Android源码分析—属性动画的工作原理

最新推荐文章于 2025-06-19 09:10:08 发布

转载最新推荐文章于 2025-06-19 09:10:08 发布 · 272 阅读

本文深入分析Android属性动画的工作原理，从源码层面解析属性动画如何利用get和set方法更新动画对象属性值，帮助开发者更好地掌握属性动画的应用。

转载请注明出处： http://blog.youkuaiyun.com/singwhatiwanna/article/details/17853275

前言

本文为Android动画系列的最后一篇文章，通过对源码的分析，能够让大家更深刻地理解属性动画的工作原理，这有助于我们更好地使用属性动画。但是，由于动画的底层实现已经深入到jni层，并且涉及到显示子系统，因此，深入地分析动画的底层实现不仅比较困难而且意义不大，因此，本文的分析到jni层为止。

Android动画系列：

android动画简介

Android动画进阶—使用开源动画库nineoldandroids

Android属性动画深入分析：让你成为动画牛人

Android源码分析—属性动画的工作原理

属性动画的原理

属性动画要求动画作用的对象提供该属性的set方法，属性动画根据你传递的该熟悉的初始值和最终值，以动画的效果多次去调用set方法，每次传递给set方法的值都不一样，确切来说是随着时间的推移，所传递的值越来越接近最终值。如果动画的时候没有传递初始值，那么还要提供get方法，因为系统要去拿属性的初始值。对于属性动画来说，其动画过程中所做的就是这么多，下面看源码分析。

源码分析

首先我们要找一个入口，就从ObjectAnimator.ofInt(mButton, “width”, 500).setDuration(5000).start()开始吧，其他动画都是类似的。

看ObjectAnimator的start方法

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@Override
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(”ObjectAnimator”, “Anim target, duration: ” + mTarget + “, ” + getDuration());
for (int i = 0; i < mValues.length; ++i) {
PropertyValuesHolder pvh = mValues[i];
ArrayList<Keyframe> keyframes = pvh.mKeyframeSet.mKeyframes;
Log.d(”ObjectAnimator”, “ Values[“ + i + “]: ” +
pvh.getPropertyName() + ”, ” + keyframes.get(0).getValue() + “, ” +
keyframes.get(pvh.mKeyframeSet.mNumKeyframes - 1).getValue());
}
}
super.start();
}

    @Override
    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("ObjectAnimator", "Anim target, duration: " + mTarget + ", " + getDuration());
            for (int i = 0; i < mValues.length; ++i) {
                PropertyValuesHolder pvh = mValues[i];
                ArrayList<Keyframe> keyframes = pvh.mKeyframeSet.mKeyframes;
                Log.d("ObjectAnimator", "   Values[" + i + "]: " +
                    pvh.getPropertyName() + ", " + keyframes.get(0).getValue() + ", " +
                    keyframes.get(pvh.mKeyframeSet.mNumKeyframes - 1).getValue());
            }
        }
        super.start();
    }

说明：上面的代码别看那么长，其实做的事情很简单，首先会判断一下，如果当前动画、等待的动画（Pending）和延迟的动画（Delay）中有和当前动画相同的动画，那么就把相同的动画给取消掉，接下来那一段是log，再接着就调用了父类的super.start()方法，因为ObjectAnimator继承了ValueAnimator，所以接下来我们看一下ValueAnimator的Start方法

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private void start(boolean playBackwards) {
if (Looper.myLooper() == null) {
throw new AndroidRuntimeException(“Animators may only be run on Looper threads”);
}
mPlayingBackwards = playBackwards;
mCurrentIteration = 0;
mPlayingState = STOPPED;
mStarted = true;
mStartedDelay = false;
mPaused = false;
AnimationHandler animationHandler = getOrCreateAnimationHandler();
animationHandler.mPendingAnimations.add(this);
if (mStartDelay == 0) {
// This sets the initial value of the animation, prior to actually starting it running
setCurrentPlayTime(0);
mPlayingState = STOPPED;
mRunning = true;
notifyStartListeners();
}
animationHandler.start();
}

    private void start(boolean playBackwards) {
        if (Looper.myLooper() == null) {
            throw new AndroidRuntimeException("Animators may only be run on Looper threads");
        }
        mPlayingBackwards = playBackwards;
        mCurrentIteration = 0;
        mPlayingState = STOPPED;
        mStarted = true;
        mStartedDelay = false;
        mPaused = false;
        AnimationHandler animationHandler = getOrCreateAnimationHandler();
        animationHandler.mPendingAnimations.add(this);
        if (mStartDelay == 0) {
            // This sets the initial value of the animation, prior to actually starting it running
            setCurrentPlayTime(0);
            mPlayingState = STOPPED;
            mRunning = true;
            notifyStartListeners();
        }
        animationHandler.start();
    }

