问题
I'm working my way through the tutorials for android development and I'm currently trying to understand how the viewpager class works. I've downloaded the demo app here and I'm trying to wrap my head around how the collection of views is created.
I understand that there's a constant, NUM_PAGES which sets the number of views generated. I would expect to find a loop or something creating NUM_PAGES worth of views at the entry point to the application but I can't find anything - it's driving me mad.
All I'm trying to work out is how 5 views are being added to the ViewPager. How is it being done?
Here is the MainActivity.java file:
/*
* Copyright 2012 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.example.android.animationsdemo;
import android.app.Activity;
import android.app.ListActivity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.ListView;
/**
* The launchpad activity for this sample project. This activity launches other activities that
* demonstrate implementations of common animations.
*/
public class MainActivity extends ListActivity {
/**
* This class describes an individual sample (the sample title, and the activity class that
* demonstrates this sample).
*/
private class Sample {
private CharSequence title;
private Class<? extends Activity> activityClass;
public Sample(int titleResId, Class<? extends Activity> activityClass) {
this.activityClass = activityClass;
this.title = getResources().getString(titleResId);
}
@Override
public String toString() {
return title.toString();
}
}
/**
* The collection of all samples in the app. This gets instantiated in {@link
* #onCreate(android.os.Bundle)} because the {@link Sample} constructor needs access to {@link
* android.content.res.Resources}.
*/
private static Sample[] mSamples;
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// Instantiate the list of samples.
mSamples = new Sample[]{
new Sample(R.string.title_crossfade, CrossfadeActivity.class),
new Sample(R.string.title_card_flip, CardFlipActivity.class),
new Sample(R.string.title_screen_slide, ScreenSlideActivity.class),
new Sample(R.string.title_zoom, ZoomActivity.class),
new Sample(R.string.title_layout_changes, LayoutChangesActivity.class),
};
setListAdapter(new ArrayAdapter<Sample>(this,
android.R.layout.simple_list_item_1,
android.R.id.text1,
mSamples));
}
@Override
protected void onListItemClick(ListView listView, View view, int position, long id) {
// Launch the sample associated with this list position.
startActivity(new Intent(MainActivity.this, mSamples[position].activityClass));
}
}
Here is the ScreenSlideActivity.java file:
/*
* Copyright 2012 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.example.android.animationsdemo;
import android.app.Fragment;
import android.app.FragmentManager;
import android.content.Intent;
import android.os.Bundle;
import android.support.v13.app.FragmentStatePagerAdapter;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.NavUtils;
import android.support.v4.view.PagerAdapter;
import android.support.v4.view.ViewPager;
import android.view.Menu;
import android.view.MenuItem;
/**
* Demonstrates a "screen-slide" animation using a {@link ViewPager}. Because {@link ViewPager}
* automatically plays such an animation when calling {@link ViewPager#setCurrentItem(int)}, there
* isn't any animation-specific code in this sample.
*
* <p>This sample shows a "next" button that advances the user to the next step in a wizard,
* animating the current screen out (to the left) and the next screen in (from the right). The
* reverse animation is played when the user presses the "previous" button.</p>
*
* @see ScreenSlidePageFragment
*/
public class ScreenSlideActivity extends FragmentActivity {
/**
* The number of pages (wizard steps) to show in this demo.
*/
private static final int NUM_PAGES = 5;
/**
* The pager widget, which handles animation and allows swiping horizontally to access previous
* and next wizard steps.
*/
private ViewPager mPager;
/**
* The pager adapter, which provides the pages to the view pager widget.
*/
private PagerAdapter mPagerAdapter;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_screen_slide);
// Instantiate a ViewPager and a PagerAdapter.
mPager = (ViewPager) findViewById(R.id.pager);
mPagerAdapter = new ScreenSlidePagerAdapter(getFragmentManager());
mPager.setAdapter(mPagerAdapter);
mPager.setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
@Override
public void onPageSelected(int position) {
// When changing pages, reset the action bar actions since they are dependent
// on which page is currently active. An alternative approach is to have each
// fragment expose actions itself (rather than the activity exposing actions),
// but for simplicity, the activity provides the actions in this sample.
invalidateOptionsMenu();
}
});
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
super.onCreateOptionsMenu(menu);
getMenuInflater().inflate(R.menu.activity_screen_slide, menu);
menu.findItem(R.id.action_previous).setEnabled(mPager.getCurrentItem() > 0);
// Add either a "next" or "finish" button to the action bar, depending on which page
// is currently selected.
MenuItem item = menu.add(Menu.NONE, R.id.action_next, Menu.NONE,
(mPager.getCurrentItem() == mPagerAdapter.getCount() - 1)
? R.string.action_finish
: R.string.action_next);
item.setShowAsAction(MenuItem.SHOW_AS_ACTION_IF_ROOM | MenuItem.SHOW_AS_ACTION_WITH_TEXT);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
switch (item.getItemId()) {
case android.R.id.home:
// Navigate "up" the demo structure to the launchpad activity.
