Android属性动画源码浅析

前言

日常开发离不开动画，属性动画更为强大，我们不仅要知道如何使用，更要知道他的原理。这样，才能得心应手。那么，今天，就从最简单的来说，了解下属性动画的原理。

ObjectAnimator
        .ofInt(mView,"width",100,500)
        .setDuration(1000)
        .start();

ObjectAnimator#ofInt

以这个为例，代码如下。

public static ObjectAnimator ofInt(Object target, String propertyName, int... values) {
    ObjectAnimator anim = new ObjectAnimator(target, propertyName);
    anim.setIntValues(values);
    return anim;
}

在这个方法中，首先会new一个ObjectAnimator对象，然后通过setIntValues方法将值设置进去，然后返回。在ObjectAnimator的构造方法中，会通过setTarget方法设置当前动画的对象，通过setPropertyName设置当前的属性名。我们重点说下setIntValues方法。

public void setIntValues(int... values) {
    if (mValues == null || mValues.length == 0) {
        // No values yet - this animator is being constructed piecemeal. Init the values with
        // whatever the current propertyName is
        if (mProperty != null) {
            setValues(PropertyValuesHolder.ofInt(mProperty, values));
        } else {
            setValues(PropertyValuesHolder.ofInt(mPropertyName, values));
        }
    } else {
        super.setIntValues(values);
    }
}

首先会判断，mValues是不是null，我们这里是null，并且mProperty也是null，所以会调用
setValues(PropertyValuesHolder.ofInt(mPropertyName, values));方法。先看PropertyValuesHolder.ofInt方法，PropertyValuesHolder这个类是holds属性和值的，在这个方法会构造一个IntPropertyValuesHolder对象并返回。

public static PropertyValuesHolder ofInt(String propertyName, int... values) {
    return new IntPropertyValuesHolder(propertyName, values);
}

IntPropertyValuesHolder的构造方法如下：

public IntPropertyValuesHolder(String propertyName, int... values) {
    super(propertyName);
    setIntValues(values);
}

在这里，首先会调用他的分类的构造方法，然后调用setIntValues方法，在他父类的构造方法中，只是设置了下propertyName。setIntValues内容如下：

public void setIntValues(int... values) {
    super.setIntValues(values);
    mIntKeyframes = (Keyframes.IntKeyframes) mKeyframes;
}

在父类的setIntValues方法中，初始化了mValueType为int.class，mKeyframes为KeyframeSet.ofInt(values)。其中KeyframeSet为关键帧集合。然后将mKeyframes赋值给mIntKeyframes。

KeyframeSet

这个类是记录关键帧的。我们看下他的ofInt方法。

public static KeyframeSet ofInt(int... values) {
    int numKeyframes = values.length;
    IntKeyframe keyframes[] = new IntKeyframe[Math.max(numKeyframes,2)];
    if (numKeyframes == 1) {
        keyframes[0] = (IntKeyframe) Keyframe.ofInt(0f);
        keyframes[1] = (IntKeyframe) Keyframe.ofInt(1f, values[0]);
    } else {
        keyframes[0] = (IntKeyframe) Keyframe.ofInt(0f, values[0]);
        for (int i = 1; i < numKeyframes; ++i) {
            keyframes[i] =
                    (IntKeyframe) Keyframe.ofInt((float) i / (numKeyframes - 1), values[i]);
        }
    }
    return new IntKeyframeSet(keyframes);
}

在这里呢？根据传入的values来计算关键帧，最后返回IntKeyframeSet。

回到ObjectAnimator里面，这里的setValues用的是父类ValueAnimator的

ValueAnimator#setValues

public void setValues(PropertyValuesHolder... values) {
    int numValues = values.length;
    mValues = values;
    mValuesMap = new HashMap<String, PropertyValuesHolder>(numValues);
    for (int i = 0; i < numValues; ++i) {
        PropertyValuesHolder valuesHolder = values[i];
        mValuesMap.put(valuesHolder.getPropertyName(), valuesHolder);
    }
    // New property/values/target should cause re-initialization prior to starting
    mInitialized = false;
}

