Add master audio balance setting

This adds an audio balance slider to accessibility allowing the end
user to adjust the audio balance between left and right channels.

Test: Change Balance through Settings, play audio
Bug: 28390736
Change-Id: I70f22d222ea310fd33ebe4f3fb054f44f69d2548

src/com/android/settings/accessibility/BalanceSeekBar.java

@@ -0,0 +1,152 @@
+/*
+ * Copyright (C) 2019 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.android.settings.accessibility;
+
+import android.content.Context;
+import android.content.res.ColorStateList;
+import android.content.res.Resources;
+import android.graphics.Canvas;
+import android.graphics.Color;
+import android.graphics.Paint;
+import android.graphics.Rect;
+import android.os.UserHandle;
+import android.provider.Settings;
+import android.util.AttributeSet;
+import android.widget.SeekBar;
+
+import com.android.settings.R;
+
+/**
+ * A custom seekbar for the balance setting.
+ *
+ * Adds a center line indicator between left and right, which snaps to if close.
+ * Updates Settings.System for balance on progress changed.
+ */
+public class BalanceSeekBar extends SeekBar {
+    private static final String TAG = "BalanceSeekBar";
+    private final Context mContext;
+    private final Object mListenerLock = new Object();
+    private OnSeekBarChangeListener mOnSeekBarChangeListener;
+    private final OnSeekBarChangeListener mProxySeekBarListener = new OnSeekBarChangeListener() {
+        @Override
+        public void onStopTrackingTouch(SeekBar seekBar) {
+            synchronized(mListenerLock) {
+                if (mOnSeekBarChangeListener != null) {
+                    mOnSeekBarChangeListener.onStopTrackingTouch(seekBar);
+                }
+            }
+        }
+
+        @Override
+        public void onStartTrackingTouch(SeekBar seekBar) {
+            synchronized(mListenerLock) {
+                if (mOnSeekBarChangeListener != null) {
+                    mOnSeekBarChangeListener.onStartTrackingTouch(seekBar);
+                }
+            }
+        }
+
+        @Override
+        public void onProgressChanged(SeekBar seekBar, int progress, boolean fromUser) {
+            if (fromUser) {
+                // Snap to centre when within the specified threshold
+                if (progress != mCenter
+                        && progress > mCenter - mSnapThreshold
+                        && progress < mCenter + mSnapThreshold) {
+                    progress = mCenter;
+                    seekBar.setProgress(progress); // direct update (fromUser becomes false)
+                }
+                final float balance = (progress - mCenter) * 0.01f;
+                Settings.System.putFloatForUser(mContext.getContentResolver(),
+                        Settings.System.MASTER_BALANCE, balance, UserHandle.USER_CURRENT);
+            }
+            // If fromUser is false, the call is a set from the framework on creation or on
+            // internal update. The progress may be zero, ignore (don't change system settings).
+
+            // after adjusting the seekbar, notify downstream listener.
+            // note that progress may have been adjusted in the code above to mCenter.
+            synchronized(mListenerLock) {
+                if (mOnSeekBarChangeListener != null) {
+                    mOnSeekBarChangeListener.onProgressChanged(seekBar, progress, fromUser);
+                }
+            }
+        }
+    };
+
+    // Percentage of max to be used as a snap to threshold
+    private static final float SNAP_TO_PERCENTAGE = 0.03f;
+    private final Paint mCenterMarkerPaint;
+    private final Rect mCenterMarkerRect;
+    // changed in setMax()
+    private float mSnapThreshold;
+    private int mCenter;
+
+    public BalanceSeekBar(Context context, AttributeSet attrs) {
+        this(context, attrs, com.android.internal.R.attr.seekBarStyle);
+    }
+
+    public BalanceSeekBar(Context context, AttributeSet attrs, int defStyleAttr) {
+        this(context, attrs, defStyleAttr, 0 /* defStyleRes */);
+    }
+
+    public BalanceSeekBar(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
+        super(context, attrs, defStyleAttr, defStyleRes);
+        mContext = context;
+        Resources res = getResources();
+        mCenterMarkerRect = new Rect(0 /* left */, 0 /* top */,
+                res.getDimensionPixelSize(R.dimen.balance_seekbar_center_marker_width),
+                res.getDimensionPixelSize(R.dimen.balance_seekbar_center_marker_height));
+        mCenterMarkerPaint = new Paint();
+        // TODO use a more suitable colour?
+        mCenterMarkerPaint.setColor(Color.BLACK);
+        mCenterMarkerPaint.setStyle(Paint.Style.FILL);
+        // Remove the progress colour
+        setProgressTintList(ColorStateList.valueOf(Color.TRANSPARENT));
+
+        super.setOnSeekBarChangeListener(mProxySeekBarListener);
+    }
+
+    @Override
+    public void setOnSeekBarChangeListener(OnSeekBarChangeListener listener) {
+        synchronized(mListenerLock) {
+            mOnSeekBarChangeListener = listener;
+        }
+    }
+
+    // Note: the superclass AbsSeekBar.setMax is synchronized.
+    @Override
+    public synchronized void setMax(int max) {
+        super.setMax(max);
+        // update snap to threshold
+        mCenter = max / 2;
+        mSnapThreshold = max * SNAP_TO_PERCENTAGE;
+    }
+
+    // Note: the superclass AbsSeekBar.onDraw is synchronized.
+    @Override
+    protected synchronized void onDraw(Canvas canvas) {
+        // Draw a vertical line at 50% that represents centred balance
+        int seekBarCenter = (canvas.getHeight() - getPaddingBottom()) / 2;
+        canvas.save();
+        canvas.translate((canvas.getWidth() - mCenterMarkerRect.right) / 2,
+                seekBarCenter - (mCenterMarkerRect.bottom / 2));
+        canvas.drawRect(mCenterMarkerRect, mCenterMarkerPaint);
+        canvas.restore();
+        super.onDraw(canvas);
+    }
+}
+