本文详细解析了Android中Selector的工作原理及其实现过程。介绍了如何通过XML文件定义不同状态下的Drawable资源，并阐述了View与Drawable间的状态同步机制。此外，还深入分析了StateListDrawable类的具体实现。
selector原理简述过程：
1.drawable = new StateListDrawable();//android默认使用selector产生的Drawable对象类型是StateListDrawable
2.drawable.inflate(r, parser, attrs);//解析xml文件把各种状态储存进入drawable对象
3.setPressed(boolean pressed);//当View被点中调用,用户点击动作之后就是view产生状态的地方,改变一个现有状态参数为点中
4.onCreateDrawableState(0);//View生成状态集传递给drawable对象,参数不为0表示有自定义状态
5.onStateChange(stateSet);//drawable通过这个方法解析出一个index可以找到之前xml文件中存入对应的图片
6.invalidateSelf()//drawable通过这个方法重新绘制,其中包含一个Drawable.Callback接口
7.invalidateDrawable()//Drawable.Callback接口会调用这个方法进行重绘，在这里这个方法被view重写，调用了view的draw方法重绘
结论：
1.我们的Drawable类对象必须将View设置为回调对象,实现invalidateDrawable(Drawable drawable),让view负责绘制
2.Drawable对象可以解析xml文件并且储存各种状态和对应的图片
3.不能直接给Drawable指定状态，要通过onCreateDrawableState(0)返回的int[],Drawable对象从中可以解析出一个index来找到对应图片，也就是说Drawable的状态是和View相关联，状态值是View传递给它，而View的状态值是由一系列不同的操作，例如点击,选中产生。
4.由上所述，自定义状态和显示对应图片的步骤:
a.自定义xml文件selector，自定义状态对应图片，传递给drawable对象
b.在View中设定一个表明状态的参数mState
c.在View中类似onclick的操作响应方法中改变mState的状态
d.在View的onCreateDrawableState方法中,根据mState的状态判断是不是要添加额外的状态进入状态数组返回给Drawable对象
e.Drawable对象会在onStateChange方法中根据传入的状态和selector中的参数和对应的图片根据一些规则指定一个正确的图片进行显示
源码分析：
1.当按钮或其他组件状态改变时，怎么实现的相应状态图片或颜色的显示呢。首先Drawable对象会通过xml载入selector的信息,这样drawable对象就有了不同的状态和图片,由于Drawable是抽象类,为了响应不同的状态需要实现Drawable类
以下代码是当用selector作为Drawable传入view中， android默认定义了StateListDrawable类来响应和储存状态
[java] view plaincopyprint?
public static Drawable createFromXmlInner(Resources r, XmlPullParser parser, AttributeSet attrs)
throws XmlPullParserException, IOException {
Drawable drawable;
final String name = parser.getName();
if (name.equals("selector")) {
drawable = new StateListDrawable();//看下这里
} else if (name.equals("level-list")) {
drawable = new LevelListDrawable();
} else if (name.equals("layer-list")) {
drawable = new LayerDrawable();
} else if (name.equals("transition")) {
drawable = new TransitionDrawable();
} else if (name.equals("color")) {
drawable = new ColorDrawable();
} else if (name.equals("shape")) {
drawable = new GradientDrawable();
} else if (name.equals("scale")) {
drawable = new ScaleDrawable();
} else if (name.equals("clip")) {
drawable = new ClipDrawable();
} else if (name.equals("rotate")) {
drawable = new RotateDrawable();
} else if (name.equals("animated-rotate")) {
drawable = new AnimatedRotateDrawable();
} else if (name.equals("animation-list")) {
drawable = new AnimationDrawable();
} else if (name.equals("inset")) {
drawable = new InsetDrawable();
} else if (name.equals("bitmap")) {
drawable = new BitmapDrawable();
if (r != null) {
((BitmapDrawable) drawable).setTargetDensity(r.getDisplayMetrics());
}
} else if (name.equals("nine-patch")) {
drawable = new NinePatchDrawable();
if (r != null) {
((NinePatchDrawable) drawable).setTargetDensity(r.getDisplayMetrics());
}
} else {
throw new XmlPullParserException(parser.getPositionDescription() +
": invalid drawable tag " + name);
}
drawable.inflate(r, parser, attrs);
return drawable;
}
2.View会在setPress后产生按下状态
[java] view plaincopyprint?
