作者：兩日的blog

Context是什么郭计，有什么用

在Android開發(fā)中霞揉，Context是一個抽象類有额，它是Android應(yīng)用程序環(huán)境的一部分竹海。它提供了訪問應(yīng)用程序資源和執(zhí)行各種操作的接口慕蔚。可以說，Context是Android應(yīng)用程序與系統(tǒng)環(huán)境進行交互的橋梁斋配。

Context的作用包括：

1. 訪問應(yīng)用程序資源：通過Context孔飒，可以獲取應(yīng)用程序的資源，如字符串许起、布局文件十偶、圖像等。這些資源可以在應(yīng)用程序的各個組件中使用园细，例如Activity惦积、Service、BroadcastReceiver等猛频。
2. 啟動組件：通過Context狮崩，可以啟動其他組件，如啟動Activity鹿寻、啟動Service睦柴、發(fā)送廣播等。它提供了訪問系統(tǒng)服務(wù)的能力毡熏，如啟動其他應(yīng)用程序坦敌、發(fā)送系統(tǒng)廣播等。
3. 獲取應(yīng)用程序的上下文：通過Context，可以獲取應(yīng)用程序的上下文狱窘，如獲取ApplicationContext杜顺，用于在整個應(yīng)用程序中共享數(shù)據(jù)或執(zhí)行全局操作。
4. 訪問系統(tǒng)服務(wù)：通過Context蘸炸，可以訪問各種系統(tǒng)服務(wù)躬络，如獲取系統(tǒng)級的服務(wù)（如傳感器服務(wù)、位置服務(wù)）搭儒、訪問設(shè)備功能（如攝像頭穷当、存儲器）、執(zhí)行網(wǎng)絡(luò)操作等淹禾。
5. 訪問應(yīng)用程序的文件：通過Context對象馁菜，可以獲取應(yīng)用程序的文件目錄，創(chuàng)建稀拐、讀取火邓、寫入和刪除文件等操作。
6. 處理資源生命周期：通過Context德撬，可以管理應(yīng)用程序資源的生命周期铲咨，如創(chuàng)建、銷毀對象蜓洪、注冊和注銷監(jiān)聽器等纤勒。它提供了一種機制，確保資源的正確使用和釋放隆檀，避免內(nèi)存泄漏等問題摇天。

public abstract AssetManager getAssets();

/**
 * Returns a Resources instance for the application's package.
 * <p>
 * <strong>Note:</strong> Implementations of this method should return
 * a Resources instance that is consistent with the AssetManager instance
 * returned by {@link #getAssets()}. For example, they should share the
 * same {@link Configuration} object.
 *
 * @return a Resources instance for the application's package
 * @see #getAssets()
 */
public abstract Resources getResources();

/** Return PackageManager instance to find global package information. */
public abstract PackageManager getPackageManager();

/** Return a ContentResolver instance for your application's package. */
public abstract ContentResolver getContentResolver();

/**
 * Return the Looper for the main thread of the current process.  This is
 * the thread used to dispatch calls to application components (activities,
 * services, etc).
 * <p>
 * By definition, this method returns the same result as would be obtained
 * by calling {@link Looper#getMainLooper() Looper.getMainLooper()}.
 * </p>
 *
 * @return The main looper.
 */
public abstract Looper getMainLooper();

/**
 * Return an {@link Executor} that will run enqueued tasks on the main
 * thread associated with this context. This is the thread used to dispatch
 * calls to application components (activities, services, etc).
 */
public Executor getMainExecutor() {
    // This is pretty inefficient, which is why ContextImpl overrides it
    return new HandlerExecutor(new Handler(getMainLooper()));
}

 
public abstract Context getApplicationContext();

public final CharSequence getText(@StringRes int resId) {
    return getResources().getText(resId);
}

