1.Drawable简介
Android系统中将可绘制对象被抽象为Drawable,不同的绘制资源对应着不同的Drawable类型。Android FrameWork提供了常用的Drawable,Android控件的绘制资源基本都是通过Drawable形式实现的。一般情况下,开发者是不会直接接触Drawable的具体实现的,Drawable资源一般都放在res/drawable目录下,用户通过图片,xml格式的Drawable资源来使用。
Android内置的比较常用的Drawable类型包括:ColorDrawable
、GradientDrawable
、ShapeDrawable
、BitmapDrawable
、 NinePatchDrawable
、InsetDrawable
、ClipDrawable
、ScaleDrawable
、RotateDrawable
、AnimationDrawable
、LayerDrawable
、LevelListDrawable
、StateListDrawable
、TransitionDrawable
。
2.Drawable 特性
Drawable 比 View 更轻量级,它只是 View 绘制中过程被调用的一个东西(背景,前景)。
自定义 View 的时候,如果只要改变背景的话,最好不要去重写 draw 函数,Drawable 这个类已经被抽象出来负责背景的绘制了。
Drawable 有许许多多的子类,但最复杂的还是 Drawable 本身,作为父类的它要考虑到方方面面。
1).setBounds
最基本的形状是什么,是矩形。
无论 Drawable 最终呈现在画布上是什么形状的,它总是被限定在一个矩形当中。
void setBackgroundBounds() {
if (mBackgroundSizeChanged && mBackground != null) {
mBackground.setBounds(0, 0, mRight - mLeft, mBottom - mTop);
mBackgroundSizeChanged = false;
rebuildOutline();
}
}
在绘制背景时,会将 View 的坐标转换为 bounds,所以 Drawable 都会被拉伸至 View 的大小。
canvas.drawRect(getBounds(), mPaint);
2).setCallBack
public static interface Callback {
public void invalidateDrawable(Drawable who);
public void scheduleDrawable(Drawable who, Runnable what, long when);
public void unscheduleDrawable(Drawable who, Runnable what);
}
View 在设置背景时,drawable 会把 callback 指向 View。
background.setCallback(this);
Drawable 在需要重绘时,会调用:
public void invalidateSelf() {
final Callback callback = getCallback();
if (callback != null) {
callback.invalidateDrawable(this);
}
}
@Override
public void invalidateDrawable(@NonNull Drawable drawable) {
if (verifyDrawable(drawable)) {
final Rect dirty = drawable.getDirtyBounds();
final int scrollX = mScrollX;
final int scrollY = mScrollY;
invalidate(dirty.left + scrollX, dirty.top + scrollY,
dirty.right + scrollX, dirty.bottom + scrollY);
rebuildOutline();
}
}
这一步说明了,Drawable 是 View 附属,它不会仅仅重画自己。
scheduleSelf 是定时重绘自己的意思,就是动画的意思。
unscheduleSelf 则是取消上一步动画的意思,很多时候我们需要让动画戛然而止。
3).setState
View 有很多 State,比较常见的是 EditText 的 foucs,以及 Button 的 press。
Drawable 是否根据 View 的状态改变而做出相应的变化,就被称为 isStateful。
触发过程如下:
view.refreshDrawableState -> view.drawableStateChanged -> bg.setState -> bg.onStateChanged
3.Drawable的定义形式
一般情况下,除了图片资源是直接放在res/drawable下(android studio中图片放在res/minmap下),其他的Drawable都是以xml格式实现的,开发者通过在xml中使用shape,selector,level-list等标签来实现对应的Drawable,从而实现相应的可绘制资源的定义,最终view通过绘制这些Drawable来实现我们想要的显示效果。
Drawable中xml标签与Drawable对象的对应关系如以下表格所示:
xml标签 | Drawable对象 |
---|---|
<selector> | StateListDrawable |
<level-list> | LevelListDrawable |
<layer-list> | LayerDrawable |
<transition> | TransitionDrawable |
<color> | ColorDrawable |
<shape> | GradientDrawable |
<scale> | ScaleDrawable |
<clip> | ClipDrawable |
<rotate> | RotateDrawable |
<anination-list> | AnimationDrawable |
<inset> | InsetDrawable |
<bitmap> | BitmapDrawable |
<nine-patch> | NinePatchDrawable |
4.从源码探索Drawable的加载过程
下面我们通过Framework的源代码来探索一下Drawable的加载过程,一般情况下,我们通过getResources().getDrawable(int id);的方式去加载。顺着这条线往下看,getDrawable->loadDrawable->loadDrawableForCookie->Drawable.createFromXml->createFromXmlInner,来看androidgraphicsdrawableDrawable.java的createFromXmlInner函数的实现代码。
public static Drawable createFromXmlInner(Resources r, XmlPullParser parser, AttributeSet attrs,
Theme theme) throws XmlPullParserException, IOException {
final Drawable drawable;
final String name = parser.getName();
if (name.equals("selector")) {
drawable = new StateListDrawable();
} else if (name.equals("animated-selector")) {
drawable = new AnimatedStateListDrawable();
