrawable(frame);
if (unschedule || animate) {
unscheduleSelf(this);
}
if (animate) {
// Unscheduling may have clobbered these values; restore them
mCurFrame = frame;
mRunning = true;
scheduleSelf(this, SystemClock.uptimeMillis() + mAnimationState.mDurations[frame]);
}
}

实现不断切换的关键：

public void scheduleSelf(Runnable what, long when) {
final Callback callback = getCallback();
if (callback != null) {
callback.scheduleDrawable(this, what, when);
}
}

@Override
public void scheduleDrawable(Drawable who, Runnable what, long when) {
if (verifyDrawable(who) && what != null) {
final long delay = when - SystemClock.uptimeMillis();
if (mAttachInfo != null) {
mAttachInfo.mViewRootImpl.mChoreographer.postCallbackDelayed(
Choreographer.CALLBACK_ANIMATION, what, who,
Choreographer.subtractFrameDelay(delay));
} else {
ViewRootImpl.getRunQueue().postDelayed(what, delay);
}
}
}