public void setPressed(boolean pressed) {
if (pressed) {
mPrivateFlags |= PRESSED;//PRESSED标志位置1
} else {
mPrivateFlags &= ~PRESSED;//PRESSED标志位置0
}
refreshDrawableState();//刷新Drawable状态
dispatchSetPressed(pressed);//分发setPress事件，Viewgroup重写了该方法，实现向子View的分发。
}
3.要实现图片的改变其实是改变View中的Drawable对象的状态,当view的状态改变,需要通知Drawable对象
[java] view plaincopyprint?
public void refreshDrawableState() {
mPrivateFlags |= DRAWABLE_STATE_DIRTY;//DRAWABLE_STATE_DIRTY标志位是干什么的呢，往下看就知道了
drawableStateChanged();//改变drawableState
ViewParent parent = mParent;
if (parent != null) {
parent.childDrawableStateChanged(this);//如果有父VIew的话，通知父View。
}
}
[java] view plaincopyprint?
protected void drawableStateChanged() {
Drawable d = mBGDrawable;//我们设置的selector对应的Drawable，也就是StateListDrawable
if (d != null && d.isStateful()) {//StateListDrawable重写了该方法，返回true。
d.setState(getDrawableState());
}
}
3.每个View通过onCreateDrawableState(0)来返回他目前的状态给Drawable对象
[java] view plaincopyprint?
public final int[] getDrawableState() {
if ((mDrawableState != null) && ((mPrivateFlags & DRAWABLE_STATE_DIRTY) == 0)) {
return mDrawableState;
} else {
mDrawableState = onCreateDrawableState(0);
mPrivateFlags &= ~DRAWABLE_STATE_DIRTY;
return mDrawableState;
}
}
//这是onCreateDrawableState(0)的源码，主要作用是确定windows的状态，view是否能被显示，是不是焦点或按下的状态，然后把所有的状态产生一个int[]值返回，参数extraSpace是表明额外的状态，所以可以自定义状态，例如CompoundButton
protected int[] onCreateDrawableState(int extraSpace) {
if ((mViewFlags & DUPLICATE_PARENT_STATE) == DUPLICATE_PARENT_STATE &&
mParent instanceof View) {
//返回父类的状态
return ((View) mParent).onCreateDrawableState(extraSpace);
}
int[] drawableState;
int privateFlags = mPrivateFlags;
//是否PRESSED状态
int viewStateIndex = (((privateFlags & PRESSED) != 0) ? 1 : 0);
//是否ENABLED状态
viewStateIndex = (viewStateIndex << 1)
+ (((mViewFlags & ENABLED_MASK) == ENABLED) ? 1 : 0);
//是否焦点状态
viewStateIndex = (viewStateIndex << 1) + (isFocused() ? 1 : 0);
//是否选择状态
viewStateIndex = (viewStateIndex << 1)
+ (((privateFlags & SELECTED) != 0) ? 1 : 0);
//view所在window是否获得焦点
final boolean hasWindowFocus = hasWindowFocus();
viewStateIndex = (viewStateIndex << 1) + (hasWindowFocus ? 1 : 0);
//通过以上的判断产生一个索引viewStateIndex,在VIEW_STATE_SETS中返回一个状态数组(所以VIEW_STATE_SETS是int[][]类型)
drawableState = VIEW_STATE_SETS[viewStateIndex];
//noinspection ConstantIfStatement
if (false) {
Log.i("View", "drawableStateIndex=" + viewStateIndex);
Log.i("View", toString()
+ " pressed=" + ((privateFlags & PRESSED) != 0)
+ " en=" + ((mViewFlags & ENABLED_MASK) == ENABLED)
+ " fo=" + hasFocus()
+ " sl=" + ((privateFlags & SELECTED) != 0)
+ " wf=" + hasWindowFocus
+ ": " + Arrays.toString(drawableState));
}
if (extraSpace == 0) {
return drawableState;
}
//如果有额外的自定义状态就把它添加到状态数组中
final int[] fullState;
if (drawableState != null) {
fullState = new int[drawableState.length + extraSpace];
System.arraycopy(drawableState, 0, fullState, 0, drawableState.length);
} else {
fullState = new int[extraSpace];
}
return fullState;
}
//这个是CompoundButton对象重写的onCreateDrawableState方法，把额外的自定义CHECKED_STATE_SET状态添加进去
protected int[] onCreateDrawableState(int extraSpace) {
final int[] drawableState = super.onCreateDrawableState(extraSpace + 1);
if (isChecked()) {
mergeDrawableStates(drawableState, CHECKED_STATE_SET);
}
return drawableState;
}
4.接下来就是Drawable对象需要通过setState接收状态，同时通过重写onStateChange方法来找到正确的图片并绘制
[java] view plaincopyprint?