// See http://developer.android.com/design/patterns/navigation.html for more.
NavUtils.navigateUpTo(this, new Intent(this, MainActivity.class));
return true;
case R.id.action_previous:
// Go to the previous step in the wizard. If there is no previous step,
// setCurrentItem will do nothing.
mPager.setCurrentItem(mPager.getCurrentItem() - 1);
return true;
case R.id.action_next:
// Advance to the next step in the wizard. If there is no next step, setCurrentItem
// will do nothing.
mPager.setCurrentItem(mPager.getCurrentItem() + 1);
return true;
}
return super.onOptionsItemSelected(item);
}
/**
* A simple pager adapter that represents 5 {@link ScreenSlidePageFragment} objects, in
* sequence.
*/
private class ScreenSlidePagerAdapter extends FragmentStatePagerAdapter {
public ScreenSlidePagerAdapter(FragmentManager fm) {
super(fm);
}
@Override
public Fragment getItem(int position) {
return ScreenSlidePageFragment.create(position);
}
@Override
public int getCount() {
return NUM_PAGES;
}
}
}
Finally, here is the ScreenSlidePageFragment.java file:
/*
* Copyright 2012 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.example.android.animationsdemo;
import android.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
/**
* A fragment representing a single step in a wizard. The fragment shows a dummy title indicating
* the page number, along with some dummy text.
*
* <p>This class is used by the {@link CardFlipActivity} and {@link
* ScreenSlideActivity} samples.</p>
*/
public class ScreenSlidePageFragment extends Fragment {
/**
* The argument key for the page number this fragment represents.
*/
public static final String ARG_PAGE = "page";
/**
* The fragment's page number, which is set to the argument value for {@link #ARG_PAGE}.
*/
private int mPageNumber;
/**
* Factory method for this fragment class. Constructs a new fragment for the given page number.
*/
public static ScreenSlidePageFragment create(int pageNumber) {
ScreenSlidePageFragment fragment = new ScreenSlidePageFragment();
Bundle args = new Bundle();
args.putInt(ARG_PAGE, pageNumber);
fragment.setArguments(args);
return fragment;
}
public ScreenSlidePageFragment() {
}
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mPageNumber = getArguments().getInt(ARG_PAGE);
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
// Inflate the layout containing a title and body text.
ViewGroup rootView = (ViewGroup) inflater
.inflate(R.layout.fragment_screen_slide_page, container, false);
// Set the title view to show the page number.
((TextView) rootView.findViewById(android.R.id.text1)).setText(
getString(R.string.title_template_step, mPageNumber + 1));
return rootView;
}
/**
* Returns the page number represented by this fragment object.
*/
public int getPageNumber() {
return mPageNumber;
}
}
Thanks
回答1:
public Fragment getItem(int position) {
return ScreenSlidePageFragment.create(position);
}
This is where your fragments are created (each one contains a view).
回答2:
I was wondering myself about the NUM_PAHES constant and where it is used. I digged a bit deeper and implemented for my project a use case with real data like shown below. I collect my data for the fragments in mTranslationObjects ArrayList so I can get the exact object I need for the given position in the pager adapter.
public class TrainerActivity extends ActionBarActivity {
/**
* The pager widget, which handles animation and allows swiping horizontally to access previous
* and next wizard steps.
*/
private ViewPager mPager;
/**
* The pager adapter, which provides the pages to the view pager widget.
*/
private PagerAdapter mPagerAdapter;
private List<TranslationObject> mTranslationObjects = new ArrayList<TranslationObject>();
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_trainer);
fillTrainerData();
// Instantiate a ViewPager and a PagerAdapter.
mPager = (VerticalViewPager) findViewById(R.id.pager);
mPagerAdapter = new ScreenSlidePagerAdapter(getSupportFragmentManager());
mPager.setAdapter(mPagerAdapter);
}
private void fillTrainerData() {
// putting objects in mTranslationObjects
...
...
...
}
private class ScreenSlidePagerAdapter extends FragmentStatePagerAdapter {
public ScreenSlidePagerAdapter(FragmentManager fm) {
super(fm);
}
@Override
public Fragment getItem(int position) {
TrainerFragment trainerFragment = new TrainerFragment();
Bundle bundle = new Bundle();
bundle.putString(TrainerFragment.BUNDLE_ORIGINAL_WORD, mTranslationObjects.get(position).originalWord);
bundle.putString(TrainerFragment.BUNDLE_TRANSLATION, mTranslationObjects.get(position).translation);
bundle.putString(TrainerFragment.BUNDLE_WORD_CONTEXT, mTranslationObjects.get(position).context);
trainerFragment.setArguments(bundle);
return trainerFragment;
}
@Override
public int getCount() {
return mTranslationObjects.size();
}
}
}
来源：https://stackoverflow.com/questions/13532954/how-are-the-views-for-this-view-pager-being-created