Android：Toast源碼分析

本來想在一篇文章中吧Toast的用法，原理以及如何自定義都跟大家說說的，但是限於篇幅，還是分開寫了，這裏主要是介紹Toast的原理。

如果大家對SDK中提供的Toast用法不是很熟悉，請移步我的上一篇blog 《Android：Toast的用法》

說句題外話，看源碼真是個不錯的習慣，大熱天一下心就哇涼哇涼的，看不懂啊，自嘲一下。

書歸正傳：上代碼

Toast.makeText(MainActivity.this, "歡迎光臨阿東的博客", Toast.LENGTH_LONG).show();

makeText的源碼：

  /**
     * Make a standard toast that just contains a text view.
     *
     * @param context  The context to use.  Usually your {@link android.app.Application}
     *                 or {@link android.app.Activity} object.
     * @param text     The text to show.  Can be formatted text.
     * @param duration How long to display the message.  Either {@link #LENGTH_SHORT} or
     *                 {@link #LENGTH_LONG}
     *
     */
    public static Toast makeText(Context context, CharSequence text, int duration) {
        Toast result = new Toast(context);

        LayoutInflater inflate = (LayoutInflater)
                context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        View v = inflate.inflate(com.android.internal.R.layout.transient_notification, null);
        TextView tv = (TextView)v.findViewById(com.android.internal.R.id.message);
        tv.setText(text);

        result.mNextView = v;
        result.mDuration = duration;

        return result;
    }

很簡單，只是實例化一個Toast，設置了文本和時間，在把Toast當做返回值返回。
看到這，下一步看show()是怎麼實現的。

    /**
     * Show the view for the specified duration.
     */
    public void show() {
        if (mNextView == null) {
            throw new RuntimeException("setView must have been called");
        }

        INotificationManager service = getService();
        String pkg = mContext.getPackageName();
        TN tn = mTN;
        tn.mNextView = mNextView;

        try {
            service.enqueueToast(pkg, tn, mDuration);
        } catch (RemoteException e) {
            // Empty
        }
    }

這裏先是做了一個判斷，如果沒有對mNextView 賦值，拋出一個異常。然後是獲取一個服務：INotificationManager，然後調用service.enqueueToast(pkg, tn, mDuration)，將Toast放到一個隊列裏面顯示。這裏enqueueToast方法傳入一個TN，下面看一下TN是個什麼東西。

    private static class TN extends ITransientNotification.Stub {
        final Runnable mShow = new Runnable() {
            @Override
            public void run() {
                handleShow();
            }
        };

        final Runnable mHide = new Runnable() {
            @Override
            public void run() {
                handleHide();
                // Don't do this in handleHide() because it is also invoked by handleShow()
                mNextView = null;
            }
        };

        private final WindowManager.LayoutParams mParams = new WindowManager.LayoutParams();
        final Handler mHandler = new Handler();    

        int mGravity;
        int mX, mY;
        float mHorizontalMargin;
        float mVerticalMargin;


        View mView;
        View mNextView;

        WindowManager mWM;

        TN() {
            // XXX This should be changed to use a Dialog, with a Theme.Toast
            // defined that sets up the layout params appropriately.
            final WindowManager.LayoutParams params = mParams;
            params.height = WindowManager.LayoutParams.WRAP_CONTENT;
            params.width = WindowManager.LayoutParams.WRAP_CONTENT;
            params.format = PixelFormat.TRANSLUCENT;
            params.windowAnimations = com.android.internal.R.style.Animation_Toast;
            params.type = WindowManager.LayoutParams.TYPE_TOAST;
            params.setTitle("Toast");
            params.flags = WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON
                    | WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE
                    | WindowManager.LayoutParams.FLAG_NOT_TOUCHABLE;
        }

        /**
         * schedule handleShow into the right thread
         */
        @Override
        public void show() {
            if (localLOGV) Log.v(TAG, "SHOW: " + this);
            mHandler.post(mShow);
        }

        /**
         * schedule handleHide into the right thread
         */
        @Override
        public void hide() {
            if (localLOGV) Log.v(TAG, "HIDE: " + this);
            mHandler.post(mHide);
        }

        public void handleShow() {
            if (localLOGV) Log.v(TAG, "HANDLE SHOW: " + this + " mView=" + mView
                    + " mNextView=" + mNextView);
            if (mView != mNextView) {
                // remove the old view if necessary
                handleHide();
                mView = mNextView;
                Context context = mView.getContext().getApplicationContext();
                if (context == null) {
                    context = mView.getContext();
                }
                mWM = (WindowManager)context.getSystemService(Context.WINDOW_SERVICE);
                // We can resolve the Gravity here by using the Locale for getting
                // the layout direction
                final Configuration config = mView.getContext().getResources().getConfiguration();
                final int gravity = Gravity.getAbsoluteGravity(mGravity, config.getLayoutDirection());
                mParams.gravity = gravity;
                if ((gravity & Gravity.HORIZONTAL_GRAVITY_MASK) == Gravity.FILL_HORIZONTAL) {
                    mParams.horizontalWeight = 1.0f;
                }
                if ((gravity & Gravity.VERTICAL_GRAVITY_MASK) == Gravity.FILL_VERTICAL) {
                    mParams.verticalWeight = 1.0f;
                }
                mParams.x = mX;
                mParams.y = mY;
                mParams.verticalMargin = mVerticalMargin;
                mParams.horizontalMargin = mHorizontalMargin;
                if (mView.getParent() != null) {
                    if (localLOGV) Log.v(TAG, "REMOVE! " + mView + " in " + this);
                    mWM.removeView(mView);
                }
                if (localLOGV) Log.v(TAG, "ADD! " + mView + " in " + this);
                mWM.addView(mView, mParams);
                trySendAccessibilityEvent();
            }
        }