/**
 * Returns a localized string from the application's package's
 * default string table.
 *
 * @param resId Resource id for the string
 * @return The string data associated with the resource, stripped of styled
 *         text information.
 */
@NonNull
public final String getString(@StringRes int resId) {
    return getResources().getString(resId);
}

/**
 * Returns a localized formatted string from the application's package's
 * default string table, substituting the format arguments as defined in
 * {@link java.util.Formatter} and {@link java.lang.String#format}.
 *
 * @param resId Resource id for the format string
 * @param formatArgs The format arguments that will be used for
 *                   substitution.
 * @return The string data associated with the resource, formatted and
 *         stripped of styled text information.
 */
@NonNull
public final String getString(@StringRes int resId, Object... formatArgs) {
    return getResources().getString(resId, formatArgs);
}

/**
 * Returns a color associated with a particular resource ID and styled for
 * the current theme.
 *
 * @param id The desired resource identifier, as generated by the aapt
 *           tool. This integer encodes the package, type, and resource
 *           entry. The value 0 is an invalid identifier.
 * @return A single color value in the form 0xAARRGGBB.
 * @throws android.content.res.Resources.NotFoundException if the given ID
 *         does not exist.
 */
@ColorInt
public final int getColor(@ColorRes int id) {
    return getResources().getColor(id, getTheme());
}

/**
 * Returns a drawable object associated with a particular resource ID and
 * styled for the current theme.
 *
 * @param id The desired resource identifier, as generated by the aapt
 *           tool. This integer encodes the package, type, and resource
 *           entry. The value 0 is an invalid identifier.
 * @return An object that can be used to draw this resource.
 * @throws android.content.res.Resources.NotFoundException if the given ID
 *         does not exist.
 */
@Nullable
public final Drawable getDrawable(@DrawableRes int id) {
    return getResources().getDrawable(id, getTheme());
}

/**
 * Returns a color state list associated with a particular resource ID and
 * styled for the current theme.
 *
 * @param id The desired resource identifier, as generated by the aapt
 *           tool. This integer encodes the package, type, and resource
 *           entry. The value 0 is an invalid identifier.
 * @return A color state list.
 * @throws android.content.res.Resources.NotFoundException if the given ID
 *         does not exist.
 */
@NonNull
public final ColorStateList getColorStateList(@ColorRes int id) {
    return getResources().getColorStateList(id, getTheme());
}

 /**
 * Set the base theme for this context.  Note that this should be called
 * before any views are instantiated in the Context (for example before
 * calling {@link android.app.Activity#setContentView} or
 * {@link android.view.LayoutInflater#inflate}).
 *
 * @param resid The style resource describing the theme.
 */
public abstract void setTheme(@StyleRes int resid);

/** @hide Needed for some internal implementation...  not public because
 * you can't assume this actually means anything. */
@UnsupportedAppUsage
public int getThemeResId() {
    return 0;
}

/**
 * Return the Theme object associated with this Context.
 */
@ViewDebug.ExportedProperty(deepExport = true)
public abstract Resources.Theme getTheme();

/**
 * Retrieve styled attribute information in this Context's theme.  See
 * {@link android.content.res.Resources.Theme#obtainStyledAttributes(int[])}
 * for more information.
 *
 * @see android.content.res.Resources.Theme#obtainStyledAttributes(int[])
 */
@NonNull
public final TypedArray obtainStyledAttributes(@NonNull @StyleableRes int[] attrs) {
    return getTheme().obtainStyledAttributes(attrs);
}

/**
 * Retrieve styled attribute information in this Context's theme.  See
 * {@link android.content.res.Resources.Theme#obtainStyledAttributes(int, int[])}
 * for more information.
 *
 * @see android.content.res.Resources.Theme#obtainStyledAttributes(int, int[])
 */
@NonNull
public final TypedArray obtainStyledAttributes(@StyleRes int resid,
        @NonNull @StyleableRes int[] attrs) throws Resources.NotFoundException {
    return getTheme().obtainStyledAttributes(resid, attrs);
}

