Add a method to read a stream without advancing it.

include/core/SkStream.h

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkStream_DEFINED
 #define SkStream_DEFINED
 
@@ skipping 45 matching lines @@
      */
     virtual size_t read(void* buffer, size_t size) = 0;
 
     /** Skip size number of bytes.
      *  @return the actual number bytes that could be skipped.
      */
     size_t skip(size_t size) {
         return this->read(NULL, size);
     }
 
+    /**
+     *  Attempt to peek at size bytes.
+     *  If this stream supports peeking, and it can peek size bytes, copy size
+     *  bytes into buffer, and return true.
+     *  If the stream does not support peeking, or cannot peek size bytes,
+     *  return false and leave buffer unchanged.
+     *  The stream is guaranteed to be in the same visible state after this
+     *  call, regardless of success or failure.
+     *  @param buffer Must not be NULL. Destination to copy bytes.

[scroggo, 2015/04/02 18:04:53]

Alternatively, we could return false if the buffer is NULL. (Though we cannot
check if the buffer is long enough...)

+     *  @param size Number of bytes to copy.
+     *  @return Whether the peek was performed.
+     */
+    virtual bool peek(void* buffer, size_t size) const { return false; }
+
     /** Returns true when all the bytes in the stream have been read.
      *  This may return true early (when there are no more bytes to be read)
      *  or late (after the first unsuccessful read).
      */
     virtual bool isAtEnd() const = 0;
 
     int8_t   readS8();
     int16_t  readS16();
     int32_t  readS32();
 
@@ skipping 236 matching lines @@
      *  The function returns the data parameter as a convenience.
      */
     SkData* setData(SkData*);
 
     void skipToAlign4();
     const void* getAtPos();
 
     size_t read(void* buffer, size_t size) override;
     bool isAtEnd() const override;
 
+    bool peek(void* buffer, size_t size) const override;
+
     bool rewind() override;
     SkMemoryStream* duplicate() const override;
 
     size_t getPosition() const override;
     bool seek(size_t position) override;
     bool move(long offset) override;
     SkMemoryStream* fork() const override;
 
     size_t getLength() const override;
 
@@ skipping 105 matching lines @@
 
 private:
     size_t fBytesWritten;
     typedef SkWStream INHERITED;
 };
 
 // for now
 typedef SkFILEStream SkURLStream;
 
 #endif