问题：
java.lang.IllegalArgumentException: pointerIndex out of range
at android.view.MotionEvent.nativeGetAxisValue (MotionEvent.java)
at android.view.MotionEvent.getX (MotionEvent.java:2139)
at android.support.v4.view.ViewPager.onInterceptTouchEvent (ViewPager.java)
at my.test.app.view.CustomViewPager.onInterceptTouchEvent (CustomViewPager.java)
at android.view.ViewGroup.dispatchTouchEvent (ViewGroup.java:2212)
at android.view.ViewGroup.dispatchTransformedTouchEvent (ViewGroup.java:2669)
at android.view.ViewGroup.dispatchTouchEvent (ViewGroup.java:2358)
at android.view.ViewGroup.dispatchTransformedTouchEvent (ViewGroup.java:2669)
at android.view.ViewGroup.dispatchTouchEvent (ViewGroup.java:2358)
at android.view.ViewGroup.dispatchTransformedTouchEvent (ViewGroup.java:2669)
at android.view.ViewGroup.dispatchTouchEvent (ViewGroup.java:2358)
at android.view.View.dispatchPointerEvent (View.java:10164)
at android.view.ViewRootImpl$ViewPostImeInputStage.processPointerEvent (ViewRootImpl.java:4490)
at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess (ViewRootImpl.java:4358)
at android.view.ViewRootImpl$InputStage.deliver (ViewRootImpl.java:3900)
at android.view.ViewRootImpl$InputStage.onDeliverToNext (ViewRootImpl.java:3953)
at android.view.ViewRootImpl$InputStage.forward (ViewRootImpl.java:3919)
at android.view.ViewRootImpl$AsyncInputStage.forward (ViewRootImpl.java:4046)
at android.view.ViewRootImpl$InputStage.apply (ViewRootImpl.java:3927)
at android.view.ViewRootImpl$AsyncInputStage.apply (ViewRootImpl.java:4103)
at android.view.ViewRootImpl$InputStage.deliver (ViewRootImpl.java:3900)
at android.view.ViewRootImpl$InputStage.onDeliverToNext (ViewRootImpl.java:3953)
at android.view.ViewRootImpl$InputStage.forward (ViewRootImpl.java:3919)
at android.view.ViewRootImpl$InputStage.apply (ViewRootImpl.java:3927)
at android.view.ViewRootImpl$InputStage.deliver (ViewRootImpl.java:3900)
at android.view.ViewRootImpl.deliverInputEvent (ViewRootImpl.java:6266)
at android.view.ViewRootImpl.doProcessInputEvents (ViewRootImpl.java:6240)
at android.view.ViewRootImpl.enqueueInputEvent (ViewRootImpl.java:6201)
at android.view.ViewRootImpl$WindowInputEventReceiver.onInputEvent (ViewRootImpl.java:6369)
at android.view.InputEventReceiver.dispatchInputEvent (InputEventReceiver.java:185)
at android.os.MessageQueue.nativePollOnce (MessageQueue.java)
at android.os.MessageQueue.next (MessageQueue.java:323)
at android.os.Looper.loop (Looper.java:136)
at android.app.ActivityThread.main (ActivityThread.java:6195)
at java.lang.reflect.Method.invoke (Method.java)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run (ZygoteInit.java:874)
at com.android.internal.os.ZygoteInit.main (ZygoteInit.java:764)
原因
在ViewPager上做View滑动是，需要禁掉ViewPager的滑动，这时创建了继承自ViewPager的CustomViewPager来实现该功能，并重写了onInterceptTouchEvent方法，并使用canScroll来决定是否允许ViewPager滑动。代码如下：
@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
if(canScroll){
return super.onInterceptTouchEvent(ev)
}else{
return false;
}
}
这段代码在运行时抛出了上文异常（非必现）。这个异常是怎么产生的呢？根据堆栈信息，我追踪到MotionEvent类的MotionEvent方法，该方法如下：
@FastNative
private static native float nativeGetAxisValue(long nativePtr,
int axis, int pointerIndex, int historyPos);
可以看到该方法被@FastNative标记，这表示可实现对 Java 原生接口 (JNI) 更快速的原生调用。也就是说该方法在jni包下实现，然后找到android_view_MotionEvent.cpp文件，得到如下函数：