这里的操作就简单了，就是把PropertyValuesHolder放入到mValuesMap中。

ObjectAnimator#start

这个方法就是动画开始的地方。

public void start() {
    // See if any of the current active/pending animators need to be canceled
    AnimationHandler handler = sAnimationHandler.get();
    if (handler != null) {
        int numAnims = handler.mAnimations.size();
        for (int i = numAnims - 1; i >= 0; i--) {
            if (handler.mAnimations.get(i) instanceof ObjectAnimator) {
                ObjectAnimator anim = (ObjectAnimator) handler.mAnimations.get(i);
                if (anim.mAutoCancel && hasSameTargetAndProperties(anim)) {
                    anim.cancel();
                }
            }
        }
        numAnims = handler.mPendingAnimations.size();
        for (int i = numAnims - 1; i >= 0; i--) {
            if (handler.mPendingAnimations.get(i) instanceof ObjectAnimator) {
                ObjectAnimator anim = (ObjectAnimator) handler.mPendingAnimations.get(i);
                if (anim.mAutoCancel && hasSameTargetAndProperties(anim)) {
                    anim.cancel();
                }
            }
        }
        numAnims = handler.mDelayedAnims.size();
        for (int i = numAnims - 1; i >= 0; i--) {
            if (handler.mDelayedAnims.get(i) instanceof ObjectAnimator) {
                ObjectAnimator anim = (ObjectAnimator) handler.mDelayedAnims.get(i);
                if (anim.mAutoCancel && hasSameTargetAndProperties(anim)) {
                    anim.cancel();
                }
            }
        }
    }
    if (DBG) {
        Log.d(LOG_TAG, "Anim target, duration: " + getTarget() + ", " + getDuration());
        for (int i = 0; i < mValues.length; ++i) {
            PropertyValuesHolder pvh = mValues[i];
            Log.d(LOG_TAG, "   Values[" + i + "]: " +
                pvh.getPropertyName() + ", " + pvh.mKeyframes.getValue(0) + ", " +
                pvh.mKeyframes.getValue(1));
        }
    }
    super.start();
}

首先呢，会获取AnimationHandler对象，如果不为空的话，就会判断是mAnimations、mPendingAnimations、mDelayedAnims中的动画，并且取消。最后调用父类的start方法。

ValueAnimator#start

private void start(boolean playBackwards) {
    if (Looper.myLooper() == null) {
        throw new AndroidRuntimeException("Animators may only be run on Looper threads");
    }
    mReversing = playBackwards;
    mPlayingBackwards = playBackwards;
    if (playBackwards && mSeekFraction != -1) {
        if (mSeekFraction == 0 && mCurrentIteration == 0) {
            // special case: reversing from seek-to-0 should act as if not seeked at all
            mSeekFraction = 0;
        } else if (mRepeatCount == INFINITE) {
            mSeekFraction = 1 - (mSeekFraction % 1);
        } else {
            mSeekFraction = 1 + mRepeatCount - (mCurrentIteration + mSeekFraction);
        }
        mCurrentIteration = (int) mSeekFraction;
        mSeekFraction = mSeekFraction % 1;
    }
    if (mCurrentIteration > 0 && mRepeatMode == REVERSE &&
            (mCurrentIteration < (mRepeatCount + 1) || mRepeatCount == INFINITE)) {
        // if we were seeked to some other iteration in a reversing animator,
        // figure out the correct direction to start playing based on the iteration
        if (playBackwards) {
            mPlayingBackwards = (mCurrentIteration % 2) == 0;
        } else {
            mPlayingBackwards = (mCurrentIteration % 2) != 0;
        }
    }
    int prevPlayingState = mPlayingState;
    mPlayingState = STOPPED;
    mStarted = true;
    mStartedDelay = false;
    mPaused = false;
    updateScaledDuration(); // in case the scale factor has changed since creation time
    AnimationHandler animationHandler = getOrCreateAnimationHandler();
    animationHandler.mPendingAnimations.add(this);
    if (mStartDelay == 0) {
        // This sets the initial value of the animation, prior to actually starting it running
        if (prevPlayingState != SEEKED) {
            setCurrentPlayTime(0);
        }
        mPlayingState = STOPPED;
        mRunning = true;
        notifyStartListeners();
    }
    animationHandler.start();
}

