四大组件的context的研究

看到同事发了一个这个图。大致有点感觉 ，但是理解的不透彻。三个月后。10.14号。等我多做点东西，对这些部件了解深了再来写一个总结吧。

今天10.14号，交作业了。这个就是安卓四大组件加一个Application可以实现的功能嘛。

其实呢，上述的这个功能呢，都与context有关。context在哪些地方用到了呢？弹出Toast、弹出dialog，启动Activity、启动Service、发送广播、操作数据库，Inflate布局等等都需要用到Context。context就是实现功能的情景。

从这个继承关系图中，contextImpl是抽象类context的具体实现类，contextwrapper是包装类，activity，application，service在初始化的时候会创建contextImpl实例实现context功能。所以activity，application，service本身来说就是context。broadcast与contentprovider用到的context是启动他们时传过来的。特殊一点就是，activity继承于ContextThemeWrapper，也是一个包装类，其内部包含了与主题（Theme）相关的接口，这里所说的主题就是指在AndroidManifest.xml中通过android：theme为Application元素或者Activity元素指定的主题。当然，只有Activity才需要主题，Service是不需要主题的，因为Service是没有界面的后台场景，所以Service直接继承于ContextWrapper，Application同理。

接下来讨论讨论app与service不能做（不推荐）的事情。

show a dialog 只可以由activity发出。为什么？最直接的谷歌爸爸设置的token的作用。本人也有写 http://www.cnblogs.com/vitabebeauty/p/7125669.html 

Start Activity  还是理解成不可以做的吧。activity的启动startActivity（），显示启动，隐式启动，都是一个activity启动另一个。就连咱们的安卓系统都可以理解成一个app应用，咱们的用的app的activity都是系统activity startActivity（）出来的。在startActivity（）中，根据启动模式，将创建/刷新的activity压入activity栈（任务栈）中，而非Activity类型的Context并没有所谓的任务栈，所以待启动的Activity就找不到栈了，这也是为什么不推荐使用Application、service启动Activity的方式。

Layout Inflaction ：

在Application和Service中去layout inflate也是合法的，但是会使用系统默认的主题样式，如果你自定义了某些样式可能不会被使用。所以这种方式也不推荐使用。
一句话总结：凡是跟UI相关的，都应该使用Activity做为Context来处理；其他的一些操作，Service,Activity,Application等实例都可以，当然了，注意Context引用的持有，防止内存泄漏。（如何防止泄露简言之，引用者要比被引用的context死的早）

 

 

 

，有一篇关于ActivityThread与AppliactionTread的文章，很不错，不是吊书袋，看了之后了解更深。

Android M 5.0,关于ActivityThread和ApplicationThread的解析.

附上contextWrapper&contextThemeWrapper（增加theme相关功能）的部分相关代码（包装类，实现依靠mBases即contextImplement实现。contextImplement在 attachBaseContext（） 方法里创建）

public class ContextWrapper extends Context {  
    Context mBase;  
      
    /** 
     * Set the base context for this ContextWrapper.  All calls will then be 
     * delegated to the base context.  Throws 
     * IllegalStateException if a base context has already been set. 
     *  
     * @param base The new base context for this wrapper. 
     */  
    protected void attachBaseContext(Context base) {  
        if (mBase != null) {  
            throw new IllegalStateException("Base context already set");  
        }  
        mBase = base;  
    }  
  
    /** 
     * @return the base context as set by the constructor or setBaseContext 
     */  
    public Context getBaseContext() {  
        return mBase;  
    }  
  
    @Override  
    public AssetManager getAssets() {  
        return mBase.getAssets();  
    }  
  
    @Override  
    public Resources getResources() {  
        return mBase.getResources();  
    }  
  
    @Override  
    public ContentResolver getContentResolver() {  
        return mBase.getContentResolver();  
    }  
  
    @Override  
    public Looper getMainLooper() {  
        return mBase.getMainLooper();  
    }  
      
    @Override  
    public Context getApplicationContext() {  
        return mBase.getApplicationContext();  
    }  
  
    @Override  
    public String getPackageName() {  
        return mBase.getPackageName();  
    }  
  
    @Override  
    public void startActivity(Intent intent) {  
        mBase.startActivity(intent);  
    }  
      
    @Override  
    public void sendBroadcast(Intent intent) {  
        mBase.sendBroadcast(intent);  
    }  
  