static jfloat android_view_MotionEvent_nativeGetAxisValue(JNIEnv* env, jclass clazz,
jlong nativePtr, jint axis, jint pointerIndex, jint historyPos) {
MotionEvent* event = reinterpret_cast(nativePtr);
size_t pointerCount = event->getPointerCount();
if (!validatePointerIndex(env, pointerIndex, pointerCount)) {
return 0;
}
if (historyPos == HISTORY_CURRENT) {
return event->getAxisValue(axis, pointerIndex);
} else {
size_t historySize = event->getHistorySize();
if (!validateHistoryPos(env, historyPos, historySize)) {
return 0;
}
return event->getHistoricalAxisValue(axis, pointerIndex, historyPos);
}
}
再从该函数找到validatePointerIndex函数，也就是该异常出处
static bool validatePointerIndex(JNIEnv* env, jint pointerIndex, size_t pointerCount) {
if (pointerIndex < 0 || size_t(pointerIndex) >= pointerCount) {
jniThrowException(env, "java/lang/IllegalArgumentException",
"pointerIndex out of range");
return false;
}
return true;
}
由代码可知，当pointerIndex 小于0或者大于pointerCount时抛出该异常。
解決方案
方案一：捕获该异常
从上文可知当pointerIndex 小于0或者大于pointerCount时抛出该异常。我并没有在代码中改变该值，所以直接捕获即可。
@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
if(canScroll){
try {
return super.onInterceptTouchEvent(ev);
} catch (Exception e) {
return false;
}
}else{
return false;
}
}
方案二：提前做判断
从上文可知当pointerIndex 小于0或者大于pointerCount时抛出该异常。那么我们可以前进行该判断来避免异常。pointerCount可以通过MotionEvent类的getPointerCount()方法获得，pointerIndex 根据堆栈信息可知得自MotionEvent.getX方法的。而ViewPager.onInterceptTouchEvent 方法中有两处MotionEvent.getX方法调用，分别如下：
//...代码省略
switch (action) {
case MotionEvent.ACTION_MOVE: {
//...代码省略
final int activePointerId = mActivePointerId;
if (activePointerId == INVALID_POINTER) {
// If we don't have a valid id, the touch down wasn't on content.
break;
}
final int pointerIndex = ev.findPointerIndex(activePointerId);
final float x = ev.getX(pointerIndex);
//...代码省略
}
case MotionEvent.ACTION_DOWN: {
/*
* Remember location of down touch.
* ACTION_DOWN always refers to pointer index 0.
*/
mLastMotionX = mInitialMotionX = ev.getX();
//...代码省略
}
//...代码省略
从代码中可以看出第一处MotionEvent.getX方法调用，其pointerIndex 来源如下：
final int activePointerId = mActivePointerId;
if (activePointerId == INVALID_POINTER) {
// If we don't have a valid id, the touch down wasn't on content.
break;
}
final int pointerIndex = ev.findPointerIndex(activePointerId)
第二处调用的是无参数的MotionEvent.getX方法，该方法传入的参数为0，因为getPointerCount()的注释中有提到该方法返回值 >= 1，所以不会抛出该异常。无参数的MotionEvent.getX方法代码如下：
public final float getX() {
return nativeGetAxisValue(mNativePtr, AXIS_X, 0, HISTORY_CURRENT);
}
综上所述，我们只需要在MotionEvent.ACTION_MOVE时参照上文获取pointerIndex 和pointerCount进行一次判断即可，这里需要注意的是mActivePointerId是私有属性，需要反射获取。INVALID_POINTER是常量，直接用-1替代即可。解决代码如下：
@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
switch (ev.getAction()) {
case MotionEvent.ACTION_MOVE:
try {
Field field = this.getClass().getSuperclass().getDeclaredField("mActivePointerId");
field.setAccessible(true);
final int mActivePointerId = (int) field.get(this);
if (mActivePointerId != -1) {
final int pointerIndex = ev.findPointerIndex(mActivePointerId);
if(pointerIndex < 0 || pointerIndex >= ev.getPointerCount()){
return false;
}
}
break;
} catch (NoSuchFieldException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
}
}
if (isCanScroll) {
return super.onInterceptTouchEvent(ev);
} else {
return false;
}
}