说明：上述代码最终会调用AnimationHandler的start方法，这个AnimationHandler并不是Handler，它是个Runnable。看下它的代码，通过代码我们发现，很快就调到了jni层，不过jni层最终还是要调回来的。它的run方法会被调用，这个 Runnable涉及到和底层的交互，我们就忽略这部分，直接看重点：ValueAnimator中的doAnimationFrame方法

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final boolean doAnimationFrame(long frameTime) {
if (mPlayingState == STOPPED) {
mPlayingState = RUNNING;
if (mSeekTime < 0) {
mStartTime = frameTime;
} else {
mStartTime = frameTime - mSeekTime;
// Now that we’re playing, reset the seek time
mSeekTime = -1;
}
}
if (mPaused) {
if (mPauseTime < 0) {
mPauseTime = frameTime;
}
return false;
} else if (mResumed) {
mResumed = false;
if (mPauseTime > 0) {
// Offset by the duration that the animation was paused
mStartTime += (frameTime - mPauseTime);
}
}
// The frame time might be before the start time during the first frame of
// an animation. The “current time” must always be on or after the start
// time to avoid animating frames at negative time intervals. In practice, this
// is very rare and only happens when seeking backwards.
final long currentTime = Math.max(frameTime, mStartTime);
return animationFrame(currentTime);
}

    final boolean doAnimationFrame(long frameTime) {
        if (mPlayingState == STOPPED) {
            mPlayingState = RUNNING;
            if (mSeekTime < 0) {
                mStartTime = frameTime;
            } else {
                mStartTime = frameTime - mSeekTime;
                // Now that we're playing, reset the seek time
                mSeekTime = -1;
            }
        }
        if (mPaused) {
            if (mPauseTime < 0) {
                mPauseTime = frameTime;
            }
            return false;
        } else if (mResumed) {
            mResumed = false;
            if (mPauseTime > 0) {
                // Offset by the duration that the animation was paused
                mStartTime += (frameTime - mPauseTime);
            }
        }
        // The frame time might be before the start time during the first frame of
        // an animation.  The "current time" must always be on or after the start
        // time to avoid animating frames at negative time intervals.  In practice, this
        // is very rare and only happens when seeking backwards.
        final long currentTime = Math.max(frameTime, mStartTime);
        return animationFrame(currentTime);
    }

注意到上述代码末尾调用了 animationFrame方法，而 animationFrame内部调用了 animateValue，下面看animateValue的代码

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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);
}
}
}

    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);
            }
        }
    }

上述代码中的calculateValue方法就是计算每帧动画所对应的属性的值，下面着重看一下到底是在哪里调用属性的get和set方法的，毕竟这个才是我们最关心的。

get方法：在初始化的时候，如果属性的初始值没有提供，则get方法将会被调用。

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private void setupValue(Object target, Keyframe kf) {
if (mProperty != null) {
kf.setValue(mProperty.get(target));
}
try {
if (mGetter == null) {
Class targetClass = target.getClass();
setupGetter(targetClass);
if (mGetter == null) {
// Already logged the error - just return to avoid NPE
return;
}
}
kf.setValue(mGetter.invoke(target));
} catch (InvocationTargetException e) {
Log.e(”PropertyValuesHolder”, e.toString());
} catch (IllegalAccessException e) {
Log.e(”PropertyValuesHolder”, e.toString());
}
}

    private void setupValue(Object target, Keyframe kf) {
        if (mProperty != null) {
            kf.setValue(mProperty.get(target));
        }
        try {
            if (mGetter == null) {
                Class targetClass = target.getClass();
                setupGetter(targetClass);
                if (mGetter == null) {
                    // Already logged the error - just return to avoid NPE
                    return;
                }
            }
            kf.setValue(mGetter.invoke(target));
        } catch (InvocationTargetException e) {
            Log.e("PropertyValuesHolder", e.toString());
        } catch (IllegalAccessException e) {
            Log.e("PropertyValuesHolder", e.toString());
        }
    }

set方法：当动画的下一帧到来的时候，PropertyValuesHolder中的setAnimatedValue方法会将新的属性值设置给对象，调用其set方法

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void setAnimatedValue(Object target) {
if (mProperty != null) {
mProperty.set(target, getAnimatedValue());
}
if (mSetter != null) {
try {
mTmpValueArray[0] = getAnimatedValue();
mSetter.invoke(target, mTmpValueArray);
} catch (InvocationTargetException e) {
Log.e(”PropertyValuesHolder”, e.toString());
} catch (IllegalAccessException e) {
Log.e(”PropertyValuesHolder”, e.toString());
}
}
}

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