Android开发培训(02)－－Android开发核心

这部分是每个android 开发者必须掌握的内容，可能对于第一次接触android的同学来说，这篇写得不是很好。

但是这个只是主要的概况一下，可以从网上搜到大量类似的文章，而且这些基本操作只要接触android之后就会掌握。

第一章　创建一个Android工程

1. 创建一个Android工程

首先下载Android Studio软件，这个ide是开发android的首选，可能会因为被墙的问题，有些东西下载不了，但是还是首选。

下载完成之后解压，进入android-studio/bin文件目录

执行　./studio.sh

就会进入到开发界面中，第一次接触的可以自己随便动手看看Android Studio有什么功能。

主要的文件夹有

app > java 

放置编写用户代码的文件夹

app > res > layout

主要放置布局文件的文件夹

还有一些其它的配置文件AndroidManifest.xml和build.gradle之类，开发接触得多了自然就知道怎么回事。

2. 运行你的app

然后点击上方绿色的手机运行

一般建议连上真实的手机，这样调试起来比较快，也比较真实。

需要打开手机的developer option开发者选项。

然后会弹出一个hello world的界面，程序完成。

也可以在模拟器上运行。

3. 新建一个简单的用户交互界面

这小节会建立一个文本输入框和一个发送按钮到下一个Activity中。

UI系统主要是由ViewGroup类和View类建立起来的，

ViewGroup规定了类的结构，View则是每个对象，下面这个关系图说明了ViewGroup和View的关系。

可以使用鼠标直接拖动的方式建立UI也，可以在xml文件中手写，因为由自动补全功能，可以两者相结合达到更好的开发速度。

原文默认的是ConstraintLayout,这里修改为LinearLayout，这个对于开发者更熟悉。

具体的xml文件

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    android:orientation="horizontal"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    xmlns:android="http://schemas.android.com/apk/res/android">

    <EditText
        android:id="@+id/edit_text"
        android:hint="@string/hint"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"/>

    <Button
        android:id="@+id/button"
        android:text="@string/send"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"/>
    
</LinearLayout>

最好把显示的string加入到String.xml文件中，这样可以进行多语言配置。

4. 开启另外一个Activity

当用户点击send按钮的时候会将信息发送给下一个界面，也就是另外一个activity。

这个时候需要用到另外一个类，Intent, 安卓系统中专门用来发送消息的类。

首先就是建立一个Intent实例

然后startActivity

具体的代码如下

package com.example.www.myfirstapp;

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.text.Editable;
import android.text.TextWatcher;
import android.util.Log;
import android.view.KeyEvent;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {
    public static final String EXTRA_MESSAGE = "com.example.myfirstapp.MESSAGE";

    Button mButton;
    EditText mEditText;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        
        mButton = (Button) findViewById(R.id.button);
        mButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                sendMessage();
            }
        });

        mEditText = (EditText) findViewById(R.id.edit_text);
    }

    /** Called when the user taps the Send button */
    private void sendMessage() {
        Intent intent = new Intent(this, DisplayMessageActivity.class);
        intent.addCategory(Intent.CATEGORY_DEFAULT);
        String message = mEditText.getText().toString();
        intent.putExtra(EXTRA_MESSAGE, message);
        startActivity(intent);
    }
}

开启另外一个Activity,接收消息，写入到TextView中。

package com.example.www.myfirstapp;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.EditText;
import android.widget.TextView;

public class DisplayMessageActivity extends AppCompatActivity {
    TextView mTextView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_display_message);
        mTextView = (TextView) findViewById(R.id.text_view);
    }


    @Override
    protected void onResume() {
        super.onResume();
        String message = getIntent().getStringExtra(MainActivity.EXTRA_MESSAGE);
        mTextView.setText(message);
    }

}

第二章　支持不同的设备

Android有很多大小和型号不同的设备，所以需要编写兼容性更好的app.

1. 支持不同的语言

需要在res资源中创建不同的res文件夹，具体需要的时候可以用到。

这部分需要熟能生巧，正常来说的话比较流行的设备不会用到这些。

2. 支持不同的屏幕，

屏幕的大小不同，也会造成资源的显示效果不好，所以需要创建不同的布局来最大话满足用户的需求。

图片的大小。

3. 支持不同的平台版本，随着Android的升级，在新版本中加入一些功能，或者删除旧版本中废弃的接口，需要做到软件的兼容性。

比如指定最小的运行sdk，编译使用的sdk等等很多问题，

需要在实践中不断地认识。

第三章　使用Fragment建立UI

为了建立动态的多窗口的用户界面，andriod允许将Activity模块化，这就需要用到Fragment类，像一个叠加的Activity，它有自己的布局文件和管理自己的生命周期。

因为fragent可以动态的配置，因为对于不同的屏幕可以很好的支持。

1. 建立一个Fragment

建立一个ActicleFragment文件，布局就是常规布局。

package com.example.www.fragmentbasic;

import android.app.Fragment;
import android.os.Bundle;
import android.support.annotation.Nullable;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

/**
 * Created by wang on 17-8-22.
 */

public class ActicleFragment extends Fragment {
    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, Bundle savedInstanceState) {
        return inflater.inflate(R.layout.article_view, container, false);
    }
}

然后需要把fragment加入到xml文件中。

Fragment 为动态的用户提交提供很大的便利，开发者可以在程序运行的时候动态的移除，添加,代替，activity.

为了展示怎么移除和添加Fragment,首先必须使用FragmentManager的类创建FragmentTransaction方法。

一个重要的规则就是当你在动态时运行需要添加或者代替fragment的时候，需要在actvity的xml文件中有一个View容器类插入你的fragment.