/**
 * Retrieve styled attribute information in this Context's theme.  See
 * {@link android.content.res.Resources.Theme#obtainStyledAttributes(AttributeSet, int[], int, int)}
 * for more information.
 *
 * @see android.content.res.Resources.Theme#obtainStyledAttributes(AttributeSet, int[], int, int)
 */
@NonNull
public final TypedArray obtainStyledAttributes(
        @Nullable AttributeSet set, @NonNull @StyleableRes int[] attrs) {
    return getTheme().obtainStyledAttributes(set, attrs, 0, 0);
}

/**
 * Retrieve styled attribute information in this Context's theme.  See
 * {@link android.content.res.Resources.Theme#obtainStyledAttributes(AttributeSet, int[], int, int)}
 * for more information.
 *
 * @see android.content.res.Resources.Theme#obtainStyledAttributes(AttributeSet, int[], int, int)
 */
@NonNull
public final TypedArray obtainStyledAttributes(@Nullable AttributeSet set,
        @NonNull @StyleableRes int[] attrs, @AttrRes int defStyleAttr,
        @StyleRes int defStyleRes) {
    return getTheme().obtainStyledAttributes(
        set, attrs, defStyleAttr, defStyleRes);
}

總之，Context在Android開發(fā)中具有重要的作用恐仑，它提供了訪問應(yīng)用程序資源泉坐、啟動組件、訪問系統(tǒng)服務(wù)以及處理資源生命周期的能力裳仆。開發(fā)者可以使用Context來實現(xiàn)各種應(yīng)用程序功能和與系統(tǒng)環(huán)境的交互腕让。

Context有哪些

Context 本身是一個抽象類，主要實現(xiàn)類為 ContextImpl歧斟，另外有子類 ContextWrapper 和 ContextThemeWrapper纯丸，另外還有其他由上述三個類引申出來的Context類，Application/Service/Activity静袖，他們的繼承關(guān)系如下：

ContextImpl/ContextWrapper/ContextThemeWrapper的區(qū)別

ContextImpl

上文說到，Context本身是一個抽象類溺蕉，主要的實現(xiàn)類就是ContextImpl伶丐，即Context的那些功能都是在ContexImpl中實現(xiàn)的，即ContextImpl實際承擔(dān)著提供應(yīng)用程序資源訪問疯特、組件啟動和系統(tǒng)服務(wù)等功能的責(zé)任哗魂。

public class ContextImpl extends Context {
    private Resources mResources;
    private Theme mTheme;
   
    void setResources(Resources r) {
        if (r instanceof CompatResources) {
            ((CompatResources) r).setContext(this);
        }
        mResources = r;
    }
    
    @Override
    public Resources getResources() {
        return mResources;
    }
    
    @Override
    public void setTheme(int resId) {
        synchronized (mSync) {
            if (mThemeResource != resId) {
                mThemeResource = resId;
                initializeTheme();
            }
        }
    }
    
    public Resources.Theme getTheme() {
        synchronized (mSync) {
            if (mTheme != null) {
                return mTheme;
            }
    
            mThemeResource = Resources.selectDefaultTheme(mThemeResource,
                    getOuterContext().getApplicationInfo().targetSdkVersion);
            initializeTheme();
    
            return mTheme;
        }
    }
    
    private void initializeTheme() {
        if (mTheme == null) {
            mTheme = mResources.newTheme();
        }
        mTheme.applyStyle(mThemeResource, true);
    }
    
    // 其他方法的實現(xiàn)省略...
}

在ContextImpl，我們重點關(guān)注一下Resource及Theme的相關(guān)實現(xiàn)漓雅，ContextImpl中提供了getResources/setResources方法录别，用于獲取Resources以及設(shè)置Resources，以提供資源的訪問邻吞。

在getTheme/setTheme用于獲取Theme以及設(shè)置Theme组题，以提供對主題的訪問.

