1
我有一個ListView與全屏圖片。 （就像一個垂直的畫廊）。我的問題是我想重寫onTouchEvent的列表視圖，以便它一次只更改一張圖片，以便在滾動後（當沒有發生任何投影時），選定的圖片會自行居中。nullpointer
我的問題是，當我調用getChildAt（getFirstVisiblePosition（））時，它返回null。我已經檢查並getFirstVisiblePosition（）返回一個當前易於使用的視圖。
我正在使用smoothScrollToPosition居中圖片。
我的代碼很長，但重要的部分在這裏:(當接收MotionEvent.ACTION_UP時）。
case MotionEvent.ACTION_UP:
{
try
{
//Calculetes the velocity if the movement
int initialVelocity = (int)ModifiedVelocityTracker.getYVelocity();*/
if (mVelocityTracker == null)
{
mVelocityTracker = VelocityTracker.obtain();
}
final VelocityTracker ModifiedVelocityTracker = mVelocityTracker;
ModifiedVelocityTracker.computeCurrentVelocity(1000);
int initialVelocity = (int)ModifiedVelocityTracker.getYVelocity();
//Detects if the motion is a fling
if ((Math.abs(initialVelocity) >
ViewConfiguration.get(mContext).getScaledMinimumFlingVelocity()) &&
(getChildCount() > 0)) // TODO FLing
{
super.onTouchEvent(getEventWithACTION_MOVE(ev));
//Send the events to cancel the superclass fling
//MotionEvent me[] = mVelocityTracker.getTrickEvents();
MotionEvent me[] = getTrickEvents(ev);
for(int i=0;i<me.length;i++)
super.onTouchEvent(me[i]);
int firstVisiblePosition = getFirstVisiblePosition();
// Pint y=0 is located on the bottom, so a negative speed means
//the finger moved up and the picture to come is the one under
if(initialVelocity < 0) //EventUp-GoDown
{
if(firstVisiblePosition != getCount() - 1) //Can Move
{
//smoothScrollToPosition(firstVisiblePosition + 1);
View currentTopView = getChildAt(firstVisiblePosition);
int botom = currentTopView.getBottom();
botom = botom - firstVisiblePosition * getHeight();
smoothScrollBy(-botom, 500);
} else
{
smoothScrollToPosition(firstVisiblePosition);
}
} else //EventDown-GoUp
{
smoothScrollToPosition(firstVisiblePosition);
}
}
else // TODO Stay in the picture that has a bigger area shown
{
onTouchEvent = super.onTouchEvent(ev);
//Center The View after the parent stops moving it
/*
* FIREST VIEW |****|/SECOND VIEW | |
*
* CASE 1 Bottom over center - Set second view the main one
* |****|
* |****| Bottom of Top View at First visible Position
* | |
* |----| center line
* | |
* | |
* | |
*
* CASE 2 Bottom under center - Set first view the main one
* |****|
* |****|
* |****|
* |----| center line
* |****|
* |****| Bottom of Top View at First visible Position
* | |
* */
int firstVisiblePosition = getFirstVisiblePosition();
View currentTopView = getChildAt(firstVisiblePosition);
int botom;
if(currentTopView != null)
{
botom = currentTopView.getBottom();
}else{
currentTopView = getSelectedView();
if(firstVisiblePosition == getPositionForView(currentTopView))
{
botom = currentTopView.getBottom();
}
else
{
botom = currentTopView.getTop();
}
}
int center = getHeight()/2;
botom = botom - firstVisiblePosition * getHeight();
if(botom < center) //Case 1 - Scroll Down
{
//Checks if the top view is the last one.
//Shouldn't happen, but just in case.
if(firstVisiblePosition != getCount() - 1) //Can Move
{
smoothScrollToPosition(firstVisiblePosition + 1);
} else
{
smoothScrollToPosition(firstVisiblePosition);
}
}
else //Case 2
{
smoothScrollToPosition(firstVisiblePosition);
}
}
onTouchEvent = true;
} catch(NullPointerException e)
{
e.printStackTrace();
}
}
坦克你提前。
由於評論。
import android.content.Context;
import android.os.SystemClock;
import android.util.Log;
import android.view.MotionEvent;
import android.view.VelocityTracker;
import android.view.View;
import android.view.ViewConfiguration;
import android.widget.ListView;
public class FixedListView extends ListView{
final static String LOGTAG = "TEST";
private Context mContext;
VelocityTracker mVelocityTracker;
public FixedListView(Context context) {
super(context);
mContext = context;
setDivider(null);
// TODO Auto-generated constructor stub
}
@Override
public boolean onTouchEvent(MotionEvent ev)
{
boolean onTouchEvent = true;
final int action = ev.getAction();
//Add the event to the ModifiedVelocityTracker used for detect if if the motion is a fling (also creates it)
Log.d(LOGTAG, "Before add to Traker: " + SystemClock.uptimeMillis());
/*if (mVelocityTracker == null)
{
mVelocityTracker = ModifiedVelocityTracker.obtain();
}
mVelocityTracker.addMovement(ev);*/
Log.d(LOGTAG, "After add to Traker: " + SystemClock.uptimeMillis());
//Detect the event action type
switch (action) {
case MotionEvent.ACTION_DOWN:
case MotionEvent.ACTION_MOVE:
{
Log.d(LOGTAG, "After eval action Traker: " + SystemClock.uptimeMillis());
onTouchEvent = super.onTouchEvent(ev);
Log.d(LOGTAG, "After super Y = " + ev.getY() +": " + SystemClock.uptimeMillis());
break;
}
case MotionEvent.ACTION_UP:
{
try
{
//Calculetes the velocity if the movement
//NOTE: I'm using my own VelocityTraker because the Android.Utils
//one can only be used once at a time and the superclass uses it.