public boolean setState(final int[] stateSet) {
if (!Arrays.equals(mStateSet, stateSet)) {
mStateSet = stateSet;
return onStateChange(stateSet);//StateListDrawable重写了onStateChange。
}
return false;
}
[java] view plaincopyprint?
@Override
//看StateListDrawable的onStateChange是怎么实现的,选择了一个正确的图片
protected boolean onStateChange(int[] stateSet) {
int idx = mStateListState.indexOfStateSet(stateSet);//根据状态集匹配一个索引值
if (idx < 0) {
idx = mStateListState.indexOfStateSet(StateSet.WILD_CARD);
}
if (selectDrawable(idx)) {//根据索引值能找到相应状态集对应的Drawable对象。
return true;
}
return super.onStateChange(stateSet);
}
[java] view plaincopyprint?
public boolean selectDrawable(int idx)
{
if (idx == mCurIndex) {
return false;
}
if (idx >= 0 && idx < mDrawableContainerState.mNumChildren) {
Drawable d = mDrawableContainerState.mDrawables[idx];
if (mCurrDrawable != null) {
mCurrDrawable.setVisible(false, false);
}
mCurrDrawable = d;//把当前状态集对应的Drawable对象赋值给mCurrDrawable
mCurIndex = idx;
if (d != null) {
d.setVisible(isVisible(), true);
d.setAlpha(mAlpha);
d.setDither(mDrawableContainerState.mDither);
d.setColorFilter(mColorFilter);
d.setState(getState());
d.setLevel(getLevel());
d.setBounds(getBounds());
}
} else {
if (mCurrDrawable != null) {
mCurrDrawable.setVisible(false, false);
}
mCurrDrawable = null;
mCurIndex = -1;
}
invalidateSelf();//最终调用这个方法。下面看看这个方法。
return true;
}
[java] view plaincopyprint?
public void invalidateSelf()
{
if (mCallback != null) {
mCallback.invalidateDrawable(this);
}
}
[java] view plaincopyprint?