重點看一下getTheme()方法，該方法抱冷，會首先獲取mThemeResource崔列，這里直接選擇的系統(tǒng)默認主題，系統(tǒng)會根據(jù)不同的sdk版本選擇不同的默認主題徘层。

@UnsupportedAppUsage
public static int selectDefaultTheme(int curTheme, int targetSdkVersion) {
    return selectSystemTheme(curTheme, targetSdkVersion,
            com.android.internal.R.style.Theme,
            com.android.internal.R.style.Theme_Holo,
            com.android.internal.R.style.Theme_DeviceDefault,
            com.android.internal.R.style.Theme_DeviceDefault_Light_DarkActionBar);
}

/** @hide */
public static int selectSystemTheme(int curTheme, int targetSdkVersion, int orig, int holo,
        int dark, int deviceDefault) {
    if (curTheme != ID_NULL) {
        return curTheme;
    }
    if (targetSdkVersion < Build.VERSION_CODES.HONEYCOMB) {
        return orig;
    }
    if (targetSdkVersion < Build.VERSION_CODES.ICE_CREAM_SANDWICH) {
        return holo;
    }
    if (targetSdkVersion < Build.VERSION_CODES.N) {
        return dark;
    }
    return deviceDefault;
}

通過ContextImpl的實例峻呕，應(yīng)用程序可以獲取到Resources對象和Theme對象，從而實現(xiàn)對資源和主題的訪問和處理趣效。需要注意的是瘦癌，這是一個簡化的，實際的ContextImpl源碼非常復(fù)雜跷敬，還涉及到處理上下文的生命周期讯私、系統(tǒng)服務(wù)的獲取等方面的邏輯。

ContextWrapper

ContextWrapper是一個包裝類，內(nèi)部包含一個mBase成員變量斤寇，所有的實現(xiàn)都是調(diào)用mBase的方法桶癣。

public class ContextWrapper extends Context {
    @UnsupportedAppUsage
    Context mBase;

    public ContextWrapper(Context base) {
        mBase = base;
    }
    
    @Override
    public void setTheme(int resid) {
        mBase.setTheme(resid);
    }
    
    /** @hide */
    @Override
    @UnsupportedAppUsage
    public int getThemeResId() {
        return mBase.getThemeResId();
    }
    
    @Override
    public Resources.Theme getTheme() {
        return mBase.getTheme();
    }
    
    @Override
    public ClassLoader getClassLoader() {
        return mBase.getClassLoader();
    }
    
    @Override
    public String getPackageName() {
        return mBase.getPackageName();
    }
    
 }

ContextThemeWrapper

ContextThemeWrapper繼承自ContextWrapper，從名字中可以看出娘锁，該類主要是跟主題相關(guān)的包裝類：

public class ContextThemeWrapper extends ContextWrapper {
    ...
    
    @Override
    public Resources getResources() {
        return getResourcesInternal();
    }
    
    private Resources getResourcesInternal() {
        if (mResources == null) {
            if (mOverrideConfiguration == null) {
                mResources = super.getResources();
            } else {
                final Context resContext = createConfigurationContext(mOverrideConfiguration);
                mResources = resContext.getResources();
            }
        }
        return mResources;
    }
    
    @Override
    public void setTheme(int resid) {
        if (mThemeResource != resid) {
            mThemeResource = resid;
            initializeTheme();
        }
    }
    
    @Override
    public Resources.Theme getTheme() {
        if (mTheme != null) {
            return mTheme;
        }
    
        mThemeResource = Resources.selectDefaultTheme(mThemeResource,
                getApplicationInfo().targetSdkVersion);
        initializeTheme();
    
        return mTheme;
    }
    
    @UnsupportedAppUsage
    private void initializeTheme() {
        final boolean first = mTheme == null;
        if (first) {
            mTheme = getResources().newTheme();
            final Resources.Theme theme = getBaseContext().getTheme();
            if (theme != null) {
                mTheme.setTo(theme);
            }
        }
        onApplyThemeResource(mTheme, mThemeResource, first);
    }
    
    ...