    @Override  
    public Intent registerReceiver(  
        BroadcastReceiver receiver, IntentFilter filter) {  
        return mBase.registerReceiver(receiver, filter);  
    }  
  
    @Override  
    public void unregisterReceiver(BroadcastReceiver receiver) {  
        mBase.unregisterReceiver(receiver);  
    }  
  
    @Override  
    public ComponentName startService(Intent service) {  
        return mBase.startService(service);  
    }  
  
    @Override  
    public boolean stopService(Intent name) {  
        return mBase.stopService(name);  
    }  
  
    @Override  
    public boolean bindService(Intent service, ServiceConnection conn,  
            int flags) {  
        return mBase.bindService(service, conn, flags);  
    }  
  
    @Override  
    public void unbindService(ServiceConnection conn) {  
        mBase.unbindService(conn);  
    }  
  
    @Override  
    public Object getSystemService(String name) {  
        return mBase.getSystemService(name);  
    }  
  
    ......  
}  

public class ContextThemeWrapper extends ContextWrapper {
    private int mThemeResource;
    private Resources.Theme mTheme;
    private LayoutInflater mInflater;
    private Configuration mOverrideConfiguration;
    private Resources mResources;

    public ContextThemeWrapper() {
        super(null);
    }

    public ContextThemeWrapper(Context base, @StyleRes int themeResId) {
        super(base);
        mThemeResource = themeResId;
    }

    public ContextThemeWrapper(Context base, Resources.Theme theme) {
        super(base);
        mTheme = theme;
    }

    @Override
    protected void attachBaseContext(Context newBase) {
        super.attachBaseContext(newBase);
    }

    /**
     * Call to set an "override configuration" on this context -- this is
     * a configuration that replies one or more values of the standard
     * configuration that is applied to the context.  See
     * {@link Context#createConfigurationContext(Configuration)} for more
     * information.
     *
     * <p>This method can only be called once, and must be called before any
     * calls to {@link #getResources()} are made.
     */
    public void applyOverrideConfiguration(Configuration overrideConfiguration) {
        if (mResources != null) {
            throw new IllegalStateException("getResources() has already been called");
        }
        if (mOverrideConfiguration != null) {
            throw new IllegalStateException("Override configuration has already been set");
        }
        mOverrideConfiguration = new Configuration(overrideConfiguration);
    }

    @Override
    public Resources getResources() {
        if (mResources != null) {
            return mResources;
        }
        if (mOverrideConfiguration == null) {
            mResources = super.getResources();
            return mResources;
        } else {
            Context resc = createConfigurationContext(mOverrideConfiguration);
            mResources = resc.getResources();
            return mResources;
        }
    }
    
    @Override
    public void setTheme(int resid) {
        if (mThemeResource != resid) {
            mThemeResource = resid;
            initializeTheme();
        }
    }
    
    /** @hide */
    @Override
    public int getThemeResId() {
        return mThemeResource;
    }

    @Override public Resources.Theme getTheme() {
        if (mTheme != null) {
            return mTheme;
        }

        mThemeResource = Resources.selectDefaultTheme(mThemeResource,
                getApplicationInfo().targetSdkVersion);
        initializeTheme();

        return mTheme;
    }

    @Override public Object getSystemService(String name) {
        if (LAYOUT_INFLATER_SERVICE.equals(name)) {
            if (mInflater == null) {
                mInflater = LayoutInflater.from(getBaseContext()).cloneInContext(this);
            }
            return mInflater;
        }
        return getBaseContext().getSystemService(name);
    }
    
    /**
     * Called by {@link #setTheme} and {@link #getTheme} to apply a theme
     * resource to the current Theme object.  Can override to change the
     * default (simple) behavior.  This method will not be called in multiple
     * threads simultaneously.
     *
     * @param theme The Theme object being modified.
     * @param resid The theme style resource being applied to <var>theme</var>.
     * @param first Set to true if this is the first time a style is being
     *              applied to <var>theme</var>.
     */
    protected void onApplyThemeResource(Resources.Theme theme, int resid, boolean first) {
        theme.applyStyle(resid, true);
    }

    private void initializeTheme() {
        final boolean first = mTheme == null;
        if (first) {
            mTheme = getResources().newTheme();
            Resources.Theme theme = getBaseContext().getTheme();
            if (theme != null) {
                mTheme.setTo(theme);
            }
        }
        onApplyThemeResource(mTheme, mThemeResource, first);
    }
}