public void setBackgroundDrawable(Drawable d) {
boolean requestLayout = false;
mBackgroundResource = 0;
if (mBGDrawable != null) {
mBGDrawable.setCallback(null);
unscheduleDrawable(mBGDrawable);
}
if (d != null) {
Rect padding = sThreadLocal.get();
if (padding == null) {
padding = new Rect();
sThreadLocal.set(padding);
}
if (d.getPadding(padding)) {
setPadding(padding.left, padding.top, padding.right, padding.bottom);
}
// Compare the minimum sizes of the old Drawable and the new. If there isn't an old or
// if it has a different minimum size, we should layout again
if (mBGDrawable == null || mBGDrawable.getMinimumHeight() != d.getMinimumHeight() ||
mBGDrawable.getMinimumWidth() != d.getMinimumWidth()) {
requestLayout = true;
}
d.setCallback(this);//这里设置了CallBack，View实现了Drawable.Callback接口。
if (d.isStateful()) {
d.setState(getDrawableState());
}
d.setVisible(getVisibility() == VISIBLE, false);
mBGDrawable = d;
if ((mPrivateFlags & SKIP_DRAW) != 0) {
mPrivateFlags &= ~SKIP_DRAW;
mPrivateFlags |= ONLY_DRAWS_BACKGROUND;
requestLayout = true;
}
} else {
/* Remove the background */
mBGDrawable = null;
if ((mPrivateFlags & ONLY_DRAWS_BACKGROUND) != 0) {
mPrivateFlags &= ~ONLY_DRAWS_BACKGROUND;
mPrivateFlags |= SKIP_DRAW;
}
requestLayout = true;
}
computeOpaqueFlags();
if (requestLayout) {
requestLayout();
}
mBackgroundSizeChanged = true;
invalidate();
}
[java] view plaincopyprint?
//View中invalidateDrawable的实现
public void invalidateDrawable(Drawable drawable) {
if (verifyDrawable(drawable)) {
final Rect dirty = drawable.getBounds();
final int scrollX = mScrollX;
final int scrollY = mScrollY;
invalidate(dirty.left + scrollX, dirty.top + scrollY,
dirty.right + scrollX, dirty.bottom + scrollY);//最终会申请失效，导致重新绘制。
}
}
[java] view plaincopyprint?
//View调用draw绘制
public void draw(Canvas canvas) {
final int privateFlags = mPrivateFlags;
final boolean dirtyOpaque = (privateFlags & DIRTY_MASK) == DIRTY_OPAQUE &&
(mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
mPrivateFlags = (privateFlags & ~DIRTY_MASK) | DRAWN;
// Step 1, draw the background, if needed
int saveCount;
if (!dirtyOpaque) {
final Drawable background = mBGDrawable;
if (background != null) {
final int scrollX = mScrollX;
final int scrollY = mScrollY;
if (mBackgroundSizeChanged) {
background.setBounds(0, 0, mRight - mLeft, mBottom - mTop);
mBackgroundSizeChanged = false;
}
if ((scrollX | scrollY) == 0) {
background.draw(canvas);//最终会调用background的draw（）
} else {
canvas.translate(scrollX, scrollY);
background.draw(canvas);
canvas.translate(-scrollX, -scrollY);
}
}
}
[java] view plaincopyprint?
//StateListDrawable的父类DrawableContainer的draw（）
@Override
public void draw(Canvas canvas) {
if (mCurrDrawable != null) {
mCurrDrawable.draw(canvas);
}
}
[java] view plaincopyprint?
//mCurrDrawable是一个BitmapDrawable类型的对象。看一下BitmapDrawable的draw()
@Override
public void draw(Canvas canvas) {
Bitmap bitmap = mBitmap;
if (bitmap != null) {
final BitmapState state = mBitmapState;
if (mRebuildShader) {
Shader.TileMode tmx = state.mTileModeX;
Shader.TileMode tmy = state.mTileModeY;
if (tmx == null && tmy == null) {
state.mPaint.setShader(null);
} else {
Shader s = new BitmapShader(bitmap,
tmx == null ? Shader.TileMode.CLAMP : tmx,
tmy == null ? Shader.TileMode.CLAMP : tmy);
state.mPaint.setShader(s);
}
mRebuildShader = false;
copyBounds(mDstRect);
}
Shader shader = state.mPaint.getShader();
if (shader == null) {
if (mApplyGravity) {
Gravity.apply(state.mGravity, mBitmapWidth, mBitmapHeight,
getBounds(), mDstRect);
mApplyGravity = false;
}
canvas.drawBitmap(bitmap, null, mDstRect, state.mPaint);//最终通过canvas绘制这张图片。
} else {
if (mApplyGravity) {
mDstRect.set(getBounds());
mApplyGravity = false;
}
canvas.drawRect(mDstRect, state.mPaint);
}
}
}
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