/*final ModifiedVelocityTracker ModifiedVelocityTracker = mVelocityTracker;
ModifiedVelocityTracker.computeCurrentVelocity(1000);
int initialVelocity = (int)ModifiedVelocityTracker.getYVelocity();*/
if (mVelocityTracker == null)
{
mVelocityTracker = VelocityTracker.obtain();
}
final VelocityTracker ModifiedVelocityTracker = mVelocityTracker;
ModifiedVelocityTracker.computeCurrentVelocity(1000);
int initialVelocity = (int)ModifiedVelocityTracker.getYVelocity();
//Detects if the motion is a fling
if ((Math.abs(initialVelocity) >
ViewConfiguration.get(mContext).getScaledMinimumFlingVelocity()) &&
(getChildCount() > 0)) // TODO FLing
{
super.onTouchEvent(getEventWithACTION_MOVE(ev));
//Send the events to cancel the superclass fling
//MotionEvent me[] = mVelocityTracker.getTrickEvents();
MotionEvent me[] = getTrickEvents(ev);
for(int i=0;i<me.length;i++)
super.onTouchEvent(me[i]);
int firstVisiblePosition = getFirstVisiblePosition();
// Pint y=0 is located on the bottom, so a negative speed means
//the finger moved up and the picture to come is the one under
if(initialVelocity < 0) //EventUp-GoDown
{
if(firstVisiblePosition != getCount() - 1) //Can Move
{
//smoothScrollToPosition(firstVisiblePosition + 1);
View currentTopView = getChildAt(firstVisiblePosition);
int botom = currentTopView.getBottom();
botom = botom - firstVisiblePosition * getHeight();
smoothScrollBy(-botom, 500);
} else
{
smoothScrollToPosition(firstVisiblePosition);
}
} else //EventDown-GoUp
{
smoothScrollToPosition(firstVisiblePosition);
}
}
else // TODO Stay in the picture that has a bigger area shown
{
onTouchEvent = super.onTouchEvent(ev);
//Center The View after the parent stops moving it
/*
* FIREST VIEW |****|/SECOND VIEW | |
*
* CASE 1 Bottom over center - Set second view the main one
* |****|
* |****| Bottom of Top View at First visible Position
* | |
* |----| center line
* | |
* | |
* | |
*
* CASE 2 Bottom under center - Set first view the main one
* |****|
* |****|
* |****|
* |----| center line
* |****|
* |****| Bottom of Top View at First visible Position
* | |
* */
int firstVisiblePosition = getFirstVisiblePosition();
View currentTopView = getChildAt(firstVisiblePosition);
int botom;
if(currentTopView != null)
{
botom = currentTopView.getBottom();
}else{
currentTopView = getSelectedView();
if(firstVisiblePosition == getPositionForView(currentTopView))
{
botom = currentTopView.getBottom();
}
else
{
botom = currentTopView.getTop();
}
}
int center = getHeight()/2;
botom = botom - firstVisiblePosition * getHeight();
if(botom < center) //Case 1 - Scroll Down
{
//Checks if the top view is the last one.
//Shouldn't happen, but just in case.
if(firstVisiblePosition != getCount() - 1) //Can Move
{
smoothScrollToPosition(firstVisiblePosition + 1);
} else
{
smoothScrollToPosition(firstVisiblePosition);
}
}
else //Case 2
{
smoothScrollToPosition(firstVisiblePosition);
}
}
onTouchEvent = true;
} catch(NullPointerException e)
{
e.printStackTrace();
}
if(mVelocityTracker != null)
mVelocityTracker.recycle();
break;
}
case MotionEvent.ACTION_CANCEL:
{
try
{
onTouchEvent = super.onTouchEvent(ev);
//Center The View after the parent stops moving it
/*
* FIREST VIEW |****|/SECOND VIEW | |
*
* CASE 1 Bottom over center - Set second view the main one
* |****|
* |****| Bottom of Top View at First visible Position
* | |
* |----| center line
* | |
* | |
* | |
*
* CASE 2 Bottom under center - Set first view the main one
* |****|
* |****|
* |****|
* |----| center line
* |****|
* |****| Bottom of Top View at First visible Position
* | |
* */
int firstVisiblePosition = getFirstVisiblePosition();
View currentTopView = getChildAt(firstVisiblePosition);
int center = getHeight()/2;
if(currentTopView.getBottom() < center) //Case 1 - Scroll Down
{
//Checks if the top view is the last one.