}

和ContextImpl相比較牙寞，ContextThemeWrapper中獲取資源以及主題的代碼有所不同，多了一個Configuration莫秆，其他行為大致一致间雀。

另外在AppCompat中，默認的主題為Theme_AppCompat_Light镊屎，

package androidx.appcompat.view;

public class ContextThemeWrapper extends ContextWrapper {
    ... 
    @Override
    public Resources.Theme getTheme() {
        if (mTheme != null) {
            return mTheme;
        }
    
        if (mThemeResource == 0) {
            mThemeResource = R.style.Theme_AppCompat_Light;
        }
        initializeTheme();
    
        return mTheme;
    }
}

App中不同Context對象的Theme

我們在開發(fā)中惹挟，經(jīng)常會用到各種Context，常用的有activity/application/applicationContext/baseContext缝驳，為了測試不同Context中Theme對象连锯，我們編寫如下代碼：

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        printLog("baseContext is ${baseContext.themeResId} baseContext is $baseContext")
        printLog("application is ${application.themeResId} application is $application")
        printLog("applicationContext is ${applicationContext.themeResId} applicationContext is $applicationContext")
        printLog("activity is ${this.themeResId}")

    }

    private fun printLog(msg: String) {
        println("MainActivity themeResId in $msg")
    }
}

我們分別獲取每個Context對應(yīng)的themeResId，即每個Context中Theme對應(yīng)的resId：

對代碼運行結(jié)果我們有如下結(jié)論：

1. getApplication和getApplicationContext得到的是同一個Application實例對象用狱；
2. Application對象中的themeResId 為0 运怖，Application其實也有主題的應(yīng)用，畢竟主題樣式都是針對UI元素的齿拂；
3. Activity****中的主題和****getBaseContext****中的主題是不一樣的驳规，具體對應(yīng)什么主題下文將進行探究
4. getBaseContext中得到的是ContextThemeWrapper，這點讓我有點意外署海，之前的理解都是Activity啟動時吗购，會新建一個ContextImpl對象，在attachBaseContext中賦予Activity中的mBase砸狞，于是仔細研究一下發(fā)現(xiàn)捻勉，其實是AppCompatActivity做了替換：

//androidx.appcompat.app.AppCompatActivity
// AppCompatActivity重寫了Activity中的attachBaseContext方法
@Override
protected void attachBaseContext(Context newBase) {
    super.attachBaseContext(getDelegate().attachBaseContext2(newBase));
}

我們看一下代理類AppCompatDelegateImpl中attachBaseContext2的實現(xiàn)：

//androidx.appcompat.app.AppCompatDelegateImpl

@NonNull
@Override
@CallSuper
public Context attachBaseContext2(@NonNull final Context baseContext) {
    mBaseContextAttached = true;

    final int modeToApply = mapNightMode(baseContext, calculateNightMode());

    // If the base context is a ContextThemeWrapper (thus not an Application context)
    // and nobody's touched its Resources yet, we can shortcut and directly apply our
    // override configuration.
    if (sCanApplyOverrideConfiguration
            && baseContext instanceof android.view.ContextThemeWrapper) {
        final Configuration config = createOverrideConfigurationForDayNight(
                baseContext, modeToApply, null);

        ContextThemeWrapperCompatApi17Impl.applyOverrideConfiguration(
                (android.view.ContextThemeWrapper) baseContext, config);
        return baseContext;
        
    }

    // Again, but using the AppCompat version of ContextThemeWrapper.
    if (baseContext instanceof ContextThemeWrapper) {
        final Configuration config = createOverrideConfigurationForDayNight(
                baseContext, modeToApply, null);
        
        ((ContextThemeWrapper) baseContext).applyOverrideConfiguration(config);
        return baseContext;
        