• 先初始化一些值
• updateScaledDuration 缩放时间，默认为1.0f
• 获取或者创建AnimationHandler，将动画加入到mPendingAnimations列表中，
• 如果没延迟，通知监听器
• animationHandler.start

在animationHandler.start中，会调用scheduleAnimation方法，在这个种，会用mChoreographerpost一个callback，最终会执行mAnimate的run方法。mChoreographerpost涉及到VSYNC，这里不多介绍。

mAnimate#run

doAnimationFrame(mChoreographer.getFrameTime());

在这里会用过doAnimationFrame设置动画帧，我们看下这个方法的代码。

void doAnimationFrame(long frameTime) {
    mLastFrameTime = frameTime;

    // mPendingAnimations holds any animations that have requested to be started
    // We're going to clear mPendingAnimations, but starting animation may
    // cause more to be added to the pending list (for example, if one animation
    // starting triggers another starting). So we loop until mPendingAnimations
    // is empty.
    while (mPendingAnimations.size() > 0) {
        ArrayList<ValueAnimator> pendingCopy =
                (ArrayList<ValueAnimator>) mPendingAnimations.clone();
        mPendingAnimations.clear();
        int count = pendingCopy.size();
        for (int i = 0; i < count; ++i) {
            ValueAnimator anim = pendingCopy.get(i);
            // If the animation has a startDelay, place it on the delayed list
            if (anim.mStartDelay == 0) {
                anim.startAnimation(this);
            } else {
                mDelayedAnims.add(anim);
            }
        }
    }

    // Next, process animations currently sitting on the delayed queue, adding
    // them to the active animations if they are ready
    int numDelayedAnims = mDelayedAnims.size();
    for (int i = 0; i < numDelayedAnims; ++i) {
        ValueAnimator anim = mDelayedAnims.get(i);
        if (anim.delayedAnimationFrame(frameTime)) {
            mReadyAnims.add(anim);
        }
    }
    int numReadyAnims = mReadyAnims.size();
    if (numReadyAnims > 0) {
        for (int i = 0; i < numReadyAnims; ++i) {
            ValueAnimator anim = mReadyAnims.get(i);
            anim.startAnimation(this);
            anim.mRunning = true;
            mDelayedAnims.remove(anim);
        }
        mReadyAnims.clear();
    }

    // Now process all active animations. The return value from animationFrame()
    // tells the handler whether it should now be ended
    int numAnims = mAnimations.size();
    for (int i = 0; i < numAnims; ++i) {
        mTmpAnimations.add(mAnimations.get(i));
    }
    for (int i = 0; i < numAnims; ++i) {
        ValueAnimator anim = mTmpAnimations.get(i);
        if (mAnimations.contains(anim) && anim.doAnimationFrame(frameTime)) {
            mEndingAnims.add(anim);
        }
    }
    mTmpAnimations.clear();
    if (mEndingAnims.size() > 0) {
        for (int i = 0; i < mEndingAnims.size(); ++i) {
            mEndingAnims.get(i).endAnimation(this);
        }
        mEndingAnims.clear();
    }

    // Schedule final commit for the frame.
    mChoreographer.postCallback(Choreographer.CALLBACK_COMMIT, mCommit, null);

    // If there are still active or delayed animations, schedule a future call to
    // onAnimate to process the next frame of the animations.
    if (!mAnimations.isEmpty() || !mDelayedAnims.isEmpty()) {
        scheduleAnimation();
    }
}

方法较长，逻辑如下：

• 从mPendingAnimations中取出动画，根据事先选择startAnimation还是加入到mDelayedAnims列表。
• 如果mDelayedAnims列表中的动画准备好了，就加入到mReadyAnims列表中
• 从mAnimations列表中取出要执行的动画，加入到mTmpAnimations列表
• 通过doAnimationFrame方法执行动画帧
• 继续执行scheduleAnimation