        private void trySendAccessibilityEvent() {
            AccessibilityManager accessibilityManager =
                    AccessibilityManager.getInstance(mView.getContext());
            if (!accessibilityManager.isEnabled()) {
                return;
            }
            // treat toasts as notifications since they are used to
            // announce a transient piece of information to the user
            AccessibilityEvent event = AccessibilityEvent.obtain(
                    AccessibilityEvent.TYPE_NOTIFICATION_STATE_CHANGED);
            event.setClassName(getClass().getName());
            event.setPackageName(mView.getContext().getPackageName());
            mView.dispatchPopulateAccessibilityEvent(event);
            accessibilityManager.sendAccessibilityEvent(event);
        }        

        public void handleHide() {
            if (localLOGV) Log.v(TAG, "HANDLE HIDE: " + this + " mView=" + mView);
            if (mView != null) {
                // note: checking parent() just to make sure the view has
                // been added...  i have seen cases where we get here when
                // the view isn't yet added, so let's try not to crash.
                if (mView.getParent() != null) {
                    if (localLOGV) Log.v(TAG, "REMOVE! " + mView + " in " + this);
                    mWM.removeView(mView);
                }

                mView = null;
            }
        }
    }

乍一看，代碼很長，一點一點分析，還是很容易能看明白的。

我們發現TN繼承了ITransientNotification.Stub，這個類的形式大家應該都有印象吧，沒有也沒關係，這個是android中AIDL（進程間通信）的寫法，這個類會對應有一個ITransientNotification.aidl的文件（注意後綴aidl）

package android.app;  

/** @hide */  
oneway interface ITransientNotification {  
    void show();  
    void hide();  
}

定義了兩個接口 show() 和 hide()，我們來看一下TN中的實現。

        /**
         * schedule handleShow into the right thread
         */
        @Override
        public void show() {
            if (localLOGV) Log.v(TAG, "SHOW: " + this);
            mHandler.post(mShow);
        }

        /**
         * schedule handleHide into the right thread
         */
        @Override
        public void hide() {
            if (localLOGV) Log.v(TAG, "HIDE: " + this);
            mHandler.post(mHide);
        }

這裏發現 Toast中用的是 handler機制。分別post一個mShow和一個mHide。我們來看看這兩個Runnable是怎麼實現的。

        final Runnable mShow = new Runnable() {
            @Override
            public void run() {
                handleShow();
            }
        };

        final Runnable mHide = new Runnable() {
            @Override
            public void run() {
                handleHide();
                // Don't do this in handleHide() because it is also invoked by handleShow()
                mNextView = null;
            }
        };

下一步肯定是看handleShow()和handleHide()的實現了。

    public void handleShow() {
            if (localLOGV) Log.v(TAG, "HANDLE SHOW: " + this + " mView=" + mView
                    + " mNextView=" + mNextView);
            if (mView != mNextView) {
                // remove the old view if necessary
                handleHide();
                mView = mNextView;
                Context context = mView.getContext().getApplicationContext();
                if (context == null) {
                    context = mView.getContext();
                }
                mWM = (WindowManager)context.getSystemService(Context.WINDOW_SERVICE);
                // We can resolve the Gravity here by using the Locale for getting
                // the layout direction
                final Configuration config = mView.getContext().getResources().getConfiguration();
                final int gravity = Gravity.getAbsoluteGravity(mGravity, config.getLayoutDirection());
                mParams.gravity = gravity;
                if ((gravity & Gravity.HORIZONTAL_GRAVITY_MASK) == Gravity.FILL_HORIZONTAL) {
                    mParams.horizontalWeight = 1.0f;
                }
                if ((gravity & Gravity.VERTICAL_GRAVITY_MASK) == Gravity.FILL_VERTICAL) {
                    mParams.verticalWeight = 1.0f;
                }
                mParams.x = mX;
                mParams.y = mY;
                mParams.verticalMargin = mVerticalMargin;
                mParams.horizontalMargin = mHorizontalMargin;
                if (mView.getParent() != null) {
                    if (localLOGV) Log.v(TAG, "REMOVE! " + mView + " in " + this);
                    mWM.removeView(mView);
                }
                if (localLOGV) Log.v(TAG, "ADD! " + mView + " in " + this);
                mWM.addView(mView, mParams);
                trySendAccessibilityEvent();
            }
        }

    public void handleHide() {
            if (localLOGV) Log.v(TAG, "HANDLE HIDE: " + this + " mView=" + mView);
            if (mView != null) {
                // note: checking parent() just to make sure the view has
                // been added...  i have seen cases where we get here when
                // the view isn't yet added, so let's try not to crash.
                if (mView.getParent() != null) {
                    if (localLOGV) Log.v(TAG, "REMOVE! " + mView + " in " + this);
                    mWM.removeView(mView);
                }

                mView = null;
            }
        }

到這裏，我們發現原來Toast是用WindowManager的addView和removeView實現的。
其中TN的構造方法中設置了WindowManager.LayoutParams ，然後在handleShow（）方法中，把我們設置的參數複製個Params，然後顯示出來。

到這裏，基本我就已經分析完了。

其實Toast的原理是這樣的，先通過makeText()實例化出一個Toast，然後調用toast.Show()方法，這時並不會馬上顯示Toast，而是通過 AIDL（進程間通信）加到 SystemService進程中的一個隊列然後通過TN的兩個預留接口來控制Toast的顯示和隱藏。在TN中進行調控Toast的顯示格式以及裏面的hide()、show()方法來控制Toast的出現以及消失，強調一下的是這個隊列是系統維護的，我們並不能干涉。

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