//Shouldn't happen, but just in case.
if(firstVisiblePosition != getCount() - 1) //Can Move
{
smoothScrollToPosition(firstVisiblePosition + 1);
} else
{
smoothScrollToPosition(firstVisiblePosition);
}
}
else //Case 2
{
if(firstVisiblePosition != 0) //Can Move
{
smoothScrollToPosition(firstVisiblePosition - 1);
} else
{
smoothScrollToPosition(firstVisiblePosition);
}
}
onTouchEvent = true;
} catch(NullPointerException e)
{
e.printStackTrace();
}
if(mVelocityTracker != null)
{
mVelocityTracker.recycle();
mVelocityTracker = null;
}
}
}
// TODO Auto-generated method stub
return onTouchEvent;
}
@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
// TODO Auto-generated method stub
return super.onInterceptTouchEvent(ev);
}
public MotionEvent[] getTrickEvents(MotionEvent ev)
{
//Detect the last touched position
final float requiredX = ev.getX();
final float requiredY = ev.getY();
//Get a time value that is longer than the last added event value by a bigger
//number than the LONGEST_PAST_TIME accepted by the VelocityTraker
//NOTE: If GOOGLE changes the LONGEST_PAST_TIME, we will have to change it too,
//I wasn't able to retrieve the original VelocityTracker LONGEST_PAST_TIME directly from it.
final long requiredPastTime = ev.getEventTime() + 201;
//Create the MotionEvents (Simulating no movement in y).
MotionEvent m1 = null;
if(requiredX == 0) //If at the left border, move one pixel to the right.
{
m1 = MotionEvent.obtain(requiredPastTime, requiredPastTime,
MotionEvent.ACTION_MOVE,
requiredX + 1, requiredY, 0);
}
else //If not at the left border, move one pixel to the left
{
m1 = MotionEvent.obtain(requiredPastTime, requiredPastTime,
MotionEvent.ACTION_MOVE,
requiredX - 1, requiredY, 0);
}
//Return to the original position after 100 time units
MotionEvent m2 = MotionEvent.obtain(requiredPastTime + 100, requiredPastTime + 100,
MotionEvent.ACTION_UP,
requiredX, requiredY, 0);
MotionEvent motEvents[] = {m1,m2};
return motEvents;
}
public MotionEvent getEventWithACTION_MOVE(MotionEvent ev)
{
//Detect the last touched position
final float requiredX = ev.getX();
final float requiredY = ev.getY();
//Get a time value that is longer than the last added event value by a bigger
//number than the LONGEST_PAST_TIME accepted by the VelocityTraker
//NOTE: If GOOGLE changes the LONGEST_PAST_TIME, we will have to change it too,
//I wasn't able to retrieve the original VelocityTracker LONGEST_PAST_TIME directly from it.
final long Time = ev.getEventTime();
//Create the MotionEvent
MotionEvent m1 = MotionEvent.obtain(Time, Time,
MotionEvent.ACTION_MOVE,
requiredX , requiredY, 0);
return m1;
}
}
和適配器
public class FixedListAdapter extends BaseAdapter
{
int pictures[];
public FixedListAdapter(int pictures[])
{
this.pictures = pictures;
}
public int getCount() {
return pictures.length;
}
public Object getItem(int position) {
return pictures[position];
}
public long getItemId(int position) {
return pictures[position];
}
public View getView(int position, View convertView, ViewGroup parent) {
LinearLayout l = new LinearLayout(GalleryTest.this);
l.setLayoutParams(new ListView.LayoutParams(
ListView.LayoutParams.MATCH_PARENT, ListView.LayoutParams.MATCH_PARENT));
l.setGravity(Gravity.CENTER);
l.setPadding(0,0,0,0);
l.setBackgroundColor(Color.BLACK);
ImageView i = new ImageView(GalleryTest.this);
i.setLayoutParams(new LinearLayout.LayoutParams(width, height));
i.setScaleType(ImageView.ScaleType.FIT_CENTER);
i.setImageResource(pictures[position]);
l.addView(i);
return l;
}
}
注：在設備和getFirstVisiblePosition上的圖像的變化（）給我的第一圖片的索引被顯示。問題是，getChildAt返回null，實際上是在屏幕上的視圖。
爲了以防萬一，我正在處理ListView的子類。 – 2011-05-02 21:11:53
如果您在該代碼中的getChildAt（）處得到null，那麼您包含的內容不是重要的部分。我們需要看到更多的上下文，因爲沒有_there_會解釋爲什麼它返回null。 – 2011-05-02 21:31:26
好的，我會複製整個班級。我也會複製適配器。感謝您查看此內容。 – 2011-05-02 21:37:25