从上面我们能看出，执行动画的关键是doAnimationFrame方法。在这个方法中，会调用animationFrame方法。

ValueAniator#animationFrame

boolean animationFrame(long currentTime) {
     boolean done = false;
     switch (mPlayingState) {
     case RUNNING:
     case SEEKED:
         float fraction = mDuration > 0 ? (float)(currentTime - mStartTime) / mDuration : 1f;
         if (mDuration == 0 && mRepeatCount != INFINITE) {
             // Skip to the end
             mCurrentIteration = mRepeatCount;
             if (!mReversing) {
                 mPlayingBackwards = false;
             }
         }
         if (fraction >= 1f) {
             if (mCurrentIteration < mRepeatCount || mRepeatCount == INFINITE) {
                 // Time to repeat
                 if (mListeners != null) {
                     int numListeners = mListeners.size();
                     for (int i = 0; i < numListeners; ++i) {
                         mListeners.get(i).onAnimationRepeat(this);
                     }
                 }
                 if (mRepeatMode == REVERSE) {
                     mPlayingBackwards = !mPlayingBackwards;
                 }
                 mCurrentIteration += (int) fraction;
                 fraction = fraction % 1f;
                 mStartTime += mDuration;
                 // Note: We do not need to update the value of mStartTimeCommitted here
                 // since we just added a duration offset.
             } else {
                 done = true;
                 fraction = Math.min(fraction, 1.0f);
             }
         }
         if (mPlayingBackwards) {
             fraction = 1f - fraction;
         }
         animateValue(fraction);
         break;
     }

     return done;
 }

• 计算fraction
• 调用animateValue方法

根据虚拟机执行引擎动态分派原则，这里会调用ObjectAnimator的animateValue方法。

ObjectAnimator#animateValue

void animateValue(float fraction) {
    final Object target = getTarget();
    if (mTarget != null && target == null) {
        // We lost the target reference, cancel and clean up.
        cancel();
        return;
    }

    super.animateValue(fraction);
    int numValues = mValues.length;
    for (int i = 0; i < numValues; ++i) {
        mValues[i].setAnimatedValue(target);
    }
}

这里主要干了两件事，

• 调用父类的animateValue方法
• 通过setAnimatedValue设置属性

其父类的方法如下：

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);
        }
    }
}

在这个方法中，会通过Interpolator得到出当前的fraction，并通过calculateValue来计算当前应该的值，这里会调用IntPropertyValuesHolder的calculateValue

void calculateValue(float fraction) {
    mIntAnimatedValue = mIntKeyframes.getIntValue(fraction);
}

我们知道，mIntKeyframes对应的是IntKeyframeSet。在这个类的getIntValue中，会通过TypeEvaluator来计算当前对应的值。不多说了。

最后，回到animateValue。计算了值之后，会调用setAnimatedValue来设置值。我们看看他的实现。

IntPropertyValuesHolder#setAnimatedValue

void setAnimatedValue(Object target) {
    if (mIntProperty != null) {
        mIntProperty.setValue(target, mIntAnimatedValue);
        return;
    }
    if (mProperty != null) {
        mProperty.set(target, mIntAnimatedValue);
        return;
    }
    if (mJniSetter != 0) {
        nCallIntMethod(target, mJniSetter, mIntAnimatedValue);
        return;
    }
    if (mSetter != null) {
        try {
            mTmpValueArray[0] = mIntAnimatedValue;
            mSetter.invoke(target, mTmpValueArray);
        } catch (InvocationTargetException e) {
            Log.e("PropertyValuesHolder", e.toString());
        } catch (IllegalAccessException e) {
            Log.e("PropertyValuesHolder", e.toString());
        }
    }
}

恩，到这里就能看到修改属性值得痕迹了，有以下四种情况

• mIntProperty不为null
• mProperty不为null
• mJniSetter不为null
• mSetter不为null

首先，我们通过String propertyName, int… values参数构造的对象，mIntProperty为null，并且mProperty也为null。那其他两个是怎么来的呢？似乎漏了什么？

还节的，在doAnimationFrame中，直接调用startAnimation么?没错，就是这里。

startAnimation

在这个方法中调用了initAnimation方法。还是根据动态分派规则，这里调用ObjectAnimator的initAnimation方法。在这里调用PropertyValuesHolder的setupSetterAndGetter方法，在这里对mSetter等进行了初始化，这里就不多说了，大家自己看代码吧。

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