動(dòng)畫分類

補(bǔ)間動(dòng)畫

旋轉(zhuǎn)咽瓷、位移崔挖、透明度贸街、縮放

屬性動(dòng)畫

同樣的屬性動(dòng)畫也可以做到對(duì)View進(jìn)行縮放、移動(dòng)狸相、旋轉(zhuǎn)以及改變透明度;除此以外,它還能改變對(duì)象的某個(gè)屬性薛匪。
ObjectAnimator.ofFloat()
ValueAnimator.start()

ObjectAnimator繼承于ValueAnimator

 public void start() {
    start(false);
}

private void start(boolean playBackwards) {
    if (Looper.myLooper() == null) {
        throw new AndroidRuntimeException("Animators may only be run on Looper threads");
    }
    //省略部分代碼...
    //為動(dòng)畫添加回調(diào)
    addAnimationCallback(0);
    //判斷動(dòng)畫開始時(shí)間是否需要延遲
    if (mStartDelay == 0 || mSeekFraction >= 0 || mReversing) {
        startAnimation();
        if (mSeekFraction == -1) {
            setCurrentPlayTime(0);
        } else {
            setCurrentFraction(mSeekFraction);
        }
    }
}

addAnimationCallback(0);

這里調(diào)用了AnimationHandler類的addAnimationFrameCallback(),最終會(huì)調(diào)用doAnimationFrame方法

 private void doAnimationFrame(long frameTime) {
        long currentTime = SystemClock.uptimeMillis();
        final int size = mAnimationCallbacks.size();
        for (int i = 0; i < size; i++) {
            final AnimationFrameCallback callback = mAnimationCallbacks.get(i);
            if (callback == null) {
                continue;
            }
            if (isCallbackDue(callback, currentTime)) {
                callback.doAnimationFrame(frameTime);
                if (mCommitCallbacks.contains(callback)) {
                    getProvider().postCommitCallback(new Runnable() {
                        @Override
                        public void run() {
                            commitAnimationFrame(callback, getProvider().getFrameTime());
                        }
                    });
                }
            }
        }
        cleanUpList();
    }

將一個(gè)callback丟到Choreographer中，當(dāng)下一個(gè)Vsync信號(hào)來時(shí)執(zhí)行doAnimationFrame()

/**
     * Callbacks that receives notifications for animation timing and frame commit timing.
     */
    interface AnimationFrameCallback {

        boolean doAnimationFrame(long frameTime);
        void commitAnimationFrame(long frameTime);
    }

doAnimationFrame()中會(huì)執(zhí)行到animateValue()

void animateValue(float fraction) {
        fraction = mInterpolator.getInterpolation(fraction);
        mCurrentFraction = fraction;
        int numValues = mValues.length;
        for (int i = 0; i < numValues; ++i) {
            mValues[i].calculateValue(fraction);
        }
        if (mUpdateListeners != null) {
            int numListeners = mUpdateListeners.size();
            for (int i = 0; i < numListeners; ++i) {
                mUpdateListeners.get(i).onAnimationUpdate(this);
            }
        }
    }

mInterpolator就是插值器TimeInterpolator脓鹃、TimeInterpolator接口提供了一個(gè)方法getInterpolation()獲取當(dāng)前屬性隨時(shí)間變化的百分比

public interface TimeInterpolator {
    float getInterpolation(float input);
}

mValues[i].calculateValue(fraction);會(huì)調(diào)用估值器逸尖，將插值器計(jì)算出的值作為輸入，計(jì)算出要變化的值設(shè)置給屬性

public interface TypeEvaluator<T> {
    public T evaluate(float fraction, T startValue, T endValue);

}

startAnimation()

startAnimation()會(huì)調(diào)用ininAnimation()方法而ObjectAnimation重寫了該方法

void initAnimation() {
    if (!mInitialized) {
        final Object target = getTarget();
        if (target != null) {
            final int numValues = mValues.length;
            for (int i = 0; i < numValues; ++i) {
                mValues[i].setupSetterAndGetter(target);
            }
        }
        super.initAnimation();
    }
}

mValues為PropertyValuesHolder類型，從這可以看出調(diào)用了PropertyValuesHolder的setupSetterAndGetter方法,這個(gè)方法中查找了相對(duì)應(yīng)屬性的set/get方法冷溶。最終會(huì)調(diào)用 getPropertyFunction()方來獲取相應(yīng)的方法,若通過反射沒有獲取到相應(yīng)的set/get方法時(shí)就會(huì)拋出 NoSuchMethodException異常

private Method getPropertyFunction(Class targetClass, String prefix, Class valueType) {
    // TODO: faster implementation...
    Method returnVal = null;
    //prefix表示set或get,然后在通過getMethodName()方法拼接出相應(yīng)的方法名
    String methodName = getMethodName(prefix, mPropertyName);
    Class args[] = null;
    //省略部分代碼
     returnVal = targetClass.getMethod(methodName, args);
            return returnVal;
     //省略部分代碼       
}

因?yàn)槲覀冃枰獙?duì)View的translationX屬性進(jìn)行設(shè)置,所以通過這里可以看出我們?cè)O(shè)置屬性時(shí)是通過View.setTranslationX()方法來設(shè)置的渐白。

Choreographer

Choreographer是動(dòng)畫原理的一個(gè)核心，搞明白動(dòng)畫工作原理之前我們需要先了解Choreographer工作過程逞频；
Choreographer工作過程簡單描述如下：在Choreographer對(duì)象中有四條鏈表宗收，分別保存著待處理的輸入事件升敲，待處理的動(dòng)畫事件，待處理的View繪制事件、待處理的post到Choreographer中事件煞躬，Android系統(tǒng)每隔16.7ms發(fā)出VSYNC信號(hào)线婚，Choreographer.FrameDisplayEventReceiver收到信號(hào)后調(diào)用onVsync方法洋措，最終會(huì)調(diào)用Choreographer.doFrame()方法剑按，在doFrame方法中處理輸入事件、動(dòng)畫事件澜驮、View繪制陷揪、post到Choreographer中的FrameCallback；我們可以使用Choreographer#postFrameCallback設(shè)置callback與Choreographer交互杂穷，設(shè)置的FrameCallCack（doFrame方法）會(huì)在下一個(gè)frame被渲染時(shí)觸發(fā)（即下一個(gè)VSYNC到來時(shí)執(zhí)行）悍缠；Choreographer更多內(nèi)容請(qǐng)參考Android Choreographer 源碼分析
Android動(dòng)畫原理簡單描述：將View的一次大的屬性變化拆分為多次小的屬性變化，在每次VSYNC信號(hào)到來時(shí)耐量，根據(jù)當(dāng)前時(shí)間和插值器來計(jì)算當(dāng)前View屬性的值飞蚓，然后給View設(shè)置該屬性值，直到動(dòng)畫執(zhí)行完畢廊蜒。其中Choreographer將動(dòng)畫拆分成一次次小的屬性變化趴拧，Choreographer是動(dòng)畫的指揮者。理想情況下山叮，屬性刷新次數(shù)（動(dòng)畫拆分為多次小的屬性變化） = 動(dòng)畫執(zhí)行時(shí)間/16.7ms 著榴。

參考
Android動(dòng)畫原理探究