} 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("ripple")) {
drawable = new RippleDrawable();
} else if (name.equals("color")) {
drawable = new ColorDrawable();
} else if (name.equals("shape")) {
drawable = new GradientDrawable();
} else if (name.equals("vector")) {
drawable = new VectorDrawable();
} else if (name.equals("animated-vector")) {
drawable = new AnimatedVectorDrawable();
} 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")) {
//noinspection deprecation
drawable = new BitmapDrawable(r);
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, theme);
return drawable;
}
上面的对应关系就从这里来的,现在是不是一下子明朗了呢。在函数结尾,Drawable通过条用inflate函数解析AttributeSet中的属性信息,并设置Drawable相应的属性。看一下BitmapDrawable吧,
public void inflate(Resources r, XmlPullParser parser, AttributeSet attrs, Theme theme)
throws XmlPullParserException, IOException {
super.inflate(r, parser, attrs, theme);
final TypedArray a = obtainAttributes(r, theme, attrs, R.styleable.BitmapDrawable);
updateStateFromTypedArray(a);
verifyState(a);
a.recycle();
}
继续看updateStateFromTypedArray
private void updateStateFromTypedArray(TypedArray a) throws XmlPullParserException {
final Resources r = a.getResources();
final BitmapState state = mBitmapState;
// Account for any configuration changes.
state.mChangingConfigurations |= a.getChangingConfigurations();
// Extract the theme attributes, if any.
state.mThemeAttrs = a.extractThemeAttrs();
final int srcResId = a.getResourceId(R.styleable.BitmapDrawable_src, 0);
if (srcResId != 0) {
final Bitmap bitmap = BitmapFactory.decodeResource(r, srcResId);
if (bitmap == null) {
throw new XmlPullParserException(a.getPositionDescription() +
": <bitmap> requires a valid src attribute");
}
state.mBitmap = bitmap;
}
state.mTargetDensity = r.getDisplayMetrics().densityDpi;
final boolean defMipMap = state.mBitmap != null ? state.mBitmap.hasMipMap() : false;
setMipMap(a.getBoolean(R.styleable.BitmapDrawable_mipMap, defMipMap));
state.mAutoMirrored = a.getBoolean(
R.styleable.BitmapDrawable_autoMirrored, state.mAutoMirrored);
state.mBaseAlpha = a.getFloat(R.styleable.BitmapDrawable_alpha, state.mBaseAlpha);
final int tintMode = a.getInt(R.styleable.BitmapDrawable_tintMode, -1);
if (tintMode != -1) {
state.mTintMode = Drawable.parseTintMode(tintMode, Mode.SRC_IN);
}
final ColorStateList tint = a.getColorStateList(R.styleable.BitmapDrawable_tint);
if (tint != null) {
state.mTint = tint;
}
final Paint paint = mBitmapState.mPaint;
paint.setAntiAlias(a.getBoolean(
R.styleable.BitmapDrawable_antialias, paint.isAntiAlias()));
paint.setFilterBitmap(a.getBoolean(
R.styleable.BitmapDrawable_filter, paint.isFilterBitmap()));
paint.setDither(a.getBoolean(R.styleable.BitmapDrawable_dither, paint.isDither()));
setGravity(a.getInt(R.styleable.BitmapDrawable_gravity, state.mGravity));
final int tileMode = a.getInt(R.styleable.BitmapDrawable_tileMode, TILE_MODE_UNDEFINED);
if (tileMode != TILE_MODE_UNDEFINED) {
final Shader.TileMode mode = parseTileMode(tileMode);
setTileModeXY(mode, mode);
}
final int tileModeX = a.getInt(R.styleable.BitmapDrawable_tileModeX, TILE_MODE_UNDEFINED);
if (tileModeX != TILE_MODE_UNDEFINED) {
setTileModeX(parseTileMode(tileModeX));
}
final int tileModeY = a.getInt(R.styleable.BitmapDrawable_tileModeY, TILE_MODE_UNDEFINED);
if (tileModeY != TILE_MODE_UNDEFINED) {
setTileModeY(parseTileMode(tileModeY));
}
// Update local properties.
initializeWithState(state, r);
}
updateStateFromTypedArray函数中,BitmapDrawable获取到src属性后,通过BitmapFactory加载图像文件,并读取用户配置的属性,设置BitmapDrawable的其他属性。其他的Drawable的实现方式类似。