接下来的layout就只有一个fragment，定义一个FrameLayout当做framgent容器。

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout
    android:id="@+id/fragment_container"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    xmlns:android="http://schemas.android.com/apk/res/android"/>

贴几个重要的类和xml

/*
 * Copyright (C) 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.fragments;

import android.os.Bundle;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentTransaction;

public class MainActivity extends FragmentActivity 
        implements HeadlinesFragment.OnHeadlineSelectedListener {

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.news_articles);

        // Check whether the activity is using the layout version with
        // the fragment_container FrameLayout. If so, we must add the first fragment
        if (findViewById(R.id.fragment_container) != null) {

            // However, if we're being restored from a previous state,
            // then we don't need to do anything and should return or else
            // we could end up with overlapping fragments.
            if (savedInstanceState != null) {
                return;
            }

            // Create an instance of ExampleFragment
            HeadlinesFragment firstFragment = new HeadlinesFragment();

            // In case this activity was started with special instructions from an Intent,
            // pass the Intent's extras to the fragment as arguments
            firstFragment.setArguments(getIntent().getExtras());

            // Add the fragment to the 'fragment_container' FrameLayout
            getSupportFragmentManager().beginTransaction()
                    .add(R.id.fragment_container, firstFragment).commit();
        }
    }

    public void onArticleSelected(int position) {
        // The user selected the headline of an article from the HeadlinesFragment

        // Capture the article fragment from the activity layout
        ArticleFragment articleFrag = (ArticleFragment)
                getSupportFragmentManager().findFragmentById(R.id.article_fragment);

        if (articleFrag != null) {
            // If article frag is available, we're in two-pane layout...

            // Call a method in the ArticleFragment to update its content
            articleFrag.updateArticleView(position);

        } else {
            // If the frag is not available, we're in the one-pane layout and must swap frags...

            // Create fragment and give it an argument for the selected article
            ArticleFragment newFragment = new ArticleFragment();
            Bundle args = new Bundle();
            args.putInt(ArticleFragment.ARG_POSITION, position);
            newFragment.setArguments(args);
            FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();

            // Replace whatever is in the fragment_container view with this fragment,
            // and add the transaction to the back stack so the user can navigate back
            transaction.replace(R.id.fragment_container, newFragment);
            transaction.addToBackStack(null);

            // Commit the transaction
            transaction.commit();
        }
    }
}

/*
 * Copyright (C) 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.fragments;

import android.app.Activity;
import android.os.Build;
import android.os.Bundle;
import android.support.v4.app.ListFragment;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.ListView;

public class HeadlinesFragment extends ListFragment {
    OnHeadlineSelectedListener mCallback;

    // The container Activity must implement this interface so the frag can deliver messages
    public interface OnHeadlineSelectedListener {
        /** Called by HeadlinesFragment when a list item is selected */
        public void onArticleSelected(int position);
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        // We need to use a different list item layout for devices older than Honeycomb
        int layout = Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB ?
                android.R.layout.simple_list_item_activated_1 : android.R.layout.simple_list_item_1;

        // Create an array adapter for the list view, using the Ipsum headlines array
        setListAdapter(new ArrayAdapter<String>(getActivity(), layout, Ipsum.Headlines));
    }

    @Override
    public void onStart() {
        super.onStart();

        // When in two-pane layout, set the listview to highlight the selected list item
        // (We do this during onStart because at the point the listview is available.)
        if (getFragmentManager().findFragmentById(R.id.article_fragment) != null) {
            getListView().setChoiceMode(ListView.CHOICE_MODE_SINGLE);
        }
    }

    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);

        // This makes sure that the container activity has implemented
        // the callback interface. If not, it throws an exception.
        try {
            mCallback = (OnHeadlineSelectedListener) activity;
        } catch (ClassCastException e) {
            throw new ClassCastException(activity.toString()
                    + " must implement OnHeadlineSelectedListener");
        }
    }

    @Override
    public void onListItemClick(ListView l, View v, int position, long id) {
        // Notify the parent activity of selected item
        mCallback.onArticleSelected(position);
        
        // Set the item as checked to be highlighted when in two-pane layout
        getListView().setItemChecked(position, true);
    }
}

/*
 * Copyright (C) 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.fragments;

import android.support.v4.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

public class ArticleFragment extends Fragment {
    final static String ARG_POSITION = "position";
    int mCurrentPosition = -1;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, 
        Bundle savedInstanceState) {

        // If activity recreated (such as from screen rotate), restore
        // the previous article selection set by onSaveInstanceState().
        // This is primarily necessary when in the two-pane layout.
        if (savedInstanceState != null) {
            mCurrentPosition = savedInstanceState.getInt(ARG_POSITION);
        }

        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.article_view, container, false);
    }

    @Override
    public void onStart() {
        super.onStart();

        // During startup, check if there are arguments passed to the fragment.
        // onStart is a good place to do this because the layout has already been
        // applied to the fragment at this point so we can safely call the method
        // below that sets the article text.
        Bundle args = getArguments();
        if (args != null) {
            // Set article based on argument passed in
            updateArticleView(args.getInt(ARG_POSITION));
        } else if (mCurrentPosition != -1) {
            // Set article based on saved instance state defined during onCreateView
            updateArticleView(mCurrentPosition);
        }
    }

    public void updateArticleView(int position) {
        TextView article = (TextView) getActivity().findViewById(R.id.article);
        article.setText(Ipsum.Articles[position]);
        mCurrentPosition = position;
    }

    @Override
    public void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);

        // Save the current article selection in case we need to recreate the fragment
        outState.putInt(ARG_POSITION, mCurrentPosition);
    }
}

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/fragment_container"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

<TextView xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/article"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:padding="16dp"
    android:textSize="18sp" />