    }

    // We can't apply the configuration directly to the existing base context, so we need to
    // wrap it. We can't create a new configuration context since the app may rely on method
    // overrides or a specific theme -- neither of which are preserved when creating a
    // configuration context. Instead, we'll make a best-effort at wrapping the context and
    // rebasing the original theme.
    if (!sCanReturnDifferentContext) {
        return super.attachBaseContext2(baseContext);
    }

    Configuration configOverlay = null;

    final Configuration config = createOverrideConfigurationForDayNight(
            baseContext, modeToApply, configOverlay);
  
      //重點1：新建ContextThemeWrapper對象將傳入的baseContext賦值給ContextWrapper中的mBase，
      // 并且ContextThemeWrapper中的主題為Theme_AppCompat_Empty
    // Next, we'll wrap the base context to ensure any method overrides or themes are left
    // intact. Since ThemeOverlay.AppCompat theme is empty, we'll get the base context's theme.
    final ContextThemeWrapper wrappedContext = new ContextThemeWrapper(baseContext,
            R.style.Theme_AppCompat_Empty);
    wrappedContext.applyOverrideConfiguration(config);

    // Check whether the base context has an explicit theme or is able to obtain one
    // from its outer context. If it throws an NPE because we're at an invalid point in app
    // initialization, we don't need to worry about rebasing under the new configuration.
    boolean needsThemeRebase;
    try {
        needsThemeRebase = baseContext.getTheme() != null;
    } catch (NullPointerException e) {
        needsThemeRebase = false;
    }

    if (needsThemeRebase) {
        // Attempt to rebase the old theme within the new configuration. This will only
        // work on SDK 23 and up, but it's unlikely that we're keeping the base theme
        // anyway so maybe nobody will notice. Note that calling getTheme() will clone
        // the base context's theme into the wrapped context's theme.
        ResourcesCompat.ThemeCompat.rebase(wrappedContext.getTheme());
    }

    
    return super.attachBaseContext2(wrappedContext);
}

//androidx.appcompat.app.AppCompatDelegate
@NonNull
@CallSuper
public Context attachBaseContext2(@NonNull Context context) {
// 重點2刀森，將上一步包裝了baseContext的ContextThemeWrapper對象進一步賦值給Activity的mBase
    attachBaseContext(context);
    return context;
}

最終AppCompatActivity中的mBase是包裝了ContextImpl的ContextThemeWrapper對象踱启，并且其主題為Theme_AppCompat_Empty

關(guān)于第三點，getBaseActivity和Activity中的主題到底是哪一個研底，我們可以根據(jù)resId和resources索引表resource.arsc（直接將apk文件拖到AndroidStudio中就可以看到該文件）找到：

2131755410和2131755474對應(yīng)16進制為0x7f100192與0x7f1001d2

可以看到埠偿，getBaseActivity和Activity中的主題分別對應(yīng)Theme_AppCompat_Empty 與我們在AndroidManifest.xml中設(shè)置的應(yīng)用主題Theme.ThemeTest

總結(jié)

Context是Android應(yīng)用程序與系統(tǒng)環(huán)境進行交互的橋梁，主要實現(xiàn)類是ContextImpl, 可以訪問應(yīng)用程序資源/啟動組件/訪問系統(tǒng)服務(wù)/訪問應(yīng)用程序的文件等榜晦，而Context可以分為三種：ContextImpl/ContextWrapper/ContextThemeWrapper冠蒋，不同ContextImpl 是Context的主要實現(xiàn)類，ContextWrapper是簡單的包裝類乾胶，所有的實現(xiàn)都由其內(nèi)部的mBase成員完成抖剿，ContextThemeWrapper繼承自ContextWrapper 朽寞，它的主要繼承者是Activity，和其他兩個Context不同的是斩郎，他內(nèi)部對應(yīng)用資源和主題有不同的行為脑融，在應(yīng)用中使用跟主題相關(guān)的Context時，最好使用activity缩宜，而不要使用getBaseContext或者applictaion.